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