385

Chapter 19

Special Senses

IntroductionA vital function of the nervous system is the gathering of sensory information

Sensory information is derived from a variety of special-ized sensory nerve endings. These include:

• Sensory endings in the skin to detect touch (fine touch, pressure), pain and temperature (see Chapter 18)

• Tendon endings and muscle spindles to detect movement and position of the limbs

• Chemoreceptive organs, such as the carotid body• Sensory endings on the tongue to detect taste• Sensory endings in the olfactory mucosa to detect

smell.In addition, information is obtained by the specialized

sensory organs, the eye and the ear; the ear and the vestibular system detect sound, acceleration and position and the eye perceives light.

EarThe ear is divided into the external ear, middle ear and inner ear

The external ear comprises the pinna and external audi-tory canal. The pinna is composed of elastic cartilage covered by hair-bearing skin. The external auditory canal is lined by hair-bearing skin. Within its subcutaneous tissues are wax-secreting ceruminous glands, which are modified sebaceous glands. The outer two-thirds of the canal is surrounded by elastic cartilage in continuity with the pinna; the inner third is surrounded by the temporal bone of the skull.

The middle ear is separated from the external ear by the tympanic membrane

The tympanic membrane marks the boundary between the external ear and the cavity of the middle ear, which is also termed the tympanic cavity (Fig. 19.1).

The tympanic membrane is a three-layered structure:

• The outer aspect is covered by stratified squamous epithelium

• The central portion is composed of fibrocollagenous support tissue containing numerous elastic fibres to provide mechanical strength

• The inner portion is lined by a low cuboidal epithe-lium which is continuous with that lining the rest of the middle ear.

The middle ear transmits sound vibrations to the inner ear

The middle ear cavity is lined by a low cuboidal epithe-lium and contains three auditory ossicles, the incus, malleus and stapes. These are:

• Composed of compact bone• Articulated by synovial joints (see Chapter 13)• Covered externally by the same low cuboidal epi-

thelium that lines the inner ear.Two small skeletal muscles, the stapedius and the

tensor tympani, are associated with the ossicles and damp motion between the bones, which occurs in response to loud noise.

The middle ear cavity communicates directly with air-filled spaces in the mastoid bone (mastoid sinuses), which are lined by low cuboidal or flattened squamous epithelium.

The auditory (Eustachian) tube equalizes pressure in the middle ear cavity

The auditory tube extends from the middle ear cavity to the nasopharynx and is lined by ciliated epithelium similar to that of the respiratory tract. Its function is to equilibrate pressure between the middle ear cavity and the atmosphere.

Normally, the auditory tube is collapsed, but it is opened by movement of muscles in the nasopharynx, such as occurs with swallowing or yawning.

The inner ear is a series of fluid-filled sacs encased in bone

The inner ear consists of fluid-filled sacs (membranous labyrinth) that lie in cavities in the temporal bone of the skull (bony or osseous labyrinth).

The membranous labyrinth comprises the cochlear duct, the saccule, the utricle and semicircular canals and the endolymphatic sac and duct, the walls of which are composed of sheets of fibrocollagenous support tissue lined by a flat epithelium. These sacs are filled with a fluid called endolymph and have epithelial and sensory specializations to detect position and sound.

The osseous labyrinth is composed of three cavities: the vestibule, the semicircular canals and the cochlea,

386 CHAPTER 19 SPECIAL SENSES

which are lined by periosteum and filled with fluid called perilymph.

Movement is detected in the inner ear by mechanoreceptors

Mechanoreceptors or hair cells are specialized epithelial cells bearing a highly organized system of microvilli (ste-reocilia) on their apical surface. Deflection of the micro-villi causes electrical depolarization of the hair cell membrane, which is transmitted to the central nervous system by the connecting axons of sensory nerve cells (Fig. 19.2).

Patches of hair cells are located in three sites:• Within the vestibular apparatus in the ampullae of

the semicircular canals to detect acceleration• Within the macula of the utricle and saccule to

perceive the direction of gravity and static position

• Within the organ of Corti of the cochlea to detect sound vibration.

At each site, the hair cell microvilli are embedded in a gelatinous matrix, which moves according to the stimu-lus it is detecting. Movement of the microvilli towards the tallest row excites (depolarizes) the hair cell mem-brane, whereas movement towards the shortest row inhibits (hyperpolarizes) it.

Hair cells are arranged in different parts of the mem-branous labyrinth (Fig. 19.3) in order to sense movement generated by different causes.

Support cells surround the hair cells and are anchored to them at their apex by occluding junctions. These junc-tions maintain ionic gradients between the endolymph and the extracellular fluid around the cells, the gradients being reversed on depolarization.

Sound is detected in the inner ear by the organ of Corti in the cochlear duct

The cochlear duct is a blind-ended tubular diverticulum filled with endolymph. It makes two and three-quarter turns within the spiral-shaped bony cochlea in the tem-poral bone, and is compressed between two other tubular spaces, the vestibular and tympanic cavities, which are filled with perilymph (Fig. 19.4).

Within the cochlear duct is the organ of Corti, which is a special adaptation of the epithelial cells lining the cochlear duct and detects sound vibration (Fig. 19.5).

Gravity and static position are detected by hair cells in the macula of the utricle and the macula of the saccule

The macula of the utricle lies in the horizontal plane, whereas the macula of the saccule lies in the vertical plane at right-angles to the macula of the utricle (see Fig. 19.3).

Each macula is histologically identical and is com-posed of the following three cell types (Fig. 19.6):

FIGURE 19.1 Anatomy of the ear. The ear consists of the external ear, middle ear and inner ear. The inner third of the external auditory canal of the external ear is surrounded by the temporal bone, and the middle ear and inner ear are contained within cavi-ties in the temporal bone. The middle ear is an air-filled cavity containing the auditory ossicles. It is connected to the nasopharynx by the auditory (Eustachian) tube and is in direct continuity with the mastoid air cells. The inner ear is a fluid-filled cavity divided into three main spaces (semicircular canal space, vestibule and cochlea). Within the inner ear are a series of interconnecting fluid-filled sacs (semicircular canals, utricle, saccule and cochlear duct). The endolymphatic duct runs from the membranous sacs to the subdural space around the brain.

petrous temporal bone

semicircularcanals

utriclevestibule

cochlea

ovalwindow

round window

auditory(Eustachian)tube

auditory ossiclesin middle ear

tympanicmembrane

externalauditory

canal

subdural space

brainendolymphatic

sac and duct

mastoid aircavities

ampullae

helicotrema

vestibular cavity

cochlear duct

tympanic cavity

saccule

EAR 387

FIGURE 19.2 Hair cell microvilli. (a) The apical surface of each hair cell bears a highly organized system of microvilli (stereocilia), which are arranged as three parallel rows in a V- or W-shaped pattern. The height of the microvilli progressively decreases from the back to the front of the hair cell to form a so-called ‘organ-pipe’ arrangement. Fine filaments link individual microvilli from each row, with the tips of the shorter microvilli being coupled to the shafts of the taller microvilli behind. Whereas the hair cell is rigidly fixed in place by support cells, the tips of the tallest row of microvilli are embedded in a gelatinous extracellular matrix, which is free to move within the fluid cavities of the inner ear or vestibular system. (b) Hair cells are supported by adjacent cells and are in contact with the axon of a sensory nerve. The support cells are anchored to the hair cells by occluding junctions. The stereocilia are embedded in a gelatinous matrix. (c) Movement of the gelatinous matrix deflects the stereocilia, causing membrane depolarization of the hair cell, which is relayed to the central nervous system via the axon of a sensory nerve. Hair cells are arranged in different patterns in the cochlea and vestibular apparatus to detect acceleration (movement), gravity (position) or sound (hearing).

gelatinous extracellular matrix

stereocilia arrangedin rows

surface ofsensory hair cell

sensory hair cell

stereocilia anchoredto matrix

filament linksindividualstereocilia rows

a

gelatinous extracellular matrix

endolymph

stereocilia

hair cell

extracellularfluid

tight junction

support cell

sensory axon

b

stereocilia bend causingmembrane depolarization

gelatinous extracellular matrixmoves within endolymph

axon fibres

signal passes to sensory neuronsof sensory ganglionc

ampullae ofsemicircular

canals

macula of utricle

macula of saccule

organ of Cortiof cochlea

FIGURE 19.3 Distribution of hair cells in the membranous labyrinth. The hair cells are arranged in patches in the ampullae of the semicircular canals to detect acceleration, in the macula of the utricle and saccule to perceive gravity direction and static posi-tion, and in the organ of Corti of the cochlea to detect sound vibration.

388 CHAPTER 19 SPECIAL SENSES

FIGURE 19.4 Cochlea. (a) The cochlea of the osseous labyrinth contains three spaces, the vestibular cavity, the cochlear duct and the tympanic cavity. These spaces are wound in a spiral within the temporal bone. The central spiral of bone within the cochlea is called the ‘modiolus’. The vestibular cavity and the tympanic cavity contain perilymph continuous with that in the vestibule (see Fig. 19.1), whereas the cochlear duct, which is con-tinuous with and part of the membranous labyrinth, is filled with endolymph. At the apex of the cochlea, the vestibular and tympanic cavities connect at an opening termed the ‘helicotrema’. The cochlear nerve emerges from the base of the cochlea and carries signals to the brain. (b) Shown here is a section through the cochlea. The vestibular membrane (Reiss-ner’s membrane) consists of two layers of flattened epithelium separated by a base-ment membrane, one cell layer being in continuity with the cells lining the vestibu-lar cavity and the other in continuity with the cells lining the cochlear duct. The cells are held together by well-developed occluding junctions to maintain different electrolyte concentrations between the endolymph and the perilymph. The stria vascularis is a specialized area of epithe-lium with a rich vascular supply in the lateral wall of the cochlear duct. Many of the cells here have ultrastructural features indicating an ion transport function, and it is thought that they secrete endolymph. The basilar membrane is thicker than the vestibular membrane and consists of col-lagen fibres as well as a basement mem-brane. On one side it is covered by cells lining the tympanic cavity, and on the other by specialized cells lining the cochlear duct. Medially, the basilar mem-brane is continuous with the organ of Corti, which is a specialized area of support cells and sensory hair cells sub-serving hearing (see Fig. 19.5). The organ of Corti is supported by a spur of bone called the ‘osseous spiral lamina’. Later-ally, the basilar membrane is attached to the spiral ligament, which is a mass of tissue developed from the endosteum of the surrounding bone. Neurons of the spiral ganglion are present adjacent to the osseous spiral lamina.

temporalbonemodiolus

vestibular cavity

cochlear duct

tympanic cavityspiral ligament

organ of Corti

cochlear nervea

organof Corti

spiralligament

nerve bundle

spiralganglion

cochlearnerve

osseousspiral lamina

stria vascularis

vestibularmembrane

vestibularcavity

cochlearduct

tympaniccavity

tectorialmembrane

b

EAR 389

FIGURE 19.5 Organ of Corti. (a) The organ of Corti is composed of epithelial support cells and sensory hair cells. Medially, it rests on the rigid bony osseous spiral lamina; laterally it is located on the deformable basilar membrane. There are two groups of hair cells, an inner group and an outer group, which are separated by a small opening at the end of the osseous spiral lamina termed the inner tunnel (tunnel of Corti). The inner group are smaller and rounder than the outer group and arranged as a single row along the cochlea. The outer group are tall and thin and arranged in three to five parallel rows, depending on the position in the length of the cochlea. The hair cells are surrounded by epithelial support cells, the inner hair cells being completely surrounded, whereas the outer hair cells are enclosed only at their extreme apex and basal portions, leaving a bare mid-zone in contact with extracellular fluid. The microvilli of the outer hair cells are attached to a sheet of gelatinous extracellular matrix braced by filamen-tous proteins (tectorial membrane), but those of the inner hair cells are free. Axons make synaptic contact with the hair cells and run to the spiral ganglion. The tectorial membrane is secreted by epithelial cells (interdental cells). There are several classes of support cell in the organ of Corti. Pillar cells contain abundant scaffolding microtubules, and surround and support the triangular cavity (inner tunnel) at the level of the lip of the osseous spiral lamina. In contrast, phalangeal cells support the hair cells and are attached to them by occluding junctions at their apices, thus isolating the basal membrane of hair cells from the endolymph and maintaining electrochemical gradients. (b) Micrograph of the organ of Corti. Note the tectorial membrane (T), the mass of pha-langeal cells bearing the hair cells (P), the basilar membrane (B), and the stria vascularis (S) on the spiral ligament (SL) within the cochlear duct (C).

cochlear duct

tympanic cavity

outer hair cells

spiralligament

basilarmembrane

innertunnel

phalangealcells

pillarcells inner

hair cell

tectorialmembrane

internalspiraltunnel

spirallimbus

osseous spirallamina

nerve

a

S

TC

P

SL

B

tympanic cavity

vestibular cavity

vestibular membrane

b

A D VA N C E D C O N C E P TDETECTION OF SOUND IN THE INNER EAR

Sound waves cause vibration of the tympanic membrane, which is then transmitted to the oval window membrane via the auditory ossicles.

Pressure waves are thence transmitted to the peri-lymph of the vestibular cavity, causing the vestibular and basilar membranes to bow inward towards the tympanic cavity, and to the round window, which bows outward.

Because the tectorial membrane remains relatively rigid, bowing of the vestibular and basilar membranes causes relative movement of the hair cell stereocilia, which results in membrane depolarization.

The signal is transmitted to the sensory nerves of the spiral ganglion and then through the cochlear cranial nerve to the brain, where it is perceived as sound.

Low-frequency sound is detected by stereocilia towards the apex of the cochlea, whereas high-frequency sounds are detected at the base.

• Support cells (sustentacular cells), which are columnar cells with short apical microvilli

• Type I hair cells, which are polygonal in shape and surrounded by a network of afferent and efferent nerve endings

• Type II hair cells, which are cylindrical in shape, with basal synaptic afferent and efferent nerve endings.

In addition to an organ-pipe arrangement of tall microvilli stereocilia on their apical surface (see Fig. 19.2), these hair cells possess a single true cilium termed a kinocilium, which is located just behind the tallest row of stereocilia.

The stereocilia and kinocilium of each new hair cell are embedded in a gelatinous plaque of extracellular matrix called the otolithic membrane, which is

C L I N I C A L E XA M P L EHEARING LOSS

Many diseases of the ear are associated with temporary or permanent hearing loss. Deafness can be divided into conductive and sensorineural types.

Conductive loss occurs when sound waves cannot be transmitted to the inner ear; common causes include blockage of the external auditory meatus (e.g. wax) or damage to the middle ear by infection (‘otitis media’).

Sensorineural loss is the result of damage to the inner ear, the nerves linking the cochlea with the brain, or in the brain itself. The most common type is called presby-cusis, which occurs in elderly people; it results from reduction in hair cells, atrophy of the stria vascularis and neuron loss in the spiral ganglia.

Recently, it has been possible to place electronic implants into the cochlea to treat deafness. Sound is detected by an external device and this causes direct stimulation of the cochlear nerve, allowing hearing.

390 CHAPTER 19 SPECIAL SENSES

FIGURE 19.7 Ampullary region of semicircular canal. Within the ampullae of the semicircular canals, hair cells are arranged over a finger-like protrusion of the lining called the ‘ampullary crista’. The hair cells are both type I and type II and are associ-ated with adjacent support cells. The axons innervating the cells emerge from the base of the ampullary crista, being derived from sensory nerve cells in the ganglion of the vestibular nerve (Scarpa’s ganglion), which connect with the vestibular nucleus in the brain stem. The hair cell stereocilia and kinocilium are embedded in the gelatinous matrix of the cupola which, unlike the tectorial membrane of the maculae, does not contain otoconia.

cupola

movement ofendolymph

sensoryhair cell

axons runningfrom sensory

cellsepitheliumof ampulla

support cells

endolymph

FIGURE 19.6 Macula. In this micrograph, the stereocilia of the hair cells of the macula are embedded in the otolithic mem-brane. The otoconia (O) are visible as numerous purple-stained particles. The stereocilia appear as pink tufts (T) arising from the cell surface, but neither individual microvilli nor type of hair cell can be identified at this magnification. The support cells (SC) appear as an epithelial sheet.

T

O

SC

suspended in the endolymph. This membrane is covered by numerous small particles composed of protein and calcium carbonate, the otoconia (otoliths).

The macula can detect the direction of gravity by sensing the direction of pull of the otolithic membrane and otoconia on the mass of hair cells that results from head movement either backwards and forwards (macula of utricle) or from side to side (macula of saccule).

Acceleration and motion are detected by hair cells in the ampullae at the end of the semicircular canals

There are three semicircular canals, which assume pos-terior, superior and horizontal positions.

Each ampulla is a 1 mm long dilated region of the membranous labyrinth and contains a patch of hair cells arranged in a tall finger-like structure (an ampullary crista). The stereocilia of the sensory hair cells are attached to a dome-shaped gelatinous matrix termed a ‘cupola’ (Fig. 19.7).

With rotary motion of the head, endolymph moves within the membranous labyrinth because of the static inertia of the fluid relative to the rest of the vestibular apparatus. Such movement causes displacement of the cupola, and the direction of this displacement is detected by the hair cells.

When integrated, perception from the three semicir-cular canals arranged in planes perpendicular to each other provides information on the direction and rate of acceleration of head movement.

EyeThe eye is designed to focus light on to specialized recep-tors that respond to light. It is composed of sclera, cornea, uvea and retina arranged around three chambers (Fig. 19.8).

The sclera proper is the outer fibrocollagenous coat of the globe of the eye

The sclera varies in thickness from 1 mm posteriorly to 0.5 mm anteriorly, and is composed of flat plates of col-lagen oriented in different directions, but parallel to the surface.

The sclera is composed of three layers:• The episclera, an external layer of loose fibrocol-

lagenous tissue running adjacent to the periorbital fat

• The stroma, the middle layer, which is composed of bundles of collagen thicker than those of the episclera

• The inner part of the sclera, adjacent to the choroid layer.

The collagen bundles of the stroma run in sweeping branching patterns, mainly looping from front to back, within the stroma. The lamina fusca contains small numbers of elastic fibres.

The blood vessels and nerves (including the optic nerve) running to and from the eye pass through the periscleral and scleral layers. The scleral stroma itself is avascular. Anteriorly, the sclera blends with the cornea in a transition zone (the limbus), which is 1 mm wide.

402 CHAPTER 19 SPECIAL SENSES

For online review questions, please visit https://studentconsult.inkling.com.

E N D O F C H A P T E R R E V I E W

True/False Answers to the MCQS, as Well as Case Answers, Can be Found in the Appendix in the Back of the Book.

1. Which of the following features are present in the ear?(a) The pinna is composed of elastic cartilage(b) The ceruminous glands are modified apocrine glands(c) The tympanic membrane is lined on one side by cuboidal epithelium(d) The middle ear cavity is lined by ciliated pseudostratified epithelium(e) The auditory ossicles articulate by syndesmoses

2. Which of the following features are seen in the inner ear?(a) Hair cells have cilia on their surface to detect movement(b) The ampullae of the semicircular canals detect acceleration(c) The macula of the saccule detects sound(d) The vestibular and tympanic cavities contain perilymph(e) The basilar membrane is continuous with the organ of Corti

3. Which of the following are present in the eye?(a) The posterior chamber lies behind the lens(b) The lens is suspended from the ciliary body(c) The inner part of the cornea is lined by endothelium(d) The aqueous humour drains through the trabecular meshwork(e) Aqueous humour is produced by the epithelium of the ciliary body

4. Which of the following features are true for the retina?(a) Retinal pigment epithelial cells are responsible for phagocytosis of photoreceptor membranes(b) Cones are concentrated at the periphery of the retina(c) The photoreceptor layer is isolated from the rest of the retina by a belt of adherent junctions forming the external

limiting membrane(d) The macula contains the fovea(e) The optic disc is devoid of photoreceptors

CASE 19.1 GRADUAL ONSET OF DEAFNESS

A 58-year-old man has been referred to a hearing assessment unit because he has experienced increasing difficulty with hearing. Physical examination of the tympanic membrane is normal and it is planned to perform audiography to determine the pattern of hearing loss.

Q. Describe the functional and structural background to this case. Concentrate on structural aspects of hearing that may be abnormal and lead to deafness.

CASE 19.2 A WOMAN WITH DETERIORATING VISION

A 63-year-old woman is seen in the clinic having been referred because of deterioration in her vision. Ophthalmoscopic examination shows some cupping of the optic disc and the intraocular pressure is elevated. A diagnosis of glaucoma is made.

Q. Describe the structural and functional background to this case. Focus on structural changes in the eye that might cause an increase in the intraocular pressure.