Ganong’s Review of Medical Physiology, 24th Edition

CHAPTER 10 Hearing & Equilibrium


OBJECTIVES

After studying this chapter, you should be able to:



image Describe the components and functions of the external, middle, and inner ear.

image Describe the way that movements of molecules in the air are converted into impulses generated in hair cells in the cochlea.

image Explain the roles of the tympanic membrane, the auditory ossicles (malleus, incus, and stapes), and scala vestibule in sound transmission.

image Explain how auditory impulses travel from the cochlear hair cells to the auditory cortex.

image Explain how pitch, loudness, and timbre are coded in the auditory pathways.

image Describe the various forms of deafness and the tests used to distinguish between them.

image Explain how the receptors in the semicircular canals detect rotational acceleration and how the receptors in the saccule and utricle detect linear acceleration.

image List the major sensory inputs that provide the information that is synthesized in the brain into the sense of position in space.


INTRODUCTION

Our ears not only let us detect sounds, but they also help us maintain balance. Receptors for two sensory modalities (hearing and equilibrium) are housed in the ear. The external ear, the middle ear, and the cochlea of the inner ear are concerned with hearing. The semicircular canals, the utricle, and the saccule of the inner ear are concerned with equilibrium. Both hearing and equilibrium rely on a very specialized type of receptor called a hair cell. There are six groups of hair cells in each inner ear: one in each of the three semicircular canals, one in the utricle, one in the saccule, and one in the cochlea. Receptors in the semicircular canals detect rotational acceleration, receptors in the utricle detect linear acceleration in the horizontal direction, and receptors in the saccule detect linear acceleration in the vertical direction.

STRUCTURE AND FUNCTION OF THE EAR

EXTERNAL & MIDDLE EAR

The external ear funnels sound waves to the external auditory meatus (Figure 10–1). In some animals, the ears can be moved like radar antennas to seek out sound. From the external auditory meatus, sound waves pass inward to the tympanic membrane (eardrum).

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FIGURE 10–1 The structures of the external, middle, and inner portions of the human ear. Sound waves travel from the external ear to the tympanic membrane via the external auditory meatus. The middle ear is an air-filled cavity in the temporal bone; it contains the auditory ossicles. The inner ear is comprised of the bony and membranous labyrinths. To make the relationships clear, the cochlea has been turned slightly and the middle ear muscles have been omitted. (From Fox SI, Human Physiology. McGraw-Hill, 2008.)

The middle ear is an air-filled cavity in the temporal bone that opens via the eustachian (auditory) tube into the nasopharynx and through the nasopharynx to the exterior. The tube is usually closed, but during swallowing, chewing, and yawning it opens, keeping the air pressure on the two sides of the eardrum equalized. The three auditory ossicles, the malleus, incus, and stapes, are located in the middle ear (Figure 10–2). The manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes. The stapes is named for its resemblance to a stirrup. Its foot plate is attached by an annular ligament to the walls of the oval window. Two small skeletal muscles, the tensor tympani and the stapedius, are also located in the middle ear. Contraction of the former pulls the manubrium of the malleus medially and decreases the vibrations of the tympanic membrane; contraction of the latter pulls the foot plate of the stapes out of the oval window. The functions of the ossicles and the muscles are considered in more detail below.

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FIGURE 10–2 The medial view of the middle ear containing three auditory ossicles malleus, incus, and stapes) and two small skeletal muscles (tensor tympani muscle and stapedius). The manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes. The foot plate of the stapes is attached by an annular ligament to the walls of the oval window. Contraction of the tensor tympani muscle pulls the manubrium medially and decreases the vibrations of the tympanic membrane; contraction of the stapedius muscle pulls the foot plate of the stapes out of the oval window. (From Fox SI, Human Physiology. McGraw-Hill, 2008.)

INNER EAR

The inner ear (labyrinth) is made up of two parts, one within the other. The bony labyrinth is a series of channels in the petrous portion of the temporal bone and is filled with a fluid called perilymph, which has a relatively low concentration of K+, similar to that of plasma or the cerebral spinal fluid. Inside these bony channels, surrounded by the perilymph, is the membranous labyrinth. The membranous labyrinth more or less duplicates the shape of the bony channels and is filled with a K+-rich fluid called endolymph. The labyrinth has three components: the cochlea (containing receptors for hearing), semicircular canals (containing receptors that respond to head rotation), and the otolith organs (containing receptors that respond to gravity and head tilt).

The cochlea is a coiled tube that, in humans, is 35 mm long and makes two and three quarter turns (Figure 10–3). The basilar membrane and Reissner’s membrane divide it into three chambers or scalae (Figure 10–4). The upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema. At the base of the cochlea, the scala vestibuli ends at the oval window, which is closed by the footplate of the stapes. The scala tympani end at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane. The scala media, the middle cochlear chamber, is continuous with the membranous labyrinth and does not communicate with the other two scalae.

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FIGURE 10–3 Schematic of the human inner ear showing the membranous labyrinth with enlargements of the structures in which hair cells are embedded. The membranous labyrinth is suspended in perilymph and filled with K+-rich endolymph, which bathes the receptors. Hair cells (darkened for emphasis) occur in different arrays characteristic of the receptor organs. The three semicircular canals are sensitive to angular accelerations that deflect the gelatinous cupula and associated hair cells. In the cochlea, hair cells spiral along the basilar membrane in the organ of Corti. Airborne sounds set the eardrum in motion, which is conveyed to the cochlea by bones of the middle ear. This flexes the membrane up and down. Hair cells in the organ of Corti are stimulated by shearing motion. The otolithic organs (saccule and utricle) are sensitive to linear acceleration in vertical and horizontal planes. Hair cells are attached to the otolithic membrane. VIII, eighth cranial nerve, with auditory and vestibular divisions. (Adapted with permission from Hudspeth AJ: How the ear’s works work. Nature 1989;341(6241):397–404.)

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FIGURE 10–4 Schematic of the cochlea and organ of Corti in the membranous labyrinth of the inner ear. Top: The cross-section of the cochlea shows the organ of Corti and the three scalae of the cochlea. Bottom: This shows the structure of the organ of Corti as it appears in the basal turn of the cochlea. DC, outer phalangeal cells (Deiters’ cells) supporting outer hair cells; IPC, inner phalangeal cell supporting inner hair cell. (Reproduced with permission from Pickels JO: An Introduction to the Physiology of Hearing, 2nd ed. Academic Press, 1988.)

The organ of Corti on the basilar membrane extends from the apex to the base of the cochlea and thus has a spiral shape. This structure contains the highly specialized auditory receptors (hair cells) whose processes pierce the tough, membranelike reticular lamina that is supported by the pillar cells or rods of Corti (Figure 10–4). The hair cells are arranged in four rows: three rows of outer hair cells lateral to the tunnel formed by the rods of Corti, and one row of inner hair cells medial to the tunnel. There are 20,000 outer hair cells and 3500 inner hair cells in each human cochlea. Covering the rows of hair cells is a thin, viscous, but elastic tectorial membrane in which the tips of the hairs of the outer but not the inner hair cells are embedded. The cell bodies of the sensory neurons that arborize around the bases of the hair cells are located in the spiral ganglion within the modiolus, the bony core around which the cochlea is wound. Ninety to 95% of these sensory neurons innervate the inner hair cells; only 5–10% innervates the more numerous outer hair cells, and each sensory neuron innervates several outer hair cells. By contrast, most of the efferent fibers in the auditory nerve terminate on the outer rather than inner hair cells. The axons of the afferent neurons that innervate the hair cells form the auditory (cochlear) division of the eighth cranial nerve.

In the cochlea, tight junctions between the hair cells and the adjacent phalangeal cells prevent endolymph from reaching the bases of the cells. However, the basilar membrane is relatively permeable to perilymph in the scala tympani, and consequently, the tunnel of the organ of Corti and the bases of the hair cells are bathed in perilymph. Because of similar tight junctions, the arrangement is similar for the hair cells in other parts of the inner ear; that is, the processes of the hair cells are bathed in endolymph, whereas their bases are bathed in perilymph.

On each side of the head, the semicircular canals are perpendicular to each other, so that they are oriented in the three planes of space. A receptor structure, the crista ampullaris, is located in the expanded end (ampulla) of each of the membranous canals. Each crista consists of hair cells and supporting (sustentacular) cells surmounted by a gelatinous partition (cupula) that closes off the ampulla (Figure 10–3). The processes of the hair cells are embedded in the cupula, and the bases of the hair cells are in close contact with the afferent fibers of the vestibular division of the eighth cranial nerve.

A pair of otolith organs, the saccule and utricle, are located near the center of the membranous labyrinth. The sensory epithelium of these organs is called the macula. The maculae are vertically oriented in the saccule and horizontally located in the utricle when the head is upright. The maculae contain supporting cells and hair cells, surrounded by an otolithic membrane in which are embedded crystals of calcium carbonate, the otoliths (Figure 10–3). The otoliths, which are also called otoconia or ear dust, range from 3 to 19 μm in length in humans. The processes of the hair cells are embedded in the membrane. The nerve fibers from the hair cells join those from the cristae in the vestibular division of the eighth cranial nerve.

SENSORY RECEPTORS IN THE EAR: HAIR CELLS

The specialized sensory receptors in the ear consist of six patches of hair cells in the membranous labyrinth. These are examples of mechanoreceptors. The hair cells in the organ of Corti signal hearing; the hair cells in the utricle signal horizontal acceleration; the hair cells in the saccule signal vertical acceleration; and a patch in each of the three semicircular canals signal rotational acceleration. These hair cells have a common structure (Figure 10–5). Each is embedded in an epithelium made up of supporting cells, with the basal end in close contact with afferent neurons. Projecting from the apical end are 30–150 rod-shaped processes, or hairs. Except in the cochlea, one of these, the kinocilium, is a true but nonmotile cilium with nine pairs of microtubules around its circumference and a central pair of microtubules. It is one of the largest processes and has a clubbed end. The kinocilium is lost from the hair cells of the cochlea in adult mammals. However, the other processes, which are called stereocilia, are present in all hair cells. They have cores composed of parallel filaments of actin. The actin is coated with various isoforms of myosin. Within the clump of processes on each cell there is an orderly structure. Along an axis toward the kinocilium, the stereocilia increase progressively in height; along the perpendicular axis, all the stereocilia are the same height.

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FIGURE 10–5 Structure of hair cell in the saccule. Left: Hair cells in the membranous labyrinth of the ear have a common structure, and each is within an epithelium of supporting cells (SC) surmounted by an otolithic membrane (OM) embedded with crystals of calcium carbonate, the otoliths (OL). Projecting from the apical end are rod-shaped processes, or hair cells (RC), in contact with afferent (A) and efferent (E) nerve fibers. Except in the cochlea, one of these, kinocilium (K), is a true but nonmotile cilium with nine pairs of microtubules around its circumference and a central pair of microtubules. The other processes, stereocilia (S), are found in all hair cells; they have cores of actin filaments coated with isoforms of myosin. Within the clump of processes on each cell there is an orderly structure. Along an axis toward the kinocilium, the stereocilia increase progressively in height; along the perpendicular axis, all the stereocilia are the same height. (Reproduced with permission from Hillman DE: Morphology of peripheral and central vestibular systems. In: Llinas R, Precht W [editors]: Frog Neurobiology. Springer, 1976.) Right:Scanning electronmicrograph of processes on a hair cell in the saccule. The otolithic membrane has been removed. The small projections around the hair cell are microvilli on supporting cells. (Courtesy of AJ Hudspeth.)

ELECTRICAL RESPONSES

Very fine processes called tip links (Figure 10–6) tie the tip of each stereocilium to the side of its higher neighbor, and at the junction are mechanically-sensitive cation channels in the taller process. When the shorter stereocilia are pushed toward the taller ones, the open time of these channels is increased. K+—the most abundant cation in endolymph—and Ca2+ enter via the channel and produce depolarization. A myosin-based molecular motor in the taller neighbor then moves the channel toward the base, releasing tension in the tip link (Figure 10–6). This causes the channel to close and permits restoration of the resting state. Depolarization of hair cells causes them to release a neurotransmitter, probably glutamate, which initiates depolarization of neighboring afferent neurons.

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FIGURE 10–6 Schematic representation of the role of tip links in the responses of hair cells. When a stereocilium is pushed toward a taller stereocilium, the tip link is stretched and opens an ion channel in its taller neighbor. The channel next is presumably moved down the taller stereocilium by a molecular motor, so the tension on the tip link is released. When the hairs return to the resting position, the motor moves back up the stereocilium. (Reproduced with permission from Hudspeth AJ, Gillespie PG: Pulling springs to tune transduction: adaptation by hair cells. Neuron 1944 Jan;12(1):1–9.)

The K+ that enters hair cells via the mechanically-sensitive cation channels is recycled (Figure 10–7). It enters supporting cells and then passes on to other supporting cells by way of tight junctions. In the cochlea, it eventually reaches the stria vascularis and is secreted back into the endolymph, completing the cycle.

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FIGURE 10–7 Ionic composition of perilymph in the scala vestibuli, endolymph in the scala media, and perilymph in the scala tympani. SL, spiral ligament. SV, stria vascularis. The dashed arrow indicates the path by which K+ recycles from the hair cells to the supporting cells to the spiral ligament and is then secreted back into the endolymph by cells in the stria vascularis.

As described above, the processes of the hair cells project into the endolymph whereas the bases are bathed in perilymph. This arrangement is necessary for the normal production of receptor potentials. The perilymph is formed mainly from plasma. On the other hand, endolymph is formed in the scala media by the stria vascularis and has a high concentration of K+ and a low concentration of Na+ (Figure 10–7). Cells in the stria vascularis have a high concentration of Na, K ATPase. In addition, it appears that a unique electrogenic K+ pump in the stria vascularis accounts for the fact that the scala media is electrically positive by 85 mV relative to the scala vestibuli and scala tympani.

The resting membrane potential of the hair cells is about –60 mV. When the stereocilia are pushed toward the kinocilium, the membrane potential is decreased to about –50 mV. When the bundle of processes is pushed in the opposite direction, the cell is hyperpolarized. Displacing the processes in a direction perpendicular to this axis provides no change in membrane potential, and displacing the processes in directions that are intermediate between these two directions produces depolarization or hyperpolarization that is proportional to the degree to which the direction is toward or away from the kinocilium. Thus, the hair processes provide a mechanism for generating changes in membrane potential proportional to the direction and distance the hair moves.

HEARING

SOUND WAVES

Sound is the sensation produced when longitudinal vibrations of the molecules in the external environment—that is, alternate phases of condensation and rarefaction of the molecules—strike the tympanic membrane. A plot of these movements as changes in pressure on the tympanic membrane per unit of time is a series of waves (Figure 10–8); such movements in the environment are generally called sound waves. The waves travel through air at a speed of approximately 344 m/s (770 mph) at 20°C at sea level. The speed of sound increases with temperature and with altitude. Other media can also conduct sound waves, but at a different speed. For example, the speed of sound is 1450 m/s at 20°C in fresh water and is even greater in salt water. It is said that the whistle of the blue whale is as loud as 188 dB and is audible for 500 miles.

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FIGURE 10–8 Characteristics of sound waves. A is the record of a pure tone. B has a greater amplitude and is louder than A. C has the same amplitude as A but a greater frequency, and its pitch is higher. D is a complex wave form that is regularly repeated. Such patterns are perceived as musical sounds, whereas waves like that shown in E, which have no regular pattern, are perceived as noise.

In general, the loudness of a sound is directly correlated with the amplitude of a sound wave. The pitch of a sound is directly correlated with the frequency (number of waves per unit of time) of the sound wave. Sound waves that have repeating patterns, even though the individual waves are complex, are perceived as musical sounds; aperiodic nonrepeating vibrations cause a sensation of noise. Most musical sounds are made up of a wave with a primary frequency that determines the pitch of the sound plus a number of harmonic vibrations (overtones) that give the sound its characteristic timbre (quality). Variations in timbre permit us to identify the sounds of the various musical instruments even though they are playing notes of the same pitch.

Although the pitch of a sound depends primarily on the frequency of the sound wave, loudness also plays a part; low tones (below 500 Hz) seem lower and high tones (above 4000 Hz) seem higher as their loudness increases. Duration also affects pitch to a minor degree. The pitch of a tone cannot be perceived unless it lasts for more than 0.01 s, and with durations between 0.01 and 0.1 s, pitch rises as duration increases. Finally, the pitch of complex sounds that include harmonics of a given frequency is still perceived even when the primary frequency (missing fundamental) is absent.

The amplitude of a sound wave can be expressed in terms of the maximum pressure change at the eardrum, but a relative scale is more convenient. The decibel scale is such a scale. The intensity of a sound in bels is the logarithm of the ratio of the intensity of that sound and a standard sound. A decibel (dB) is 0.1 bel. The standard sound reference level adopted by the Acoustical Society of America corresponds to 0 dB at a pressure level of 0.000204 × dyne/cm2, a value that is just at the auditory threshold for the average human. A value of 0 dB does not mean the absence of sound but a sound level of an intensity equal to that of the standard. The 0–140 dB range from threshold pressure to a pressure that is potentially damaging to the organ of Corti actually represents a 107 (10 million)-fold variation in sound pressure. Put another way, atmospheric pressure at sea level is 15 lb/in2 or 1 bar, and the range from the threshold of hearing to potential damage to the cochlea is 0.0002–2000 μbar.

A range of 120–160 dB (eg, firearms, jackhammer, jet plane on take off) is classified as painful; 90–110 dB (eg, subway, bass drum, chain saw, lawn mower) is classified as extremely high; 60–80 dB (eg, alarm clock, busy traffic, dishwasher, conversation) is classified as very loud; 40–50 dB (eg, moderate rainfall, normal room noise) is moderate; and 30 dB (eg, whisper, library) is faint. Prolonged or frequent exposure to sounds greater than 85 dB can cause hearing loss.

The sound frequencies audible to humans range from about 20 to a maximum of 20,000 cycles per second (cps, Hz). In bats and dogs, much higher frequencies are audible. The threshold of the human ear varies with the pitch of the sound (Figure 10–9), the greatest sensitivity being in the 1000- to 4000-Hz range. The pitch of the average male voice in conversation is about 120 Hz and that of the average female voice about 250 Hz. The number of pitches that can be distinguished by an average individual is about 2000, but trained musicians can improve on this figure considerably. Pitch discrimination is best in the 1000- to 3000-Hz range and is poor at high and low pitches.

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FIGURE 10–9 Human audibility curve. The middle curve is that obtained by audiometry under the usual conditions. The lower curve is that obtained under ideal conditions. At about 140 db (top curve), sounds are felt as well as heard.

The presence of one sound decreases an individual’s ability to hear other sounds, a phenomenon known as masking. It is believed to be due to the relative or absolute refractoriness of previously stimulated auditory receptors and nerve fibers to other stimuli. The degree to which a given tone masks others is related to its pitch. The masking effect of the background noise in all but the most carefully soundproofed environments raises the auditory threshold by a definite and measurable amount.

SOUND TRANSMISSION

The ear converts sound waves in the external environment into action potentials in the auditory nerves. The waves are transformed by the eardrum and auditory ossicles into movements of the foot plate of the stapes. These movements set up waves in the fluid of the inner ear (Figure 10–10). The action of the waves on the organ of Corti generates action potentials in the nerve fibers.

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FIGURE 10–10 Schematic representation of the auditory ossicles and the way their movement translates movements of the tympanic membrane into a wave in the fluid of the inner ear. The wave is dissipated at the round window. The movements of the ossicles, the membranous labyrinth, and the round window are indicated by dashed lines. The waves are transformed by the eardrum and auditory ossicles into movements of the foot plate of the stapes. These movements set up waves in the fluid of the inner ear. In response to the pressure changes produced by sound waves on its external surface, the tympanic membrane moves in and out to function as a resonator that reproduces the vibrations of the sound source. The motions of the tympanic membrane are imparted to the manubrium of the malleus, which rocks on an axis through the junction of its long and short processes, so that the short process transmits the vibrations of the manubrium to the incus. The incus moves so that the vibrations are transmitted to the head of the stapes. Movements of the head of the stapes swing its foot plate.

In response to the pressure changes produced by sound waves on its external surface, the tympanic membrane moves in and out. The membrane therefore functions as a resonator that reproduces the vibrations of the sound source. It stops vibrating almost immediately when the sound wave stops. The motions of the tympanic membrane are imparted to the manubrium of the malleus. The malleus rocks on an axis through the junction of its long and short processes, so that the short process transmits the vibrations of the manubrium to the incus. The incus moves in such a way that the vibrations are transmitted to the head of the stapes. Movements of the head of the stapes swing its foot plate to and fro like a door hinged at the posterior edge of the oval window. The auditory ossicles thus function as a lever system that converts the resonant vibrations of the tympanic membrane into movements of the stapes against the perilymph-filled scala vestibuli of the cochlea (Figure 10–10). This system increases the sound pressure that arrives at the oval window, because the lever action of the malleus and incus multiplies the force 1.3 times and the area of the tympanic membrane is much greater than the area of the foot plate of the stapes. Some sound energy is lost as a result of resistance, but it has been calculated that at frequencies below 3000 Hz, 60% of the sound energy incident on the tympanic membrane is transmitted to the fluid in the cochlea.

Contraction of the tensor tympani and stapedius muscles of the middle ear cause the manubrium of the malleus to be pulled inward and the footplate of the stapes to be pulled outward (Figure 10–2). This decreases sound transmission. Loud sounds initiate a reflex contraction of these muscles called the tympanic reflex. Its function is protective, preventing strong sound waves from causing excessive stimulation of the auditory receptors. However, the reaction time for the reflex is 40–160 ms, so it does not protect against brief intense stimulation such as that produced by gunshots.

BONE & AIR CONDUCTION

Conduction of sound waves to the fluid of the inner ear via the tympanic membrane and the auditory ossicles, the main pathway for normal hearing, is called ossicular conduction. Sound waves also initiate vibrations of the secondary tympanic membrane that closes the round window. This process, unimportant in normal hearing, is air conduction. A third type of conduction, bone conduction, is the transmission of vibrations of the bones of the skull to the fluid of the inner ear. Considerable bone conduction occurs when tuning forks or other vibrating bodies are applied directly to the skull. This route also plays a role in transmission of extremely loud sounds.

TRAVELING WAVES

The movements of the foot plate of the stapes set up a series of traveling waves in the perilymph of the scala vestibuli. A diagram of such a wave is shown in Figure 10–11. As the wave moves up the cochlea, its height increases to a maximum and then drops off rapidly. The distance from the stapes to this point of maximum height varies with the frequency of the vibrations initiating the wave. High-pitched sounds generate waves that reach maximum height near the base of the cochlea; low-pitched sounds generate waves that peak near the apex. The bony walls of the scala vestibuli are rigid, but Reissner’s membrane is flexible. The basilar membrane is not under tension, and it is also readily depressed into the scala tympani by the peaks of waves in the scala vestibuli. Displacements of the fluid in the scala tympani are dissipated into air at the round window. Therefore, sound produces distortion of the basilar membrane, and the site at which this distortion is maximal is determined by the frequency of the sound wave. The tops of the hair cells in the organ of Corti are held rigid by the reticular lamina, and the hairs of the outer hair cells are embedded in the tectorial membrane (Figure 10–4). When the stapes moves, both membranes move in the same direction, but they are hinged on different axes, so a shearing motion bends the hairs. The hairs of the inner hair cells are not attached to the tectorial membrane, but they are apparently bent by fluid moving between the tectorial membrane and the underlying hair cells.

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FIGURE 10–11 Traveling waves. Top: The solid and the short-dashed lines represent the wave at two instants of time. The long-dashed line shows the “envelope” of the wave formed by connecting the wave peaks at successive instants. Bottom: Displacement of the basilar membrane by the waves generated by stapes vibration of the frequencies shown at the top of each curve.

FUNCTIONS OF THE OUTER HAIR CELLS

The inner hair cells are the primary sensory receptors that generate action potentials in the auditory nerves and are stimulated by the fluid movements noted above. The outer hair cells, on the other hand, respond to sound like the inner hair cells, but depolarization makes them shorten and hyperpolarization makes them lengthen. They do this over a very flexible part of the basal membrane, and this action somehow increases the amplitude and clarity of sounds. Thus, outer hair cells amplify sound vibrations entering the inner ear from the middle ear. These changes in outer hair cells occur in parallel with changes in prestin, a membrane protein, and this protein may well be the motor protein of outer hair cells.

The olivocochlear bundle is a prominent bundle of efferent fibers in each auditory nerve that arises from both ipsilateral and contralateral superior olivary complexes and ends primarily around the bases of the outer hair cells of the organ of Corti. The activity in this nerve bundle modulates the sensitivity of these hair cells via the release of acetylcholine. The effect is inhibitory, and it may function to block background noise while allowing other sounds to be heard.

ACTION POTENTIALS IN AUDITORY NERVE FIBERS

The frequency of the action potentials in single auditory nerve fibers is proportional to the loudness of the sound stimuli. At low sound intensities, each axon discharges to sounds of only one frequency, and this frequency varies from axon to axon depending on the part of the cochlea from which the fiber originates. At higher sound intensities, the individual axons discharge to a wider spectrum of sound frequencies, particularly to frequencies lower than that at which threshold simulation occurs.

The major determinant of the pitch perceived when a sound wave strikes the ear is the place in the organ of Corti that is maximally stimulated. The traveling wave set up by a tone produces peak depression of the basilar membrane, and consequently maximal receptor stimulation, at one point. As noted above, the distance between this point and the stapes is inversely related to the pitch of the sound, with low tones producing maximal stimulation at the apex of the cochlea and high tones producing maximal stimulation at the base. The pathways from the various parts of the cochlea to the brain are distinct. An additional factor involved in pitch perception at sound frequencies of less than 2000 Hz may be the pattern of the action potentials in the auditory nerve. When the frequency is low enough, the nerve fibers begin to respond with an impulse to each cycle of a sound wave. The importance of this volley effect, however, is limited; the frequency of the action potentials in a given auditory nerve fiber determines principally the loudness, rather than the pitch, of a sound.

CENTRAL PATHWAY

The afferent fibers in the auditory division of the eighth cranial nerve end in dorsal and ventral cochlear nuclei (Figure 10–12). From there, auditory impulses pass by various routes to the inferior colliculi, the centers for auditory reflexes, and via the medial geniculate body in the thalamus to the auditory cortex located on the superior temporal gyrus of the temporal lobe. Information from both ears converges on each superior olive, and beyond this, most of the neurons respond to inputs from both sides. In humans, low tones are represented anterolaterally and high tones posteromedially in the auditory cortex.

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FIGURE 10–12 Simplified diagram of main auditory (left) and vestibular (right) pathways superimposed on a dorsal view of the brain stem. Cerebellum and cerebral cortex have been removed. For the auditory pathway, eighth cranial nerve afferent fibers form the cochlea end in dorsal and ventral cochlear nuclei. From there, most fibers cross the midline and terminate in the contralateral inferior colliculus. From there, fibers project to the medial geniculate body in the thalamus and then to the auditory cortex located on the superior temporal gyrus of the temporal lobe. For the vestibular pathway, the vestibular nerve terminates in the ipsilateral vestibular nucleus. Most fibers from the semicircular canals terminate in the superior and medial divisions of the vestibular nucleus and project to nuclei controlling eye movement. Most fibers from the utricle and saccule terminate in the lateral division, which then projects to the spinal cord. They also terminate on neurons that project to the cerebellum and the reticular formation. The vestibular nuclei also project to the thalamus and from there to the primary somatosensory cortex. The ascending connections to cranial nerve nuclei are concerned with eye movements.

The responses of individual second-order neurons in the cochlear nuclei to sound stimuli are like those of the individual auditory nerve fibers. The frequency at which sounds of the lowest intensity evoke a response varies from unit to unit; with increased sound intensities, the band of frequencies to which a response occurs becomes wider. The major difference between the responses of the first- and second-order neurons is the presence of a sharper “cutoff” on the low-frequency side in the medullary neurons. This greater specificity of the second-order neurons is probably due to an inhibitory process in the brain stem. In the primary auditory cortex, most neurons respond to inputs from both ears, but strips of cells are stimulated by input from the contralateral ear and inhibited by input from the ipsilateral ear.

The increasing availability of positron emission tomography (PET) scanning and functional magnetic resonance imaging (fMRI) has greatly improved our level of knowledge about auditory association areas in humans. The auditory pathways in the cortex resemble the visual pathways in that increasingly complex processing of auditory information takes place along them. An interesting observation is that although the auditory areas look very much the same on the two sides of the brain, there is marked hemispheric specialization. For example, Wernicke’s area (see Figure 8–7) is concerned with the processing of auditory signals related to speech. During language processing, this area is much more active on the left side than on the right side. Wernicke’s area on the right side is more concerned with melody, pitch, and sound intensity. The auditory pathways are also very plastic, and, like the visual and somatosensory pathways, they are modified by experience. Examples of auditory plasticity in humans include the observation that in individuals who become deaf before language skills are fully developed, sign language activates auditory association areas. Conversely, individuals who become blind early in life are demonstrably better at localizing sound than individuals with normal eyesight.

Musicians provide additional examples of cortical plasticity. In these individuals, the size of the auditory areas activated by musical tones is increased. In addition, violinists have altered somatosensory representation of the areas to which the fingers they use in playing their instruments project. Musicians also have larger cerebellums than nonmusicians, presumably because of learned precise finger movements.

A portion of the posterior superior temporal gyrus known as the planum temporale, which is located between Heschl’s gyrus (transverse temporal gyrus) and the sylvian fissure (Figure 10–13) is regularly larger in the left than in the right cerebral hemisphere, particularly in right-handed individuals. This area appears to be involved in language-related auditory processing. A curious observation is that the planum temporale is even larger than normal on the left side in musicians and others who have perfect pitch.

image

FIGURE 10–13 Left and right planum temporale in a brain sectioned horizontally along the plane of the sylvian fissure. Plane of section shown in the insert at the bottom. (Reproduced with permission from Kandel ER, Schwartz JH, Jessel TM [editors]: Principles of Neural Science, 3rd ed. McGraw-Hill, 1991.)]

SOUND LOCALIZATION

Determination of the direction from which a sound emanates in the horizontal plane depends on detecting the difference in time between the arrival of the stimulus in the two ears and the consequent difference in phase of the sound waves on the two sides; it also depends on the fact that the sound is louder on the side closest to the source. The detectable time difference, which can be as little as 20 μs, is said to be the most important factor at frequencies below 3000 Hz and the loudness difference the most important at frequencies above 3000 Hz. Neurons in the auditory cortex that receive input from both ears respond maximally or minimally when the time of arrival of a stimulus at one ear is delayed by a fixed period relative to the time of arrival at the other ear. This fixed period varies from neuron to neuron.

Sounds coming from directly in front of the individual differ in quality from those coming from behind because each pinna (the visible portion of the exterior ear) is turned slightly forward. In addition, reflections of the sound waves from the pinnal surface change as sounds move up or down, and the change in the sound waves is the primary factor in locating sounds in the vertical plane. Sound localization is markedly disrupted by lesions of the auditory cortex.

DEAFNESS

Deafness can be divided into two major categories: conductive (or conduction) and sensorineural hearing loss. Conductive deafness refers to impaired sound transmission in the external or middle ear and impacts all sound frequencies. Among the causes of conduction deafness are plugging of the external auditory canals with wax (cerumen) or foreign bodies, otitis externa (inflammation of the outer ear, “swimmer’s ear”) and otitis media (inflammation of the middle ear) causing fluid accumulation, perforation of the eardrum, and osteosclerosis in which bone is resorbed and replaced with sclerotic bone that grows over the oval window.

Sensorineural deafness is most commonly the result of loss of cochlear hair cells but can also be due to problems with the eighth cranial nerve or within central auditory pathways. It often impairs the ability to hear certain pitches while others are unaffected. Aminoglycoside antibiotics such as streptomycin and gentamicin obstruct the mechanosensitive channels in the stereocilia of hair cells (especially outer hair cells) and can cause the cells to degenerate, producing sensorineural hearing loss and abnormal vestibular function. Damage to the hair cells by prolonged exposure to noise is also associated with hearing loss (see Clinical Box 10–1). Other causes include tumors of the eighth cranial nerve and cerebellopontine angle and vascular damage in the medulla.


CLINICAL BOX 10–1



Hearing Loss

Hearing loss is the most common sensory defect in humans. According to the World Health Organization, over 270 million people worldwide have moderate to profound hearing loss, with one fourth of these cases beginning in childhood. According to the National Institutes of Health, ˜15% of Americans between 20 and 69 years of age have high frequency hearing loss due to exposure to loud sounds or noise at work or in leisure activities (noise-induced hearing loss, NIHL). Both inner and outer hair cells are damaged by excessive noise, but outer hair cells appear to be more vulnerable. The use of various chemicals also causes hearing loss; such chemicals are called ototoxins.These include some antibiotics (streptomycin), loop diuretics (furosemide), and platinum-based chemotherapy agents (cisplatin). These ototoxic agents damage the outer hair cells or the stria vascularis. Presbycusis, the gradual hearing loss associated with aging, affects more than one-third of those over 75 and is probably due to gradual cumulative loss of hair cells and neurons. In most cases, hearing loss is a multifactorial disorder caused by both genetic and environmental factors. Single-gene mutations have been shown to cause hearing loss. This type of hearing loss is a monogenic disorder with an autosomal dominant, autosomal recessive, X-linked, or mitochondrial mode of inheritance. Monogenic forms of deafness can be defined as syndromic (hearing loss associated with other abnormalities) or nonsyndromic (only hearing loss). About 0.1% of newborns have genetic mutations leading to deafness. Nonsyndromic deafness due to genetic mutations can first appear in adults rather than in children and may account for many of the 16% of all adults who have significant hearing impairment. It is now estimated that the products of 100 or more genes are essential for normal hearing, and deafness loci have been described in all but 5 of the 24 human chromosomes. The most common mutation leading to congenital hearing loss is that of the protein connexin 26. This defect prevents the normal recycling of K+ through the sustenacular cells. Mutations in three nonmuscle myosins also cause deafness. These are myosin-VIIa, associated with the actin in the hair cell processes; myosin-Ib, which is probably part of the “adaptation motor” that adjusts tension on the tip links; and myosin-VI, which is essential in some way for the formation of normal cilia. Deafness is also associated with mutant forms of α-tectin, one of the major proteins in the tectorial membrane. An example of syndromic deafness is Pendred syndrome, in which a mutant multifunctional anion exchanger causes deafness and goiter. Another example is one form of the long QT syndrome in which one of the K+ channel proteins, KVLQT1, is mutated. In the stria vascularis, the normal form of this protein is essential for maintaining the high K+ concentration in endolymph, and in the heart it helps maintain a normal QT interval. Individuals who are homozygous for mutant KVLQT1 are deaf and predisposed to the ventricular arrhythmias and sudden death that characterize the long QT syndrome. Mutations of the membrane protein barttin can cause deafness as well as the renal manifestations of Bartter syndrome.


THERAPEUTIC HIGHLIGHTS

Cochlear implants are used to treat both children and adults with severe hearing loss. The U.S. Food and Drug Administration has reported that, as of April 2009, approximately 188,000 people worldwide have received cochlear implants. They may be used in children as young as 12 months old. These devices consist of a microphone (picks up environmental sounds), a speech processor (selects and arranges these sounds), a transmitter and receiver/stimulator (converts these sounds into electrical impulses), and an electrode array (sends the impulses to the auditory nerve). Although the implant cannot restore normal hearing, it provides a useful representation of environmental sounds to a deaf person. Those with adult-onset deafness that receive cochlear implants can learn to associate the signals it provides with sounds they remember. Children that receive cochlear implants in conjunction with intensive therapy have been able to acquire speech and language skills. Research is also underway to develop cells that can replace the hair cells in the inner ear. For example, researchers at Stanford University were able to generate cells resembling mechanosensitive hair cells from mouse embryonic and pluripotent stem cells.



Auditory acuity is commonly measured with an audiometer. This device presents the subject with pure tones of various frequencies through earphones. At each frequency, the threshold intensity is determined and plotted on a graph as a percentage of normal hearing. This provides an objective measurement of the degree of deafness and a picture of the tonal range most affected.

Conduction and sensorineural deafness can be differentiated by simple tests with a tuning fork. Three of these tests, named for the individuals who developed them, are outlined in Table 10–1. The Weber and Schwabach tests demonstrate the important masking effect of environmental noise on the auditory threshold.

images

TABLE 10–1 Common tests with a tuning fork to distinguish between sensorineural and conduction deafness.

VESTIBULAR SYSTEM

The vestibular system can be divided into the vestibular apparatus and central vestibular nuclei. The vestibular apparatus within the inner ear detects head motion and position and transduces this information to a neural signal (Figure 10–3). The vestibular nuclei are primarily concerned with maintaining the position of the head in space. The tracts that descend from these nuclei mediate head-on-neck and head-on-body adjustments.

CENTRAL PATHWAY

The cell bodies of the 19,000 neurons supplying the cristae and maculae on each side are located in the vestibular ganglion. Each vestibular nerve terminates in the ipsilateral four-part vestibular nucleus (Figure 10–12) and in the flocculonodular lobe of the cerebellum (not shown in the figure). Fibers from the semicircular canals terminate primarily in the superior and medial divisions of the vestibular nucleus; neurons in this region project mainly to nuclei controlling eye movement. Fibers from the utricle and saccule project predominantly to the lateral division (Deiters nucleus) of the vestibular nucleus which then projects to the spinal cord (lateral vestibulospinal tract). Fibers from the utricle and saccule also terminate on neurons that project to the cerebellum and the reticular formation. The vestibular nuclei also project to the thalamus and from there to two parts of the primary somatosensory cortex. The ascending connections to cranial nerve nuclei are largely concerned with eye movements.

RESPONSES TO ROTATIONAL ACCELERATION

Rotational acceleration in the plane of a given semicircular canal stimulates its crista. The endolymph, because of its inertia, is displaced in a direction opposite to the direction of rotation. The fluid pushes on the cupula, deforming it. This bends the processes of the hair cells (Figure 10–3). When a constant speed of rotation is reached, the fluid spins at the same rate as the body and the cupula swings back into the upright position. When rotation is stopped, deceleration produces displacement of the endolymph in the direction of the rotation, and the cupula is deformed in a direction opposite to that during acceleration. It returns to mid position in 25–30 s. Movement of the cupula in one direction commonly causes an increase in the firing rate of single nerve fibers from the crista, whereas movement in the opposite direction commonly inhibits neural activity (Figure 10–14).

image

FIGURE 10–14 Ampullary responses to rotation. Average time course of impulse discharge from the ampulla of two semicircular canals during rotational acceleration, steady rotation, and deceleration. Movement of the cupula in one direction increases the firing rate of single nerve fibers from the crista, and movement in the opposite direction inhibits neural activity. (Reproduced with permission from Adrian ED: Discharge from vestibular receptors in the cat. J Physiol [Lond] 1943;101:389.)

Rotation causes maximal stimulation of the semicircular canals most nearly in the plane of rotation. Because the canals on one side of the head are a mirror image of those on the other side, the endolymph is displaced toward the ampulla on one side and away from it on the other. The pattern of stimulation reaching the brain therefore varies with the direction as well as the plane of rotation. Linear acceleration probably fails to displace the cupula and therefore does not stimulate the cristae. However, there is considerable evidence that when one part of the labyrinth is destroyed, other parts take over its functions. Clinical Box 10–2 describes the characteristic eye movements that occur during a period of rotation.


CLINICAL BOX 10–2



Nystagmus

The characteristic jerky movement of the eye observed at the start and end of a period of rotation is called nystagmus. It is actually a reflex that maintains visual fixation on stationary points while the body rotates, although it is not initiated by visual impulses and is present in blind individuals. When rotation starts, the eyes move slowly in a direction opposite to the direction of rotation, maintaining visual fixation (vestibuloocular reflex, VOR). When the limit of this movement is reached, the eyes quickly snap back to a new fixation point and then again move slowly in the other direction. The slow component is initiated by impulses from the vestibular labyrinths; the quick component is triggered by a center in the brain stem. Nystagmus is frequently horizontal (ie, the eyes move in the horizontal plane), but it can also be vertical (when the head is tipped sideways during rotation) or rotatory (when the head is tipped forward). By convention, the direction of eye movement in nystagmus is identified by the direction of the quick component. The direction of the quick component during rotation is the same as that of the rotation, but the postrotatory nystagmus that occurs owing to displacement of the cupula when rotation is stopped is in the opposite direction. When nystagmus is seen at rest, it is a sign of a pathology. Two examples of this are congenital nystagmus that is seen at birth and acquired nystagmus that occurs later in life. In these clinical cases, nystagmus can persist for hours at rest. Acquired nystagmus can be seen in patients with acute temporal bone fracture affecting semicircular canalsor after damage to the flocculonodular lobe or midline structures such as the fastigial nucleus. It can also occur as a result of stroke, multiple sclerosis, head injury, and brain tumors. Some drugs (especially antiseizure drugs), alcohol, and sedatives can cause nystagmus.

Nystagmus can be used as a diagnostic indicator of the integrity of the vestibular system. Caloric stimulation can be used to test the function of the vestibular labyrinth. The semicircular canals are stimulated by instilling warm (40°C) or cold (30°C) water into the external auditory meatus. The temperature difference sets up convection currents in the endolymph, with consequent motion of the cupula. In healthy subjects, warm water causes nystagmus that bears toward the stimulus, whereas cold water induces nystagmus that bears toward the opposite ear. This test is given the mnemonic COWS (Cold water nystagmus is Opposite sides, Warm water nystagmus is Same side). In the case of a unilateral lesion in the vestibular pathway, nystagmus is reduced or absent on the side of the lesion. To avoid nystagmus, vertigo, and nausea when irrigating the ear canals in the treatment of ear infections, it is important to be sure that the fluid used is at body temperature.


THERAPEUTIC HIGHLIGHTS

There is no cure for acquired nystagmus and treatment is dependent upon the cause. Correcting the underlying cause (stopping drug usage, surgical removal of a tumor) is often the treatment of choice. Also, rectus muscle surgery has been used successfully to treat some cases of acquired nystagmus. Short-term correction of nystagmus can result from injections of botulinum toxin (Botox) to paralyze the ocular muscles.



RESPONSES TO LINEAR ACCELERATION

The utriclar macula responds to horizontal acceleration, and the saccular macula responds to vertical acceleration. The otoliths in the surrounding membrane are denser than the endolymph, and acceleration in any direction causes them to be displaced in the opposite direction, distorting the hair cell processes and generating activity in the nerve fibers. The maculae also discharge tonically in the absence of head movement, because of the pull of gravity on the otoliths.

The impulses generated from these receptors are partly responsible for labyrinth righting reflexes. These reflexes are a series of responses integrated for the most part in the nuclei of the midbrain. The stimulus for the reflex is tilting of the head, which stimulates the otolithic organs; the response is compensatory contraction of the neck muscles to keep the head level. In cats, dogs, and primates, visual cues can initiate optical righting reflexes that right the animal in the absence of labyrinthine or body stimulation. In humans, the operation of these reflexes maintains the head in a stable position and the eyes fixed on visual targets despite movements of the body and the jerks and jolts of everyday life. The responses are initiated by vestibular stimulation, stretching of neck muscles, and movement of visual images on the retina, and the responses are the vestibulo-ocular reflex and other remarkably precise reflex contractions of the neck and extraocular muscles.

Although most of the responses to stimulation of the maculae are reflex in nature, vestibular impulses also reach the cerebral cortex. These impulses are presumably responsible for conscious perception of motion and supply part of the information necessary for orientation in space. Vertigo is the sensation of rotation in the absence of actual rotation and is a prominent symptom when one labyrinth is inflamed.

SPATIAL ORIENTATION

Orientation in space depends in part on input from the vestibular receptors, but visual cues are also important. Pertinent information is also supplied by impulses from proprioceptors in joint capsules, which supply data about the relative position of the various parts of the body, and impulses from cutaneous exteroceptors, especially touch and pressure receptors. These four inputs are synthesized at a cortical level into a continuous picture of the individual’s orientation in space. Clinical Box 10–3 describes some common vestibular disorders.


CLINICAL BOX 10–3



Vestibular Disorders

Vestibular balance disorders are the ninth most common reason for visits to a primary care physician. It is one of the most common reasons elderly people seek medical advice. Patients often describe balance problems in terms of vertigo, dizziness, lightheadedness, and motion sickness. Neither lightheadedness nor dizziness is necessarily a symptom of vestibular problems, but vertigo is a prominent symptom of a disorder of the inner ear or vestibular system, especially when one labyrinth is inflamed. Benign paroxysmal positional vertigo (BPPV) is the most common vestibular disorder characterized by episodes of vertigo that occur with particular changes in body position (eg, turning over in bed, bending over). One possible cause is that otoconia from the utricle separate from the otolith membrane and become lodged in the canal or cupula of the semicircular canal. This causes abnormal deflections when the head changes position relative to gravity.

Ménière disease is an abnormality of the inner ear causing vertigo or severe dizziness, tinnitus, fluctuating hearing loss, and the sensation of pressure or pain in the affected ear lasting several hours. Symptoms can occur suddenly and recur daily or very rarely. The hearing loss is initially transient but can become permanent. The pathophysiology likely involves an immune reaction. An inflammatory response can increase fluid volume within the membranous labyrinth, causing it to rupture and allowing the endolymph and perilymph to mix together. The worldwide prevalence for Ménière’s disease is ˜12 per 1000 individuals. It is diagnosed most often between the ages of 30 and 60; and it affects both genders similarly.

The nausea, blood pressure changes, sweating, pallor, and vomiting that are the well-known symptoms of motion sickness are produced by excessive vestibular stimulation and occurs when conflicting information is fed into the vestibular and other sensory systems. The symptoms are probably due to reflexes mediated via vestibular connections in the brain stem and the flocculonodular lobe of the cerebellum. Space motion sickness (ie, the nausea, vomiting, and vertigo experienced by astronauts) develops when they are first exposed to microgravity and often wears off after a few days of space flight. It can then recur with reentry, as the force of gravity increases again. It is believed to be due to mismatches in neural input created by changes in the input from some parts of the vestibular apparatus and other gravity sensors without corresponding changes in the other spatial orientation inputs.


THERAPEUTIC HIGHLIGHTS

Symptoms of BPPV often subside over weeks or months, but if treatment is needed one option is a procedure called canalith repositioning. This consists of simple and slow maneuvers to position your head to move the otoconia from the semicircular canals back into the vestibule that houses the utricle. There is no cure for Ménière disease, but the symptoms can be controlled by reducing the fluid retention through dietary changes (low-salt or salt-free diet, no caffeine, no alcohol) or medications such as diuretics (eg, hydrochlorothiazide). Individuals with Ménière disease often respond to drugs used to alleviate the symptoms of vertigo. Vestibulosuppressants such as meclizine (an antihistamine drug) decrease the excitability of the middle ear labyrinth and block conduction in middle ear vestibular-cerebellar pathway. Motion sickness commonly can be prevented with the use of antihistamines or scopolamine,cholinergic muscarinic receptor antagonist.



CHAPTER SUMMARY

image The external ear funnels sound waves to the external auditory meatus and tympanic membrane. From there, sound waves pass through three auditory ossicles (malleus, incus, and stapes) in the middle ear. The inner ear contains the cochlea and organ of Corti.

image The hair cells in the organ of Corti signal hearing. The stereocilia provide a mechanism for generating changes in membrane potential proportional to the direction and distance the hair moves. Sound is the sensation produced when longitudinal vibrations of air molecules strike the tympanic membrane.

image The pressure changes produced by sound waves cause the tympanic membrane to move in and out; thus it functions as a resonator to reproduce the vibrations of the sound source. The auditory ossicles serve as a lever system to convert the vibrations of the tympanic membrane into movements of the stapes against the perilymph-filled scala vestibuli of the cochlea.

image The activity within the auditory pathway passes from the eighth cranial nerve afferent fibers to the dorsal and ventral cochlear nuclei to the inferior colliculi to the thalamic medial geniculate body and then to the auditory cortex.

image Loudness is correlated with the amplitude of a sound wave, pitch with the frequency, and timbre with harmonic vibrations.

image Conductive deafness is due to impaired sound transmission in the external or middle ear and impacts all sound frequencies. Sensorineural deafness is usually due to loss of cochlear hair cells but can result from damage to the eighth cranial nerve or central auditory pathway. Conduction and sensorineural deafness can be differentiated by simple tests with a tuning fork.

image Rotational acceleration stimulates the crista in the semicircular canals, displacing the endolymph in a direction opposite to the direction of rotation, deforming the cupula and bending the hair cell. The utricle responds to horizontal acceleration and the saccule to vertical acceleration. Acceleration in any direction displaces the otoliths, distorting the hair cell processes and generating neural activity.

image Spatial orientation is dependent on input from vestibular receptors, visual cues, proprioceptors in joint capsules, and cutaneous touch and pressure receptors.

MULTIPLE-CHOICE QUESTIONS

For all questions, select the single best answer unless otherwise directed.

1. A 45-year-old woman visited her physician after experiencing sudden onset of vertigo, tinnitus and hearing loss in her left ear, nausea, and vomiting. This was the second episode in the past few months. She was referred to an otolaryngologist to rule out Ménière’s disease. Which of the following statements correctly describe the functions of the external, middle, or inner ear?

A. Sound waves are funneled through the external ear to the external auditory meatus and then they pass inward to the tympanic membrane.

B. The cochlea of the inner ear contains receptors for hearing, semicircular canals contain receptors that respond to head tilt, and the otolith organs contain receptors that respond to rotation.

C. Contraction of the tensor tympani and stapedius muscles of the middle ear cause the manubrium of the malleus to be pulled outward and the footplate of the stapes to be pulled inward.

D. Sound waves are transformed by the eardrum and auditory ossicles into movements of the foot plate of the malleus.

E. The semicircular canals, the utricle, and the saccule of the middle ear are concerned with equilibrium.

2. A 45-year-old male with testicular cancer underwent chemotherapy treatment with cisplatin. He reported several adverse side effects including changes in taste, numbness and tingling in his fingertips, and reduced sound clarity. When the damage to the outer hair cells is greater than the damage to the inner hair cells,

A. perception of vertical acceleration is disrupted.

B. K+ concentration in endolymph is decreased.

C. K+ concentration in perilymph is decreased.

D. there is severe hearing loss.

E. affected hair cells fail to shorten when exposed to sound.

3. Which of the following statements is correct?

A. The motor protein for inner hair cells is prestin.

B. The auditory ossicles function as a lever system to convert the resonant vibrations of the tympanic membrane into movements of the stapes against the endolymph-filled scala tympani.

C. The loudness of a sound is directly correlated with the amplitude of a sound wave, and pitch is inversely correlated with the frequency of the sound wave.

D. Conduction of sound waves to the fluid of the inner ear via the tympanic membrane and the auditory ossicles is called bone conduction.

E. High-pitched sounds generate waves that reach maximum height near the base of the cochlea; low-pitched sounds generate waves that peak near the apex.

4. A 40-year-old male, employed as a road construction worker for nearly 20 years, went to his physician to report that he recently began to notice difficulty hearing during normal conversations. A Weber test showed that sound from a vibrating tuning fork was localized to the right ear. A Schwabach test showed that bone conduction was below normal. A Rinne test showed that both air and bone conduction were abnormal, but air conduction lasted longer than bone conduction. The diagnosis was:

A. sensorial deafness in both ears.

B. conduction deafness in the right ear.

C. sensorial deafness in the right ear.

D. conduction deafness in the left ear.

E. sensorineural deafness in the left ear.

5. What would the diagnosis be if a patient had the following test results? Weber test showed that sound from a vibrating tuning fork was louder than normal; Schwabach test showed that bone conduction was better than normal; and Rinne test showed that air conduction did not outlast bone conduction.

A. Sensorial deafness in both ears.

B. Conduction deafness in both ears.

C. Normal hearing.

D. Both sensorial and conduction deafness.

E. A possible tumor on the eighth cranial nerve.

6. The auditory pathway

A. and vestibular pathway contains a synapse in the cerebellum.

B. and vestibular pathway project to the same regions of the cerebral cortex.

C. is comprised of afferent fibers of the eighth cranial nerve, the dorsal and ventral cochlear nuclei, the superior colliculi, the lateral geniculate body, and the auditory cortex.

D. is comprised of afferent fibers of the eighth cranial nerve, the dorsal and ventral cochlear nuclei, the inferior colliculi, the medial geniculate body, and the auditory cortex.

E. is not subject to plasticity like the visual pathways.

7. A healthy male medical student volunteered to undergo evaluation of the function of his vestibular system for a class demonstration. The direction of his nystagmus is expected to be vertical when he is rotated

A. after warm water is put in one of his ears.

B. with his head tipped backward.

C. after cold water is put in both of his ears.

D. with his head tipped sideways.

E. with his head tipped forward.

8. In the utricle, tip links in hair cells are involved in

A. formation of perilymph.

B. depolarization of the stria vascularis.

C. movements of the basement membrane.

D. perception of sound.

E. regulation of distortion-activated ion channels.

9. Postrotatory nystagmus is caused by continued movement of

A. aqueous humor over the ciliary body in the eye.

B. cerebrospinal fluid over the parts of the brain stem that contain the vestibular nuclei.

C. endolymph in the semicircular canals, with consequent bending of the cupula and stimulation of hair cells.

D. endolymph toward the helicotrema.

E. perilymph over hair cells that have their processes embedded in the tectorial membrane.

10. A patient enters the hospital for evaluation of deafness. He is found to also have an elevated plasma renin, although his blood pressure is 118/75 mm Hg. Mutation of what single gene may explain these findings?

A. The gene for barttin

B. The gene for Na+ channel

C. The gene for renin

D. The gene for cystic fibrosis transmembrane conductance regulator

E. The gene for tyrosine hydroxylase

CHAPTER RESOURCES

Angelaki DE, Cullen KE: Vestibular system: The many facets of a multimodal sense. Annu Rev Neurosci 2008;31:125.

Ashmore J: Cochlear outer hair cell motility. Physiol Rev 2008;88:173.

Baloh RW, Halmagyi M: Disorders of the Vestibular System. Oxford University Press, 1996.

Eatock RA, Songer JE: Mammalian vestibular hair cells and primary afferents: Channeling motion signals. Annu Rev Neurosci 2011;34:

Highstein SM, Fay RR, Popper AN (editors): The Vestibular System. Springer, 2004.

Hudspeth AJ: How the ear’s works work. Nature 1989;341:397.

Oertel D, Fay RR, Popper AN (editors): Integrative Functions in the Mammalian Auditory Pathway. Springer, 2002.

Oshima K, Shin K, Diensthuber M, Peng AW, Ricci AJ, Heller S. Mechanosensitive hair cell-like cells from embryonic and induced pluripotent stem cells. Cell 2010;141:704.

Pickles JO: An Introduction to the Physiology of Hearing, 2nd ed. Academic Press, 1988.

Richardson GP, Boutet-de Monvel J, Petit C: How the genetics of deafness illuminates auditory physiology. Annu Rev Physiol 2011;73:311.

Robles L, Ruggero MA: Mechanics of the mammalian cochlea. Physiol Rev 2001;81:1305.

Vollrath MA, Kwan KY, Corey DP: The micromachinery of mechanotransduction in hair cells. Annu Rev Neurosci 2007;30:339.

Willems PJ: Genetic causes of hearing loss. NE J Med 2000;342:1101.