BARR'S The Human Nervous System: An anatomical viewpoint, 9th Edition

PART 3 - Review of the Major Systems

Chapter 21

Auditory System

Important Facts

  • The ossicles of the middle ear transfer vibrations from the air to the perilymph. Movement of the ossicles is restrained by the tensor tympani and stapedius muscles, innervated by cranial nerves V and VII, respectively.
  • In the cochlea, the oscillations of the basilar membrane are detected by the inner and outer hair cells of the organ of Corti. The outer hair cells respond with movement, which is transmitted to the tectorial membrane and thence to the inner hair cells, increasing the sensitivity of the latter to sound. The inner hair cells respond by releasing their excitatory transmitter and stimulating the sensory terminals of the cochlear division of cranial nerve VIII.
  • The primary sensory neurons have their somata in the spiral ganglion of the cochlea. Their axons end in the dorsal and ventral cochlear nuclei.
  • Axons from the dorsal cochlear nucleus cross the midline, travel rostrally in the lateral lemniscus, and end in the inferior colliculus.
  • Axons from the ventral cochlear nucleus end in the superior olivary nuclei of both sides. The convergence of signals from the left and right sides allows neurons in the superior olivary nucleus to respond to the different times of arrival of sound in the two ears, thus providing the ability to determine the direction of the source. The neurons in each superior olivary nucleus have axons that travel in the lateral lemniscus and end in the inferior colliculus.
  • The inferior colliculus projects (through the inferior brachium) to the medial geniculate body, which projects to the primary auditory area of the cerebral cortex.
  • The primary auditory cortex is located on the superior surface of the temporal lobe. It is connected with auditory association cortex of the superior temporal gyrus and nearby parts of the parietal lobe. In the left cerebral hemisphere (of most people), these regions are coextensive with the receptive language area.
  • Descending pathways modify transmission in the central auditory system. The sensitivity of the organ of Corti is actively inhibited by efferent (olivocochlear) fibers in the cochlear nerve. These inhibit both the outer hair cells and the sensory terminals on the inner hair cells.
  • Destructive lesions rostral to the cochlear nuclei do not cause unilateral deafness.

Hearing is second in importance among the special senses of humans, yielding first place only to sight. Their role in language accounts, to a large extent, for the reliance placed on these special senses. The auditory system consists of the external ear, middle ear, cochlea of the internal ear, cochlear nerve, and pathways in the central nervous system (CNS).

External and Middle Ear

The external ear consists of the auricle or pinna and the external acoustic meatus, with the latter being separated from the middle ear by the tympanic membrane. The function of the external ear is to collect sound waves, which cause vibration of the tympanic membrane. The vibration is transmitted across the middle ear by a chain of ossicles (little bones): the malleus, incus, and stapes. The malleus is attached to the tympanic membrane and articulates with the incus, which articulates in turn with the stirrup-shaped stapes. The footplate of the stapes occupies the fenestra vestibuli (oval window) in the wall between the middle and internal ears; the rim of the foot plate is attached to the margin of the fenestra vestibuli by the annular ligament, composed of elastic connective tissue. The ossicles constitute a bent lever with the longer of the two arms attached to the

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tympanic membrane, and the area of the foot plate of the stapes is considerably less than the area of the tympanic membrane. With this arrangement, the vibratory force of the tympanic membrane is magnified about 15 times at the fenestra vestibuli; the substantial increase in force is important because the sound waves are transferred from air to a liquid.

Protection against the effect of sudden, excessive noise is provided by reflex contraction of the tensor tympani and stapedius muscles, which are inserted on the malleus and stapes, respectively. The tensor tympani is innervated by the trigeminal nerve, and the stapedius is innervated by the facial nerve (see Chapter 8).

Inner Ear

The internal ear, which has two functions, consists of the membranous labyrinth encased in the bony labyrinth. Certain parts of the internal ear contain sensory areas for the vestibular system, which is discussed in Chapter 22. The cochlea is the part of the internal ear that contains the organ of Corti (spiral organ). This sense organ detects the sound waves produced in the fluid in the cochlea by vibration of the stapes and sends action potentials centrally in the cochlear division of the vestibulocochlear nerve. A central pathway with several synaptic relays leads to the primary auditory area of the cerebral cortex. Other central connections in the brain stem cause reflex responses.

 

FIGURE 21-1 (A) Base of the skull showing the squamous (blue) and petrous (yellow) parts of the right temporal bone and the position of the labyrinth. (Reprinted with permission from Moore KL, Dalley AF. Clinically Oriented Anatomy, 5th ed. Philadelphia: Lippincott, Williams & Wilkins, 2006.) (B) Anterolateral view of the right bony labyrinth.

For more detailed descriptions of the cochlea and organ of Corti, see the CD-ROM that accompanies the printed book.

BONY AND MEMBRANOUS LABYRINTHS

The bony labyrinth (Fig. 21-1) is located in the petrous part of the temporal bone, which forms a prominent oblique ridge between the middle and posterior cranial fossae. The labyrinth is a system of tunnels within the bone. A preparation such as that represented in Figure 21-1 is made by chipping away the surrounding cancellous bone until only the walls of the tunnels (which are composed of compact bone) remain. The fenestra vestibuli or oval window, in which the foot plate of the stapes fits, is in the wall of the vestibule, the middle part of the bony labyrinth. The fenestra cochleae (round window) is located below the fenestra vestibuli; it is closed by a thin membrane that

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makes pressure waves possible in the fluid in the internal ear. The fluid would otherwise be completely enclosed in a rigid “box,” except for the source of the waves at the fenestra vestibuli. Three semicircular canals extend posterolaterally from the vestibule, and the cochlea constitutes the anteromedial part of the bony labyrinth. The cochlea has the shape of a snail shell; its base abuts against the deep end of the internal acoustic meatus, which opens into the posterior cranial fossa.

The cochlear and vestibular divisions of the vestibulocochlear nerve leave the internal acoustic meatus and are attached to the lateral aspect of the brain stem at the junction of the medulla and pons. Within the internal meatus, the vestibulocochlear nerve is accompanied by the two divisions of the facial nerve (see Chapter 8) and the labyrinthine arteryand vein (see Chapter 25).

The delicate membranous labyrinth conforms, for the most part, to the contours of the bony labyrinth (Fig. 21-2). There are, however, two dilations, the utricle and the saccule, in the vestibule of the bony labyrinth. Three semicircular ducts arise from the utricle. A patch of sensory epithelium is present on the inner surface of the utricle, the saccule, and each semicircular duct. The saccule is continuous with the cochlear duct through a narrow channel known as the ductus reuniens. The cochlear duct contains, along its entire length, the organ of Corti.

 

FIGURE 21-2 Anterolateral view of the right membranous labyrinth.

Whereas the lumen of the membranous labyrinth is filled with endolymph, the interval between the membranous and bony labyrinths is filled with perilymph. The vestibular part of the membranous labyrinth is suspended within the bony labyrinth by trabeculae of connective tissue. The cochlear duct is firmly attached along two sides to the bony wall of the cochlear canal.

COCHLEA

The cochlear canal makes 2.5 turns around a bony pillar or core, the modiolus, where channels for blood vessels and branches of the cochlear

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nerve are present. The cochlea is most conveniently described as if it were resting on its base (Fig. 21-3), although its base actually faces posteromedially.

The cochlear canal, the cavity of this part of the bony labyrinth, is divided by two partitions into three spiral spaces. The middle of these is the cochlear duct (scala media), which contains endolymph. The cochlear duct is firmly fixed to the inner and outer walls of the cochlear canal. The remaining spiral spaces are the scala vestibuli and the scala tympani, which contain perilymph. The thin unspecialized wall of the cochlear duct apposing the scala vestibuli is called the vestibular or Reissner's membrane, and the thicker wall apposing the scala tympani constitutes the specialized basilar membrane, on which the organ of Corti rests.

The basilar membrane is of special importance in the physiology of hearing because it responds to vibration of the stapes in the following manner. As shown in Figure 21-4, vibration of the foot plate of the stapes produces corresponding waves in the perilymph, beginning with that of the vestibule. Sound waves propagate through the scala vestibuli, Reissner's membrane, the endolymph in the cochlear duct, and the basilar membrane to the scala tympani. These same waves create a vibration of the membrane closing the fenestra cochleae at the base of the scala tympani; this is essential to eliminate the damping of pressure waves that would otherwise occur in bone-encased fluid.

 

FIGURE 21-3 Section through the cochlea.

The perilymph filling the scala vestibuli and scala tympani is a watery fluid, similar in composition to cerebrospinal fluid. In fact, there is a communication, the tiny cochlear canaliculus, between the scala tympani and the subarachnoid space.

The spiral ganglion consists of cells in a spiral configuration at the periphery of the modiolus (see Fig. 21-3). The primary sensory neurons of both divisions of the vestibulocochlear nerve are bipolar,

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rather than unipolar as in other cerebrospinal nerves, retaining this embryonic characteristic of primary sensory neurons. The two neurites, which are functionally both axons, are myelinated. The distal axons reach the organ of Corti by traversing openings in the osseous spiral lamina projecting from the modiolus, where myelin sheaths terminate. The central axons traverse channels in the modiolus, enter the internal acoustic meatus from the base of the cochlea, and continue in the cochlear nerve. Within the external acoustic meatus, a small anastomotic connection, the Oort anastomosis, carries efferent axons from the vestibular nerve into the cochlear nerve.

 

FIGURE 21-4 Schematic representation of the manner in which sound waves in the perilymph and endolymph cause vibration of the basilar membrane.

COCHLEAR DUCT

Vibration of the basilar membrane (Fig. 21-5) is essential in the transduction of mechanical stimuli (sound waves) to neural signals in the organ of Corti. The inner edge of the basilar membrane is attached to the osseous spiral lamina, which projects from the modiolus like the thread on a screw. The outer edge of the membrane is attached to the outer wall of the cochlear canal. The basilar membrane contains collagen and elastic fibers, mostly directed across the width of the membrane. The width of the basilar membrane steadily increases from the base to the apex of the cochlea; this is made possible by a progressive narrowing of the osseous spiral lamina. The width of the membrane at any point determines the pitch of sound to which it resonates maximally. High tones, therefore, cause maximal vibration in the basal turn of the cochlea, and low tones cause maximal vibration near the apex. The range of audible frequencies in the human ear is from 20 to 20,000 Hz. The range extends over 11 octaves, of which seven are used in musical instruments such as the piano. Ordinary conversation

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falls within the range of 300 to 3000 Hz. With advancing age, a gradual decrease in the perception of high frequencies takes place.

CLINICAL NOTE

High-Tone Deafness

Persistent exposure to loud sounds causes degenerative changes in the organ of Corti at the base of the cochlea, causing high-tone deafness. This is prone to occur in workers exposed to the sound of compression engines or jet engines and in those working for long hours on farm tractors. High-tone deafness was formerly encountered most frequently among workmen in boiler factories and is still sometimes called “boilermakers' disease.”

 

FIGURE 21-5 Structure of the cochlear duct and the spiral organ of Corti.

The vestibular or Reissner's membrane consists of two layers of simple squamous epithelium separated by a trace of connective tissue.

The outer wall of the cochlear duct is specialized as the stria vascularis, which consists of cuboidal epithelium overlying vascular connective tissue. The stria vascularis produces endolymph. This is similar to intracellular fluid in respect to its high concentration of potassium ions and low concentration of sodium ions. Endolymph fills the membranous labyrinth; absorption takes place into venules surrounding the endolymphatic sac in the dura mater on the posterior surface of the petrous part of the temporal bone. This sac is an expansion of the endolymphatic duct, which arises from the communication between the saccule and the utricle (see Fig. 21-2).

The epithelial lining of the membranous labyrinth, including the specialized sensory areas for the auditory and vestibular systems, is ectodermal in origin. The epithelium differentiates from the cells lining the otic vesicle. This is formed by an invagination of ectoderm at the level of the hindbrain of the early embryo.

ORGAN OF CORTI

The organ of Corti or spiral organ (see Fig. 21-5) consists of supporting cells and sensory cells. Supporting cells (pillar cells and phalangeal cells) form the sides and roof of thetunnel of Corti. The fluid in the tunnel of Corti has a chemical composition similar to that of perilymph rather than endolymph. The high concentration of potassium ions in endolymph would prevent impulse conduction by the neurites that cross the tunnel of Corti to reach the outer hair cells. Sensory hair cells are located on either side of the tunnel of Corti and are flanked by border cells on the inner aspect and by cells of Hensen at the outer edge of the basilar membrane.

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The tectorial membrane is a ribbon-like structure of gelatinous consistency attached to the spiral limbus, a thickening of the periosteum on the osseous spiral lamina. The tectorial membrane extends over the organ of Corti, and the tips of the hairs of the outer hair cells are embedded in the membrane.

SUPPORTING CELLS OF THE ORGAN OF CORTI—MORE DETAILS

SENSORY CELLS

The sensory cells are called hair cells because of the hair-like projections from their free ends. There is a single row of about 7,000 inner hair cells; the 25,000 or so outer hair cells are arranged in three rows in the basal turn of the cochlea, increasing to five rows at the apex. The hairs are microvilli of an unusual type: they are rigid and of different lengths. Each hair has its tip joined by a linking protein molecule to an ion channel embedded in the cell membrane that forms the side of the adjacent hair. The mechanical stimulus of a vibration moves the whole bundle of hairs, which bend only at their points of attachment to the body of the cell; this applies tension to the link at the tip of each hair, which pulls on and opens the ion channel in the side of the adjacent hair. Entry of potassium and calcium ions from the endolymph depolarizes the cell membrane and initiates synaptic signaling to the innervating neurite.

The inner hair cells are the principal sensory elements. Each one synapses with the neurites of up to 10 rapidly conducting neurons whose myelinated axons make up at least 90% of the fibers of the cochlear nerve. No neuron is contacted by more than one inner hair cell. The outer hair cells synapse with branches of unmyelinated axons, which account for 5% to 10% of the fibers of the cochlear nerve. The zone of outer hair cells receives most of the efferent fibers of the cochlear nerve, which are described later. The outer hair cells are motile. Their microvilli move in response to transduced sound and produce corresponding vibrations of the tectorial membrane. This has the effect of lowering the threshold of excitation of the inner hair cells.

It is basic to the physiology of the cochlea that a particular region of the basilar membrane, depending on the pitch of sound, responds by maximal vibration. Bending of the hairs reduces the membrane potential of the hair cells, causing increased release of their chemical transmitter and initiation of action potentials in the sensory nerve endings. Regardless of the pitch of sound, vibration of the basilar membrane begins at the base of the cochlea and travels along the membrane with increasing magnitude to a point determined by the pitch. At this point, the vibration suddenly dies away, and impulses reaching the brain from the place of maximal stimulation of the organ of Corti are interpreted as a particular pitch of sound. An increase in the intensity of sound causes maximal vibration in a larger region of the basilar membrane, thereby activating more hair cells and neurons. Tonotopic localization is sharpened by lateral inhibition (see Chapter 19) in the nuclei of the ascending pathway to the auditory cortex and by various descending connections, including centrifugal fibers in the vestibulocochlear nerve.

Auditory Pathways

The cochlear nerve consists principally of axons of cells in the spiral ganglion, most of which are myelinated. It traverses the internal acoustic meatus in the petrous part of the temporal bone alongside the vestibular nerve, the two roots of the facial nerve (Chapter 8), and the labyrinthine artery (Chapter 25). On emerging from the internal meatus, the vestibulocochlear and facial nerves traverse the subarachnoid space in the cerebellopontine angle, a region between the middle and inferior cerebellar peduncles. The cochlear fibers enter the brain stem at this level and bifurcate, with one branch ending in the dorsal cochlear nucleus and the other branch in the ventral cochlear nucleus (Fig. 21-6). The cochlear nuclei are situated superficially in the rostral end of the medulla adjacent to the base of the inferior cerebellar peduncle (see Fig. 7-7). A tonotopic pattern of axonal endings has been demonstrated in both nuclei in laboratory animals and probably exists in humans. The dorsal and ventral cochlear nuclei differ in their contributions to the central pathways.

PATHWAY TO THE AUDITORY CORTEX

The pathway to the cerebral cortex is characterized by variable numbers of synaptic relays between the cochlear nuclei and the specific

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thalamic nucleus for hearing, the medial geniculate body (see Fig. 21-6). There is a relay in the inferior colliculus, and additional synaptic interruptions may occur in the superior olivary nucleus and in the nucleus of the lateral lemniscus. The pathway also includes a

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significant ipsilateral projection to the cortex. The transmission of acoustic data to the cortex can best be described after certain components of the pathway in the brain stem have been identified.

 

FIGURE 21-6 Ascending auditory pathway.

CLINICAL NOTE

Acoustic Neuroma

One of the more common types of intracranial neoplasm is a benign tumor derived from the neuroglial cells (Schwann cells) of the vestibular division of the eighth cranial nerve, within the internal auditory meatus. The correct name for the tumor is vestibular schwannoma (or neurilemmoma), but the older term acoustic neuroma is still in widespread use. Vertigo, the principal effect of damage to the vestibular system (see Chapter 22), occurs in some patients, but in most, the first symptom is slowly increasing hearing loss in the affected ear. This is due to pressure on the cochlear nerve, which is squeezed between the growing tumor and the bony wall of the meatus. In the early stages, there may also be tinnitus (a ringing or buzzing sound) due to abnormal stimulation of the sensory axons.

The tumor causes enlargement of the internal auditory meatus, a useful radiological sign, and expands into the subarachnoid space of the cerebellopontine angle. There, with further enlargement, the tumor presses upon and stretches the roots of nearby cranial nerves. The facial nerve, despite its close proximity to the vestibulocochlear, is surprisingly resistant to stretching, and the next symptom to develop is usually a tingling sensation in the face, with sensory impairment that can be detected on examination. A decreased corneal reflex (Chapter 8) is often an early sign of involvement of the trigeminal nerve. With downward growth, the tumor impinges on glossopharyngeal rootlets, causing sensory impairment in the pharynx and posterior third of the tongue, with reduction or loss of the gag reflex. The clinical course of the disease is long (years) because of the slow growth of the tumor and the availability of a space—the cerebellopontine angle—that the tumor can occupy before it impinges on the brain stem. A large acoustic neuroma eventually presses on the medulla, obstructing the flow of cerebrospinal fluid through and out of the fourth ventricle, with resultant hydrocephalus (Chapter 26) and symptoms and signs of raised intracranial pressure (headache, vomiting, papilledema). Death ensues from loss of cardiovascular control and other vital functions of the medulla.

With early diagnosis, an acoustic neuroma can sometimes be removed without permanently damaging the cochlear nerve, but in most cases, the surgery is followed by permanent deafness. Severe vertigo is experienced postoperatively. Permanent facial paralysis and diminished function of the trigeminal and glossopharyngeal nerves frequently follow removal of larger tumors from the pontocerebellar angle.

The superior olivary nucleus is situated in the ventrolateral corner of the tegmentum of the pons at the level of the motor nucleus of the facial nerve (see Fig. 7-8). (Although considered here as a unit, the nucleus is a complex of four nuclei, whose connections differ in detail.) Auditory fibers that cross the pons in the ventral part of the tegmentum constitute the trapezoid body (see Fig. 7-8). The lateral lemniscus, the ascending auditory tract, extends from the region of the superior olivary nucleus, through the lateral part of the pontine tegmentum, and close to the surface of the brain stem in the isthmus region between the pons and midbrain (see Fig. 7-9).

The projection from the cochlear nuclei to the inferior colliculus and then to the medial geniculate nucleus, through the components of the pathway just identified, is as follows (seeFig. 21-6). Axons from the ventral cochlear nucleus proceed to the region of the ipsilateral superior olivary nucleus, in which some of the fibers terminate. The majority of the axons continue across the pons, with a slight forward slope; these constitute the trapezoid body. On reaching the region of the superior olivary nucleus on the other side of the brain stem, the fibers either continue into the lateral lemniscus or terminate in the superior olivary nucleus, from which fibers are added to the lateral lemniscus. Fibers from the dorsal cochlear nucleus pass over the base of the inferior cerebellar peduncle, continue obliquely to the region of the contralateral superior olivary nucleus, and then turn rostrally in the lateral lemniscus. They end in the inferior colliculus.

Signals conveyed by the lateral lemniscus reach the inferior colliculus in the midbrain.

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The complexity of neuronal organization in the inferior colliculus indicates integrative activity at this level. Ascending axons from the inferior colliculus traverse the inferior brachium (see Fig. 6-3) and end in the medial geniculate body.

The last link in the auditory pathway consists of the auditory radiation in the sublentiform part of the internal capsule, through which the medial geniculate body projects to theprimary auditory cortex of the temporal lobe. This primary auditory area, corresponding to Brodmann's areas 41 and 42, is located in the floor of the lateral sulcus, extending only slightly onto the lateral surface of the hemisphere. A landmark is provided by the anterior transverse temporal gyri (Heschl's convolutions) on the dorsal surface of the superior temporal gyrus (see Fig. 15-3). The area receives afferent fibers from the tonotopically organized ventral part of the medial geniculate body. The tonotopic pattern in the auditory area is such that whereas fibers for low-frequency sounds end in the anterolateral part of the area, fibers for high-frequency sounds go to its posteromedial part. Some of the columns of neurons (see Chapter 14) within the primary auditory cortex occur in bands recognizable by virtue of their higher cytochrome oxidase activity. These columns may be involved in comprehension of speech.

Analysis of acoustic stimuli at a higher neural level, notably the recognition and interpretation of sounds on the basis of past experience, occurs in the auditory association cortex of the temporal lobe, which is located posteriorly to the primary auditory area. In addition to its afferents from the primary auditory area, the association cortex also receives projections from regions of the medial geniculate nucleus other than its tonotopically organized ventral part. In the cerebral hemisphere dominant for language (the left, in most people), the auditory association cortex is known as Wernicke's area (see Chapter 15), and along with the adjacent parietal lobe cortex, it is essential for the understanding of spoken and written language.

Above the level of the cochlear nuclei, the auditory pathway is both crossed and uncrossed because many axons ascend in the lateral lemniscus of the same side. In addition, the inferior colliculi of the two sides are connected by commissural fibers. Consequently, any loss of hearing that results from a unilateral cortical lesion is so slight as to make detection difficult in audiometric testing. Most lesions in the vicinity of the auditory cortex also involve Wernicke's area and cause receptive aphasia when the dominant hemisphere for language is involved (see Chapter 15). The latter disability obscures any slight auditory deficiency.

The directions and distances of sources of sound are determined by the discrepancy in times of arrival of the stimulus in the left and right ears. Results obtained from investigations with animals indicate that the different inputs to the brain from the two cochleae are compared and analyzed in the superior olivary nuclei, although the auditory cortex is necessary if the coded information transmitted rostrally from the medulla is to have any meaning. The most severe loss of ability to judge the sources of sounds is that caused by unilateral deafness resulting from disease of the ear. The condition is equivalent to the loss of binocular vision that results from blindness in one eye.

TESTING IMPAIRED HEARING

DESCENDING PROJECTIONS IN THE AUDITORY PATHWAY

Parallel with the flow of information from the organ of Corti to the auditory cortex, some neurons with descending axons conduct in the reverse direction. The descending connections consist of the following: corticogeniculate fibers, which originate in the auditory and adjoining cortical areas and terminate in all parts of the medial geniculate body; corticocollicular fibers from the same cortical areas to the inferior colliculi of both sides; colliculo-olivary fibers from the inferior colliculus to the superior olivary nucleus; and colliculocochleonuclear fibers from the inferior colliculus to the dorsal and ventral cochlear nuclei. Except for the corticocollicular projection, which includes both crossed and uncrossed fibers, these descending pathways are ipsilateral.

As indicated earlier, control is exerted by the CNS over the initiation of auditory neural signals in the organ of Corti. Olivocochlear fibers, constituting the olivocochlear bundle of Rasmussen, are the axons of cholinergic neurons in the superior olivary nuclei. The axons leave the brain stem in the vestibular division of the vestibulocochlear nerve and then cross over

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into the cochlear division in a branch, the Oort anastomosis, located in the internal acoustic meatus.

The endings of the olivocochlear axons are applied to the outer hair cells (where their synaptic terminals outnumber those of afferent fibers) and to the preterminal parts of the sensory neurites that innervate the inner hair cells. The efferent axons are inhibitory to both the receptor cells and the sensory axons. Inhibition of the outer hair cells reduces the amplitude of the vibrations of the tectorial membrane, thereby raising the threshold of excitation of the inner hair cells. Thus, the efferent fibers of the cochlear nerve reduce the sensitivity of the ear.

The central transmission of data from the sensory hair cells is therefore far more than just a relay to the cortex. In the various cell stations of the pathway, a complex processing of acoustic data takes place that provides for refinement of such qualities as pitch, timbre, and volume of sound perception. In particular, feedback inhibition sharpens the perception of pitch, especially through the olivocochlear bundle. This is accomplished by inhibition in the organ of Corti except for the region in which the basilar membrane is responding by maximal vibration to a particular frequency of sound waves (auditory sharpening). Central inhibition probably suppresses background noise when attention is being concentrated on a particular sound.

AUDITORY REFLEXES

A few acoustic fibers from the inferior colliculus pass forward to the superior colliculus, which influences motor neurons of the cervical region of the spinal cord through the tectospinal tract. The superior colliculus also influences neurons of the oculomotor, trochlear, and abducens nuclei through indirect connections in the brain stem (see Chapter 8). These pathways provide for reflex turning of the head and eyes toward the source of a sudden loud sound.

Some axons from the superior olivary nucleus terminate in the motor nuclei of the trigeminal and facial nerves for reflex contraction of the tensor tympani and stapedius muscles, respectively. Contraction of these muscles in response to loud sounds reduces the vibration of the tympanic membrane and the stapes, thereby protecting the delicate structures in the cochlea from mechanical damage.

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