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