Topographical Anatomy and the Appendicular Skeleton

Every palpation, every orthopedic test and every film you read rests on a shared map of the body: its planes, its regions, its surface landmarks and the bones and joints of the limbs. General anatomy carries a fifth of the Part I weight, and the appendicular skeleton supplies a large share of those items because it is so easy to test with a single landmark, a single fracture or a single ligament. Learn this chapter well and the muscles, nerves and vessels of the next chapter will have somewhere to attach.

Section 1Planes, terminology and the body's regions

The anatomical position and the cardinal planes

All anatomical description assumes the anatomical position: the body stands erect, the head and eyes face forward, the upper limbs hang at the sides with the palms facing anteriorly, and the feet point forward and sit slightly apart. Whatever position a patient actually occupies on the table, terms such as anterior, medial and proximal are always read as if the patient stood this way. That is why the radius is lateral and the thumb is lateral, even when the forearm is pronated.

Three planes, each at right angles to the other two, slice the body. A sagittal plane is vertical and runs from front to back, dividing right from left. The single sagittal plane through the midline is the median (midsagittal) plane; any other is a parasagittal plane. A frontal (coronal) plane is also vertical but runs from side to side, dividing anterior from posterior. A transverse (horizontal, axial) plane divides superior from inferior; this is the plane of the axial CT and MRI slice, conventionally viewed from the patient's feet so the patient's right lies on the viewer's left.

Each plane has a perpendicular axis about which movement in that plane occurs. Flexion and extension happen in the sagittal plane about a coronal (frontal, mediolateral) axis. Abduction and adduction happen in the frontal plane about a sagittal (anteroposterior) axis. Rotation happens in the transverse plane about a vertical (longitudinal) axis.

Terms of direction and movement

Directional terms come in opposing pairs: superior and inferior (cranial and caudal), anterior and posterior (ventral and dorsal), medial and lateral, proximal and distal, superficial and deep, and ipsilateral and contralateral. Palmar (volar) and plantar replace anterior and inferior in the hand and foot.

Movement terms pair the same way. Flexion decreases the angle between segments in the sagittal plane and extension increases it; at the ankle these are dorsiflexion and plantarflexion, and the thumb flexes in the frontal plane because it sits rotated 90 degrees from the fingers. Abduction moves a part away from the midline (for fingers, away from the middle finger; for toes, away from the second toe) and adduction returns it. Medial and lateral rotation turn a limb about its long axis, and circumduction traces a cone. Pronation crosses the radius over the ulna to turn the palm back; supination uncrosses it. Inversion and eversion turn the sole medially and laterally. The scapula adds elevation, depression, protraction, retraction and upward and downward rotation, and the thumb adds opposition.

Regions and surface landmarks

Topographical anatomy links what you can see and feel at the surface to what lies beneath. The board expects you to know the vertebral level of the commonly used landmarks, because those levels let you locate viscera, vessels and spinal segments from the outside.

Head and neck. The external occipital protuberance (inion) marks the midline at the back of the skull, and the mastoid process lies behind the ear. The sternocleidomastoid divides the neck into an anterior triangle (bounded by the midline, the mandible and the anterior border of sternocleidomastoid; subdivided into submental, submandibular, carotid and muscular triangles) and a posterior triangle (bounded by sternocleidomastoid, trapezius and the middle third of the clavicle; it contains the spinal accessory nerve, the trunks of the brachial plexus and the third part of the subclavian artery). In the anterior neck, the hyoid lies at C3, the thyroid cartilage at C4 to C5 (the common carotid bifurcates at its upper border, about C4), and the cricoid cartilage at C6, where the larynx becomes the trachea and the pharynx becomes the esophagus. The anterior tubercle of the C6 transverse process (the carotid tubercle) lies deep to the cricoid.

Back. The spinous process of C7 (vertebra prominens) is usually the first clearly palpable spinous process, though T1 is often equally prominent; flexion and extension of the neck help separate them because C7 glides forward under the palpating finger with extension while T1 stays fixed. With the arms at the sides, the root of the spine of the scapula lies at T3, the inferior angle at T7 and the superior angle at T2. A line joining the highest points of the iliac crests (the supracristal or intercristal line) crosses the L4 spinous process or the L4 to L5 interspace, the classic landmark for lumbar puncture. The skin dimples over the posterior superior iliac spines mark S2, the level of the termination of the dural sac and the middle of the sacroiliac joints.

Thorax. The sternal angle (angle of Louis), where the manubrium meets the body of the sternum, is the most useful landmark in the chest: the second costal cartilage articulates here, so rib counting starts here, and a horizontal plane through it reaches the T4 to T5 disc. That plane marks the tracheal bifurcation, the beginning and end of the aortic arch, the junction of the superior and inferior mediastinum and the entry of the azygos vein into the superior vena cava. The jugular (suprasternal) notch lies at T2 to T3, the xiphisternal junction at T9, and the male nipple at the fourth intercostal space, in the T4 dermatome. The apex beat lies in the left fifth intercostal space at the midclavicular line.

Abdomen. Two vertical midclavicular lines and two horizontal planes (subcostal above, intertubercular below) divide the abdomen into nine regions: right and left hypochondriac, epigastric, right and left lumbar (lateral), umbilical, right and left inguinal (iliac) and hypogastric (pubic). Clinicians more often use four quadrants. The transpyloric plane, halfway between the jugular notch and the pubic symphysis, lies at L1 and crosses the pylorus, the neck of the pancreas, the duodenojejunal flexure, the hila of the kidneys, the origin of the superior mesenteric artery and approximately the end of the spinal cord. The subcostal plane (lowest point of the tenth costal cartilage) lies at L3. The umbilicus lies at the L3 to L4 disc in a slim adult and sits in the T10 dermatome. The aorta bifurcates at L4, just below and to the left of the umbilicus. McBurney point lies one third of the way from the anterior superior iliac spine to the umbilicus and overlies the usual base of the appendix.

Pelvis and perineum. The anterior superior iliac spine (ASIS), the iliac crest and its tubercle, the pubic tubercle and the pubic symphysis are palpable anteriorly; the inguinal ligament runs from ASIS to pubic tubercle. Posteriorly the ischial tuberosities bear weight in sitting and the coccyx can be felt in the natal cleft. The diamond-shaped perineum is bounded by the pubic symphysis, the ischiopubic rami, the ischial tuberosities, the sacrotuberous ligaments and the coccyx. A line between the ischial tuberosities divides it into an anterior urogenital triangle and a posterior anal triangle.

Section 2The integument

The skin is the body's largest organ, roughly 15 percent of body weight and close to two square meters in an adult. Together with its appendages (hair, nails and glands) it forms the integumentary system. It has two layers of its own, the epithelial epidermis and the connective tissue dermis, and rests on the hypodermis (subcutaneous tissue, superficial fascia), which anatomists do not count as skin but which shares its blood supply and receptors.

The epidermis

The epidermis is keratinized stratified squamous epithelium. It is avascular and is nourished by diffusion from capillary loops in the dermal papillae, which is why a shallow scrape that does not bleed has not reached the dermis. Cells are born in the deepest layer and migrate toward the surface over roughly four weeks, flattening, filling with keratin and finally dying.

Four cell types matter. Keratinocytes are the majority and produce keratin. Melanocytes, derived from neural crest, sit in the basal layer and transfer melanin in melanosomes to surrounding keratinocytes, where it caps the nucleus against ultraviolet damage; people of all skin tones have similar numbers of melanocytes, and differences in color come from the amount, type and distribution of melanin. Langerhans cells are bone marrow derived antigen presenting cells. Merkel cells pair with sensory nerve endings to form slowly adapting touch receptors.

Thick skin (palms and soles) has a stratum lucidum and many sweat glands but no hair; thin skin carries hair follicles and sebaceous glands.

The dermis and hypodermis

The dermis has two layers. The superficial papillary dermis is loose areolar connective tissue that projects upward as dermal papillae, interlocking with epidermal ridges to resist shear; it holds capillary loops and Meissner corpuscles. The deeper and much thicker reticular dermis is dense irregular connective tissue rich in type I collagen and elastic fibers; it gives skin its strength and elasticity and houses hair follicles, glands, larger vessels and nerves. The predominant orientation of collagen bundles produces cleavage (tension) lines, often called Langer lines; incisions made parallel to them gape less and scar more neatly.

The hypodermis is loose connective tissue and adipose tissue that anchors skin to deep fascia, insulates and stores energy. It contains the deep Pacinian (lamellated) corpuscles and the superficial veins and cutaneous nerves that clinicians meet during venipuncture and nerve blocks.

Appendages

Hair grows from a follicle, an epidermal down-growth into the dermis. The hair bulb at its base surrounds a vascular dermal papilla, and its matrix cells divide to form the shaft. The arrector pili is a band of smooth muscle from the papillary dermis to the follicle; its contraction, driven by sympathetic fibers acting on alpha-1 receptors, raises the hair and dimples the skin ("goose bumps").

Sebaceous glands open into hair follicles and secrete sebum by holocrine secretion (the whole cell disintegrates). They are androgen sensitive, which explains acne at puberty. Eccrine sweat glands are coiled tubular glands distributed over almost the entire body and densest on palms, soles and forehead; they open directly onto the skin, secrete a watery fluid by merocrine exocytosis and are key to thermoregulation. Their innervation is a classic exception: they are supplied by sympathetic postganglionic fibers that release acetylcholine onto muscarinic receptors. Apocrine sweat glands in the axilla, areola and anogenital region open into hair follicles, become active at puberty and secrete a protein-rich product that bacteria break down into body odor.

Nails are plates of hard keratin made by the nail matrix, visible as the pale lunula. Clubbing, spooning, pitting and splinter hemorrhages are findings worth noticing on every hand you touch.

Cutaneous receptors

The skin surface and the spinal nerves

Every patch of skin is supplied by a spinal (or trigeminal) nerve, so the skin surface is a readable map of the spinal cord. A dermatome is the area of skin supplied by the sensory fibers of a single spinal nerve root. Dermatomes overlap considerably, so section of one root alone produces a smaller deficit than the map suggests; testing the center of the dermatome (the autonomous zone) is most reliable. A dermatome is not the same as the territory of a peripheral nerve: the median nerve, for example, carries fibers from C6, C7 and C8, and a lesion of it produces a peripheral nerve pattern, not a dermatomal band. Distinguishing the two patterns is the heart of the neurologic exam.

Skin as an organ

The skin is a barrier (physical, chemical and immunologic), a thermoregulator through sweating and sympathetically controlled dermal blood flow, a sensory organ, and an endocrine organ: ultraviolet B converts 7-dehydrocholesterol to cholecalciferol (vitamin D3), which the liver and kidney then hydroxylate.

Burns illustrate depth: superficial (epidermis, red and painful), partial thickness (into dermis, blistered) and full thickness (dermis and nerve endings destroyed, leathery and painless). The adult rule of nines gives 9 percent to the head, 9 to each upper limb, 18 to each side of the trunk, 18 to each lower limb and 1 to the perineum.

Section 3Osteology of the upper limb

Each upper limb contains 32 bones: the pectoral girdle (clavicle and scapula), the humerus, the radius and ulna, eight carpals, five metacarpals and fourteen phalanges. The girdle attaches to the axial skeleton only at the sternoclavicular joint; everything else is held by muscle. That design buys a huge range of motion at the price of stability, a trade-off that explains most shoulder pathology.

Clavicle

The clavicle is an S-shaped strut, convex forward in its medial two thirds and concave forward in its lateral third. It holds the limb away from the trunk and transmits force to the axial skeleton. The underside carries the conoid tubercle and trapezoid line for the two parts of the coracoclavicular ligament, and a groove for subclavius. The clavicle is the first bone to begin ossifying (fifth to sixth fetal week), largely by intramembranous ossification, and its medial epiphysis is the last to fuse, in the mid twenties. It is the most commonly fractured bone in the body, usually at the junction of its middle and lateral thirds after a fall on the shoulder or outstretched hand. Sternocleidomastoid lifts the medial fragment while the limb drags the lateral fragment down.

Scapula

The scapula is a flat triangular bone lying over ribs 2 to 7. The subscapular fossa faces the ribs; posteriorly the spine separates a small supraspinous fossa from a larger infraspinous fossa; the spine ends laterally as the acromion, the bony roof of the shoulder. The hooked coracoid process projects anteriorly and gives attachment to pectoralis minor, coracobrachialis and the short head of biceps, as well as the coracoclavicular, coracoacromial and coracohumeral ligaments. The shallow glenoid cavity faces laterally and slightly upward and forward; above it the supraglenoid tubercle anchors the long head of biceps, and below it the infraglenoid tubercle anchors the long head of triceps. The superior border carries the suprascapular notch, bridged by the superior transverse scapular ligament: the suprascapular artery passes above the ligament and the suprascapular nerve passes below it. The spinoglenoid notch at the lateral base of the spine is a second entrapment point where only infraspinatus is affected.

Humerus

The anatomical neck is the groove just around the head, where the capsule attaches; the surgical neck is the narrower region below the tubercles and is the common fracture site. The greater tubercle lies laterally and receives supraspinatus, infraspinatus and teres minor; the lesser tubercle lies anteriorly and receives subscapularis. Between them the intertubercular (bicipital) groove carries the tendon of the long head of biceps under the transverse humeral ligament, with pectoralis major inserting on its lateral lip, teres major on its medial lip and latissimus dorsi in its floor. Midshaft laterally lies the deltoid tuberosity, and posteriorly the radial (spiral) groove carries the radial nerve and profunda brachii artery.

Distally the bone flattens into medial and lateral supracondylar ridges ending in the medial epicondyle (prominent; common flexor origin; the ulnar nerve runs in a groove behind it) and the lateral epicondyle (common extensor origin). The articular surfaces are the rounded capitulum laterally for the radial head and the spool-shaped trochlea medially for the ulna. The coronoid and radial fossae (anterior) and olecranon fossa (posterior) receive processes in flexion and extension.

Radius and ulna

The ulna is the stabilizing bone of the elbow and is larger proximally. Its olecranon forms the point of the elbow and receives triceps; the coronoid process projects anteriorly below it and receives brachialis at the ulnar tuberosity just below; the trochlear notch between them grips the trochlea; and the radial notch on the lateral side of the coronoid receives the radial head. Distally the small ulnar head and styloid process are separated from the carpus by the triangular fibrocartilage.

The radius is the rotating bone and is larger distally. Its disc-shaped head articulates with the capitulum above and spins within the annular ligament against the radial notch. Below the neck lies the radial tuberosity for biceps. Distally the broad end carries the styloid process (which reaches about 1 cm further distally than the ulnar styloid), the ulnar notch for the distal radioulnar joint, and the dorsal (Lister) tubercle, around which the extensor pollicis longus tendon turns. The interosseous membrane transfers load from the radius to the ulna.

Two eponymous distal radius fractures are favorites. A Colles fracture follows a fall on an outstretched hand, usually in an older woman with osteoporosis; the distal fragment is displaced dorsally and radially, producing the "dinner fork" deformity. A Smith fracture is the reverse, with volar displacement after a fall on a flexed wrist.

Carpals, metacarpals and phalanges

The eight carpals sit in two rows. From lateral (thumb side) to medial, the proximal row is the scaphoid, lunate, triquetrum and pisiform, and the distal row is the trapezium, trapezoid, capitate and hamate. A common memory aid is "Some Lovers Try Positions That They Can't Handle." The proximal row (minus the pisiform) articulates with the radius and the articular disc; the distal row articulates with the metacarpals. The pisiform is a sesamoid bone within the flexor carpi ulnaris tendon. The capitate is the largest carpal and the first to ossify (about age 1); the pisiform is the last (about age 9 to 12).

The five metacarpals each have a base, shaft and head; the heads form the knuckles. The thumb has two phalanges and each finger three (proximal, middle and distal), giving fourteen.

Palpable landmarks of the upper limb

Palpable landmarks include the clavicle, acromion and AC joint line, the coracoid in the deltopectoral groove, the scapular spine, medial border and inferior angle, the greater tubercle below the acromion, the intertubercular groove during rotation, the epicondyles and olecranon (a straight line in extension and a triangle in flexion; a disrupted relationship suggests dislocation), the radial head spinning distal to the lateral epicondyle, both styloids, Lister tubercle, the scaphoid in the snuffbox, the pisiform and the hook of the hamate.

Section 4Osteology of the lower limb

Each lower limb has 31 bones: the hip bone (os coxae), femur, patella, tibia, fibula, seven tarsals, five metatarsals and fourteen phalanges. Where the upper limb is built for reach and dexterity, the lower limb is built for weight bearing and locomotion, so its bones are heavier, its joints deeper and its ligaments stronger.

The hip bone and the pelvis

The hip bone forms from three bones, the ilium, ischium and pubis, which meet at a Y-shaped (triradiate) cartilage in the acetabulum and fuse at about 15 to 17 years. The ilium and ischium each supply about two fifths of the acetabulum and the pubis about one fifth. The acetabulum's articular lunate surface is horseshoe shaped; its inferior gap, the acetabular notch, is bridged by the transverse acetabular ligament, and the nonarticular acetabular fossa holds a fat pad and the ligament of the head of the femur.

The ilium carries the iliac crest, the ASIS (sartorius, inguinal ligament), the AIIS (rectus femoris, iliofemoral ligament), the posterior superior and inferior spines, the iliac fossa and the auricular surface for the sacrum, with the greater sciatic notch below.

The ischium has a body, a ramus, the ischial spine (sacrospinous ligament; the pudendal nerve crosses it, the landmark for pudendal block) and the ischial tuberosity (hamstrings, sacrotuberous ligament, the hamstring part of adductor magnus; the bone we sit on). Between spine and tuberosity is the lesser sciatic notch. The sacrospinous and sacrotuberous ligaments convert the notches into the greater and lesser sciatic foramina.

The pubis has a body with the pubic crest and pubic tubercle (inguinal ligament), a superior ramus with the pectineal line (pecten pubis), and an inferior ramus that joins the ischial ramus as the ischiopubic ramus. The two pubic bodies meet at the pubic symphysis. The pubis and ischium enclose the obturator foramen, whose canal transmits the obturator nerve and vessels.

The two hip bones, sacrum and coccyx form the pelvis. The pelvic brim separates the greater (false) pelvis from the lesser (true) pelvis. Sex differences reflect adaptation for childbirth.

Femur

The femur is the longest and strongest bone. Its head forms about two thirds of a sphere and carries a small pit, the fovea capitis, for the ligament of the head. The neck joins the shaft at an angle of inclination of about 125 degrees in the adult (about 150 degrees at birth). A smaller angle is coxa vara and a larger one coxa valga. The neck is also twisted forward relative to the line of the femoral condyles, an angle of anteversion (torsion) of about 10 to 15 degrees in adults.

The greater trochanter receives gluteus medius and minimus, piriformis, obturator internus and the gemelli; the lesser trochanter receives iliopsoas. The intertrochanteric line (anterior) marks the capsule and iliofemoral ligament, and the intertrochanteric crest lies posteriorly. The linea aspera on the back of the shaft anchors the adductors, vasti and short head of biceps, continuing up as the gluteal tuberosity. Distally the medial and lateral condyles are separated by the intercondylar fossa, and the adductor tubercle receives the hamstring part of adductor magnus.

The blood supply of the femoral head comes mainly from the medial circumflex femoral artery through retinacular branches that run up the neck under the synovium, with a smaller contribution from the lateral circumflex femoral artery and from the artery of the ligament of the head (a branch of the obturator artery that matters chiefly in children). A displaced intracapsular neck fracture tears the retinacular vessels and risks avascular necrosis of the head, whereas extracapsular intertrochanteric fractures do not. A hip fracture in an older adult classically leaves the limb shortened and laterally rotated.

Patella

The patella is the largest sesamoid bone, embedded in the quadriceps tendon. Its posterior surface has a larger lateral facet and a smaller medial facet that glide on the femoral patellar surface. The Q angle, between a line from ASIS to the center of the patella and a line from the patella to the tibial tuberosity, is normally about 13 degrees in men and 18 degrees in women; a larger angle increases lateral pull, which helps explain why lateral patellar dislocation and patellofemoral pain are more common in women.

Tibia and fibula

The tibia is the weight-bearing bone of the leg. Its proximal end has medial and lateral condyles whose flat upper surfaces (the tibial plateau) are separated by the intercondylar eminence with its medial and lateral tubercles, and by anterior and posterior intercondylar areas for the cruciate ligaments and menisci. The tibial tuberosity receives the patellar ligament. Gerdy tubercle on the anterolateral condyle receives the iliotibial tract. The shaft is triangular, its anterior border (shin) and medial surface subcutaneous; posteriorly the soleal line marks the soleus origin. Distally the medial malleolus projects downward and the fibular notch receives the fibula.

The slender fibula bears little body weight (roughly a sixth or less) but anchors muscles and forms the lateral wall of the ankle mortise. Its head receives biceps femoris and the lateral collateral ligament; the common fibular nerve winds around its neck, where it is exposed to fracture, tight casts and leg crossing. The lateral malleolus extends further distally and lies more posteriorly than the medial malleolus, which is why inversion is freer than eversion.

Tarsals, metatarsals and phalanges

Seven tarsals form the hindfoot and midfoot. The talus sits between the malleoli; its trochlea (body) articulates with the tibia and fibula, its head with the navicular, and its undersurface with the calcaneus. It has no muscle attachments, and its blood supply enters mainly through the neck and runs backward, so neck fractures risk avascular necrosis of the body. The calcaneus is the largest tarsal; its tuberosity receives the calcaneal (Achilles) tendon and its sustentaculum tali supports the talar head, with flexor hallucis longus grooving its underside. The navicular lies medially in front of the talus; its tuberosity receives most of tibialis posterior. The cuboid lies laterally and has a groove for the fibularis longus tendon. The three cuneiforms (medial, intermediate, lateral) articulate with the first three metatarsals; the recessed intermediate cuneiform locks the base of the second metatarsal in place like a keystone.

The five metatarsals have bases, shafts and heads. Under the first metatarsal head lie two sesamoids within the flexor hallucis brevis tendons. The tuberosity at the base of the fifth receives fibularis brevis; an avulsion fracture here follows inversion injury, whereas a Jones fracture at the metaphyseal diaphyseal junction a little further distal heals poorly because of a watershed blood supply. The second and third metatarsal shafts are the usual sites of stress (march) fractures. The phalanges number fourteen, two in the great toe and three in each lesser toe.

Palpable landmarks of the lower limb

Routinely palpated are the iliac crests (compared for pelvic obliquity), ASIS and PSIS, the pubic tubercle (inguinal hernias lie above and medial to it, femoral hernias below and lateral), the ischial tuberosity, the greater trochanter (level with the pubic tubercle and hip joint center), the femoral condyles and joint line, the tibial tuberosity and Gerdy tubercle, the fibular head and neck, both malleoli, the sustentaculum tali, the navicular tuberosity and the base of the fifth metatarsal.

Section 5Arthrology and syndesmology of the appendicular skeleton

Arthrology is the study of joints and syndesmology the study of ligaments: shape sets which movements are possible, while ligaments, capsule and muscle decide how much is allowed.

Classifying joints

Structurally, fibrous joints are united by dense connective tissue: sutures, syndesmoses (distal tibiofibular joint, interosseous membranes) and gomphoses (teeth). Cartilaginous joints are either synchondroses (primary; hyaline cartilage, usually temporary, such as the growth plate and first sternocostal joint) or symphyses (secondary; fibrocartilage, midline, such as the pubic symphysis and intervertebral discs). Synovial joints have a cavity. Functionally these correspond roughly to synarthroses (immovable), amphiarthroses (slightly movable) and diarthroses (freely movable).

A synovial joint has hyaline articular cartilage (fibrocartilage in the TMJ and SC joint), a fibrous capsule, a synovial membrane lining everything except cartilage, and synovial fluid rich in hyaluronic acid and lubricin that nourishes the avascular cartilage. Accessory structures include ligaments, discs, menisci, labra and bursae. Hilton law: the nerves to the muscles moving a joint also supply the joint, which is why hip disease can present as knee pain.

Two clinical ideas recur. The close-packed position is the position of maximal congruence and ligament tension, where the joint is most stable and least able to move further (glenohumeral: abduction with lateral rotation; hip: extension with medial rotation and abduction; knee: full extension; ankle: dorsiflexion). The capsular pattern is the characteristic proportional loss of motion when the whole capsule is inflamed or contracted (glenohumeral: lateral rotation lost most, then abduction, then medial rotation; hip: medial rotation, flexion, abduction; knee: flexion more than extension; elbow: flexion more than extension).

The shoulder complex

The shoulder complex has four articulations. The sternoclavicular (SC) joint is saddle-shaped with an articular disc; the disc and the strong costoclavicular ligament are its main stabilizers. It is the only bony link between upper limb and trunk.

The acromioclavicular (AC) joint is a plane joint. The AC ligament gives horizontal stability and the coracoclavicular ligament (lateral trapezoid and medial conoid parts) gives vertical stability, suspending the limb. In a complete AC separation both are torn and the clavicle appears to step up.

The glenohumeral joint is a ball and socket joint with a large head on a shallow glenoid, deepened by the glenoid labrum. Its loose capsule is reinforced by the superior, middle and inferior glenohumeral ligaments and the coracohumeral ligament; the coracoacromial arch roofs it, with the subacromial bursa between arch and supraspinatus. Stability is mainly dynamic, from the rotator cuff. The weak anteroinferior region explains why more than 95 percent of dislocations are anterior, risking the axillary nerve, an anteroinferior labral tear (Bankart) and a posterolateral head impaction (Hill-Sachs). Posterior dislocation follows seizures or electric shock.

The scapulothoracic articulation is physiologic, not anatomic. In elevation the glenohumeral joint and scapula move in about a 2 to 1 ratio (scapulohumeral rhythm): of 180 degrees, about 120 occur at the glenohumeral joint and 60 from scapular upward rotation, and the humerus must rotate laterally so the greater tubercle clears the acromion.

The elbow and forearm

The elbow combines the humeroulnar (hinge), humeroradial and proximal radioulnar (pivot) joints in one capsule. The ulnar collateral ligament resists valgus stress and is injured in throwers; the radial collateral ligament resists varus and blends with the annular ligament around the radial head. A sudden pull on a young child's hand slips the head out of the annular ligament (nursemaid's elbow). The carrying angle is about 5 to 15 degrees, greater in women; a malunited supracondylar fracture can reverse it (cubitus varus). Pronation and supination also use the distal radioulnar joint, stabilized by the triangular fibrocartilage complex.

The wrist and hand

The radiocarpal joint is condyloid: the radius and the articular disc of the triangular fibrocartilage complex (TFCC) receive the scaphoid, lunate and triquetrum; the ulna does not touch the carpus. The midcarpal joint adds much flexion and radial deviation.

The first carpometacarpal joint is a saddle permitting opposition and is the hand's commonest site of osteoarthritis. Metacarpophalangeal joints are condyloid; their collateral ligaments are tight in flexion, so hands are splinted with MCPs flexed. Interphalangeal joints are hinges with a palmar (volar) plate that blocks hyperextension.

The hip

The hip is a deep ball and socket joint stabilized mainly by bone and ligament. The acetabular labrum and transverse acetabular ligament complete the socket. Three capsular ligaments spiral so that they tighten in extension: the iliofemoral (Y ligament of Bigelow, AIIS to intertrochanteric line), the strongest in the body, limiting extension; the pubofemoral, limiting abduction; and the ischiofemoral, limiting medial rotation. The ligament of the head adds little stability but carries a small artery. Most dislocations are posterior (dashboard injury), leaving the limb shortened, flexed, adducted and medially rotated and endangering the sciatic nerve.

The knee

The knee comprises the medial and lateral tibiofemoral joints and the patellofemoral joint. It is a modified hinge with poor bony fit, so it depends on menisci, ligaments and muscle.

The menisci are fibrocartilage wedges that deepen the tibial surfaces and spread load; only their peripheral third is vascular. The medial meniscus is C-shaped and bound to the capsule and MCL, so it is less mobile and more often torn. The lateral meniscus is nearly circular, more mobile and free of the LCL.

The cruciate ligaments are intracapsular but extrasynovial and named for their tibial attachments. The ACL runs from the anterior intercondylar area to the medial side of the lateral femoral condyle and prevents anterior translation of the tibia; noncontact pivoting tears it, and the Lachman test is the most sensitive sign. The stronger PCL runs from the posterior intercondylar area to the lateral side of the medial femoral condyle and prevents posterior translation; a dashboard blow tears it.

The flat MCL, blended with capsule and medial meniscus, resists valgus; the cordlike LCL, from lateral epicondyle to fibular head, resists varus. A lateral blow to a planted knee produces the "unhappy triad" of ACL, MCL and medial meniscus injury.

In terminal extension the tibia rotates laterally on the femur and locks the knee (the screw-home mechanism); popliteus unlocks it by rotating the femur laterally on the planted tibia.

The suprapatellar bursa communicates with the joint, so effusions ballot the patella. The prepatellar bursa inflames with kneeling (housemaid's knee), the superficial infrapatellar with upright kneeling, and a popliteal (Baker) cyst is usually a distended semimembranosus or gastrocnemius bursa.

The ankle and foot

The proximal tibiofibular joint is a plane joint; the distal one is a syndesmosis whose injury is a "high ankle sprain."

The talocrural joint is a hinge in which the talar trochlea sits in the malleolar mortise. The trochlea is wider anteriorly, so dorsiflexion is close-packed and stable, while plantarflexion is loose; most sprains therefore occur in plantarflexion with inversion.

Of the three lateral ligaments, the anterior talofibular (ATFL) is weakest and most often sprained, the calcaneofibular fails next and the posterior talofibular rarely tears. The fan-shaped medial deltoid ligament is so strong that eversion often avulses the medial malleolus instead.

The subtalar joint produces most inversion and eversion; the transverse tarsal (Chopart) joint (talonavicular plus calcaneocuboid) adds to it. The Lisfranc ligament binds the medial cuneiform to the second metatarsal base. The spring (plantar calcaneonavicular) ligament cradles the talar head, the long and short plantar ligaments support the lateral column, and the plantar aponeurosis tightens as the toes dorsiflex (windlass mechanism). Foot pronation combines eversion, abduction and dorsiflexion; supination the reverse.

Words worth knowing

Appendicular Muscles, Nerves and Vessels

Muscles move the bones of Chapter 1, nerves tell them when, and vessels keep them alive. The board tests this material with relentless precision (one muscle, one nerve, one root, one lesion), and the same facts drive your daily examination: every manual muscle test, reflex and pulse you check is a question about this chapter.

Section 1Muscles of the shoulder girdle and arm

Shoulder muscles fall into three groups: axioappendicular muscles that tie the limb to the trunk and move the scapula, scapulohumeral muscles that move the humerus on the scapula, and the arm muscles, divided by intermuscular septa into an anterior flexor compartment and a posterior extensor compartment.

Axioappendicular muscles: the scapular stabilizers

The scapula has no ligamentous attachment to the thorax, so its position depends on muscle. Trapezius and serratus anterior form a force couple that upwardly rotates the scapula during arm elevation; levator scapulae, the rhomboids and pectoralis minor rotate it downward.

Weakness of serratus anterior causes medial winging: the medial border lifts off the chest when the patient pushes against a wall. The long thoracic nerve runs on the outer surface of the muscle in the midaxillary line, where surgery or a heavy shoulder strap can injure it. Weakness of trapezius after spinal accessory injury (often during lymph node surgery in the posterior triangle) causes a drooping shoulder, weak shrug and a milder lateral winging on abduction.

Scapulohumeral muscles: deltoid, teres major and the rotator cuff

The deltoid arises from the lateral third of the clavicle, the acromion and the scapular spine (mirroring trapezius) and inserts on the deltoid tuberosity. Its anterior fibers flex and medially rotate, its multipennate middle fibers abduct from about 15 to 90 degrees, and its posterior fibers extend and laterally rotate. Teres major runs from the inferior angle to the medial lip of the intertubercular groove and adducts and medially rotates.

The four rotator cuff muscles (SITS) blend with the capsule and press the humeral head into the glenoid, resisting the upward shear of the deltoid. Without them, deltoid contraction would simply jam the head against the acromion.

Supraspinatus is the cuff tendon most often torn and impinged, because it passes through the narrow subacromial space beneath the coracoacromial arch. A painful arc between roughly 60 and 120 degrees of abduction suggests impingement or tendinopathy; inability to hold the arm at 90 degrees (positive drop arm) suggests a full thickness tear.

Muscles of the arm

The anterior compartment is supplied by the musculocutaneous nerve (C5 to C7). Biceps brachii has a long head from the supraglenoid tubercle, whose tendon runs inside the joint capsule and through the intertubercular groove, and a short head from the coracoid; it inserts on the radial tuberosity and, through the bicipital aponeurosis, into forearm fascia. It is the most powerful supinator when the elbow is flexed and a strong elbow flexor. Brachialis, from the distal anterior humerus to the coronoid process and ulnar tuberosity, is the prime flexor of the elbow in every forearm position (its lateral part also receives radial nerve fibers). Coracobrachialis runs from coracoid to midshaft humerus, flexes and adducts the arm, and is pierced by the musculocutaneous nerve.

The posterior compartment is supplied by the radial nerve. Triceps brachii has a long head from the infraglenoid tubercle (it crosses the shoulder and helps extend and adduct it), a lateral head above the radial groove and a medial head below it; all insert on the olecranon. Tiny anconeus assists elbow extension. Reflexes test the roots: biceps C5 (and C6), brachioradialis C6, triceps C7.

Section 2Muscles of the forearm and hand

The forearm has an anterior (flexor-pronator) compartment arising largely from the medial epicondyle and a posterior (extensor-supinator) compartment arising largely from the lateral epicondyle. The anterior compartment is supplied by the median nerve except for one and a half muscles supplied by the ulnar nerve (flexor carpi ulnaris and the medial half of flexor digitorum profundus). The entire posterior compartment is supplied by the radial nerve, mostly its deep branch (the posterior interosseous nerve).

The anterior compartment

The median nerve passes between the two heads of pronator teres and the ulnar nerve between the two heads of flexor carpi ulnaris (the cubital tunnel), both sites of entrapment. The anterior interosseous nerve is purely motor to the deep layer; its palsy prevents a round "OK" sign (the thumb and index tips cannot flex) without sensory loss. Overuse at the common flexor origin causes medial epicondylalgia (golfer's elbow).

The posterior compartment

The superficial extensors are brachioradialis and extensor carpi radialis longus (both from the lateral supracondylar ridge and supplied by the radial nerve proper), then, from the common extensor origin on the lateral epicondyle, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi and extensor carpi ulnaris. Brachioradialis is the exception that proves the rule: it lies in the extensor compartment and is supplied by the radial nerve but flexes the elbow, most strongly in the midprone position. The deep extensors are supinator (the posterior interosseous nerve passes through it under the arcade of Frohse), abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis. Lateral epicondylalgia (tennis elbow) is a tendinopathy chiefly of the extensor carpi radialis brevis origin, reproduced by resisted wrist extension (Cozen test).

At the wrist the extensor retinaculum creates six dorsal compartments: (1) abductor pollicis longus and extensor pollicis brevis; (2) extensor carpi radialis longus and brevis; (3) extensor pollicis longus, turning around Lister tubercle; (4) extensor digitorum and extensor indicis; (5) extensor digiti minimi; (6) extensor carpi ulnaris. Tenosynovitis of the first compartment is de Quervain tenosynovitis, provoked by ulnar deviation with the thumb tucked (Finkelstein test). The anatomical snuffbox is bounded laterally by abductor pollicis longus and extensor pollicis brevis and medially by extensor pollicis longus; its floor is the scaphoid and trapezium, and the radial artery crosses it.

The intrinsic muscles of the hand

The median nerve supplies the muscles remembered as LOAF (lateral two Lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis); the ulnar nerve supplies every other intrinsic muscle. Because the lumbricals and interossei flex the MCP and extend the IP joints, their paralysis produces a claw posture. Froment sign tests adductor pollicis: holding paper between thumb and index, the patient flexes the thumb IP joint with flexor pollicis longus (median) to compensate.

The carpal tunnel and Guyon canal

The carpal tunnel is an osseofibrous passage whose floor and walls are the concave carpal bones and whose roof is the flexor retinaculum (transverse carpal ligament), attached laterally to the scaphoid tubercle and trapezium and medially to the pisiform and hook of the hamate. It contains ten structures: four tendons of flexor digitorum superficialis, four of flexor digitorum profundus, flexor pollicis longus and the median nerve. Flexor carpi radialis runs in its own groove in the trapezium, and palmaris longus, the ulnar nerve and artery and the palmar cutaneous branch of the median nerve pass superficial to the retinaculum.

Carpal tunnel syndrome is the most common entrapment neuropathy. Anything that crowds the tunnel (tenosynovitis from repetitive work, pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, a lunate dislocation) compresses the median nerve, producing night paresthesia of the lateral three and a half digits, then weakness and atrophy of the thenar muscles. Sensation over the thenar eminence is spared because the palmar cutaneous branch leaves the nerve proximal to the tunnel and passes over it. Phalen (sustained wrist flexion), Tinel (tapping) and carpal compression tests provoke symptoms; nerve conduction studies confirm.

Guyon canal (the ulnar canal) lies between the pisiform medially and the hook of the hamate laterally, roofed by the palmar carpal ligament and floored by the flexor retinaculum and pisohamate ligament. It transmits the ulnar nerve and artery. Compression here (cyclists leaning on handlebars, hook of hamate fracture, ganglion) weakens the intrinsics and numbs the palmar little finger but spares flexor carpi ulnaris, the medial flexor digitorum profundus and the dorsal cutaneous branch (dorsal ulnar hand sensation), all of which leave the nerve in the forearm. That sparing distinguishes it from cubital tunnel syndrome at the elbow.

Section 3Brachial plexus and the vessels of the upper limb

Roots, trunks, divisions, cords, branches

The brachial plexus is formed by the ventral rami of C5 to T1. The roots emerge between the anterior and middle scalenes; the trunks cross the posterior triangle of the neck; the divisions lie behind the clavicle; the cords surround the second part of the axillary artery and are named for their relation to it; the terminal branches enter the arm. ("Randy Travis Drinks Cold Beer.") C5 and C6 join as the upper trunk, C7 continues as the middle trunk, and C8 and T1 join as the lower trunk. Each trunk splits into an anterior division (flexor territory) and a posterior division (extensor territory). The three posterior divisions form the posterior cord; the anterior divisions of the upper and middle trunks form the lateral cord; the anterior division of the lower trunk continues as the medial cord.

The five terminal nerves and their lesions

The musculocutaneous nerve pierces coracobrachialis, supplies the arm flexors and ends as the lateral cutaneous nerve of the forearm. The axillary nerve passes through the quadrangular space around the surgical neck to deltoid and teres minor and supplies the skin over the lower deltoid (the "regimental badge" area). The radial nerve descends in the spiral groove, enters the forearm in front of the lateral epicondyle and divides into the sensory superficial branch and the motor deep branch (posterior interosseous nerve). The median nerve gives no branches in the arm; it supplies most forearm flexors, the anterior interosseous nerve and, through the carpal tunnel, LOAF and the palmar lateral three and a half digits. The ulnar nerve also gives no branches in the arm; it passes behind the medial epicondyle, supplies flexor carpi ulnaris and medial profundus, gives a dorsal cutaneous branch in the forearm and supplies most intrinsic hand muscles and the medial one and a half digits.

The ulnar paradox explains the last two rows: a proximal lesion also paralyzes the medial half of profundus, so the fingers cannot flex at the DIP joints and claw less than after a distal lesion, where the intact profundus pulls the fingers into a deeper claw.

Arteries of the upper limb

The subclavian artery (from the brachiocephalic trunk on the right, the aortic arch on the left) passes between anterior and middle scalenes with the plexus and becomes the axillary artery at the lateral border of the first rib. Pectoralis minor divides the axillary artery into three parts that give one, two and three branches: part 1, superior thoracic; part 2, thoracoacromial and lateral thoracic; part 3, subscapular (the largest) and the anterior and posterior circumflex humeral arteries. The axillary becomes the brachial artery at the inferior border of teres major.

The brachial artery runs medially in the arm with the median nerve, gives the profunda brachii (with the radial nerve in the spiral groove) and the superior and inferior ulnar collateral arteries, and divides in the cubital fossa at the level of the radial neck into the radial and ulnar arteries. The ulnar artery gives the common interosseous artery (anterior and posterior interosseous branches) and enters the hand superficial to the flexor retinaculum through Guyon canal to form most of the superficial palmar arch. The radial artery is palpable lateral to the flexor carpi radialis tendon at the wrist, crosses the snuffbox, passes between the heads of the first dorsal interosseous and forms most of the deep palmar arch, which lies more proximally. Common and proper palmar digital arteries arise from the superficial arch. The Allen test checks that each artery alone can fill the hand.

A rich scapular anastomosis links the suprascapular and dorsal scapular arteries (from the subclavian) with the circumflex scapular branch of the subscapular artery, so the limb survives ligation of the axillary artery above the subscapular origin. The cubital fossa contains, from lateral to medial, the biceps tendon, the brachial artery and the median nerve (the radial nerve lies at its lateral edge under brachioradialis).

Veins and lymph

The superficial veins begin in the dorsal venous network of the hand. The cephalic vein runs up the lateral (radial) side, through the deltopectoral groove, pierces the clavipectoral fascia and joins the axillary vein. The basilic vein runs up the medial side, pierces the deep fascia in mid arm and joins the brachial veins to form the axillary vein at the lower border of teres major. The median cubital vein links them across the cubital fossa and is the usual venipuncture site; the bicipital aponeurosis shields the brachial artery and median nerve beneath it. The axillary vein becomes the subclavian vein at the first rib.

Superficial lymph vessels follow the veins. Those with the basilic vein reach the cubital (epitrochlear) nodes above the medial epicondyle; those with the cephalic vein may drain directly to apical nodes. The axillary nodes form five groups: pectoral (anterior), humeral (lateral), subscapular (posterior), central and apical; all drain to the apical group and then the subclavian trunk. They also drain about three quarters of the breast, so an enlarged axillary node demands a breast and limb exam.

Section 4Muscles of the hip, thigh, leg and foot

The gluteal region

The gluteal muscles are organized in layers. Gluteus maximus (posterior ilium, sacrum, coccyx and sacrotuberous ligament to the iliotibial tract and gluteal tuberosity; inferior gluteal nerve, L5 to S2) is the powerful hip extensor and lateral rotator used in climbing stairs, running and rising from a chair, but is quiet during level walking. Gluteus medius and minimus (outer ilium to the greater trochanter; superior gluteal nerve, L4 to S1) abduct and medially rotate the hip and, most importantly, keep the pelvis level when the opposite foot is off the ground. Tensor fasciae latae (ASIS to iliotibial tract; superior gluteal nerve) flexes, abducts and medially rotates.

The deep lateral rotators are piriformis, superior gemellus, obturator internus, inferior gemellus, quadratus femoris and, from the front, obturator externus. Piriformis (anterior sacrum through the greater sciatic foramen to the greater trochanter; nerve to piriformis, S1 and S2) is the landmark of the region: the superior gluteal nerve and vessels exit above it, while the sciatic nerve, inferior gluteal nerve and vessels, posterior femoral cutaneous nerve, pudendal nerve and internal pudendal vessels and the nerves to obturator internus and quadratus femoris exit below it.

Iliopsoas, the strongest hip flexor, is formed by psoas major (T12 to L5 vertebral bodies, discs and transverse processes; ventral rami L1 to L3) and iliacus (iliac fossa; femoral nerve), inserting together on the lesser trochanter.

The thigh

Sartorius (ASIS to the medial tibia) flexes, abducts and laterally rotates the hip and flexes the knee, the cross-legged "tailor's" position. Rectus femoris arises from the AIIS and above the acetabulum, so it is the only quadriceps head that crosses the hip; it is most easily strained in kicking and sprinting. All four heads insert via the quadriceps tendon, patella and patellar ligament on the tibial tuberosity; the oblique lower fibers of vastus medialis resist lateral patellar tracking. The patellar reflex tests L4 (L3 and L4).

Adductor magnus is a hybrid: its adductor part (ischiopubic ramus to linea aspera) is supplied by the obturator nerve, its hamstring part (ischial tuberosity to adductor tubercle) by the tibial division of sciatic. The adductor hiatus between its two parts lets the femoral vessels pass into the popliteal fossa. Gracilis is the only adductor that crosses the knee.

The hamstrings arise from the ischial tuberosity (except the short head of biceps) and cross both hip and knee. Semitendinosus and semimembranosus insert medially and medially rotate the flexed knee; biceps femoris inserts on the fibular head and laterally rotates it. Sartorius, gracilis and semitendinosus share the pes anserinus on the medial tibia, each from a different compartment and nerve (femoral, obturator, tibial).

The leg

Deep fascia, the interosseous membrane and intermuscular septa divide the leg into four compartments. Their tight walls make them vulnerable to compartment syndrome, most often anterior.

Gastrocnemius (two heads from the femoral condyles) and soleus (fibular head and soleal line) join as the calcaneal (Achilles) tendon; the ankle reflex tests S1. A rupture leaves a palpable gap and a positive Thompson test (squeezing the calf fails to plantarflex the foot). Behind the medial malleolus, in the tarsal tunnel under the flexor retinaculum, lie (anterior to posterior) Tibialis posterior, flexor Digitorum longus, posterior tibial Artery and Veins, tibial Nerve and flexor Hallucis longus: "Tom, Dick And Very Nervous Harry." Behind the lateral malleolus run fibularis longus and brevis. Fibularis longus crosses the sole to the medial cuneiform and first metatarsal base, forming a sling with tibialis anterior.

The foot and its arches

The dorsum carries extensor digitorum brevis and extensor hallucis brevis (deep fibular nerve). The sole has four layers: (1) abductor hallucis, flexor digitorum brevis, abductor digiti minimi; (2) quadratus plantae, lumbricals, and the long flexor tendons; (3) flexor hallucis brevis, adductor hallucis, flexor digiti minimi brevis; (4) the interossei and the tendons of fibularis longus and tibialis posterior. The medial plantar nerve supplies abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and the first lumbrical (like the median nerve in the hand); the lateral plantar nerve supplies the rest (like the ulnar).

The arches distribute weight between heel and forefoot, absorb shock and make the foot a rigid lever at push-off. Pes planus (flatfoot) may be flexible and harmless or, in adults, due to tibialis posterior dysfunction, with the "too many toes" sign and inability to perform a single-leg heel raise. Pes cavus (high arch) can signal a neuromuscular disorder such as Charcot-Marie-Tooth disease.

Section 5Lumbosacral plexus and the vessels of the lower limb

The lumbar plexus (L1 to L4)

The lumbar plexus forms within psoas major from the ventral rami of L1 to L4 (often with a small T12 contribution). Its branches emerge lateral to, through, or medial to psoas. The lumbosacral trunk (part of L4 with L5) descends over the sacral ala to join the sacral plexus.

A helpful pattern: the femoral nerve comes from posterior divisions and supplies extensors of the knee; the obturator nerve comes from anterior divisions. The saphenous nerve, the longest cutaneous branch of the femoral nerve, runs in the adductor canal and then with the great saphenous vein down the medial leg to the medial foot, so a femoral lesion numbs the medial leg as well as the anterior thigh.

The sacral plexus (L4 to S4)

The sacral plexus lies on piriformis in the posterior pelvic wall. Its main branches are the superior gluteal (L4 to S1), inferior gluteal (L5 to S2), sciatic (L4 to S3), posterior femoral cutaneous (S1 to S3) and pudendal (S2 to S4) nerves, plus small nerves to piriformis, obturator internus and quadratus femoris. The sciatic nerve, the largest nerve in the body, is two nerves in one sheath: the tibial division (anterior divisions, L4 to S3) and the common fibular division (posterior divisions, L4 to S2). It leaves the pelvis below piriformis (in a minority of people the fibular division pierces or passes above the muscle), descends between the greater trochanter and ischial tuberosity, supplies the hamstrings, and usually divides at the apex of the popliteal fossa.

The tibial nerve runs through the popliteal fossa and deep posterior compartment, passes through the tarsal tunnel and divides into the medial and lateral plantar nerves; its sural contribution (joined by a fibular communicating branch) supplies the lateral foot. The common fibular nerve follows the biceps tendon to wind around the fibular neck, then divides into the deep fibular nerve (anterior compartment, dorsum muscles, first web space skin) and the superficial fibular nerve (lateral compartment, most of the dorsum of the foot). The pudendal nerve leaves through the greater sciatic foramen, hooks around the ischial spine and sacrospinous ligament, re-enters through the lesser sciatic foramen into the pudendal canal and supplies the perineum and the external anal and urethral sphincters.

Nerve lesions and gait

Distinguish a common fibular palsy from an L5 radiculopathy: both cause foot drop and dorsal foot numbness, but L5 root lesions also weaken inversion (tibialis posterior, tibial nerve) and hip abduction (gluteus medius, superior gluteal nerve), may include back pain and positive straight leg raise, and spare nothing at the fibular head because the lesion is proximal.

Arteries of the lower limb

The external iliac artery becomes the femoral artery as it passes under the inguinal ligament at the mid-inguinal point (halfway between ASIS and pubic symphysis), where its pulse is felt. In the femoral triangle (bounded by the inguinal ligament, the medial border of sartorius and the medial border of adductor longus; floored by iliopsoas, pectineus and adductor longus) the contents lie, lateral to medial, as Nerve, Artery, Vein, Empty space (femoral canal) and Lymphatics: NAVEL. The femoral sheath encloses the artery, vein and canal but not the nerve. A femoral hernia passes through the femoral canal below and lateral to the pubic tubercle and is more common in women.

About 4 cm below the ligament the femoral artery gives the profunda femoris (deep femoral) artery, the main supply of the thigh, which gives the medial and lateral circumflex femoral arteries and three or four perforating arteries. The femoral artery continues through the adductor (Hunter) canal under sartorius, with the femoral vein, saphenous nerve and nerve to vastus medialis, then passes through the adductor hiatus to become the popliteal artery. In the popliteal fossa the artery is the deepest structure (tibial nerve most superficial, popliteal vein between). At the lower border of popliteus it divides into the anterior tibial artery, which pierces the interosseous membrane, descends in the anterior compartment and becomes the dorsalis pedis at the ankle, and the tibioperoneal (tibiofibular) trunk, which gives the fibular (peroneal) artery and continues as the posterior tibial artery. The posterior tibial artery passes behind the medial malleolus and divides into medial and lateral plantar arteries; the lateral plantar artery and a deep branch of dorsalis pedis form the plantar arch.

The gluteal region is supplied by the superior and inferior gluteal arteries from the internal iliac, and the obturator artery sends the small artery of the ligament of the femoral head. The cruciate and trochanteric anastomoses around the hip link the gluteal, circumflex femoral and first perforating arteries and supply collateral flow.

Veins and lymph

The great saphenous vein begins at the medial end of the dorsal venous arch, passes anterior to the medial malleolus (a reliable cutdown site), ascends the medial leg with the saphenous nerve, passes behind the medial femoral condyle and pierces the cribriform fascia at the saphenous opening, about 3 to 4 cm inferolateral to the pubic tubercle, to join the femoral vein. It is the longest vein in the body and the usual graft for coronary bypass. The small saphenous vein begins laterally, passes behind the lateral malleolus with the sural nerve and ascends the midline of the calf to join the popliteal vein. Deep veins accompany the arteries as paired venae comitantes below the knee. Valved perforating veins carry blood from superficial to deep, and the calf muscle pump (the soleal sinuses) drives it upward. Incompetent valves produce varicose veins.

Deep vein thrombosis arises from Virchow triad (stasis, endothelial injury, hypercoagulability) and presents with unilateral calf swelling, warmth and tenderness, though often silently; its danger is pulmonary embolism. Superficial lymphatics follow the great saphenous vein to the superficial inguinal nodes, which also drain the lower abdominal wall below the umbilicus, the gluteal region, the perineum, the external genitalia (but not the testes, which drain to lumbar nodes) and the anal canal below the pectinate line. Vessels following the small saphenous vein reach the popliteal nodes. Both drain to the deep inguinal nodes along the femoral vein (the highest lies in the femoral canal) and then to the external iliac nodes.

Words worth knowing

Visceral Anatomy: Cardiovascular, Respiratory, Digestive, Urogenital and Endocrine Systems

The viscera are where the spine meets the rest of the patient. Pain from the heart, gallbladder, kidney, pancreas or uterus can arrive in a chiropractic office dressed as back, shoulder or neck pain, and the board expects you to know exactly which organ sits where, what supplies it and how its sensory fibers reach the cord. This chapter walks through the cardiovascular, respiratory, digestive, urogenital and endocrine systems with an eye on relations, blood supply and the landmarks that let you find each organ on the body surface and on film.

Section 1The heart, great vessels and lymphatic system

The mediastinum

The mediastinum is the central compartment of the thorax between the two pleural cavities, extending from the superior thoracic aperture to the diaphragm and from the sternum to the bodies of the thoracic vertebrae. An imaginary transverse thoracic plane passes from the sternal angle (manubriosternal joint, the second costal cartilage) back to the T4 to T5 intervertebral disc. Above it is the superior mediastinum; below it the inferior mediastinum, which is subdivided by the pericardium into anterior, middle and posterior parts.

The sternal angle plane is one of the most heavily tested landmarks in thoracic anatomy because so many things happen there: the aortic arch begins and ends, the trachea bifurcates (the carina is at about T4 to T5 in the cadaver, a little lower in the living upright subject), the azygos vein arches over the right main bronchus into the superior vena cava, the thoracic duct crosses from right to left, the left recurrent laryngeal nerve hooks under the arch, and the ligamentum arteriosum connects the pulmonary trunk to the arch.

Pericardium

The heart sits in a double sac. The outer fibrous pericardium is tough, fuses with the central tendon of the diaphragm below and the adventitia of the great vessels above, and prevents sudden overdistension. Inside it, the serous pericardium has a parietal layer lining the fibrous sac and a visceral layer (epicardium) on the heart surface, with a thin film of fluid in the pericardial cavity between them. The fibrous and parietal layers are supplied by the phrenic nerve (C3 to C5), so pericarditis refers pain to the shoulder and supraclavicular region. A rapid pericardial effusion causes cardiac tamponade (hypotension, distended neck veins and muffled heart sounds, Beck triad).

External features and chambers

The heart is a cone lying obliquely, with its apex pointing anteriorly, inferiorly and to the left, usually at the left fifth intercostal space at the midclavicular line (about 9 cm from the midline), where the apex beat is palpated. The base faces posteriorly and is formed mostly by the left atrium. The sternocostal (anterior) surface is mainly right ventricle; the diaphragmatic (inferior) surface is mainly left ventricle with some right ventricle; the left (pulmonary) surface is left ventricle.

The coronary (atrioventricular) sulcus encircles the heart between atria and ventricles and carries the right coronary artery and the circumflex branch and the coronary sinus. The anterior and posterior interventricular sulci mark the septum and carry the anterior interventricular (left anterior descending) and posterior interventricular arteries.

Valves and the fibrous skeleton

The four valves sit in a dense fibrous skeleton of four rings and two trigones that anchors the valve leaflets, gives the myocardium attachment and, crucially, electrically insulates atria from ventricles so that the only normal electrical path between them is the AV bundle. The atrioventricular valves (tricuspid on the right, mitral on the left) have cusps tethered by chordae tendineae to papillary muscles, which contract during systole to prevent the cusps from everting into the atria. The semilunar valves (pulmonary and aortic) have three pocket-shaped cusps without chordae; the spaces behind the aortic cusps are the aortic sinuses, and the right and left coronary arteries arise from the right and left aortic sinuses respectively.

Valve sounds are heard best not over the valve itself but downstream where the turbulent flow is carried. Remember the auscultation points with the phrase "All People Enjoy Time Magazine": aortic (right second intercostal space at the sternal border), pulmonary (left second intercostal space), Erb point (left third), tricuspid (left fourth or fifth at the lower sternal border) and mitral (apex, left fifth intercostal space at the midclavicular line).

Coronary circulation

The myocardium is supplied by the right and left coronary arteries, the first branches of the aorta. Coronary flow to the left ventricle occurs mainly during diastole, because systolic contraction compresses intramural vessels; tachycardia shortens diastole and so reduces subendocardial perfusion.

Dominance is defined by which artery gives the posterior interventricular (posterior descending) artery. About 70% of people are right dominant, about 10% left dominant and the rest codominant. Venous blood returns mainly through the great cardiac vein (alongside the LAD), the middle cardiac vein (alongside the posterior interventricular artery) and the small cardiac vein, all draining into the coronary sinus in the posterior coronary sulcus, which opens into the right atrium.

Conduction system

Specialized cardiac muscle cells, not nerves, generate and spread the heartbeat. The sinoatrial (SA) node lies in the wall of the right atrium at the upper end of the crista terminalis near the SVC opening and is the normal pacemaker (about 60 to 100 beats per minute). The impulse spreads through atrial muscle to the atrioventricular (AV) node in the interatrial septum near the opening of the coronary sinus (within the triangle of Koch). The AV node delays conduction (about 0.1 second), allowing the atria to finish filling the ventricles. The impulse then passes through the fibrous skeleton by the AV bundle (bundle of His), which divides at the top of the muscular septum into right and left bundle branches; the right bundle runs partly in the moderator band to reach the anterior papillary muscle early. Terminal Purkinje fibers spread the impulse through the subendocardial ventricular myocardium from apex to base, so the ventricles contract upward and wring blood into the outflow tracts.

The heart's rate is modified by autonomic nerves through the cardiac plexus: sympathetic fibers from the cervical and upper thoracic ganglia (T1 to T4) increase rate and force; parasympathetic fibers from the vagus slow the SA node and AV conduction.

The great arteries

The ascending aorta gives only the coronary arteries. The aortic arch begins and ends at the sternal angle plane and gives three branches from right to left: the brachiocephalic trunk (which divides behind the right sternoclavicular joint into the right common carotid and right subclavian), the left common carotid and the left subclavian. The descending thoracic aorta runs in the posterior mediastinum to the left of the vertebral bodies, gives posterior intercostal arteries (3rd to 11th), subcostal, bronchial, esophageal and superior phrenic branches, and passes through the aortic hiatus at T12 behind the median arcuate ligament to become the abdominal aorta.

The abdominal aorta lies on the left anterior surface of the lumbar bodies and bifurcates at about L4 (the level of the iliac crests, the supracristal plane) into the common iliac arteries. Its branches fall into three groups.

The great veins

The brachiocephalic veins form behind each sternoclavicular joint from the union of the internal jugular and subclavian veins; the left one is longer and crosses obliquely behind the manubrium. They join behind the right first costal cartilage to form the superior vena cava, which descends to enter the right atrium at the level of the third costal cartilage and receives the azygos vein just before it enters the pericardium. The inferior vena cava forms at L5 from the common iliac veins, ascends to the right of the aorta, receives the lumbar, right gonadal, renal, right suprarenal, hepatic and inferior phrenic veins, and pierces the central tendon of the diaphragm at T8. The left gonadal and left suprarenal veins drain into the left renal vein, which crosses in front of the aorta just below the origin of the superior mesenteric artery.

The azygos system drains the posterior thoracic wall and provides an important collateral route between the IVC and SVC. The azygos ascends to the right of the thoracic vertebral bodies; the hemiazygos and accessory hemiazygos on the left cross behind the aorta at about T8 and T7 to join it. Because these veins connect with the valveless internal and external vertebral venous plexuses (Batson plexus), tumors of the prostate, breast, lung, kidney and thyroid can seed the vertebrae, a fact every chiropractor should remember when an older patient has unexplained night pain.

The lymphatic system

Lymphatic vessels collect the protein-rich interstitial fluid that escapes capillaries (about 2 to 3 liters a day), carry absorbed fat from the gut as chyle, and filter both through lymph nodes before returning them to the venous system.

• The cisterna chyli is a dilated sac anterior to L1 to L2, to the right of the aorta, under the right crus of the diaphragm. It receives the intestinal and lumbar lymph trunks.
• The thoracic duct leaves the cisterna, passes through the aortic hiatus at T12, ascends in the posterior mediastinum between the aorta and azygos vein behind the esophagus, crosses to the left at about T4 to T5, and empties into the junction of the left internal jugular and left subclavian veins (the left venous angle). It drains everything except the right upper quadrant of the body.
• The right lymphatic duct drains the right side of the head and neck, right upper limb and right side of the thorax into the right venous angle.

The testes drain to para-aortic nodes at L1 to L2 because they descended from there, while the scrotal skin drains to the superficial inguinal nodes. The spleen is the largest lymphoid organ and filters blood rather than lymph; the thymus is the site of T lymphocyte maturation and is described with the endocrine glands.

Section 2The respiratory system

Upper airway and larynx

Air enters through the nasal cavity, where three conchae (superior, middle, inferior) increase surface area to warm, humidify and filter it. The pharynx is divided into nasopharynx (pharyngeal tonsil, opening of the auditory tube), oropharynx (palatine tonsils) and laryngopharynx, which continues into the esophagus at C6.

The larynx lies at C3 to C6 and guards the lower airway. Its unpaired cartilages are the thyroid (the laryngeal prominence), the cricoid (the only complete ring, at C6) and the epiglottis; the paired cartilages are the arytenoids (on which the vocal ligaments pivot), corniculates and cuneiforms. The vocal folds (true cords) bound the rima glottidis, the narrowest part of the adult airway; the vestibular folds (false cords) lie above them.

Sensation above the vocal folds is carried by the internal laryngeal nerve (a branch of the superior laryngeal), which mediates the cough reflex; below the folds it is carried by the recurrent laryngeal. The left recurrent laryngeal nerve loops under the aortic arch at the ligamentum arteriosum and the right loops under the right subclavian artery; both ascend in the tracheoesophageal groove, where they are at risk in thyroid surgery and can be compressed by a left atrial enlargement, an aortic aneurysm or an apical (Pancoast) lung tumor, producing hoarseness.

Trachea and bronchial tree

The trachea runs from C6 to its bifurcation at the sternal angle (T4 to T5), held open by 16 to 20 C-shaped hyaline cartilage rings whose open posterior ends are bridged by the trachealis muscle, adjacent to the esophagus. The carina is the internal ridge at the bifurcation and is highly sensitive to cough. The right main bronchus is wider, shorter and more vertical than the left, so aspirated objects and a misplaced endotracheal tube usually go right, typically into the posterior segment of the right upper lobe or the superior segment of the right lower lobe when the patient is supine.

Each main bronchus divides into lobar (secondary) bronchi (three on the right, two on the left) and then segmental (tertiary) bronchi, each supplying a bronchopulmonary segment: a surgically resectable pyramidal unit with its own segmental bronchus and segmental artery centrally and intersegmental veins at its margins. There are about 10 segments on the right and 8 to 10 on the left. Branching continues to bronchioles (no cartilage, no glands), terminal bronchioles (end of the conducting zone), respiratory bronchioles, alveolar ducts and alveoli (the respiratory zone).

Lungs and pleura

The pleura is a serous membrane: the visceral pleura covers the lung surface and is insensitive to pain (autonomic supply), while the parietal pleura lines the thoracic wall (costal), the diaphragm (diaphragmatic), the mediastinum (mediastinal) and the apex (cervical pleura or cupula). The costal parietal pleura is supplied by intercostal nerves, so pleurisy produces sharp, localized chest wall pain; the mediastinal and central diaphragmatic pleura are supplied by the phrenic nerve, referring pain to the shoulder. The cupula rises about 2 to 3 cm above the medial third of the clavicle, so a needle or penetrating wound at the root of the neck can cause pneumothorax. The costodiaphragmatic recess is the lowest part of the pleural cavity and is where fluid collects first (blunting of the costophrenic angle on an upright film).

Surface projections

Use the "rule of twos" style pattern: the pleura reaches lower than the lung by about two rib spaces.

The oblique fissure runs from about the T3 spinous process posteriorly to the sixth costal cartilage anteriorly, roughly following the medial border of the abducted scapula. The horizontal fissure of the right lung follows the fourth costal cartilage from the sternum to meet the oblique fissure in the midaxillary line. As a result, the lower lobes are heard mostly from behind and the right middle lobe from the front. Because the pleura dips below the twelfth rib medially, a posterior approach to the kidney or a poorly placed needle at the thoracolumbar junction can enter the pleural cavity.

Diaphragm and the muscles of respiration

The diaphragm is the principal muscle of quiet inspiration. It arises from the xiphoid (sternal part), the inner surfaces of the lower six costal cartilages (costal part) and the lumbar vertebrae by two crura (right crus from L1 to L3, left from L1 to L2) and the medial and lateral arcuate ligaments, and inserts into its own central tendon. It is innervated by the phrenic nerve (C3, C4, C5), which "keeps the diaphragm alive", so a cord injury above C3 requires ventilatory support. The lower costal margins receive sensory fibers from the lower intercostal nerves.

Memory aid: "I 8 10 Eggs At 12" (IVC 8, esophagus 10, aorta 12). Quiet inspiration uses the diaphragm (descends, increasing vertical diameter) with the external intercostals and the interchondral part of the internal intercostals elevating the ribs (the upper ribs move in a "pump handle" fashion that increases anteroposterior diameter; the lower ribs move in a "bucket handle" fashion that increases transverse diameter). Forced inspiration recruits the scalenes (which fix and elevate ribs 1 and 2), sternocleidomastoid, pectoralis minor, serratus posterior superior and, with the arms fixed, pectoralis major and latissimus dorsi. Quiet expiration is passive, driven by elastic recoil of the lungs and chest wall. Forced expiration uses the abdominal muscles (rectus abdominis, external and internal obliques, transversus abdominis), the internal intercostals (interosseous part) and the quadratus lumborum, which anchors the twelfth rib.

Section 3The digestive system

Oral cavity to stomach

The parotid duct (Stensen) pierces the buccinator to open opposite the upper second molar; the submandibular duct (Wharton) opens on the sublingual papilla beside the frenulum. The anterior two thirds of the tongue receive general sensation from the lingual nerve (V3) and taste from the chorda tympani (VII); the posterior third receives both from the glossopharyngeal (IX). All tongue muscles are supplied by the hypoglossal nerve (XII) except palatoglossus (vagus).

The esophagus is about 25 cm long, from C6 to the cardia of the stomach at about T11. Its upper third is skeletal muscle, the middle third mixed and the lower third smooth muscle. It has three classic constrictions: at the cricopharyngeus (C6), where it is crossed by the aortic arch and left main bronchus (T4 to T5), and where it passes the diaphragm (T10). These are sites where swallowed objects lodge and strictures form.

The stomach has a cardia, fundus, body and pyloric part (antrum, canal and sphincter), with a lesser curvature facing up and right and a greater curvature down and left. The stomach lies mostly in the left upper quadrant and epigastrium and is related posteriorly to the stomach bed: the pancreas, left kidney and adrenal, spleen, splenic artery and transverse mesocolon, all behind the lesser sac.

Small and large intestine

The duodenum (about 25 cm) is C-shaped around the head of the pancreas and is mostly retroperitoneal. Its four parts are the superior (first, L1, the duodenal cap or bulb, intraperitoneal at its start and the usual site of peptic ulcers), descending (second, L1 to L3, receiving the bile and pancreatic ducts at the major duodenal papilla through the hepatopancreatic ampulla and sphincter of Oddi), horizontal (third, L3, crossed anteriorly by the superior mesenteric vessels) and ascending (fourth, L2 to L3, ending at the duodenojejunal flexure, suspended by the suspensory muscle or ligament of Treitz). The ligament of Treitz separates upper from lower GI bleeding.

The large intestine includes the cecum and appendix, ascending, transverse, descending and sigmoid colon, rectum and anal canal. The appendix arises from the posteromedial cecum where the three teniae converge; its base projects to McBurney point, one third of the way from the anterior superior iliac spine to the umbilicus. Early appendicitis pain is periumbilical (midgut visceral afferents to T10) and then shifts to the right lower quadrant once the parietal peritoneum is irritated. The rectum begins at S3, has no teniae and lies in the curve of the sacrum. The anal canal is divided by the pectinate line: above it, the epithelium is endodermal, supplied by the superior rectal artery, drained to the portal system and internal iliac nodes and innervated by painless autonomic fibers (internal hemorrhoids are painless); below it, the epithelium is ectodermal, supplied by the inferior rectal artery, drained systemically and to the superficial inguinal nodes and innervated by the somatic inferior rectal (pudendal) nerve (external hemorrhoids and fissures are painful).

The peritoneum

The parietal peritoneum lines the abdominal wall and is somatically innervated and precisely localized; the visceral peritoneum covers organs and refers poorly localized pain. Organs are either intraperitoneal (suspended by a mesentery) or retroperitoneal. The greater omentum hangs from the greater curvature of the stomach like an apron; the lesser omentum runs from the lesser curvature and duodenum to the liver and has two parts, the hepatogastric and hepatoduodenal ligaments. The free edge of the hepatoduodenal ligament contains the portal triad: the portal vein (posterior), the common bile duct (right anterior) and the proper hepatic artery (left anterior). Behind it lies the omental (epiploic) foramen of Winslow, the opening from the greater sac into the lesser sac (omental bursa) behind the stomach.

Liver, gallbladder, pancreas and spleen

The liver is the largest gland, in the right upper quadrant under the diaphragm, protected by ribs 7 to 11. Anatomically it has right, left, caudate and quadrate lobes, divided on its anterior surface by the falciform ligament (carrying the ligamentum teres, the remnant of the umbilical vein). Functionally, it is divided by the plane through the gallbladder fossa and IVC (Cantlie line) into right and left lobes and into eight Couinaud segments, each with its own portal triad branch. The bare area on its posterior surface touches the diaphragm without peritoneum, bounded by the coronary ligaments. The liver receives about 75% of its blood from the portal vein and 25% from the hepatic artery; it drains by three hepatic veins into the IVC.

The gallbladder lies in a fossa on the visceral surface of the liver; its fundus projects to the tip of the right ninth costal cartilage where the linea semilunaris meets the costal margin (the site of Murphy sign). Bile flows from the right and left hepatic ducts into the common hepatic duct, joins the cystic duct to form the common bile duct, which runs behind the first part of the duodenum and through the pancreatic head to join the main pancreatic duct at the ampulla. The cystic artery usually arises from the right hepatic artery within the hepatobiliary (Calot) triangle, bounded by the cystic duct, common hepatic duct and inferior liver surface.

The pancreas lies retroperitoneally across L1 to L2: the head in the duodenal C (with the uncinate process hooking behind the superior mesenteric vessels), then the neck, body and the tail, which reaches the splenic hilum in the splenorenal ligament (the only intraperitoneal part). The main pancreatic duct (Wirsung) joins the bile duct; the accessory duct (Santorini) opens at the minor papilla. Pancreatic pain is deep epigastric pain boring through to the back at T12 to L1, often eased by leaning forward.

The spleen sits in the left upper quadrant along ribs 9 to 11 in the midaxillary line, behind the stomach and above the left kidney and colic flexure. A normal spleen is not palpable. It is held by the gastrosplenic and splenorenal ligaments and is the most commonly injured abdominal organ in blunt trauma, especially with left lower rib fractures; referred pain to the left shoulder (Kehr sign) follows diaphragmatic irritation by blood.

Blood supply and the portal system

The marginal artery links the SMA and IMA along the colon.

The hepatic portal vein forms behind the neck of the pancreas at about L2 by the union of the superior mesenteric vein and the splenic vein (the inferior mesenteric vein usually joins the splenic vein). It carries nutrient-rich blood from the gut, spleen, pancreas and gallbladder to the liver sinusoids. When flow through the liver is obstructed (cirrhosis), portal hypertension diverts blood through portocaval anastomoses.

Section 4The urogenital system

Kidneys and ureters

The kidneys lie retroperitoneally on the posterior abdominal wall against the psoas major, quadratus lumborum and diaphragm, from about T12 to L3. The right kidney sits lower than the left because of the liver. The hilum faces medially at about L1 (the transpyloric plane) and contains, from anterior to posterior, the renal vein, renal artery and renal pelvis (VAP). Each kidney is wrapped in a fibrous capsule, then perirenal fat, then the renal (Gerota) fascia, then pararenal fat. Rib 12 crosses the posterior surface of both kidneys, and the costovertebral angle (between rib 12 and the erector spinae) is where renal tenderness is elicited by percussion (Murphy punch).

Internally, the outer cortex contains renal corpuscles and convoluted tubules; the inner medulla forms 8 to 18 renal pyramids whose apices (papillae) empty into minor calyces, which join to form two or three major calyces and the renal pelvis. Each renal artery divides into five segmental arteries that are end arteries (no anastomoses), then interlobar, arcuate, interlobular (cortical radiate) and afferent arterioles.

The ureter (about 25 cm) descends on the psoas major, crosses the pelvic brim anterior to the bifurcation of the common iliac artery, runs along the lateral pelvic wall and enters the bladder obliquely. It is crossed anteriorly by the gonadal vessels in the abdomen and, in the pelvis, passes under the uterine artery in females ("water under the bridge") and under the ductus deferens in males. Stones lodge at three constrictions: the ureteropelvic junction, the pelvic brim (crossing the iliac vessels) and the ureterovesical junction. Renal colic travels with T11 to L2 afferents and radiates from loin to groin, into the scrotum or labia and the medial thigh.

Bladder and urethra

The urinary bladder lies behind the pubic symphysis in the pelvis when empty and rises into the abdomen above the pubis when full, where it can be ruptured intraperitoneally by a blow. The trigone is a smooth triangle on the posterior wall between the two ureteric openings and the internal urethral orifice; it is mesodermal in origin and a common site of infection. The detrusor muscle is supplied by pelvic splanchnic nerves (S2 to S4, parasympathetic), which contract it for voiding; sympathetic fibers (T11 to L2) relax the detrusor and contract the internal sphincter, and the somatic pudendal nerve (S2 to S4) controls the voluntary external urethral sphincter. "S2, 3, 4 keeps the pee off the floor."

The female urethra is about 4 cm, opening in the vestibule anterior to the vagina, so ascending infection is common. The male urethra (about 20 cm) has preprostatic, prostatic (receives ejaculatory ducts and prostatic ducts), membranous (through the external sphincter, the narrowest and least distensible part, ruptured in pelvic fractures) and spongy (penile) parts, the last receiving the bulbourethral gland ducts.

Reproductive organs

The uterosacral ligaments run from the cervix to the front of the sacrum. Uterine, ovarian and cervical conditions such as endometriosis, dysmenorrhea and retroversion can therefore cause sacral and low back pain, which the chiropractor must distinguish from sacroiliac dysfunction by history (cyclic timing) and examination. The rectouterine pouch (of Douglas) is the lowest point of the female peritoneal cavity and collects blood or pus.

Pelvic floor and perineum

The pelvic diaphragm is formed by the levator ani (puborectalis, pubococcygeus and iliococcygeus) and the coccygeus (ischiococcygeus). It supports the pelvic viscera, resists increases in intra-abdominal pressure, and the puborectalis sling maintains the anorectal angle for fecal continence. It is supplied by the nerve to levator ani (S3, S4) and the inferior rectal branch of the pudendal nerve. The coccygeus overlies the sacrospinous ligament and attaches to the coccyx, so coccygeal manipulation and coccydynia involve this muscle.

The perineum is the diamond-shaped region below the pelvic diaphragm, bounded by the pubic symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligaments and coccyx. A line between the ischial tuberosities divides it into the anterior urogenital triangle (external genitalia, perineal membrane, deep and superficial perineal pouches) and the posterior anal triangle (anal canal, external anal sphincter and the fat-filled ischioanal fossae). The perineal body is a fibromuscular node at the center where many muscles attach; tearing it in childbirth weakens pelvic floor support.

The pudendal nerve (S2 to S4) leaves the pelvis through the greater sciatic foramen below piriformis, hooks around the sacrospinous ligament near the ischial spine and re-enters through the lesser sciatic foramen to run in the pudendal (Alcock) canal on the obturator internus fascia. It supplies the external anal and urethral sphincters and the skin of the perineum.

Section 5The endocrine glands

Endocrine glands are ductless and richly vascular; their anatomy matters clinically for surgery, for the mass effects of enlargement and for the structures they lie against.

Pituitary and hypothalamus

The anterior pituitary (adenohypophysis) develops from Rathke pouch, an upgrowth of oral ectoderm, and secretes GH, prolactin, TSH, ACTH, FSH and LH under control of hypothalamic releasing and inhibiting hormones delivered by the portal system. The posterior pituitary (neurohypophysis) is a downgrowth of the diencephalon; it stores and releases ADH and oxytocin made in the supraoptic and paraventricular nuclei and carried down the hypothalamohypophyseal tract in the infundibulum. A pituitary adenoma growing upward compresses the decussating nasal retinal fibers in the optic chiasm, producing bitemporal hemianopia; lateral growth into the cavernous sinus affects the ocular motor nerves.

Thyroid and parathyroids

The thyroid develops from the floor of the pharynx at the foramen cecum of the tongue and descends along the thyroglossal duct; remnants form a midline neck cyst that moves upward when the tongue protrudes, and a pyramidal lobe may persist. Follicular cells make T3 and T4; parafollicular (C) cells, from the ultimobranchial body, make calcitonin. The relationship of the thyroid arteries to the laryngeal nerves is the key surgical fact: the superior thyroid artery is ligated close to the gland to spare the external laryngeal nerve (cricothyroid, voice pitch), and the inferior thyroid artery is ligated away from the gland to spare the recurrent laryngeal nerve. Inadvertent removal of the parathyroids during thyroidectomy causes hypocalcemia with tetany (Chvostek and Trousseau signs).

Adrenal glands

Each adrenal has two embryologically distinct parts. The cortex is mesodermal and has three zones: glomerulosa (aldosterone), fasciculata (cortisol) and reticularis (androgens), remembered as "salt, sugar, sex: the deeper you go, the sweeter it gets." The medulla is derived from neural crest; its chromaffin cells are modified postganglionic sympathetic neurons that receive preganglionic fibers directly from the thoracic splanchnic nerves (about T10 to L1) and secrete epinephrine and norepinephrine. A pheochromocytoma produces episodic headache, sweating, palpitations and hypertension.

Islets, gonads, pineal and thymus

The islets of Langerhans make up about 1 to 2% of pancreatic mass: alpha cells (glucagon) at the periphery, beta cells (insulin), the most numerous, in the core, and delta cells (somatostatin). Islet venous blood drains into the portal system, so the liver sees the highest concentration of insulin. The pineal gland secretes melatonin in darkness, regulated by a retinohypothalamic pathway through the suprachiasmatic nucleus and the superior cervical sympathetic ganglion; its calcification makes it a useful midline marker on skull films and CT. The thymus develops from the third pharyngeal pouch, educates T lymphocytes and secretes thymic hormones; thymoma is associated with myasthenia gravis.

Words worth knowing

The Vertebral Column: Bones, Joints, Ligaments and Muscles

The vertebral column is the chiropractor's home ground, and Spinal Anatomy carries more weight on Part I than any other domain. This chapter builds the spine from its bones up: the features that identify every region, the joints and ligaments that allow and limit motion, the muscles that move and stabilize it, and the developmental story that explains the variants you will meet on every second radiograph.

Section 1Osteology of the axial skeleton

The typical vertebra

The adult column usually has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral and about 4 coccygeal (3 to 5). Every typical vertebra has two functional parts. The vertebral body anteriorly is a short cylinder of cancellous bone inside a thin cortical shell that bears compressive load; its superior and inferior surfaces are covered by cartilaginous endplates and edged by a raised ring apophysis. The vertebral (neural) arch posteriorly is formed by two pedicles, which project back from the body, and two laminae, which meet in the midline. Body and arch enclose the vertebral foramen; stacked, these form the vertebral canal that houses the cord, meninges and cauda equina.

Seven processes arise from the arch: one spinous process where the laminae meet, two transverse processes at the pedicle-lamina junction, and four articular processes (two superior, two inferior) carrying the facets of the zygapophyseal joints. The region of lamina between the superior and inferior articular processes is the pars interarticularis, the site of spondylolysis. The superior and inferior vertebral notches above and below each pedicle form, with the adjacent vertebra, the intervertebral foramen.

Regional features

The atypical vertebrae

C1, the atlas, has no body and no spinous process. It is a ring of an anterior arch (with an anterior tubercle and a facet for the dens on its posterior surface), a posterior arch (with a posterior tubercle and, on its upper surface, a groove for the vertebral artery and the C1 nerve), and two lateral masses. The superior articular facets are concave, kidney shaped and face up and medially to cradle the occipital condyles; the inferior facets are flat to slightly concave and nearly circular. In some people a bony bridge over the vertebral artery groove forms a posterior ponticle (arcuate foramen, Kimmerle anomaly).

C2, the axis, carries the dens (odontoid process), which projects up from the body to articulate with the anterior arch of the atlas, held by the transverse ligament. Its superior articular facets sit on the body and pedicles, lateral to the dens, rather than behind the pedicles; this means the C1 to C2 joints lie anterior to the C2 to C3 joints. C2 has a large, bifid spinous process, the first prominent spinous process palpable below the occiput, and its pars region (isthmus) is where a hangman fracture occurs.

C7, the vertebra prominens, has a long, non-bifid spinous process that is usually the most prominent in the lower neck (though T1 is sometimes more prominent; flexing and extending the neck helps, because C6 slides forward under the palpating finger while C7 stays put). Its transverse foramina are small and usually transmit only accessory vertebral veins, because the vertebral artery normally enters the transverse foramen of C6. The anterior tubercle of C6 is large and called the carotid tubercle, against which the common carotid artery can be compressed. A cervical rib may articulate with C7.

The L5 vertebra has the largest body, which is wedge shaped (taller anteriorly), contributing to the lumbosacral angle. Its stout transverse processes arise from the pedicles and the body and anchor the iliolumbar ligaments. Its inferior facets face forward more than other lumbar facets (more coronal), which resists forward slip of L5 on S1.

The sacrum and coccyx

The sacrum is formed by five fused vertebrae and transmits body weight from L5 to the pelvis through the sacroiliac joints. Its base faces upward and carries the S1 body, whose projecting anterior lip is the sacral promontory, and the two alae (fused transverse and costal elements). Its concave anterior (pelvic) surface has four pairs of anterior sacral foramina for the ventral rami; the dorsal surface has four pairs of posterior sacral foramina for the dorsal rami. Down the dorsal midline runs the median sacral crest (fused spinous processes); lateral to it are the intermediate crests (fused articular processes) and, beyond the foramina, the lateral crests (fused transverse processes). The laminae of S5 (and often S4) fail to fuse, leaving the sacral hiatus, flanked by the sacral cornua and used for caudal epidural injection. Laterally, the ear-shaped auricular surface (spanning S1 to S3) articulates with the ilium, and behind it the rough sacral tuberosity anchors the interosseous ligament. The dural sac ends at about S2.

The coccyx has three to five rudimentary segments; the first has coccygeal cornua that articulate with the sacral cornua. It anchors the gluteus maximus, coccygeus, levator ani (anococcygeal body) and the sacrotuberous and sacrospinous ligaments.

Ribs, sternum and the skull base

There are 12 pairs of ribs: true ribs 1 to 7 join the sternum by their own costal cartilages; false ribs 8 to 10 join the cartilage above; floating ribs 11 and 12 end free. A typical rib (3 to 9) has a head with two facets (for its own vertebra and the one above, separated by a crest that the intra-articular ligament anchors to the disc), a neck, a tubercle articulating with the transverse process of its own vertebra, an angle and a costal groove sheltering the intercostal vein, artery and nerve (VAN, from above down). Rib 1 is short, broad and flat, with a single head facet, a scalene tubercle for anterior scalene, and grooves for the subclavian vein in front and subclavian artery behind. Rib 2 has a tuberosity for serratus anterior. Ribs 10 to 12 have a single head facet, and 11 and 12 lack necks and tubercles.

The sternum has a manubrium (jugular notch at about T2 to T3, clavicular notches, rib 1 cartilage), a body (ribs 2 to 7, with the sternal angle at rib 2) and a xiphoid process (xiphisternal joint at about T9).

The upper cervical spine is inseparable from the occipital bone. The occipital condyles flank the foramen magnum, which transmits the medulla, the vertebral arteries, the anterior and posterior spinal arteries, the spinal roots of the accessory nerve and the tectorial membrane. The hypoglossal canal pierces the base of each condyle, and the jugular foramen (CN IX, X, XI, internal jugular vein) lies lateral to it. The basiocciput (clivus) receives the apical ligament and tectorial membrane; the external occipital protuberance (inion) and the superior and inferior nuchal lines anchor the nuchal ligament and posterior neck muscles; the mastoid process anchors SCM, splenius capitis and longissimus capitis.

Section 2Arthrology of the spine

Adjacent vertebrae from C2 to S1 are joined at three points: an anterior intervertebral disc (a secondary cartilaginous joint, or symphysis) and two posterior zygapophyseal (facet) joints (synovial plane joints). Together with the ligaments they form the motion segment (functional spinal unit), the smallest unit that shows the mechanical behavior of the whole column. In the cervical spine, small uncovertebral joints add two more contacts.

The intervertebral disc

There are 23 discs, the first between C2 and C3 and the last between L5 and S1; there is no disc between the occiput and C1 or between C1 and C2. Discs make up roughly a quarter of the length of the presacral column, which is why height decreases through the day as they lose water under load. They are thickest in the lumbar region and thinnest in the upper thoracic region; cervical and lumbar discs are thicker anteriorly, which helps create the lordoses.

The disc is innervated only in the outer third of the annulus. Posteriorly the supply is the recurrent meningeal (sinuvertebral) nerve, which also supplies the posterior longitudinal ligament, the anterior dura and the vertebral periosteum; laterally and anteriorly fibers come from the ventral rami and gray rami communicantes. Painful internal disc disruption and the ingrowth of nerves along radial fissures in degenerated discs explain discogenic pain without nerve root compression.

Zygapophyseal joints

The facet joints are true synovial joints with hyaline cartilage, a fibrous capsule lined by synovium, and intra-articular meniscoid folds (fibroadipose wedges at the joint poles) that can become entrapped and are one proposed basis for acute locked back. Each joint is supplied by the medial branch of the dorsal ramus from the level above and the level of the joint, the basis of medial branch blocks and radiofrequency neurotomy. Their orientation is the main determinant of regional motion.

Uncovertebral joints

The uncinate processes rise from the lateral margins of the superior surfaces of C3 to C7 (and sometimes T1) bodies and contact the beveled inferior lateral edges of the body above. The cleft that develops here in childhood (around age 10) is the uncovertebral joint (joint of Luschka). Whether it is a true synovial joint or a degenerative fissure in the disc is debated, but its clinical significance is clear: the uncinates limit lateral translation and lateral flexion, guide flexion and extension, and form the anteromedial boundary of the cervical intervertebral foramen. Uncovertebral hypertrophy is a leading cause of cervical foraminal stenosis, best seen on oblique films.

The intervertebral foramen

Each foramen is bounded above and below by pedicles, anteriorly by the posterolateral vertebral bodies and the disc (and the uncinate in the neck), and posteriorly by the zygapophyseal joint capsule and the lateral ligamentum flavum. It transmits the spinal nerve (or its roots) with its dural sleeve, the dorsal root ganglion (in the foramen in most of the lumbar spine), the spinal branch of a segmental artery, the intervertebral veins linking internal and external plexuses, the recurrent meningeal nerve, lymphatics and fat. The nerve occupies about a third to a half of the foramen. Foraminal size falls with extension and ipsilateral lateral flexion, the basis of the Spurling maneuver.

Cervical nerves C1 to C7 exit above the vertebra of the same number, and C8 exits between C7 and T1; from T1 down, each nerve exits below its numbered vertebra. Because the lumbar roots cross the disc above the foramen they exit through, a typical posterolateral L4 to L5 herniation compresses the traversing L5 root, while a far lateral (foraminal) herniation at the same disc compresses the exiting L4 root. A C5 to C6 cervical disc compresses the C6 root.

The upper cervical joints

The atlanto-occipital joints are paired synovial condyloid (ellipsoid) joints between the convex occipital condyles and the concave superior facets of the atlas. Their main motion is flexion and extension (nodding, the "yes" joint, about 15 to 25 degrees), with a few degrees of lateral flexion and negligible rotation.

The atlantoaxial articulation comprises three synovial joints: a median pivot (trochoid) joint between the dens and the anterior arch of the atlas in front and the transverse ligament behind (with a bursa between ligament and dens), and two lateral atlantoaxial joints between the lateral masses, which are planar and biconvex in their cartilage. Their chief motion is rotation (the "no" joint), about 40 degrees to each side, roughly half of all cervical rotation. Because the lateral joint surfaces are biconvex, the atlas descends slightly as it rotates, which tightens the capsules and helps explain why end-range rotation stresses the vertebral arteries.

Costovertebral joints

Each typical rib makes two synovial joints with the spine. The joint of the head of the rib links the two head facets with the demifacets of adjacent bodies and the disc between, divided into two cavities by the intra-articular ligament and reinforced by the radiate ligament (ribs 1, 10, 11 and 12 have a single cavity). The costotransverse joint links the tubercle with the transverse costal facet (ribs 1 to 10 only), reinforced by the costotransverse, lateral and superior costotransverse ligaments. The axis through the two joints determines rib motion: in the upper ribs it is near coronal, giving pump-handle elevation of the sternum; in the lower ribs it is closer to sagittal, giving bucket-handle lateral expansion.

The sacroiliac joint

The sacroiliac joint joins the auricular surfaces of the sacrum and ilium. Its anterior and inferior portion is a synovial joint whose sacral surface is covered by thicker hyaline cartilage and whose iliac surface by thinner fibrocartilage-like cartilage; the posterior and superior portion is a syndesmosis held by the massive interosseous ligament. The surfaces are irregular, with an interlocking ridge and furrow that increase with age, providing form closure; the ligaments, thoracolumbar fascia and muscles (gluteus maximus, latissimus via the fascia, biceps femoris via the sacrotuberous ligament, multifidus) compress it and provide force closure.

Motion is small, a few degrees of rotation and one or two millimeters of translation. Nutation is the sacral base moving anteriorly and inferiorly while the coccyx moves posteriorly (occurs when standing, in trunk flexion and in late pregnancy); it tightens the sacrotuberous, sacrospinous and interosseous ligaments and is the more stable, close-packed position. Counternutation is the reverse and is resisted by the long dorsal sacroiliac ligament. Sacroiliitis is an early feature of the seronegative spondyloarthropathies, especially ankylosing spondylitis, and affects the synovial (lower) part first.

Curves and regional motion

The fetal column is one C-shaped curve concave forward. The thoracic and sacral kyphoses are primary curves retained from this posture. The cervical lordosis develops as the infant holds up the head (around 3 to 4 months) and the lumbar lordosis as the child sits, stands and walks (about 9 to 18 months); these are secondary curves, produced mainly by wedge-shaped discs. Typical adult values: cervical lordosis about 30 to 40 degrees, thoracic kyphosis about 20 to 45 degrees, lumbar lordosis about 40 to 60 degrees. The lumbosacral (Ferguson) angle, between the top of the sacrum and the horizontal on a standing lateral film, averages about 41 degrees; an increased angle increases shear at L5 to S1.

Section 3Syndesmology: the ligaments of the spine and pelvis

The long ligaments

The anterior longitudinal ligament (ALL) is a broad, strong band running down the anterior surfaces of the vertebral bodies and discs from the basiocciput and anterior tubercle of the atlas (where it continues as the anterior atlanto-occipital membrane) to the pelvic surface of the sacrum. It is widest in the lumbar spine, firmly attached to the bodies and their margins and less firmly to the discs, and it limits extension.

The posterior longitudinal ligament (PLL) runs inside the vertebral canal on the posterior surfaces of the bodies and discs from C2 (above which it continues as the tectorial membrane) to the sacrum. It is broad in the cervical region but becomes narrow in the lumbar region, with a serrated (denticulate) outline that flares over each disc and narrows over each body. It is attached more firmly to the discs than to the bodies, leaving room behind the bodies for the basivertebral veins. It limits flexion, is richly innervated by the sinuvertebral nerve, and its narrowness in the lumbar spine leaves the posterolateral annulus unreinforced, which is why most lumbar herniations are posterolateral. Ossification of the PLL (OPLL), especially in the cervical spine, causes myelopathy.

The short ligaments

The craniovertebral ligaments

The occiput, atlas and axis are held together by an internal set of ligaments that, from anterior (deep) to posterior (superficial) inside the canal, are the apical ligament, the alar ligaments, the cruciform ligament and the tectorial membrane, with the dura behind them.

The ligaments of the pelvis

The iliolumbar ligament runs from the tip of the L5 transverse process (often also L4) to the posterior inner lip of the iliac crest. It is the key ligamentous link between the lumbar spine and pelvis, limiting forward slip, lateral flexion and rotation of L5 on S1, and it is a recognized pain source in low back pain over the iliac crest.

The greater sciatic foramen is divided by piriformis. Above it pass the superior gluteal nerve and vessels; below it pass the sciatic nerve, inferior gluteal nerve and vessels, posterior femoral cutaneous nerve, nerves to obturator internus and quadratus femoris, and the pudendal nerve and internal pudendal vessels. The lesser sciatic foramen transmits the obturator internus tendon and readmits the pudendal nerve and internal pudendal vessels to the perineum.

Section 4Myology of the axial skeleton

The back muscles fall into three layers. The superficial (extrinsic) layer (trapezius, latissimus dorsi, levator scapulae, rhomboids) moves the upper limb and is supplied by ventral rami or the accessory nerve. The intermediate layer (serratus posterior superior and inferior) is respiratory and supplied by intercostal nerves. The deep (intrinsic, true) back muscles move and stabilize the column and are all supplied by dorsal rami of spinal nerves, a rule worth memorizing because it lets you answer innervation questions about any of them.

The suboccipital region

Four small muscles lie deep to semispinalis capitis between the occiput, atlas and axis. They are all supplied by the suboccipital nerve (dorsal ramus of C1), which is purely motor. They have a high density of muscle spindles and act mostly as fine postural adjusters and proprioceptive sensors of head position.

The suboccipital triangle is bounded by rectus capitis posterior major (superomedially), obliquus capitis superior (superolaterally) and obliquus capitis inferior (inferiorly). Its roof is semispinalis capitis, and its floor is the posterior atlanto-occipital membrane and posterior arch of the atlas. It contains the third part of the vertebral artery in its groove on the atlas and the suboccipital nerve. The greater occipital nerve (dorsal ramus of C2) emerges below obliquus capitis inferior, curves up across the triangle and pierces semispinalis capitis and trapezius to supply the posterior scalp; entrapment produces occipital neuralgia. The third occipital nerve (C3) supplies the C2 to C3 facet joint, a common source of cervicogenic headache.

The deep back muscles

The splenius capitis and cervicis form a bandage over the deeper muscles of the neck, arising from the nuchal ligament and spinous processes of C7 to T6 and inserting into the mastoid process and superior nuchal line (capitis) or the transverse processes of C1 to C3 (cervicis). Acting alone, they rotate the head and neck to the same side.

The erector spinae is the large intermediate column lateral to the spinous processes. It arises by a broad common tendon from the posterior iliac crest, dorsal sacrum, sacroiliac ligaments and lumbar spinous processes and divides into three columns, lateral to medial: iliocostalis (to the angles of the ribs and cervical transverse processes), longissimus (to transverse processes and, as longissimus capitis, the mastoid process) and spinalis (spinous to spinous, best developed in the thorax); "I Love Spaghetti". Bilaterally it extends the column and controls flexion eccentrically; unilaterally it laterally flexes to the same side. It falls electrically silent at full flexion (the flexion relaxation phenomenon), when ligaments and fascia carry the load; loss of this silence is a marker of chronic low back pain.

The transversospinalis group runs obliquely upward and medially, so acting on one side it rotates the column to the opposite side; bilaterally it extends. The deepest segmental muscles are the interspinales (spinous to spinous, cervical and lumbar), the intertransversarii (transverse to transverse; the anterior cervical and lateral lumbar parts are supplied by ventral rami) and the levatores costarum (transverse processes C7 to T11 to the rib below), which elevate the ribs.

Prevertebral and scalene muscles

The prevertebral muscles lie on the front of the cervical spine and are supplied by ventral rami: longus colli (from T3 to the atlas, the main deep neck flexor and a cervical lordosis stabilizer), longus capitis (C3 to C6 transverse processes to the basiocciput, flexes the head), rectus capitis anterior (C1 lateral mass to basiocciput, flexes the head) and rectus capitis lateralis (C1 transverse process to the jugular process, laterally flexes).

The roots and trunks of the brachial plexus and the subclavian artery pass between anterior and middle scalenes through the interscalene triangle, while the subclavian vein passes in front of the anterior scalene. Scalene hypertrophy or a cervical rib narrowing this space produces neurogenic or arterial thoracic outlet syndrome (Adson test).

Abdominal wall, quadratus lumborum, psoas and fascia

The aponeuroses of the three flat muscles form the rectus sheath. Above the arcuate line (about a third of the way from umbilicus to pubis), the internal oblique aponeurosis splits around the rectus; below it, all three aponeuroses pass in front, leaving only transversalis fascia behind the muscle.

The thoracolumbar fascia in the lumbar region has three layers. The posterior layer arises from the spinous processes and supraspinous ligaments and covers the erector spinae; the latissimus dorsi and, through it, the contralateral gluteus maximus attach to it, creating a diagonal "posterior oblique sling" across the sacroiliac joints. The middle layer attaches to the tips of the transverse processes and separates erector spinae from quadratus lumborum. The anterior layer covers the front of quadratus lumborum. The posterior and middle layers fuse at the lateral raphe, where transversus abdominis and internal oblique attach. Tension from these muscles and from the expanding erector spinae within the fascial envelope (the hydraulic amplifier effect) stiffens the lumbar spine and pelvis for load transfer.

Section 5Development and variants of the spine

Somites, sclerotomes and resegmentation

During the third and fourth weeks, the paraxial mesoderm on each side of the neural tube and notochord segments into paired blocks called somites, beginning in the occipital region around day 20 and reaching about 42 to 44 pairs. Each somite differentiates into a ventromedial sclerotome, which forms the vertebrae and ribs, and a dorsolateral dermomyotome, which gives the dermis of the back (dermatome) and skeletal muscle (myotome). The epaxial part of the myotome forms the intrinsic back muscles (hence their dorsal ramus supply), and the hypaxial part forms the limb and body wall muscles (ventral rami).

Sclerotome cells migrate to surround the notochord and neural tube. Each sclerotome then splits into a loosely packed cranial half and a densely packed caudal half, and in the process of resegmentation the caudal half of one sclerotome fuses with the cranial half of the next to form one vertebral body. This explains three adult facts: each vertebra is intersegmental, built from two adjacent somites; the spinal nerves, which grew out to their own myotomes, now pass between vertebrae through the intervertebral foramina; and the segmental myotomes span two vertebrae, so the deep muscles can move one bone on the next. Because the first cervical sclerotome half joins the occiput, there are eight cervical nerves but only seven cervical vertebrae.

The notochord regresses where it is surrounded by the developing bodies but persists between them, expanding as the nucleus pulposus; the surrounding mesenchyme of the dense sclerotome half forms the annulus fibrosus. Notochordal remnants also form the apical ligament of the dens, and rare malignant chordomas arise from such remnants, most often in the sacrococcygeal region and clivus.

Chondrification and ossification

The mesenchymal (membranous) vertebra chondrifies from about the sixth week, and ossification begins around the eighth week. A typical vertebra has three primary ossification centers: one for the centrum and one for each half of the neural arch. Note that the centrum is not the whole adult body: the posterolateral corners of the body come from the arch centers, joined at the neurocentral synchondroses, which fuse between about 3 and 6 years. The two halves of the arch fuse posteriorly in the first year or two (in the lumbar region first). At puberty, secondary centers appear at the tips of the spinous and transverse processes, at the mammillary processes in the lumbar spine and as the ring apophyses on the rims of the bodies; these fuse by about age 25.

Failures of formation and segmentation

• Block vertebra (failure of segmentation): two or more bodies fused because the intervening disc did not form. Congenital blocks, most common at C2 to C3 and C5 to C6, show a rudimentary disc remnant, a narrowed anteroposterior "wasp waist" at the old disc level and often fused posterior elements, which distinguish them from acquired fusion (infection, ankylosing spondylitis, surgery). Motion concentrates at adjacent segments, accelerating their degeneration.
• Klippel-Feil syndrome: multiple congenital cervical fusions with the classic (but often incomplete) triad of short neck, low posterior hairline and restricted motion; associated with Sprengel deformity (undescended scapula, sometimes with an omovertebral bone), renal anomalies, hearing loss and scoliosis.
• Hemivertebra (failure of formation): one lateral half of a body fails to develop, leaving a wedge that causes congenital scoliosis; a posterior hemivertebra causes congenital kyphosis.
• Butterfly vertebra: failure of fusion of the two lateral chondrification centers, with a sagittal cleft giving a butterfly shape on the frontal film; usually incidental.
• Agenesis or hypoplasia of a pedicle, with compensatory sclerosis of the opposite pedicle, which must be distinguished from a lytic metastasis destroying a pedicle (the "winking owl" sign).

Neural tube and arch defects

Spina bifida occulta is failure of the laminae to fuse in the midline, most often at L5 or S1, with intact skin and normal neural tissue; it is seen in a large minority of lumbosacral films and is usually incidental, though a hair tuft, dimple or lipoma over it may signal an underlying tethered cord. Spina bifida cystica includes the meningocele (meninges and CSF herniate through the defect) and the far more serious myelomeningocele (cord or roots in the sac), with paralysis, sphincter dysfunction and often Chiari II malformation and hydrocephalus. These neural tube defects are reduced by periconceptional folic acid, and screened for by maternal serum alpha fetoprotein.

Transitional segments and other variants on film

Spondylolysis, a defect of the pars interarticularis, is not a true congenital anomaly but a fatigue fracture in a genetically susceptible pars, most often at L5 (about 85 to 90% of cases), common in athletes who repeatedly hyperextend (gymnasts, linemen). Seen on an oblique film as the collar on the "Scottie dog" neck. Bilateral defects allow spondylolisthesis, graded by the Meyerding system in quarters of the sacral endplate (grade 1 up to 25%, through grade 4 up to 100%, and spondyloptosis beyond). The dysplastic (congenital) type arises from deficient upper sacral or L5 facets and carries a higher risk of progression and of cauda equina symptoms.

Words worth knowing

The Spinal Cord and Central Nervous System

Every adjustment, every neurological screen and every red flag decision rests on knowing where a lesion sits inside the central nervous system. This chapter walks from the coverings of the cord to the cortex, linking each structure to the deficit it produces when damaged, because that is exactly how the board asks the question and how a patient presents in your office.

Section 1Meninges, cerebrospinal fluid and the ventricles

Three coverings, three spaces

The brain and spinal cord are wrapped by three connective tissue membranes. From outside in they are the dura mater, the arachnoid mater and the pia mater. A simple memory aid is "PAD" read from the inside out (pia, arachnoid, dura).

The cranial dura has two layers. The outer periosteal (endosteal) layer is simply the periosteum of the inner skull and is firmly attached at the sutures and the skull base. The inner meningeal layer is continuous with the spinal dura. Where the two layers separate they enclose the dural venous sinuses (superior and inferior sagittal, straight, transverse, sigmoid, cavernous and others), which drain venous blood and absorb CSF. The meningeal layer also folds inward to form the dural reflections: the falx cerebri between the hemispheres, the tentorium cerebelli between the occipital lobes and the cerebellum, the falx cerebelli between the cerebellar hemispheres, and the diaphragma sellae over the pituitary. The free edge of the tentorium forms the tentorial notch through which the midbrain passes; a supratentorial mass can push the medial temporal lobe (uncus) through this notch, compressing the oculomotor nerve and producing a "blown" ipsilateral pupil.

The spinal dura is a single meningeal layer. It is separated from the periosteum of the vertebral canal by the epidural space, which in the spine is a real space filled with fat and the internal vertebral (Batson) venous plexus. This valveless plexus communicates with pelvic, abdominal and thoracic veins and is the route by which prostate, breast and lung cancers seed the vertebral bodies. The spinal dura is anchored above to the margin of the foramen magnum and to the posterior surfaces of C2 and C3, and the dural sac ends at about S2. Below that the filum terminale picks up a dural covering (the coccygeal ligament, or filum terminale externum) and attaches to the dorsum of the coccyx. Each spinal nerve root carries a sleeve of dura into the intervertebral foramen, where it blends with the epineurium.

In the skull, by contrast, the epidural space is only a potential space because the dura is fused to bone. An epidural hematoma is usually arterial (classically the middle meningeal artery, torn by a temporal bone fracture at the pterion), forms a lens shaped (biconvex) collection that does not cross sutures, and often follows a lucid interval. A subdural hematoma forms between dura and arachnoid when bridging veins tear, is crescent shaped, crosses sutures, and is common in the elderly and in alcoholics with brain atrophy. A subarachnoid hemorrhage from a ruptured berry aneurysm produces the "worst headache of my life" with neck stiffness.

The arachnoid mater is a thin avascular membrane held against the dura by CSF pressure (the subdural space is normally only potential). Delicate trabeculae cross the subarachnoid space to the pia. This space contains CSF, the major arteries and veins of the brain surface, and the cranial and spinal nerve roots. Where the brain surface pulls away from the arachnoid, the space widens into cisterns: the cerebellomedullary cistern (cisterna magna), the pontine cistern, the interpeduncular cistern, the quadrigeminal cistern and, in the spine, the lumbar cistern between about L2 and S2, which contains the cauda equina and is the target of lumbar puncture.

The pia mater is a delicate vascular layer adherent to the surface of the brain and cord, following every sulcus. In the spine the pia forms two specializations. The denticulate ligaments are 21 or so pairs of tooth like lateral extensions that pass between the dorsal and ventral roots and attach to the dura, suspending the cord in the middle of the dural sac; the first is at the foramen magnum and the last lies between T12 and L1 roots. The filum terminale (internum) is a strand of pia that continues from the tip of the conus medullaris to the end of the dural sac at S2.

Cerebrospinal fluid: production, flow and absorption

The adult has roughly 150 mL of CSF at any moment and produces about 500 mL per day, so the whole volume turns over three to four times daily. Most CSF is secreted by the choroid plexus, tufts of fenestrated capillaries covered by a modified ependymal epithelium that projects into the lateral, third and fourth ventricles. The choroid epithelial cells are joined by tight junctions, and these form the blood CSF barrier (the capillaries themselves are leaky; the barrier is the epithelium). Secretion is an active process: Na+/K+ ATPase on the apical surface pumps sodium into the ventricle, carbonic anhydrase supplies bicarbonate, and chloride and water follow. This is why acetazolamide, a carbonic anhydrase inhibitor, reduces CSF production and is used in idiopathic intracranial hypertension.

Compared with plasma, normal CSF is clear and colorless, has much less protein (about 15 to 45 mg/dL), glucose about two thirds of serum glucose (roughly 45 to 80 mg/dL), slightly higher chloride and magnesium, lower potassium and calcium, and fewer than 5 white cells per microliter (all mononuclear). Normal opening pressure in the lateral decubitus position is about 70 to 180 mm H2O (some sources accept up to 200 to 250).

The flow pathway is a favorite board sequence:

1. Lateral ventricles (one in each hemisphere; anterior horn, body, atrium, posterior and inferior horns).
2. Through the paired interventricular foramina (of Monro) into the third ventricle, a midline slit between the two thalami.
3. Through the cerebral aqueduct (of Sylvius) in the midbrain to the fourth ventricle, which lies dorsal to the pons and upper medulla and ventral to the cerebellum.
4. Out of the fourth ventricle through the single midline median aperture (foramen of Magendie) and the two lateral apertures (foramina of Luschka) into the subarachnoid space (cisterna magna and pontine cistern). A memory aid: Magendie is Medial, Luschka is Lateral. A small amount also passes into the central canal of the cord.
5. Around the brain and cord in the subarachnoid space, then absorbed by the arachnoid granulations (villi), which herniate into the superior sagittal sinus and act as one way pressure dependent valves into venous blood.

Hydrocephalus and the ependyma

The ventricles are lined by ependymal cells, ciliated cuboidal glia whose cilia help move CSF. Hydrocephalus is an excess of CSF with ventricular enlargement. In noncommunicating (obstructive) hydrocephalus flow is blocked within the ventricular system, most often at the narrow cerebral aqueduct (aqueductal stenosis) or at the fourth ventricle outlets (as in Dandy Walker malformation or a posterior fossa tumor). Only the ventricles upstream of the block dilate. In communicating hydrocephalus CSF reaches the subarachnoid space but is poorly absorbed, typically because the arachnoid granulations are scarred by prior meningitis or subarachnoid hemorrhage; all ventricles enlarge. Normal pressure hydrocephalus in older adults causes the triad "wet, wobbly and wacky" (urinary incontinence, a magnetic broad based gait, and dementia) because the enlarged lateral ventricles stretch the corona radiata fibers that serve the legs and bladder.

Section 2Gross and internal anatomy of the spinal cord

Shape, length and enlargements

The spinal cord begins at the foramen magnum, continuous with the medulla, and in the adult ends as the cone shaped conus medullaris at about the L1 to L2 disc level. In the newborn it ends at about L3. The cord is roughly 42 to 45 cm long, about the thickness of a finger, and slightly flattened front to back. It has 31 segments defined by the 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. There are 8 cervical nerves but only 7 cervical vertebrae because C1 exits above the atlas and C8 exits below C7.

Two swellings mark the limb segments. The cervical enlargement spans roughly C4 to T1 and supplies the brachial plexus; the lumbosacral (lumbar) enlargement spans roughly L1 (or L2) to S3 and supplies the lumbosacral plexus. They are wider because they hold many more lower motor neurons and interneurons for the limbs.

Because the vertebral column grows longer than the cord during fetal life (the "ascent" of the cord), lower roots must travel downward inside the dural sac before reaching their foramina. Below the conus the lumbar, sacral and coccygeal roots form the cauda equina ("horse's tail") floating in the lumbar cistern, along with the filum terminale.

Segment to vertebra relations

Since the cord is shorter than the canal, a given cord segment lies above the vertebra of the same number. The classic working rule:

The other rule to master is root exit. In the cervical spine each nerve exits above the vertebra of the same number (C6 exits between C5 and C6), and C8 exits between C7 and T1. From T1 downward each nerve exits below its numbered vertebra (L4 exits the L4 to L5 foramen). A posterolateral disc herniation at L4 to L5 usually compresses the traversing L5 root, not the exiting L4 root, because the exiting root has already left high in the foramen; a far lateral (foraminal) herniation at the same level catches the exiting L4.

Surface features

The cord surface shows a deep anterior median fissure (with the anterior spinal artery in it), a shallow posterior median sulcus, and paired posterolateral sulci where the dorsal rootlets enter and anterolateral sulci where the ventral rootlets leave. Above T6 a posterior intermediate sulcus separates the fasciculus gracilis medially from the fasciculus cuneatus laterally. Each segment gives off dorsal (sensory) and ventral (motor) rootlets that fuse into roots and join just distal to the dorsal root ganglion in the intervertebral foramen to form the mixed spinal nerve.

Gray matter: horns, laminae and nuclei

In cross section the central gray matter forms an H or butterfly. Its parts are the dorsal (posterior) horn, which is sensory; the ventral (anterior) horn, which is motor; and, from T1 to L2 and from S2 to S4, an intermediate zone with autonomic cell columns. The lateral horn (intermediolateral cell column) is present only from T1 to L2 (or L3) and contains preganglionic sympathetic neurons. The sacral parasympathetic nucleus sits in a similar position at S2 to S4. The gray commissure around the central canal connects the two sides, and the anterior white commissure ventral to it carries crossing fibers, most importantly the spinothalamic fibers.

Bror Rexed divided the gray matter into ten layers, the Rexed laminae, that recur at every level:

Within lamina IX the motor neurons are arranged somatotopically: medial groups supply axial and trunk muscles and are present at all levels; lateral groups, found only in the enlargements, supply the limbs. Within the lateral group, flexor motor neurons lie dorsal to extensor ones, and those for distal muscles lie lateral to those for proximal muscles.

White matter: three funiculi

The white matter is divided by the roots into a posterior (dorsal) funiculus between the posterior median sulcus and the dorsal roots, a lateral funiculus between the dorsal and ventral roots, and an anterior funiculus between the ventral roots and the anterior median fissure. The posterior funiculus carries the dorsal columns (gracilis and cuneatus). The lateral funiculus carries the lateral corticospinal, rubrospinal, spinocerebellar and lateral spinothalamic tracts. The anterior funiculus carries the anterior corticospinal, vestibulospinal, tectospinal, reticulospinal and anterior spinothalamic tracts and the medial longitudinal fasciculus. Short propriospinal fibers, the fasciculus proprius, hug the gray matter at every level, and Lissauer's dorsolateral tract caps the dorsal horn, carrying pain fibers up or down one or two segments before they synapse.

The proportion of white to gray matter changes with level, and the board asks you to identify a section. The cervical cord has the most white matter (every tract is at full size), is oval, has large ventral horns in the enlargement and shows both gracilis and cuneatus. The thoracic cord is small and round with slender horns, a visible lateral horn and a prominent Clarke nucleus. The lumbar cord is round with large ventral and dorsal horns and relatively less white matter. The sacral cord is small with gray matter dominating and very little white matter.

Blood supply of the cord

Three longitudinal arteries run the length of the cord. The single anterior spinal artery forms from the union of branches of both vertebral arteries at the level of the foramen magnum, runs in the anterior median fissure and supplies the anterior two thirds of the cord: the ventral and lateral horns, the corticospinal and spinothalamic tracts. The paired posterior spinal arteries arise from the vertebral or posterior inferior cerebellar arteries and supply the posterior one third, mainly the dorsal columns and dorsal horns.

These longitudinal channels are too small to supply the whole cord alone, so they are reinforced by segmental medullary (radicular) arteries from the vertebral, ascending and deep cervical, posterior intercostal, lumbar and lateral sacral arteries. The largest is the great anterior segmental medullary artery of Adamkiewicz, usually arising on the left side between T9 and T12 (range T8 to L2). It supplies most of the lower two thirds of the cord. The upper thoracic cord (about T1 to T4) is a watershed zone with sparse collateral supply, so it is vulnerable to hypotension. Clamping or injuring the artery of Adamkiewicz during aortic aneurysm repair causes anterior spinal artery syndrome: bilateral paralysis and loss of pain and temperature below the lesion with sparing of vibration and position sense.

Section 3Ascending and descending tracts

General plan of a sensory pathway

Conscious sensory pathways use a chain of three neurons. The first order neuron has its cell body in a dorsal root ganglion (or a cranial nerve ganglion) and carries the signal into the CNS. The second order neuron lies in the dorsal horn or a brainstem nucleus, and its axon decussates (crosses the midline) and ascends to the thalamus. The third order neuron in the thalamus (ventral posterolateral nucleus, VPL, for the body; ventral posteromedial, VPM, for the face) projects through the posterior limb of the internal capsule to the primary somatosensory cortex in the postcentral gyrus. Where the second order axon crosses determines the side of the deficit, and that single fact solves most lesion questions.

Dorsal column medial lemniscus (DCML)

The DCML carries fine (discriminative) touch, two point discrimination, vibration, conscious proprioception (joint position sense) and stereognosis. Large myelinated A beta fibers from Meissner and Pacinian corpuscles, muscle spindles and joint receptors enter the medial division of the dorsal root and ascend ipsilaterally without synapsing in the cord. Fibers from below T6 (legs and lower trunk) form the medially placed fasciculus gracilis; fibers from T6 and above (arms and upper trunk) add laterally as the fasciculus cuneatus. The dorsal columns are therefore somatotopically layered with the sacral fibers most medial.

The first order neurons synapse in the nucleus gracilis and nucleus cuneatus in the lower medulla. Second order axons sweep across the midline as the internal arcuate fibers (the sensory decussation) and ascend as the medial lemniscus to VPL. Third order neurons go to areas 3, 1 and 2 of the postcentral gyrus. A lesion of the dorsal column in the cord produces ipsilateral loss; a lesion of the medial lemniscus above the decussation produces contralateral loss.

Testing: vibration with a 128 Hz tuning fork, joint position sense at the great toe, two point discrimination, and the Romberg test. A positive Romberg (stable with eyes open, falls with eyes closed) indicates loss of proprioceptive input, a sensory ataxia from dorsal column or large fiber peripheral nerve disease, not a cerebellar problem.

Anterolateral system: spinothalamic tracts

The lateral spinothalamic tract carries pain and temperature; the anterior spinothalamic tract carries crude (nondiscriminative) touch and pressure. First order neurons are small A delta (sharp, fast pain, cold) and C fibers (slow burning pain, warmth). They enter through the lateral division of the dorsal root, may travel one or two segments in Lissauer's tract, and synapse in the dorsal horn (laminae I, II and V). The transmitter is glutamate, with substance P and CGRP for C fibers. Second order axons cross within one or two segments through the anterior white commissure and ascend in the contralateral anterolateral funiculus to VPL. The tract is laminated with sacral fibers most lateral (dorsolateral) and cervical fibers most medial; this explains sacral sparing when an intramedullary lesion expands outward from the center.

Companion anterolateral pathways, the spinoreticular (arousal and the unpleasant quality of pain) and spinomesencephalic tracts, reach the reticular formation and periaqueductal gray, which drives descending pain inhibition via the raphe nuclei and locus ceruleus.

Spinocerebellar tracts: unconscious proprioception

These two neuron pathways inform the cerebellum about limb position and movement and end ipsilaterally in the cerebellum (some cross twice, so the net effect is still ipsilateral). The posterior (dorsal) spinocerebellar tract arises from Clarke nucleus (C8 to L3), ascends ipsilaterally in the lateral funiculus and enters through the inferior cerebellar peduncle; it carries leg and lower trunk information from individual muscle spindles and tendon organs. Its upper limb equivalent is the cuneocerebellar tract from the accessory (lateral) cuneate nucleus, also via the inferior peduncle. The anterior (ventral) spinocerebellar tract crosses in the cord, ascends, enters through the superior cerebellar peduncle and crosses back, reporting the activity of spinal interneurons and whole limb movement. The rostral spinocerebellar tract is its upper limb counterpart. Friedreich ataxia degenerates the spinocerebellar tracts, dorsal columns and corticospinal tracts together.

Descending motor tracts

The corticospinal tract is the principal voluntary motor pathway, especially for skilled distal movements. About a third of its fibers arise from the primary motor cortex (area 4, precentral gyrus, including the giant Betz cells), a third from premotor and supplementary motor areas (area 6), and a third from parietal areas 3, 1, 2 and 5. Fibers descend through the corona radiata, the posterior limb of the internal capsule, the middle three fifths of the cerebral peduncle (crus cerebri), the basilar pons and the medullary pyramids. At the medulla to cord junction about 85 to 90 percent cross in the pyramidal decussation to form the lateral corticospinal tract in the lateral funiculus, which synapses on lateral (limb) motor neurons. The remaining 10 to 15 percent continue uncrossed as the anterior corticospinal tract and cross at segmental levels through the anterior white commissure to reach axial motor neurons. In the lateral corticospinal tract the leg fibers are lateral and the arm fibers medial. The corticobulbar tract does the same job for cranial nerve motor nuclei; most of these receive bilateral input, the key exceptions being the lower face (facial nucleus portion for the lower face receives contralateral input only) and, in many people, the hypoglossal nucleus.

These tracts explain posturing. In decorticate posturing (lesion above the red nucleus) the rubrospinal tract is intact, so the arms flex while the legs extend: think "cor" means the hands come to the heart. In decerebrate posturing (lesion between the red nucleus and the vestibular nuclei, at the midbrain or upper pons) the rubrospinal influence is lost and unopposed vestibulospinal and pontine reticulospinal drive extends all four limbs. Decerebrate carries a worse prognosis.

Upper versus lower motor neuron lesions

Classic cord lesion patterns

Brown Sequard syndrome is a hemisection of the cord, often from a stab wound, lateral disc or tumor. Below the lesion it produces ipsilateral spastic weakness (lateral corticospinal), ipsilateral loss of vibration, position and fine touch (dorsal column) and contralateral loss of pain and temperature starting one or two segments below the lesion (spinothalamic, already crossed). At the level of the lesion there is ipsilateral lower motor neuron weakness and a band of ipsilateral total anesthesia from destroyed roots and dorsal horn. A lesion above T1 can also damage descending hypothalamospinal fibers, causing ipsilateral Horner syndrome.

Central cord syndrome, the most common incomplete cord injury, typically follows hyperextension of a spondylotic cervical spine in an older adult. The central gray and the medially placed arm fibers of the corticospinal tracts suffer most, so the arms are weaker than the legs, there is variable sensory loss below the lesion and often sacral sparing. Syringomyelia, a fluid cavity in the central cord commonly associated with Chiari I malformation, first damages the crossing spinothalamic fibers in the anterior white commissure, giving a bilateral cape like loss of pain and temperature over the shoulders and arms with preserved touch (dissociated sensory loss); as it expands it destroys ventral horn cells (hand wasting) and then the lateral tracts.

Section 4The brainstem and cranial nerve nuclei

Three parts and their landmarks

The brainstem connects the spinal cord to the diencephalon and contains ascending and descending tracts, the nuclei of cranial nerves III to XII, the reticular formation and vital cardiorespiratory centers. Each level has a basal part (descending motor fibers), a tegmentum (cranial nerve nuclei, reticular formation, ascending tracts) and, in the midbrain, a tectum (colliculi) dorsal to the aqueduct.

The medulla oblongata runs from the foramen magnum to the pontomedullary junction. Ventrally it shows the pyramids (corticospinal fibers) beside the anterior median fissure and the oval olives (inferior olivary nucleus, which sends climbing fibers to the cerebellum) lateral to them. Dorsally the closed lower medulla shows the gracile and cuneate tubercles; the open upper medulla forms the lower half of the floor of the fourth ventricle. The medulla contains the respiratory centers and cardiovascular (vasomotor) center, the nucleus of the solitary tract and the area postrema, a chemoreceptor trigger zone for vomiting that lacks a blood brain barrier.

The pons has a bulging basilar part of transverse pontocerebellar fibers that form the middle cerebellar peduncle, with corticospinal bundles scattered among the pontine nuclei. Its dorsal tegmentum forms the upper floor of the fourth ventricle, where the facial colliculus marks the place the facial nerve fibers loop around the abducens nucleus. The pons houses the pneumotaxic and apneustic respiratory areas and the locus ceruleus.

The midbrain (mesencephalon) surrounds the cerebral aqueduct. Ventrally the cerebral peduncles (crus cerebri) carry corticospinal and corticobulbar fibers, with the interpeduncular fossa between them where CN III exits. Behind the crus lies the pigmented substantia nigra (dopaminergic pars compacta). The tegmentum contains the red nucleus, the oculomotor and trochlear nuclei and the periaqueductal gray. The dorsal tectum holds the superior colliculi (visual reflexes) and inferior colliculi (auditory relay); CN IV is the only nerve to exit the dorsal brainstem, just below the inferior colliculus, and it is the only cranial nerve whose fibers fully decussate.

Organization of cranial nerve nuclei

In the embryo the sulcus limitans divides the alar plate (sensory, dorsolateral) from the basal plate (motor, ventromedial). In the floor of the fourth ventricle this pattern persists: motor nuclei lie medial and sensory nuclei lateral to the sulcus limitans. The nuclei form functional columns: general somatic efferent (III, IV, VI, XII) closest to the midline; special visceral (branchial) efferent (motor V, VII, nucleus ambiguus for IX, X and cranial XI) displaced ventrolaterally; general visceral efferent (Edinger Westphal, superior and inferior salivatory, dorsal motor nucleus of X); then general and special visceral afferent (solitary nucleus); then general somatic afferent (trigeminal); and special somatic afferent (vestibular and cochlear) most laterally.

The reticular formation

The reticular formation is a diffuse network of neurons in the core of the tegmentum, from the medulla to the midbrain. Its ascending component, the ascending reticular activating system, projects through the intralaminar thalamic nuclei to the cortex and maintains wakefulness; bilateral damage causes coma. Its descending reticulospinal output modulates muscle tone, posture and pain. Neurochemically distinct groups sit within or beside it: the raphe nuclei (serotonin, sleep and pain modulation), the locus ceruleus in the pons (norepinephrine, arousal and stress) and the ventral tegmental area and substantia nigra (dopamine). Medullary reticular neurons form the respiratory rhythm generator and the vasomotor center.

Brainstem vascular syndromes

Brainstem strokes produce crossed (alternating) findings: a cranial nerve deficit on the side of the lesion and a long tract deficit (hemiplegia or hemianesthesia) on the opposite body, because the cranial nerve has not yet crossed while the long tracts already have or will. The cranial nerve involved localizes the level.

Section 5Cerebrum, diencephalon, cerebellum and their blood supply

Lobes and functional areas

Each cerebral hemisphere has a frontal, parietal, temporal and occipital lobe, plus the insula buried beneath the lateral sulcus and the limbic lobe on the medial surface. The central sulcus separates frontal from parietal; the lateral (Sylvian) fissure separates temporal below from frontal and parietal above; the parieto-occipital sulcus marks the occipital lobe medially, and the calcarine sulcus holds primary visual cortex.

Broca and Wernicke areas are connected by the arcuate fasciculus; its lesion causes conduction aphasia with poor repetition. White matter fibers are classified as association fibers (within one hemisphere), commissural fibers (corpus callosum, anterior commissure, joining hemispheres) and projection fibers (to and from lower centers, through the internal capsule).

Internal capsule

The internal capsule is a compact band of projection fibers between the caudate and thalamus medially and the lentiform nucleus laterally. Its anterior limb (between caudate head and lentiform) carries frontopontine and thalamocortical fibers. The genu carries corticobulbar fibers. The posterior limb (between thalamus and lentiform) carries corticospinal fibers (arm anterior, leg posterior) and thalamocortical somatosensory fibers; its retrolenticular and sublenticular parts carry the optic radiations and auditory radiations. Because so many fibers converge in so small a space, a small lacunar infarct of the lenticulostriate branches of the middle cerebral artery can cause a pure motor hemiparesis of face, arm and leg equally.

Diencephalon: thalamus and hypothalamus

The thalamus is the relay station for nearly all sensory input to the cortex (smell is the exception) and for motor loops from the basal ganglia and cerebellum.

The hypothalamus, below the hypothalamic sulcus, controls autonomic, endocrine and homeostatic functions. The supraoptic nucleus makes mainly ADH and the paraventricular nucleus mainly oxytocin and CRH; both travel down the infundibulum to the posterior pituitary. The lateral nucleus drives hunger (lesion: anorexia), the ventromedial nucleus produces satiety (lesion: hyperphagia and obesity), the anterior hypothalamus promotes cooling and parasympathetic activity, the posterior promotes heat conservation and sympathetic activity, the suprachiasmatic nucleus sets circadian rhythm from retinal input, and the mammillary bodies participate in memory and are damaged in Wernicke Korsakoff syndrome (thiamine deficiency).

Basal ganglia and limbic system

The basal ganglia include the caudate and putamen (together the striatum), the globus pallidus (putamen plus globus pallidus form the lentiform nucleus), the subthalamic nucleus and the substantia nigra. They modulate movement through a loop: cortex to striatum to globus pallidus internus to VA and VL thalamus and back to cortex. The direct pathway disinhibits the thalamus and facilitates movement; the indirect pathway (via globus pallidus externus and subthalamic nucleus) inhibits it. Dopamine from the substantia nigra pars compacta excites the direct pathway through D1 receptors and inhibits the indirect pathway through D2 receptors, so loss of nigral dopamine in Parkinson disease produces bradykinesia, rigidity, resting tremor and postural instability. Huntington disease (CAG repeat, autosomal dominant) degenerates the caudate, causing chorea and dementia. A lesion of the subthalamic nucleus causes contralateral hemiballismus.

The limbic system governs emotion, memory and drives. Its core structures are the hippocampus (memory consolidation), amygdala (fear and emotional memory; bilateral damage gives Kluver Bucy syndrome), cingulate and parahippocampal gyri, fornix, mammillary bodies and anterior thalamus.

Cerebellum

The cerebellum coordinates movement, balance and motor learning. It has a midline vermis and two hemispheres, divided into three functional regions. The vestibulocerebellum (flocculonodular lobe) controls balance and eye movements; the spinocerebellum (vermis and paravermis) controls axial and proximal limb tone and posture; the cerebrocerebellum (lateral hemispheres) plans and times skilled limb movement. The cortex has three layers (molecular, Purkinje, granular). Purkinje cells are the only output of the cortex and are inhibitory (GABA) onto the deep nuclei: from medial to lateral, fastigial, globose, emboliform and dentate ("Fat Girls Eat Doughnuts"). Inputs arrive as climbing fibers from the inferior olive and mossy fibers from everywhere else. The inferior peduncle carries mostly input from the cord and vestibular system, the middle peduncle carries pontocerebellar input from the opposite cortex, and the superior peduncle carries most output, from the dentate to the contralateral red nucleus and VL thalamus.

Because the output crosses once to the opposite cortex and the corticospinal tract crosses back, a cerebellar hemisphere controls the ipsilateral body. Hemisphere lesions give ipsilateral limb ataxia, dysmetria (positive finger to nose), intention tremor, dysdiadochokinesia and hypotonia; vermis lesions give truncal ataxia and a wide based gait, typical of alcoholic degeneration.

Blood supply of the brain

Two internal carotid arteries (anterior circulation) and two vertebral arteries (posterior circulation) supply the brain. The vertebral arteries arise from the subclavians, ascend through the transverse foramina of C6 to C1, loop behind the lateral mass of the atlas in the suboccipital triangle, pierce the posterior atlanto-occipital membrane and dura, enter the foramen magnum and unite at the pontomedullary junction to form the basilar artery. Before joining they give the anterior spinal artery and the posterior inferior cerebellar artery (PICA). The basilar gives the anterior inferior cerebellar artery (AICA), the labyrinthine artery (often from AICA), pontine branches and the superior cerebellar artery, then divides into the two posterior cerebral arteries (PCA).

The circle of Willis joins the systems around the optic chiasm and pituitary stalk: the anterior communicating artery links the two anterior cerebral arteries (ACA), and each posterior communicating artery joins the internal carotid to the PCA. The most common site of a berry aneurysm is the anterior communicating artery; a posterior communicating aneurysm compresses CN III and dilates the pupil.

Words worth knowing

Peripheral and Autonomic Nervous Systems and the Special Senses

The peripheral nervous system is where chiropractic lives: the intervertebral foramen, the spinal nerve, its rami and the reflex arcs you test on every new patient. This chapter connects those spinal nerves to the twelve cranial nerves, the autonomic outflow that runs every organ, and the special senses of vision, hearing, balance, taste and smell, all at the level of detail the board expects.

Section 1Spinal nerves, rami, dermatomes and myotomes

Formation of a spinal nerve

Each of the 31 pairs of spinal nerves forms from two roots. The dorsal (posterior) root carries afferent fibers whose cell bodies are pseudounipolar neurons in the dorsal root ganglion (DRG). The ventral (anterior) root carries efferent fibers from alpha and gamma motor neurons in the ventral horn and, from T1 to L2 and S2 to S4, preganglionic autonomic fibers. The two roots unite just distal to the DRG, within or immediately beyond the intervertebral foramen (IVF), to form a short mixed spinal nerve about 1 cm long. Almost immediately it divides into a dorsal (posterior) ramus and a ventral (anterior) ramus, both of which are mixed nerves.

The DRG usually sits in the IVF itself, directly beneath the pedicle. In the cervical spine the ganglia lie somewhat lateral to the foramen on the groove of the transverse process; in the lower lumbar and sacral regions they may lie more medially, and the S1 and S2 ganglia sit within the sacral canal. The DRG is exquisitely sensitive to mechanical compression and to inflammatory mediators from a degenerating disc, which is one reason a small foraminal protrusion can produce severe radicular pain.

The intervertebral foramen and its contents

The IVF is bounded above and below by the pedicles of adjacent vertebrae, anteriorly by the posterolateral vertebral bodies and the intervertebral disc (and in the cervical spine the uncovertebral joint), and posteriorly by the zygapophyseal (facet) joint capsule and the lateral edge of the ligamentum flavum. It is largest in the upper lumbar region and is shaped like an inverted teardrop. Flexion opens it and extension, lateral flexion and rotation toward the same side narrow it, the basis of foraminal compression tests such as the Spurling (maximum cervical foraminal compression) test.

Rami and branches

The dorsal ramus passes backward around the articular pillar and divides into a medial branch and a lateral branch (with an intermediate branch in the lumbar region). It supplies the deep (true, intrinsic) back muscles (erector spinae, transversospinalis and segmental muscles), the zygapophyseal joints and the skin of the back near the midline. Each facet joint receives medial branch input from two levels, the level above and the level of the joint, which is why medial branch blocks and radiofrequency neurotomy must target two nerves. Named dorsal rami include the suboccipital nerve (C1 dorsal ramus, motor to the suboccipital muscles, usually no cutaneous branch), the greater occipital nerve (C2 medial branch, piercing semispinalis capitis and trapezius to supply the posterior scalp) and the third occipital nerve (C3, supplying the C2 to C3 facet). The lateral branches of L1 to L3 dorsal rami become the superior cluneal nerves over the iliac crest, and S1 to S3 give the middle cluneal nerves.

The ventral ramus is larger. In the thoracic region it continues as an intercostal nerve (T1 to T11) or the subcostal nerve (T12), supplying intercostal and abdominal wall muscles and the skin of the anterolateral trunk. Elsewhere ventral rami join to form the cervical plexus (C1 to C4), brachial plexus (C5 to T1), lumbar plexus (L1 to L4), sacral plexus (L4 to S4) and coccygeal plexus. The phrenic nerve, from C3, C4 and C5 ventral rami ("keeps the diaphragm alive"), is the most important branch of the cervical plexus.

Rami communicantes connect ventral rami to the sympathetic chain. The white ramus communicans is myelinated, carries preganglionic sympathetic fibers into the chain and exists only from T1 to L2. The gray ramus communicans is unmyelinated, carries postganglionic sympathetic fibers back to every spinal nerve (all 31) for sweat glands, piloerector muscles and blood vessels, and lies medial to the white ramus.

The recurrent meningeal (sinuvertebral) nerve of Luschka arises from the ventral ramus just beyond the DRG, receives a sympathetic contribution from a gray ramus, and re-enters the canal through the IVF. It supplies the posterior longitudinal ligament, the anterior dura, the outer third of the posterior annulus fibrosus, the periosteum and the epidural vessels, with branches extending one or two levels up and down. It is the principal pain pathway for discogenic and ligamentous pain, explaining why a torn annulus can hurt without any root compression. The anterior and lateral annulus and the anterior longitudinal ligament receive branches from the ventral rami and gray rami.

Dermatomes and myotomes

A dermatome is the strip of skin supplied by the sensory fibers of one spinal nerve (one DRG); a myotome is the group of muscles supplied by the motor fibers of one segment. Because adjacent dermatomes overlap considerably, sectioning one root produces only a small area of reduced sensation, while loss of a whole dermatome usually means two or three roots are involved. Pain and temperature dermatomes overlap less than touch dermatomes, so pinprick is the more sensitive screen. C1 typically has no dermatome. Most muscles are supplied by two or more segments, so a single radiculopathy weakens but does not paralyze.

Useful landmarks: C2 posterior scalp, C3 neck, C4 shoulder cap and clavicle, C5 lateral arm (deltoid patch), C6 lateral forearm, thumb and index finger, C7 middle finger, C8 ring and little fingers and medial forearm, T1 medial arm, T2 axilla, T4 nipple, T6 xiphoid, T10 umbilicus, L1 inguinal ligament, L3 medial knee, L4 medial leg and medial malleolus, L5 dorsum of foot and great toe, S1 lateral foot, little toe and sole, S2 posterior thigh, S3 to S5 concentric saddle rings around the anus.

Deep tendon reflexes and the radiculopathy grid

A deep tendon (muscle stretch) reflex is a monosynaptic arc: the tap stretches muscle spindles, Ia afferents enter through the dorsal root and synapse directly on alpha motor neurons of the same muscle, while inhibitory interneurons relax the antagonist (reciprocal inhibition). A lesion anywhere in the arc (sensory root, ventral horn, motor root, peripheral nerve, neuromuscular junction, muscle) diminishes the reflex. Loss of descending inhibition (upper motor neuron lesion) exaggerates it. Reflexes are graded 0 (absent), 1+ (diminished), 2+ (normal), 3+ (brisk, may spread) and 4+ (with sustained clonus).

Superficial (polysynaptic) reflexes include the abdominal reflexes (upper T8 to T10, lower T10 to T12), the cremasteric reflex (L1, L2: genitofemoral nerve), the plantar reflex (L5, S1) and the anal wink (S2 to S4). Superficial reflexes are lost with upper motor neuron lesions, while the Babinski response emerges.

Section 2The cranial nerves

Functional components

Cranial nerves carry seven possible fiber types. General somatic afferent (GSA) fibers carry touch, pain, temperature and proprioception (mostly V). General visceral afferent (GVA) fibers carry sensation from viscera (IX, X). Special visceral afferent (SVA) fibers carry taste (VII, IX, X) and smell (I). Special somatic afferent (SSA) fibers carry vision (II), hearing and balance (VIII). General somatic efferent (GSE) fibers supply muscles derived from somites: extraocular muscles (III, IV, VI) and tongue (XII). General visceral efferent (GVE) fibers are parasympathetic (III, VII, IX, X). Special visceral (branchial) efferent (SVE) fibers supply pharyngeal arch muscles (V, VII, IX, X, XI). A classic memory aid for the sensory, motor or both classification is "Some Say Marry Money, But My Brother Says Big Brains Matter More" (I sensory, II sensory, III motor, IV motor, V both, VI motor, VII both, VIII sensory, IX both, X both, XI motor, XII motor).

Two nerves are exceptions to the rule that cranial nerves are peripheral nerves. CN I (olfactory) and CN II (optic) are outgrowths of the forebrain: the optic nerve is a CNS tract wrapped in meninges and myelinated by oligodendrocytes, which is why it is affected by multiple sclerosis (optic neuritis) and why raised intracranial pressure reaches the optic disc (papilledema).

Notes on individual nerves

Oculomotor (III). A compressive lesion (posterior communicating aneurysm, uncal herniation) affects the parasympathetic fibers on the outside of the nerve first, so the pupil dilates early. An ischemic lesion (diabetes) damages the core and typically spares the pupil. A complete palsy leaves the eye "down and out" (unopposed lateral rectus and superior oblique), with ptosis and a fixed dilated pupil.

Trochlear (IV). The longest intracranial course and the only dorsal exit. A palsy gives vertical diplopia worst when looking down and in (reading, descending stairs), and the patient tilts the head away from the affected side.

Trigeminal (V). The trigeminal (semilunar) ganglion in Meckel cave holds the sensory cell bodies. V1 carries the afferent limb of the corneal reflex; the efferent limb is VII. The spinal trigeminal tract descends to about C2 to C3 and overlaps the upper cervical dorsal horn, forming the trigeminocervical nucleus, the anatomical basis of cervicogenic headache referred from C1 to C3 structures to the forehead and orbit. Trigeminal neuralgia causes brief lancinating pain, usually in V2 or V3.

Abducens (VI). A long course along the clivus makes it the nerve most often affected by raised intracranial pressure (a "false localizing sign"). Palsy causes horizontal diplopia and inability to abduct.

Facial (VII). The motor root and the nervus intermedius (taste and parasympathetic) enter the internal acoustic meatus, pass the geniculate ganglion (taste cell bodies) and give the greater petrosal nerve (lacrimal gland via the pterygopalatine ganglion), the nerve to stapedius, and the chorda tympani (taste from the anterior two thirds of the tongue and parasympathetic to the submandibular and sublingual glands via the submandibular ganglion) before exiting the stylomastoid foramen to supply the facial muscles through five branches (temporal, zygomatic, buccal, marginal mandibular, cervical). Bell palsy (a lower motor neuron lesion, often viral) paralyzes the whole half of the face including the forehead. A cortical stroke (upper motor neuron) causes contralateral weakness of the lower face only, because the forehead receives bilateral cortical input. Losing the stapedius produces hyperacusis.

Glossopharyngeal (IX), vagus (X) and accessory (XI). All exit the jugular foramen, so a jugular foramen tumor (Vernet syndrome) affects all three. IX carries the afferent limb of the gag reflex and the carotid sinus reflex; X carries the efferent limb. The recurrent laryngeal nerve loops under the aortic arch on the left and the subclavian artery on the right and supplies all intrinsic laryngeal muscles except the cricothyroid (superior laryngeal nerve); damage causes hoarseness. The spinal accessory nerve arises from C1 to C5, ascends through the foramen magnum, then exits the jugular foramen; it crosses the posterior triangle superficially, where it is vulnerable to lymph node biopsy, producing trapezius weakness and lateral scapular winging.

Hypoglossal (XII). A lower motor neuron lesion causes ipsilateral tongue atrophy and fasciculations with deviation toward the lesion on protrusion.

Section 3The autonomic nervous system

General plan: a two neuron chain

The autonomic nervous system controls smooth muscle, cardiac muscle and glands without conscious effort. Unlike somatic motor output, which uses one neuron from the CNS to skeletal muscle, autonomic output uses two neurons. The preganglionic neuron has its cell body in the brainstem or spinal cord and sends a thinly myelinated B fiber to an autonomic ganglion. The postganglionic neuron has its cell body in that ganglion and sends an unmyelinated C fiber to the effector. The two divisions, sympathetic and parasympathetic, differ in where their preganglionic neurons sit, where their ganglia lie, and which transmitters they use. The enteric nervous system is often counted as a third division.

Sympathetic outflow and the chain

Preganglionic sympathetic neurons lie in the intermediolateral cell column from T1 to L2. Their axons leave through the ventral root, enter the spinal nerve and pass through a white ramus communicans into the sympathetic trunk (chain), a beaded cord of paravertebral ganglia that runs from the base of the skull to the coccyx along the sides of the vertebral bodies. In the chain a preganglionic fiber has four options:

1. Synapse at the same level and return through a gray ramus to the spinal nerve (sweat glands, arrector pili, vessels of the body wall and limbs).
2. Ascend or descend in the chain to synapse at a higher or lower ganglion. This is how the cervical and the lumbosacral ganglia, which have no white rami, are supplied.
3. Synapse in the chain and leave as a visceral branch, such as the cardiac and pulmonary branches from the cervical and upper thoracic ganglia.
4. Pass through the chain without synapsing as a splanchnic nerve to a prevertebral ganglion in the abdomen.

There are usually three cervical ganglia: the superior cervical ganglion (C1 to C4 level, supplying the head: pupil dilator, superior tarsal muscle, sweat glands and vessels of the face, salivary glands), the small middle cervical ganglion (C6) and the inferior cervical ganglion, which commonly fuses with the first thoracic ganglion to form the cervicothoracic (stellate) ganglion in front of the neck of the first rib and the C7 transverse process. Cervical ganglia give cardiac nerves. There are about 11 thoracic, 4 lumbar and 4 sacral ganglia, and the two chains unite in front of the coccyx at the single ganglion impar.

The splanchnic nerves carry preganglionic fibers to the prevertebral ganglia around the branches of the abdominal aorta:

The adrenal medulla is the exception to the two neuron rule. Its chromaffin cells are modified postganglionic neurons, receive preganglionic fibers directly from the splanchnic nerves, and release about 80 percent epinephrine and 20 percent norepinephrine into the blood.

Interrupting the sympathetic supply to the head anywhere along the three neuron chain (hypothalamus, descending through the brainstem and cervical cord to T1, then the chain and superior cervical ganglion, then along the internal carotid) causes Horner syndrome: ptosis (superior tarsal muscle), miosis (pupil dilator) and anhidrosis of the face, often with apparent enophthalmos. Causes include lateral medullary infarct, cervical cord injury, a Pancoast tumor at the lung apex, carotid dissection and trauma to the lower brachial plexus (Klumpke palsy with T1 involvement).

Parasympathetic outflow

The cranial parasympathetic outflow uses four nerves and four head ganglia, plus the vagus for the body:

The sacral parasympathetic outflow arises from S2 to S4 and leaves as the pelvic splanchnic nerves (nervi erigentes). These supply the hindgut beyond the splenic flexure, the bladder detrusor (contraction for voiding), and erectile tissue ("point and shoot": parasympathetic for erection, sympathetic for emission and ejaculation). Note the important trap: the sympathetic supply to the gut travels in splanchnic nerves from the thoracolumbar region, while the parasympathetic supply to the pelvis travels in the pelvic splanchnic nerves from S2 to S4.

The enteric nervous system

The gut wall contains its own network of roughly as many neurons as the spinal cord. The myenteric (Auerbach) plexus, between the circular and longitudinal muscle layers, controls motility; the submucosal (Meissner) plexus controls secretion and local blood flow. The enteric system can run peristalsis independently, while sympathetic input slows and parasympathetic input speeds it. Absence of ganglion cells in the distal colon (failure of neural crest migration) causes Hirschsprung disease; destruction of the myenteric plexus in the esophagus causes achalasia.

Neurotransmitters and receptors

Cholinergic neurons release acetylcholine: all preganglionic neurons of both divisions, all parasympathetic postganglionic neurons, sympathetic postganglionic neurons to sweat glands, and somatic motor neurons. Adrenergic neurons release norepinephrine: most sympathetic postganglionic neurons. Sympathetic fibers to renal vessels release dopamine.

Most organs receive dual innervation with opposing effects. The main exceptions with sympathetic supply only are the sweat glands, arrector pili muscles, adrenal medulla and most blood vessels. Vascular tone is therefore controlled by increasing or decreasing a single sympathetic signal.

Visceral afferents and referred pain

Visceral pain fibers travel back with the sympathetic nerves to the spinal segments that supplied the organ's sympathetic outflow (T1 to L2), while reflex afferents (stretch, chemoreceptors) and pelvic pain from structures below the pelvic pain line travel with the parasympathetic nerves. In the dorsal horn, visceral afferents converge on the same lamina V neurons as somatic afferents from the dermatome of that segment. The brain, accustomed to somatic input, misattributes the pain to the body wall: referred pain.

Section 4The eye and vision

The bony orbit

The orbit is a four sided pyramid whose apex points posteromedially toward the optic canal. Seven bones contribute: frontal, zygomatic, maxilla, ethmoid, sphenoid, lacrimal and palatine. The roof is the orbital plate of the frontal bone with the lesser wing of the sphenoid; the lateral wall, the strongest, is the zygomatic bone and greater wing of the sphenoid; the floor is mainly the maxilla; the medial wall, thinnest of all, is the paper thin lamina papyracea of the ethmoid with the lacrimal bone. A blow to the eye raises orbital pressure and fractures the weak floor into the maxillary sinus (blowout fracture), trapping the inferior rectus and limiting upward gaze. The optic canal transmits CN II and the ophthalmic artery; the superior orbital fissure transmits III, IV, V1 branches, VI and the superior ophthalmic vein; the inferior orbital fissure transmits the infraorbital nerve and vessels (V2).

The globe

The eyeball has three coats. The outer fibrous tunic is the opaque white sclera posteriorly and the transparent, avascular cornea anteriorly, which provides about two thirds of the eye's refractive power. The middle vascular tunic (uvea) consists of the choroid (pigmented vascular layer nourishing the outer retina), the ciliary body (ciliary muscle for accommodation and ciliary processes that secrete aqueous humor) and the iris with its two muscles: the sphincter (constrictor) pupillae, parasympathetic via CN III, and the dilator pupillae, sympathetic via alpha 1 receptors. The inner neural tunic is the retina.

Aqueous humor is secreted by the ciliary processes into the posterior chamber, flows through the pupil into the anterior chamber and drains through the trabecular meshwork into the canal of Schlemm (scleral venous sinus) at the iridocorneal angle. Impaired drainage raises intraocular pressure: open angle glaucoma is chronic and painless with gradual peripheral field loss and cupping of the optic disc; closed (narrow) angle glaucoma is an emergency with a painful red eye, blurred vision, halos and a mid dilated pupil, and drugs that dilate the pupil can precipitate it. The lens, held by the suspensory ligaments (zonule fibers), separates the aqueous compartment from the gel like vitreous body. Clouding of the lens is a cataract.

The retina

Light passes through the inner retina to reach the photoreceptors at the back, next to the retinal pigment epithelium. The signal then travels forward through three neurons: photoreceptors (rods and cones), bipolar cells and ganglion cells, whose axons form the optic nerve. Horizontal and amacrine cells provide lateral processing. Rods (about 120 million) contain rhodopsin, are very sensitive, mediate night and peripheral vision, and are absent from the fovea. Cones (about 6 million) provide color and high acuity in bright light and are concentrated in the fovea centralis of the macula lutea, where the inner layers are pushed aside. The optic disc, about 3 mm medial (nasal) to the fovea, has no photoreceptors and creates the physiological blind spot. The central retinal artery (from the ophthalmic artery, a branch of the internal carotid) supplies the inner retina; the choroid supplies the photoreceptors. Vitamin A deficiency impairs rhodopsin regeneration and causes night blindness.

Extraocular muscles

Six muscles move each eye. The four recti arise from the common tendinous ring (annulus of Zinn) around the optic canal; the superior oblique arises from the sphenoid above the ring and turns through the trochlea, a fibrocartilage pulley on the frontal bone; the inferior oblique arises from the orbital floor near the lacrimal fossa. Because the orbit points laterally about 23 degrees from the line of sight, the superior and inferior recti also adduct, and the obliques, which insert behind the equator, also abduct.

Coordinated horizontal gaze depends on the paramedian pontine reticular formation (PPRF) and abducens nucleus, which drive the ipsilateral lateral rectus and, through the medial longitudinal fasciculus (MLF), the contralateral medial rectus. A lesion of the MLF, classic in young adults with multiple sclerosis, causes internuclear ophthalmoplegia: on attempted lateral gaze the eye on the side of the lesion fails to adduct while the abducting eye shows nystagmus; convergence is preserved.

Visual pathway and field defects

Each eye sees a visual field; the image falls inverted and reversed on the retina, so the temporal field falls on the nasal retina and the upper field falls on the lower retina. Ganglion cell axons leave as the optic nerve. At the optic chiasm, above the pituitary, fibers from the nasal halves of each retina cross; temporal fibers stay ipsilateral. Each optic tract therefore carries information about the opposite visual field from both eyes and ends mainly in the lateral geniculate nucleus, with collaterals to the pretectal area (light reflex), superior colliculus (reflex orientation) and suprachiasmatic nucleus. From the LGN the optic radiations (geniculocalcarine tract) pass to the primary visual cortex. Fibers serving the lower retina (upper field) loop forward through the temporal lobe as Meyer loop; fibers serving the upper retina (lower field) take a direct course through the parietal lobe. All end on the banks of the calcarine sulcus: upper retina on the cuneus above, lower retina on the lingual gyrus below, with the macula at the occipital pole.

Pupillary light reflex and accommodation

The pupillary light reflex uses CN II as the afferent limb and CN III as the efferent limb. Light on one retina sends signals through the optic nerve and tract (bypassing the LGN) to the pretectal nucleus in the midbrain. Each pretectal nucleus projects to both Edinger Westphal nuclei (crossing through the posterior commissure), whose preganglionic fibers travel in CN III to the ciliary ganglion, and short ciliary nerves then constrict the sphincter pupillae. Because of this bilateral projection, light in one eye constricts both pupils: the direct and consensual responses.

• Lesion of the left optic nerve (afferent): shining light in the left eye constricts neither pupil, while light in the right eye constricts both. The swinging flashlight test reveals a relative afferent pupillary defect (Marcus Gunn pupil): the affected pupil paradoxically dilates when the light swings to it.
• Lesion of the left oculomotor nerve (efferent): the left pupil stays dilated whichever eye is lit, while the right pupil constricts to light in either eye.

The accommodation (near) response has three parts: convergence of the eyes (medial recti), pupillary constriction, and thickening of the lens. To focus near, the parasympathetically driven ciliary muscle contracts, which relaxes tension on the zonule fibers and lets the elastic lens round up and increase its refractive power. The pathway runs through the visual cortex to the midbrain near response center, Edinger Westphal and the oculomotor complex. Loss of lens elasticity with age is presbyopia. The Argyll Robertson pupil of neurosyphilis is small, irregular, and reacts to accommodation but not to light, a light near dissociation from a pretectal lesion.

Section 5The ear, balance, taste and smell

External and middle ear

The external ear consists of the elastic cartilage auricle (pinna) and the external acoustic meatus, about 2.5 cm long, its outer third cartilaginous and inner two thirds bony, lined by skin with ceruminous glands. Sensory supply is mixed: the auriculotemporal nerve (V3), the great auricular nerve (C2, C3), and small branches of VII and the auricular branch of X, which explains why cleaning the ear canal can trigger a cough. The canal ends at the tympanic membrane, which is angled, concave outward, and attached at its center to the handle of the malleus (the umbo). On otoscopy a cone of light points anteroinferiorly.

The middle ear (tympanic cavity) is an air filled space in the petrous temporal bone. It connects anteriorly to the nasopharynx through the auditory (pharyngotympanic, Eustachian) tube, which equalizes pressure and is opened by the tensor veli palatini during swallowing; in children it is shorter and more horizontal, favoring otitis media. Posteriorly it opens into the mastoid antrum and air cells. Its roof is the thin tegmen tympani, separating it from the middle cranial fossa; its floor lies over the internal jugular vein; its medial wall shows the promontory (basal turn of the cochlea), the oval window and the round window.

Three ossicles form a lever chain: the malleus (attached to the tympanic membrane), the incus and the stapes, whose footplate sits in the oval window. Because the tympanic membrane is about 17 to 20 times larger than the footplate and the ossicles add a lever advantage, sound pressure is amplified roughly 20 times (about 25 to 30 dB), matching the impedance of air to the fluid of the inner ear. Two small muscles damp loud sounds: the tensor tympani (V3) on the malleus and the stapedius (VII) on the stapes, the smallest skeletal muscle in the body. The chorda tympani crosses the middle ear between the malleus and incus.

Inner ear

The inner ear lies in the petrous temporal bone and has two parts: a bony labyrinth filled with perilymph (like extracellular fluid, high sodium) and, suspended within it, a membranous labyrinth filled with endolymph (unusual extracellular fluid with high potassium and low sodium, secreted by the stria vascularis). The bony labyrinth includes the vestibule, three semicircular canals and the cochlea; the membranous labyrinth includes the utricle and saccule in the vestibule, three semicircular ducts and the cochlear duct (scala media).

The cochlea makes about 2.5 turns around a central bony pillar, the modiolus, which contains the spiral ganglion (bipolar first order neurons). In cross section it has three channels: the scala vestibuli (perilymph, beginning at the oval window), the scala media (endolymph) and the scala tympani (perilymph, ending at the round window); the two perilymph channels connect at the apex through the helicotrema. The scala media is separated from the scala vestibuli by Reissner (vestibular) membrane and from the scala tympani by the basilar membrane, on which sits the organ of Corti with one row of inner hair cells (the true sensory receptors, about 95 percent of afferent fibers) and three rows of outer hair cells (which amplify by changing length), their stereocilia touching the overlying tectorial membrane.

Stapes movement creates a pressure wave that displaces the basilar membrane. The membrane is narrow and stiff at the base near the oval window, where it responds best to high frequencies, and wide and flexible at the apex, where it responds to low frequencies: this is tonotopic organization (place theory). Bending the stereocilia toward the tallest one opens mechanically gated channels, letting K+ flow in from the endolymph and depolarizing the hair cell. Noise induced and age related hearing loss (presbycusis) first damages the basal, high frequency region.

Auditory pathway and hearing tests

First order bipolar neurons in the spiral ganglion form the cochlear nerve and synapse in the dorsal and ventral cochlear nuclei at the pontomedullary junction. From there fibers ascend bilaterally, many crossing in the trapezoid body to the superior olivary nuclei (which compare timing and intensity between ears to localize sound), then through the lateral lemniscus to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally the primary auditory cortex in the transverse temporal gyri of Heschl. Because input ascends bilaterally above the cochlear nuclei, a unilateral lesion central to the cochlear nuclei does not cause deafness in one ear; only cochlear, nerve or cochlear nucleus lesions do.

Vestibular apparatus and pathways

The vestibular system senses head position and movement. The utricle and saccule (otolith organs) contain a macula whose hair cells are covered by a gelatinous membrane studded with calcium carbonate crystals (otoconia). They detect linear acceleration and static head tilt relative to gravity: the utricle mainly horizontal, the saccule mainly vertical. The three semicircular ducts (anterior, posterior and lateral), set at right angles to one another, each have a swelling, the ampulla, with a crista ampullaris whose hair cells are embedded in a gelatinous cupula. When the head rotates, endolymph lags behind and bends the cupula, so the canals detect angular (rotational) acceleration. Each canal pairs with a canal on the opposite side in the same plane. Displaced otoconia that drift into a semicircular canal, usually the posterior canal, cause benign paroxysmal positional vertigo (BPPV): brief vertigo with head position change, diagnosed by the Dix Hallpike maneuver (torsional upbeating nystagmus) and treated with the Epley canalith repositioning maneuver.

First order neurons in the vestibular (Scarpa) ganglion in the internal acoustic meatus project to the four vestibular nuclei (superior, inferior, medial and lateral) in the floor of the fourth ventricle and directly to the flocculonodular lobe of the cerebellum. The vestibular nuclei project to:

• The extraocular motor nuclei (III, IV, VI) through the MLF, producing the vestibulo-ocular reflex, which keeps the eyes fixed on a target while the head moves. Its integrity is tested with the head impulse test, doll's eye maneuver and caloric testing (cold water produces nystagmus with its fast phase to the opposite side, warm to the same side: "COWS").
• The spinal cord through the lateral vestibulospinal tract (extensor antigravity tone) and the medial vestibulospinal tract (neck muscles for head stabilization).
• The cerebellum, and through the thalamus to the cortex (posterior insula and parietal areas) for conscious perception of motion.

Taste

Taste receptor cells sit in taste buds on fungiform, foliate and circumvallate papillae (filiform papillae have none) and on the palate, pharynx and epiglottis. The five basic tastes are sweet, salty, sour, bitter and umami; sweet, bitter and umami use G protein coupled receptors (bitter through gustducin), while salty and sour act through ion channels. Taste is carried by three nerves: VII (chorda tympani) from the anterior two thirds of the tongue and the greater petrosal nerve from the palate; IX from the posterior third including the circumvallate papillae; and X from the epiglottis and pharynx. General sensation of the anterior two thirds is V3 (lingual nerve), while IX supplies both general sensation and taste to the posterior third. All taste fibers enter the solitary tract and synapse in the rostral (gustatory) part of the nucleus of the solitary tract, ascend ipsilaterally in the central tegmental tract to VPM of the thalamus, and reach the gustatory cortex in the anterior insula and frontal operculum.

Smell

The olfactory epithelium lines the roof of the nasal cavity, the upper septum and the superior conchae. Its olfactory receptor neurons are bipolar neurons that, unusually, are continuously replaced from basal stem cells. Each expresses one of several hundred odorant receptors (G protein coupled, using Golf, cAMP and cyclic nucleotide gated channels). Their unmyelinated axons form about 20 bundles, the true olfactory nerves (CN I), which pass through the cribriform plate to synapse with mitral and tufted cells in the glomeruli of the olfactory bulb. The olfactory tract runs back and divides into lateral and medial striae. The lateral stria reaches the primary olfactory cortex (piriform cortex, periamygdaloid region, uncus and entorhinal area of the medial temporal lobe) and the amygdala, tying smell to emotion and memory. Smell is the only sense that reaches the cortex without a mandatory thalamic relay (a secondary route reaches the orbitofrontal cortex through the dorsomedial thalamus). Fracture of the cribriform plate in head trauma can shear the olfactory nerves (anosmia) and leak CSF from the nose (CSF rhinorrhea).

Words worth knowing

Neurophysiology and Muscle Physiology

Every reflex you test with a hammer, every muscle you grade on a five point scale and every patient who describes burning, tingling or weakness is telling you something about excitable membranes. This chapter builds neuromuscular physiology from the ion up, through synapses, receptors and reflex arcs, to the contraction of skeletal, smooth and cardiac muscle. Physiology carries a large share of Part I, and these mechanisms return again in pathology, pharmacology and every neurological exam you will ever perform.

Section 1Membranes, resting potential and the action potential

The membrane as a capacitor with gates

The plasma membrane is a phospholipid bilayer that ions cannot cross on their own. Charged particles move only through proteins: channels (passive, fast, down an electrochemical gradient), carriers (facilitated diffusion), and pumps (primary active transport using ATP). Because the bilayer separates two conducting solutions, it behaves as a capacitor. Only a tiny number of ions need to cross to change the voltage across it, which is why the bulk concentrations inside and outside a neuron barely change during a single action potential.

The key concentration gradients in a typical mammalian neuron are shown below. Sodium, chloride and calcium are high outside; potassium and organic anions are high inside.

Equilibrium potentials: the Nernst idea

If a membrane were permeable to only one ion, that ion would diffuse down its concentration gradient until the electrical force pulling it back exactly balanced the chemical force pushing it out. The voltage at which this balance occurs is the ion's equilibrium (Nernst) potential. At body temperature the Nernst equation simplifies to E = (61.5 / z) log ([ion]outside / [ion]inside), where z is the ion's valence. For potassium, a roughly 30 fold higher concentration inside gives about -90 mV. For sodium, about 10 fold higher outside gives about +60 mV. Note the sign convention: the inside is always reported relative to the outside.

Two consequences matter clinically. First, the potassium equilibrium potential depends heavily on extracellular potassium, because extracellular potassium is low and small absolute changes alter the ratio greatly. Hyperkalemia makes E(K) less negative and depolarizes cells; hypokalemia hyperpolarizes them. Second, the driving force on any ion is the difference between the membrane potential and that ion's equilibrium potential (Vm minus E ion). The bigger the difference, the stronger the push when a channel opens.

The resting membrane potential: the Goldman idea

Real membranes are permeable to several ions at once. The Goldman Hodgkin Katz equation weights each ion's concentration gradient by its permeability. At rest, neurons have many open potassium leak channels (two pore domain and inward rectifier types), so potassium permeability dominates and the resting potential (about -70 mV in neurons, about -90 mV in skeletal and ventricular muscle) sits close to E(K). It is a little less negative than E(K) because a small sodium leak pulls it toward E(Na). Whichever ion has the highest permeability at a moment drags the membrane toward its own equilibrium potential: that single principle explains the resting potential, the upstroke and repolarization.

The Na+/K+ ATPase moves 3 Na+ out and 2 K+ in per ATP. It maintains the gradients that leak channels use, and because it moves net positive charge out it is electrogenic, contributing only a few millivolts directly. Its main job is long term: block it with digitalis (ouabain, digoxin) or starve it of ATP in ischemia, and the gradients slowly run down, cells depolarize and swell.

Graded potentials

Graded potentials are local changes in membrane potential at dendrites, cell bodies and sensory endings: excitatory and inhibitory postsynaptic potentials, receptor (generator) potentials and the end plate potential. They vary in size with stimulus strength, can be depolarizing or hyperpolarizing, decay with distance (decremental conduction) and can summate. How far they spread is set by the length constant (lambda), which increases with higher membrane resistance and lower internal (axial) resistance. The time constant (resistance times capacitance) sets how quickly they rise and fall.

The action potential

The action potential is an all or none, regenerative, non decremental depolarization that travels along axons and muscle fibers. It begins when graded depolarization at the axon initial segment (or axon hillock), which has the highest density of voltage gated sodium channels, reaches threshold (about -55 mV in neurons).

1. Upstroke (depolarization). Voltage gated Na+ channels open their activation (m) gates. Sodium entry depolarizes the membrane, which opens more sodium channels: a positive feedback loop (the Hodgkin cycle). The membrane overshoots zero and approaches, but never reaches, E(Na), peaking near +30 to +40 mV.
2. Peak and inactivation. Within about a millisecond the slower inactivation (h) gates close the sodium channels. They cannot reopen until the membrane repolarizes and the h gate resets.
3. Repolarization. Voltage gated (delayed rectifier) K+ channels open more slowly than sodium channels. Potassium leaves, returning the membrane toward rest.
4. Afterhyperpolarization (undershoot). Potassium channels close slowly, so for a few milliseconds total potassium conductance exceeds that at rest and the membrane dips toward E(K).

Threshold is the voltage at which inward sodium current just exceeds outward potassium and leak current. A stimulus that depolarizes slowly can fail to fire because sodium channels inactivate as they open, a phenomenon called accommodation. The pump plays no direct role in a single spike: an action potential still fires for thousands of cycles after the pump is poisoned.

Refractory periods

During the absolute refractory period no stimulus, however strong, can trigger a second action potential, because most sodium channels are inactivated. It lasts from threshold until repolarization is largely complete, and it limits maximum firing frequency (roughly 1 kHz for large neurons). During the relative refractory period, which overlaps the afterhyperpolarization, a stronger than normal stimulus can fire the cell: some sodium channels have recovered, but potassium conductance is still elevated and the membrane is farther from threshold. Refractoriness also forces one way propagation in the normal axon, because the patch just behind the advancing spike cannot fire again.

Conduction and myelination

An action potential spreads because local currents flow from the active region to adjacent resting membrane and depolarize it to threshold. Conduction velocity rises with axon diameter (lower internal resistance) and with myelination (higher membrane resistance and lower capacitance, so current spreads farther and charges the membrane faster). Myelin is formed by oligodendrocytes in the CNS (one cell wraps many axons) and Schwann cells in the PNS (one cell per internode). Voltage gated sodium channels cluster at the nodes of Ranvier, so the impulse jumps from node to node: saltatory conduction. It is faster and also metabolically cheaper, because ion exchange occurs only at nodes.

Demyelination slows or blocks conduction. In multiple sclerosis (CNS) and Guillain Barré syndrome (PNS), current leaks through the bare internode and may fail to reach threshold at the next node. Warmth worsens conduction in demyelinated fibers (Uhthoff phenomenon) because sodium channels inactivate faster at higher temperature.

Nerve fiber classification

Two systems coexist. The Erlanger Gasser (letter) system applies to all fibers, sensory and motor, and sorts them by conduction velocity. The Lloyd Hunt (Roman numeral) system applies to sensory fibers from muscle and skin. Boards test the matches.

Susceptibility to block differs by mechanism. Local anesthetics block small fibers first (C and B, then A delta), so pain and autonomic function disappear before touch and motor power. Pressure and ischemia affect large myelinated fibers first, which is why a "dead leg" from sitting on the sciatic nerve loses proprioception and motor function before pain. Hypoxia affects B fibers most.

Section 2Synapses and neurotransmitters

Electrical and chemical synapses

An electrical synapse is a gap junction: connexin proteins (six form a connexon, two connexons span the gap) create a pore that lets ions and small molecules pass directly from cell to cell. Transmission is nearly instantaneous, usually bidirectional, and synchronizes groups of cells. Gap junctions couple cardiac myocytes at intercalated discs, single unit smooth muscle cells, some brainstem and hypothalamic neurons, and glia. A chemical synapse releases a transmitter across a cleft of about 20 to 40 nm. It is slower (a synaptic delay of about 0.5 ms), strictly one way, and can amplify, invert (inhibit) and modulate signals. Nearly all synapses tested on boards are chemical.

Release: from spike to vesicle fusion

1. The action potential invades the presynaptic terminal and opens voltage gated calcium channels (N and P/Q types at most neuronal and neuromuscular terminals).
2. Calcium enters and binds synaptotagmin, the calcium sensor on docked vesicles.
3. The SNARE complex zips together: v SNARE synaptobrevin on the vesicle pairs with t SNAREs syntaxin and SNAP 25 on the membrane, pulling the membranes into fusion.
4. Transmitter is released by exocytosis in discrete packets, or quanta, each the contents of one vesicle. A spontaneous single quantum at the neuromuscular junction produces a miniature end plate potential.
5. Vesicle membrane is retrieved by clathrin mediated endocytosis (dynamin pinches it off) and refilled by vesicular transporters driven by a proton gradient.

Release depends steeply on calcium: roughly the fourth power of calcium concentration in the terminal. Botulinum toxin and tetanus toxin are proteases that cleave SNARE proteins. Botulinum acts at peripheral cholinergic terminals and causes descending flaccid paralysis. Tetanus toxin travels retrograde to the spinal cord and blocks release of glycine and GABA from inhibitory Renshaw and interneurons, causing spastic paralysis, trismus and opisthotonus. Lambert Eaton syndrome produces antibodies against presynaptic voltage gated calcium channels; weakness improves with repeated effort as calcium accumulates.