Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

9. Ear and Vestibular Apparatus

9.1 Ear, Overview

A Auditory and vestibular apparatus in situ

a Coronal section through the right ear, anterior view, b Main parts of the auditory apparatus: external ear (yellow), middle ear (blue), and inner ear (green).

The auditory and vestibular apparatus are located deep in the petrous part of the temporal bone (petrous bone). The auditory apparatus consists of the external ear, middle ear, and inner ear (seeb). Sound waves are captured by the external ear (auricle, see B) and travel through the external auditory canal to the tympanic membrane, which marks the lateral boundary of the middle ear. The sound waves set the tympanic membrane into motion, and these mechanical vibrations are transmitted by the chain of auditory ossicles in the middle ear to the oval window, which leads into the inner ear (seep. 144). The ossicular chain induces vibrations in the membrane covering the oval window, and these in turn cause a fluid column in the inner ear to vibrate, setting receptor cells in motion (see p. 150). The transformation of sound waves into electrical impulses takes place in the inner ear, which is the actual organ of hearing. The external ear and middle ear, on the other hand, constitute the sound conduction apparatus. The organ of balance is the vestibular apparatus, which is also located in the auditory apparatus and will be described after the units that deal with the auditory apparatus. It contains the semicircular canals for the perception of angular acceleration (rotational head movements) and the saccule and utricle for the perception of linear acceleration. Diseases of the vestibular apparatus produce dizziness (vertigo).

В Right auricle

The auricle of the ear encloses a cartilaginous framework (auricular cartilage) that forms a funnel-shaped receptor for acoustic vibrations.

C Cartilage and muscles of the auricle

a Lateral view of the external surface, b Medial view of the posterior surface of the right ear.

The skin (removed here) is closely applied to the elastic cartilage of the auricle (shown in light blue). The muscles of the ear are classified as muscles of facial expression and, like the other members of this group, are supplied by the facial nerve. Prominent in other mammals, the auricular muscles are vestigial in humans, with no significantfunction.

D Arterial supply of the right auricle

Lateral view (a) and posterior view (b).

The proximal and medial portions of the laterally directed anterior surface of the ear are supplied by the anterior auricular arteries, which arise from the superficial temporal artery (see p.59). The other parts of the ear are supplied by branches of the posterior auricular artery, which arises from the external carotid artery. These vessels are linked by extensive anastomoses, so operations on the external ear are unlikely to compromise the auricular blood supply. The copious blood flowthrough the auricle contributes to temperature regulation: dilation of the vessels helps dissipate heat through the skin. The lack of insulating fat predisposes the ear to frostbite, which is particularly common in the upper third of the auricle. The lymphatic drainage and innervation of the auricle are covered in the next unit.

9.2 External Ear: Auricle, Auditory Canal, and Tympanic Membrane

В Sensory innervation of the auricle

Right ear, lateral view (a) and posterior view (b). The auricular region has a complex nerve supply because, developmentally, it is located at the boundary between the cranial nerves (pharyngeal arch nerves) and branches of the cervical plexus. Four cranial nerves contribute to the innervation of the auricle:

* Trigeminal nerve (CN V)

* Facial nerve (CN VII; the skin area that receives sensory innervation from the facial nerve is not precisely known)

* Glossopharyngeal nerve (CN IX) and vagus nerve (CN X)

Two branches of the cervical plexus are involved:

 Lesser occipital nerve (C2)

 Great auricular nerve (C2, C3)

Note: Because the vagus nerve contributes to the innervation of the external auditory canal (auricular branch, see below), mechanical cleaning of the ear canal (by inserting an aural speculum or by irrigating the ear) may evoke coughing and nausea. The auricular branch of the vagus nerve passes through the mastoid canaliculus and through a space between the mastoid process and the tympanic part of the temporal bone (tympanomastoid fissure, seep.23) to the external ear and external auditory canal. The ear canal receives sensory fibers from the glossopharyngeal nerve through its communicating branch with the vagus nerve.

C External auditory canal, tympanic membrane, and tympanic cavity

Right ear, coronal section, anterior view. The tympanic membrane (eardrum, seeE) separates the external auditory canal from the tympanic cavity, which is part of the middle ear (seep. 144). The external auditory canal is an S-shaped tunnel (see D) that is approximately 3 cm long with an average diameter of 0.6 cm. The outer third of the ear canal is cartilaginous. The inner two-thirds of the canal are osseous, the wall being formed by the tympanic part of the temporal bone. The cartilage nous part in particular bears numerous sebaceous and cerumen glands beneath the keratinized stratified squamous epithelium. The cerumen glands produce a watery secretion that combines with the sebum and sloughed epithelial cells to form a protective barrier (cerumen, “ear- wax”) that screens out foreign bodies and keeps the epithelium from drying out. If the cerumen absorbs water (e.g., water in the ear canal after swimming), it may obstruct the ear canal (cerumen impaction), temporarily causing a partial loss of hearing.

D Curvature of the external auditory canal

Right ear, anterior view (a) and transverse section (b).

The external auditory canal is most curved in its cartilaginous portion. It is important for the clinician to know how the ear canal is curved. When the tympanic membrane is inspected with an otoscope, the auricle should be pulled backward and upward in order to straighten the cartilaginous part of the ear canal so that the speculum of the otoscope can be introduced (c).

Note the proximity of the cartilaginous anterior wall of the external auditory canal to the temporomandibular joint. This allows the examiner to palpate movements of the mandibular head by inserting the small finger into the outer part of the ear canal.

E Tympanic membrane

Right tympanic membrane, lateral view. The healthy tympanic membrane has a pearly gray color and an oval shape with an average surface area of approximately 75 mm2. It consists of a lax portion, the pars flaccida (ShrapnelI membrane), and a larger taut portion, the porstenso, which is drawn inward at its center to form the umbo (“navel”). The umbo marks the lower tip of the handle (manubrium) of the malleus, which is attached to the tympanic membrane all along its length. It is visible through the pars tensa as a light-colored streak (malleolar stria). The tympanic membrane is divided into four quadrants in a clockwise direction: anterosuperior (I), anteroinferior (II), posteroinferior (III), pos- terosuperior (IV). The boundary lines of the quadrants are the malleolar stria and a line intersecting it perpendicularly at the umbo. The quadrants of the tympanic membrane are clinically important because they are used in describing the location of lesions. The function of the tympanic membrane is reviewed on pp. 140 and 146. A triangular area of reflected light can be seen in the anteroinferior quadrant of a normal tympanic membrane. The location of this “cone of light” is helpful in evaluating the tension of the tympanic membrane.

9.3 Middle Ear: Tympanic Cavity and Pharyngotympanic Tube

A The middle ear and associated structures

Right petrous bone, superior view. The middle ear (light blue) is located within the petrous part of the temporal bone between the external ear (yellow) and inner ear (green). The tympanic cavity of the middle ear contains the chain of auditory ossicles, of which the malleus (hammer) and incus (anvil) are visible here. The tympanic cavity communicates anteriorly with the pharynx via the pharyngotympanic (auditory) tube, and it communicates posteriorly with the mastoid air cells. Infections can spread from the phyynx to the mastoid cells by this route (see C).

В Walls of the tympanic cavity

Anterior view with the anterior wall removed. The tympanic cavity is a slightly oblique space that is bounded by six walls:

 Lateral (membranous) wall: boundary with the external ear; formed largely by the tympanic membrane.

 Medial (labyrinthine) wall: boundary with the inner ear; formed largely by the promontory, or the bony eminence, overlying the basal turn of the cochlea.

 Inferior (jugular) wall: forms the floor of the tympanic cavity and borders on the bulb of the jugular vein.

 Posterior (mastoid) wall: borders on the air cells of the mastoid process, communicating with the cells through the aditus (inlet) of the mastoid antrum.

 Superior (tegmental) wall: forms the roof of the tympanic cavity.

 Anterior (carotid) wall (removed here): includes the opening to the pharyngotympanic (auditory) tube and borders on the carotid canal.

C Tympanic cavity: clinically important anatomical relationships

Oblique sagittal section showing the medial wall of the tympanic cavity (seeB). The anatomical relationships of the tympanic cavity are particularly important in treating chronic suppurative otitis media. During this inflammation of the middle ear, pathogenic bacteria may spread upward to adjacent regions. For example, bacteria may spread upward through the roof of the tympanic cavity into the middle cranial fossa (inciting meningitis or a cerebral abscess, especially of the temporal lobe); they may invade the mastoid air cells (mastoiditis) or sigmoid sinus (sinus thrombosis); they may pass through the air cells of the petrous apex and enter the CSF space, causing abducent paralysis, trigeminal nerve irritation, or visual disturbances (Gradenigo syndrome); or they may invade the facial nerve canal, resulting in facial paralysis.

D Pharyngotympanic (auditory) tube

Medial view of the right half of the head. The pharyngotympanic tube (auditory tube) creates an open channel between the middle ear and pharynx. One-third of the tube is bony and two-thirds are cartilaginous. The bony part of the tube is located in the petrous bone, and the cartilaginous part continues onward to the pharynx, where it expands into a funnel-shaped orifice. As it expands, it forms a kind of hook (hamulus) which is attached to a membranous part (membranous lamina) that enlarges toward the pharynx. The pharyngotympanic tube also opens during swallowing. Air passing through the tube serves to equalize the air pressure on the two sides of the tympanic membrane. This equalization is essential for maintaining normal tympanic membrane mobility, which, in turn, is necessary for normal hearing. The pharyngotympanic tube is opened by the muscles of the soft palate (tensor veli palatini and levator veli palatini) and by the salpingopharyngeus, which is part of the superior pharyngeal muscle. The fibers of the tensor veli palatini arising from the membranous lamina of the pharyngotympanic tube are of special significance: When the tensor veli palatini tenses the soft palate during swallowing, its fibers attached to the membranous lamina simultaneously open the pharyngotympanic tube. The tube is lined with ciliated respiratory epithelium whose cilia beat toward the pharynx, thus inhibiting the passage of microorganisms into the middle ear. If this nonspecific protective mechanism fails, bacteria may migrate up the tube and incite a purulent middle ear infection (see C).

9.4 Middle Ear: Auditory Ossicles and Tympanic Cavity

A Auditory ossicles

The auditory ossicles of the left ear. The ossicular chain consists of three small bones in the middle ear (chain function is described in B). It establishes an articular connection from the tympanic membrane to the oval window and consists of the following bones:

 Malleus (“hammer”)

 Incus ("anvil”)

 Stapes (“stirrup”)

a, b Malleus: posterior view and anterior view c, d Incus: medial view and anterolateral view e,f Stapes: superior view and medial view g Medial view of the ossicular chain

Note the articulations between the malleus and incus (incudomalleolar joint) and between the incus and stapes (incudostapedial joint).

В Function of the ossicular chain

Anterior view.

a Sound waves (periodic pressure fluctuations in the air) set the tympanic membrane into vibration. The ossicular chain transmits the vibrations of the tympanic membrane (and thus the sound waves) to the oval window, which in turn communicates them to an aqueous medium, the perilymph. While sound waves encounter very little resistance in air, they encounter considerably higher impedance when they reach the fluid interface of the inner ear (perilymph). The sound waves must therefore be amplified (“impedance matching”). The difference in surface area between the tympanic membrane and oval window increases the sound pressure by a factor of 17, and this is augmented by the 1 .3-fold mechanical advantage of the lever action of the ossicular chain. Thus, in passing from the tympanic membrane to the inner ear, the sound pressure is amplified by a factor of 22. If the ossicular chain fails to transform the sound pressure between the tympanic membrane and stapes base (footplate), the patient will experience conductive hearing loss of magnitude approximately 20dB.

b,c Sound waves impinging on the tympanic membrane induce motion in the ossicular chain, causing a tilting movement of the stapes (b normal position, c tilted position). The movements of the stapes base against the membrane of the oval window (stapedial membrane) induce corresponding waves in the fluid column in the inner ear.

d The movements of the ossicular chain are essentially rocking movements (the dashed line indicates the axis of the movements, the arrows indicate their direction). Two muscles affect the mobility of the ossicular chain: the tensor tympani and the stapedius (see C).

C Ossicular chain in the tympanic cavity

Lateral view of the right ear. The joints and their stabilizing ligaments can be seen. The two muscles of the middle ear—the stapedius and tensor tympani—can also be identified. The stapedius (innervated by the stapedial branch of the facial nerve) inserts on the stapes. When it contracts, it stiffens the sound conduction apparatus and decreases sound transmission to the inner ear. This filtering function is believed to be particularly important at high sound frequencies (“high-pass filter”). When sound is transmitted into the middle ear through a probe placed in the external ear canal, one can measure the action of the stapedius (stapedius reflex test) by measuring the change in acoustic impedance (i.e., the amplification of the sound waves). Contraction of the tensor tympani (innervated by the trigeminal nerve via the medial pterygoid nerve) stiffens the tympanic membrane, thereby reducing the transmission of sound. Both muscles undergo a reflex contraction in response to loud acoustic stimuli.

Note: The chorda tympani, which contains gustatory fibers for the anterior two-thirds of the tongue, passes through the middle ear without a bony covering (making it susceptible to injury during otological surgery).

D Mucosal lining of the tympanic cavity

Posterolateral view with the tympanic membrane partially removed. The tympanic cavity and the structures it contains (ossicular chain, tendons, nerves) are covered with mucosa that is raised into folds and deepened into depressions conforming to the covered surfaces. The epithelium consists mainly of a simple squamous type, with areas of ciliated columnar cells and goldet cells. Because the tympanic cavity communicates directly with the respiratory tract through the pharyngotympanic tube, it can also be interpreted as a specialized paranasal sinus. Like the sinuses, it is susceptible to frequent infections (otitis media).

E Clinically important levels of the tympanic cavity

The tympanic cavity is divided into three levels in relation to the tympanic membrane:

* The epitympanum (epitympanic recess, attic) above the tympanic membrane

* The mesotympanum medial to the tympanic membrane

* The hypotympanum (hypotympanic recess) below the tympanic membrane

The epitympanum communicates with the mastoid air cells, and the hypotympanum communicates with the pharyngotympanic tube.

9.5 Inner Ear, Overview

A Schematic diagram of the inner ear

The inner ear is embedded within the petrous part of the temporal bone (seeB) and contains the auditory and vestibular apparatus for hearing and balance (seep. 150 ff). It comprises a membranous labyrinth contained within a similarly shaped bony labyrinth. The auditory apparatus consists of the cochlear labyrinth with the membranous cochlear duct. The membranous duct and its bony shell make up the cochlea, which contains the sensory epithelium of the auditory apparatus (organ of Corti). The vestibular apparatus includes the vestibular labyrinth with three semicircular canals (semicircular ducts), a saccule, and a utricle, each of which contains sensory epithelium. While each of the membranous semicircular ducts is encased in its own bony shell (semicircular canal), the utricle and saccule are contained in a common bony capsule, the vestibule. The cavity of the bony labyrinth is filled with perilymph (perilym-phaticspace, beige), whose composition reflects its being an ultrafiltrate of blood. The perilymphatic space is connected to the subarachnoid space by the cochlear aqueduct (= perilymphatic duct). It ends at the posterior surface of the petrous part of the temporal bone below the internal acoustic meatus. The membranous labyrinth “floats” in the bony labyrinth, being loosely attached to it by connective-tissue fibers. It is filled with endolymph (endolymphatic space, blue-green), whose ionic composition corresponds to that of intracellular fluid. The endolymphatic spaces of the auditory and vestibular apparatus communicate with each other through the ductus reuniens and are connected by the endolymphatic duct to the endolymphatic sac, an epidural pouch on the posterior surface of the petrous bone in which the endolymph is reabsorbed.

В Projection of the Inner ear onto the bony skull

a Superior view of the petrous part of the temporal bone, b Right lateral view of the squamous part of the temporal bone.

The apex of the cochlea is directed anteriorly and laterally—not upward as one might intuitively expect. The bony semicircular canals are oriented at an approximately 45” angle to the cardinal body planes (coronal, transverse, and sagittal). It is important to know this arrangement when interpreting thin-slice CT scans of the petrous bone.

Note: The location of the semicircular canals is of clinical importance in thermal function tests of the vestibular apparatus. The lateral (horizontal) semicircular canal is directed 30° forward and upward (see b). If the head of the supine patient is elevated by 30°, the horizontal semicircular canal will assume a vertical alignment. Since warm fluids tend to rise, irrigating the auditory canal with warm (44° C) or cool (30° C) water (relative to the normal body temperature) can induce a thermal current in the endolymph of the semicircular canal, causing the patient to manifest vestibular nystagmus (jerky eye movements, vestibulo-ocular reflex). Because head movements always stimulate both vestibular apparatuses, caloric testing is the only method of separately testing the function of each vestibular apparatus (important in the diagnosis of unexplained vertigo).

C Innervation of the membranous labyrinth

Right ear, anterior view. Afferent impulses from the receptor organs of the utricle, saccule, and semicircular canals (i.e., the vestibular apparatus) are first relayed by dendritic (peripheral) processes to the two-part vestibular ganglion (superior and inferior parts), which contains the cell bodies (perikarya) of the afferent neurons (bipolar ganglion cells). Their central processes form the vestibular part of the vestibulocochlear nerve through the internal acoustic meatus and the cerebellopontine angle to the brainstem.

Afferent impulses from the receptor organs of the cochlea (i.e., the auditory apparatus) are first transmitted by dendritic (peripheral) processes to the spiral ganglia, which contain the cell bodies of the bipolar ganglion cells. They are located in the central bony core of the cochlea (modiolus). Their central processes form the cochlear part of the vestibulocochlear nerve.

Note: also the section of the facial nerve with its parasympathetic fibers (nervus intermedius) within the internal auditory canal (see D).

D Passage of cranial nerves through the right internal acoustic meatus

Posterior oblique view of the fundus of the internal acoustic meatus. The approximately 1-cm-long internal auditory canal begins at the internal acoustic meatus on the posterior wall of the petrous bone. It contains:

* the vestibulocochlear nerve with its cochlear and vestibular parts,

* the markedly thinner facial nerve with its parasympathetic fibers (nervus intermedius), and

* the labyrinthine artery and vein (not shown).

Given the close proximity of the vestibulocochlear nerve and facial nerve in the bony canal, a tumor of the vestibulocochlear nerve (acoustic neuroma) may exert pressure on the facial nerve, leading to peripheral facial paralysis (see also p. 79). Acoustic neuroma is a benign tumor that originates from the Schwann cells of vestibular fibers, and so it would be more accurate to call it a vestibular schwannoma (see also p. 82). Tumor growth always begins in the internal auditory canal; as the tumor enlarges it may grow into the cerebellopontine angle. Acute, unilateral inner ear dysfunction with hearing loss (sudden sensorineural hearing loss), often accompanied by tinnitus, typically reflects an underlying vascular disturbance (vasospasm of the labyrinthine artery causing decreased blood flow).

9.6 Ear: Auditory Apparatus

A Location and structure of the cochlea

a Cross-section through the cochlea in the petrous bone, b The three compartments of the cochlear canal, c Cochlear turn with sensory apparatus.

The bony canal of the cochlea (spiral canal) is approximately 30-35 mm long in the adult. It makes 2V2 turns around its bony axis, the modiolus, which is permeated by branched cavities and contains the spiral ganglion (peri-karya of the afferent neurons). The base of the cochlea is directed toward the internal acoustic meatus (a). A cross-section through the cochlear canal displays three membranous compartments arranged in three levels (b). The upper and lower compartments, the scala vestibuli and scala tympani, each contain perilymph, while the middle level, the cochlear duct (scala media), contains endolymph. The perilymphatic spaces are interconnected at the apex by the helicotrema, while the endolymphatic space ends blindly at the apex. The cochlear duct, which is triangular in cross-section, is separated from the scala vestibuli by the vestibular (Reissner) membrane and from the scala tympani by the basilar membrane. The basilar membrane represents a bony projection of the modiolus (spiral lamina) and widens steadily from the base of the cochlea to the apex. High frequencies (up to 20,000 Hz) are perceived by the narrow portions of the basilar membrane while low frequencies (down to about 200 Hz) are perceived by its broader portions (tonotop/c organization). The basilar membrane and bony spiral lamina thus form the floor of the cochlear duct, upon which the actual organ of hearing, the organ of Corti, is located. This organ consists of a system of sensory cells and supporting cells covered by an acellular gelatinous flap, the tectorial membrane. The sensory cells (inner and outer hair cells) are the receptors of the organ of Corti (c). These cells bear approximately 50-100 stereocilia, and on their apical surface synapse on their basal side with the endings of afferent and efferent neurons. They have the ability to transform mechanical energy into electrochemical potentials (see below). A magnified cross-sectional view of a cochlear turn (c) also reveals the stria vascularis, a layer of vascularized epithelium in which the endolymph is formed. This endolymph fills the membranous labyrinth (appearing here as the cochlear duct, which is part of the labyrinth). The organ of Corti is located on the basilar membrane. It transforms the energy of the acoustic traveling wave into electrical impulses, which are then carried to the brain by the cochlear nerve. The principal cell of signal transduction is the inner hair cell. The function of the basilar membrane is to transmit acoustic waves to the inner hair cell, which transforms them into impulses that are received and relayed by the cochlear ganglion.

В Sound conduction during hearing

a Sound conduction from the middle ear to the inner ear: Sound waves in the air deflect the tympanic membrane, whose vibrations are conducted by the ossicular chain to the oval window. The sound pressure induces motion of the oval window membrane, whose vibrations are, in turn, transmitted through the perilymph to the basilar membrane of the inner ear (see b). The round window equalizes pressures between the middle and inner ear.

b Formation of a traveling wave in the cochlea: The sound wave begins at the oval window and travels up the scala vestibuli to the apex of the cochlea (“traveling wave”). The amplitude of the traveling wave gradually increases as a function of the sound frequency and reaches a maximum value at particular sites (shown greatly exaggerated in the drawing). These are the sites where the receptors of the organ of Corti are stimulated and signal transduction occurs. To understand this process, one must first grasp the structure of the organ of Corti (the actual organ of hearing), which is depicted in C.

C Organ of Corti at rest (a) and deflected by a traveling wave (b)

The traveling wave is generated by vibrations of the oval window membrane (seeBb). At each site that is associated with a particular sound frequency, the traveling wave causes a maximum deflection of the basilar membrane and thus of the tectorial membrane, setting up shearing movements between the two membranes. These shearing move ments cause the stereocilia on the outer hair cells to bend. In response, the hair cells actively change their length, thereby increasing the local amplitude of the traveling wave. This additionally bends the stereocilia of the inner hair cells, stimulating the release of glutamate at their basal pole. The release of this substance generates an excitatory potential on the afferent nerve fibers, which is transmitted to the brain.

9.7 Inner Ear: Vestibular Apparatus

A Structure of the vestibular apparatus

The vestibular apparatus is the organ of balance. It consists of the membranous semicircular ducts, which contain sensory ridges (ampullary crests) in their dilated portions (ampullae), and of the saccule and utricle with their macular organs (their location in the petrous bone is shown in B, p. 148). The sensory organs in the semicircular ducts respond to angular acceleration while the macular organs, which have an approximately vertical and horizontal orientation, respond to horizontal (utricular macula) and vertical (saccular macula) linear acceleration, as well as to gravitational forces.

В Structure of the ampulla and am pu Mary crest

Cross-section through the ampulla of a semicircular canal. Each canal has a bulbous expansion at one end (ampulla) that is traversed by a connective-tissue ridge with sensory epithelium (ampullary crest). Extending above the ampullary crest is a gelatinous cupula, which is attached to the roof of the ampulla. Each of the sensory cells of the ampullary crest (approximately 7000 in all) bears on its apical pole one long kinocilium and approximately 80 shorter stereocilia, which project into the cupula. When the head is rotated in the plane of a particular semicircular canal, the inertial lag of the endolymph causes a deflection of the cupula, which in turn causes a bowing of the stereocilia. The sensory cells are either depolarized (excitation) or hyperpolarized (inhibition), depending on the direction of ciliary displacement (see details in E).

C Structure of the utricular and saccular maculae

The maculae are thickened oval areas in the epithelial lining of the utricle and saccule, each averaging 2 mm in diameter and containing arrays of sensory and supporting cells. Like the sensory cells of the ampullary crest, the sensory cells of the macular organs bear specialized stereocilia, which project into an otolithic membrane. The latter consists of a gelatinous layer, similar to the cupula, but it has calcium carbonate crystals or otoliths (statoliths) embedded in its surface. With their high specific gravity, these crystals exert traction on the gelatinous mass in response to linear acceleration, and this induces shearing movements of the cilia. The sensory cells are either depolarized or hyperpolarized by the movement, depending on the orientation of the cilia. There are two distinct categories of vestibular hair cells (type I and type II); type I cells (light red) are goldet shaped.

D Stimulus transduction in the vestibular sensory cells

Each of the sensory cells of the maculae and ampullary crest bears on its apical surface one long kinocilium and approximately 80 stereocilia of graduated lengths, forming an array that resembles a pipe organ. This arrangement results in a polar differentiation of the sensory cells. The cilia are straight while in a resting state. When the stereocilia are deflected toward the kinocilium, the sensory cell depolarizes and the frequency of action potentials (discharge rate of impulses) is increased (right side of diagram). When the stereocilia are deflected away from the kinocilium, the cell hyperpolarizes and the discharge rate is decreased (left side of diagram). This mechanism regulates the release of the transmitter glutamate at the basal pole of the sensory cell, thereby controlling the activation of the afferent nerve fiber (depolarization stimulates glutamate release, and hyperpolarization inhibits it). In this way the brain receives information on the magnitude and direction of movements and changes of position.

E Specialized orientations of the stereocilia in the vestibular apparatus (ampullary crest and maculae)

Because the stimulation of the sensory cells by deflection of the stereocilia away from or toward the kinocilium is what initiates signal transduction, the spatial orientation of the cilia must be specialized to ensure that every position in space and every movement of the head stimulates or inhibits certain receptors. The ciliary arrangement shown here ensures that every direction in space will correlate with the maximum sensitivity of a particular receptor field. The arrows indicate the polarity of the cilia, i.e., each of the arrowheads points in the direction of the kinocilium in that particular field.

Note that the sensory cells show an opposite, reciprocal arrangement in the sensory fields of the utricle and saccule.

F Interaction of contralateral semicircular canals during head rotation

When the head rotates to the right (red arrow), the endolymph flows to the left because of its inertial mass (solid blue arrow, taking the head as the reference point). Owing to the alignment of the stereocilia, the left and right semicircular canals are stimulated in opposite fashion. On the right side, the stereocilia are deflected toward the kinocilium (dotted arrow; the discharge rate increases). On the left side, the stereocilia are deflected away from the kinocilium (dotted arrow; the discharge rate decreases). This arrangement heightens the sensitivity to stimuli by increasing the stimulus contrast between the two sides. In other words, the difference between the decreased firing rate on one side and the increased firing rate on the other side enhances the perception of the kinetic stimulus.

9.8 Ear: Blood Supply

A Origin of the principal arteries of the tympanic cavity

Artery

Origin

Distribution

Except for the caroticotympanic arteries, which arise from the petrous part of the internal carotid artery, all of the vessels that supply blood to the tympanic cavity arise from the external carotid artery. The vessels have many anastomoses with one another and reach the auditory ossicles, for example, through folds of mucosa. The ossicles are also traversed by intraosseous vessels.

Caroticotympanic arteries

Stylomastoid artery

Inferior tympanic artery

Deep auricular artery

Internal carotid artery

Posterior auricular artery

Ascending pharyngeal artery

Maxillary artery

Pharyngotympanic (auditory) tube and anterior wall of the tympanic cavity

Posterior wall of the tympanic cavity, mastoid air cells, stapedius muscle, stapes

Floor of the tympanic cavity, promontory

Tympanic membrane, floor of the tympanic cavity

Posterior tympanic artery

Stylomastoid artery

Chorda tympani, tympanic membrane, malleus

Superior tympanic artery

Middle meningeal artery

Tensor tympani, roof of the tympanic cavity, stapes

Anterior tympanic artery

Maxillary artery

Tympanic membrane, mastoid antrum, malleus, incus

В Arteries of the tympanic cavity and mastoid air cells

Right petrous bone, anterior view. The malleus, incus, portions of the chorda tympani, and the anterior tympanic artery have been removed.

C Vascular supply of the ossicular chain and tympanic membrane

Medial view of the right tympanic membrane. This region receives most of its blood supply from the anterior tympanic artery. With inflammation of the tympanic membrane, the arteries may become so dilated that their course in the tympanic membrane can be seen, as illustrated here.

D Blood supply of the labyrinth

Right anterior view. The labyrinth receives all of its arterial blood supply from the internal auditory artery, a branch of the anterior inferior cerebellar artery. The labyrinthine artery occasionally arises directly from the basilar artery.