Medical Physiology, 3rd Edition

Vestibular and Auditory Transduction: Hair Cells

Balancing on one foot and listening to music both involve sensory systems that have similar transduction mechanisms. Sensation in both the vestibular and auditory systems begins with the inner ear, and both use a highly specialized kind of receptor called the hair cell. Common structure and function often suggest a common origin, and indeed, the organs of mammalian hearing and balance both evolved from the lateral line organs present in all aquatic vertebrates. The lateral line consists of a series of pits or tubes along the flanks of an animal. Within each indentation are clusters of sensory cells that are similar to hair cells. These cells have microvilli-like structures that project into a gelatinous material that in turn is in contact with the water in which the animal swims. The lateral line is exquisitely sensitive to vibrations or pressure changes in the water in many animals, although it is also sensitive to temperature or electrical fields in some species. Reptiles abandoned the lateral line during their evolution, but they retained the hair cell–centered sensory structures of the inner ear that evolved from the lateral line.

The vestibular system generates our sense of balance and the auditory system provides our sense of hearing. Vestibular sensation operates constantly while we are awake and communicates to the brain the head's orientation and changes in the head's motion. Such information is essential for generating muscle contractions that will put our body where we want it to be, reorienting the body when something pushes us aside (vestibulospinal reflexes), and moving our eyes continually so that the visual world stays fixed on our retinas even though our head may be nodding about (vestibulo-ocular reflexes). imageN15-4 Vestibular dysfunction can make it impossible to stabilize an image on our moving retinas, and it causes the disconcerting feeling that the world is uncontrollably moving around—vertigo. Walking and standing can be difficult or impossible. With time, compensatory adjustments are made as the brain learns to substitute more visual and proprioceptive cues to help guide smooth and accurate movements.

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Vestibulo-ocular Reflexes

Contributed by Philine Wangemann

Through vestibulospinal reflexes, the vestibular system influences body posture, which is essential for balancing our body, preventing it from falling, and—when falling—lifting our head to prevent it from impact injury. Further, through vestibulo-ocular reflexes, the vestibular system influences movements of the eyes, which stabilize images on our retinas during head movements.

Since visual processing in the retina is relatively slow, it is necessary to stabilize the images of the world on the retina. Stabilizing reflexes, collectively called vestibulo-ocular reflexes, enable you to read this text while shaking your head. Note that it is much more difficult to read while shaking the book! The vestibular system measures head movements and elicits compensatory movements of the eyes (eFig. 15-1). The position of each eye is controlled by three pairs of muscles that control horizontal, vertical and rotational eye movements. Vestibulo-ocular reflexes are linked to all five vestibular organs to enable compensatory eye movements in every direction. Muscles that control horizontal eye movements are linked to the horizontal semicircular canals and the utricle. Muscles that control vertical eye movements are linked to the anterior and posterior semicircular canals and the saccule. Finally, muscles that control rotational eye movements are linked to the anterior and posterior semicircular canals and the utricle. Vestibulo-ocular reflexes can be suppressed during the observation of moving targets—for example, watching a bird or a ball flying by.

During large rotations, for example during spinning, the eye movements required for stabilizing an image on the retina exceed the limits of the orbit. Under these conditions, vestibulo-ocular reflexes elicit fast reset movements of the eyes. Alternations between slow movements of the eyes intended to stabilize images on the retina and fast reset motions are called nystagmus (from the Greek nystagmos [tired or sleepy, like the nodding movement of the head just before falling asleep]). Nystagmus can occur in all directions and is named by the direction of the fast reset phase (e.g., rightward nystagmus). Lesions of the vestibular system, for example in head trauma or stroke, can lead to a spontaneous nystagmus when altered neuronal activity is falsely interpreted as head movements. Nystagmus induced by body rotations or by a caloric test is used clinically to evaluate vestibular function because lesions alter or eliminate nystagmus. The caloric test consists of introducing cold (30°C) or warm (40°C) water into the external ear canal. Temperature changes induce convective movements of endolymph that are interpreted as head rotations. Comparisons of the caloric responses of the left and right ear can be used to localize lesions.

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EFIGURE 15-1 Example of a vestibulo-ocular reflex. A, Head rotation to the left causes endolymph to push and pull on the cupulae of the left and right horizontal canals, respectively. B, Movement of the cupula tilts hair bundles in the ampulla of the left horizontal canal in the stimulatory direction, leading to excitation of afferent dendrites and an increase in the frequency of action potentials. Conversely, movement of the cupula tilts hair bundles in the ampulla of the right horizontal canal in the inhibitory direction, which leads to a cessation of afferent stimulation and a decrease in the frequency of action potentials. C, Head rotations have a linear component that results from centrifugal forces against gravity. Centrifugal forces move the otolith membranes of the left and right macula utricle to the left. Displacements of the otolith membranes causes stimulation and inhibition of afferent activity depending on the orientation of the hair bundles. Note that the simple left head movement is coded by an intricate pattern of increased and decreased neuronal activity. The pattern is analyzed by the vestibular nuclei and used to elicit compensatory movements of the eyes to ensure that images remain stable on the retina during head movements. (Courtesy Philine Wangemann.)

Auditory sensation is often at the forefront of our conscious experience, unlike vestibular information, which we rarely notice unless something goes wrong. Hearing is an exceptionally versatile process that allows us to detect things in our environment, to precisely identify their nature, to localize them well at a distance, and, through language, to communicate with speed, complexity, nuance, and emotion.

Bending the stereovilli of hair cells along one axis causes cation channels to open or to close

Hair cells are mechanoreceptors that are specialized to detect minuscule movement along one particular axis. The hair cell is an epithelial cell (see pp. 43–45); the hair bundles project from the apical end, whereas synaptic contacts occur at the basal end. Hair cells are somewhat different in the vestibular and auditory systems. In this section, we illustrate concepts mainly with the vestibular hair cell (Fig. 15-15A), which comes in two subtypes. Vestibular type I cells have a bulbous basal area, surrounded by a calyx-shaped afferent nerve terminal (see Fig. 15-15B, left). Vestibular type II hair cells are more cylindrical and have several simple, bouton-shaped afferent nerve terminals (see Fig. 15-15B, right). Auditory hair cells also come in two varieties, inner hair cells and outer hair cells (see pp. 378–380). However, all hair cells sense movement in basically the same way.

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FIGURE 15-15 Vestibular hair cells. A, Scanning electron micrograph of a bullfrog hair cell from the sensory epithelium of the saccule. B, Type I and type II cells. (A, From Corey DP, Assad JA: In Corey DP, Roper SD [eds]: Sensory Transduction. New York, Rockefeller University Press, 1992;B, data from Philine Wangemann, Kansas State University.)

As part of their hair bundles, vestibular hair cells (see Fig. 15-15B) have one large kinocilium, which is a true cilium with the characteristic 9 + 2 pattern of microtubules (see Fig. 2-11C). The role of the kinocilium is unknown. In mammals, auditory hair cells lose their kinocilium with maturity.

Both vestibular and auditory hair cells have 50 to 150 stereovilli, which are filled with actin and are more akin to microvilli. The stereovilli—often called stereocilia, although they lack the typical 9 + 2 pattern of true cilia—are 0.2 to 0.8 µm in diameter and are generally 4 to 10 µm in height. These “hairs” are arranged in a neat array. In the vestibular system, the kinocilium stands tallest along one side of the bundle and the stereovilli fall away in height to the opposite side (see Fig. 15-15B). Stereovilli are narrower at their base and insert into the apical membrane of the hair cell, where they make a sort of hinge before connecting to a cuticular plate. Within the bundle, stereovilli are connected one to the next, but they can slide with respect to each other as the bundle is deflected side to side. The ends of the stereovilli are interconnected with very fine strands called tip links, which are visible by electron microscopy.

The epithelium of which the hair cells are a part separates perilymph from endolymph. The perilymph bathes the basolateral side of the hair cells. In composition (i.e., relatively low [K+], high [Na+]), perilymph is similar to CSF. Its voltage is zero—close to that of most other extracellular fluids in the body. The basolateral resting potential of vestibular hair cells and auditory inner hair cells is about −40 mV (see Fig. 15-15B). The endolymph bathing the stereovilli is singular in composition. It has a very high [K+] (150 mM) and a very low [Na+] (1 mM), more like cytoplasm than extracellular fluid. It also has a relatively high [image] (30 mM). The voltage of the vestibular endolymph is ~0 mV relative to perilymph. Across the apical membrane of vestibular hair cells, the chemical gradient for K+ is small. However, the electrical gradient is fairly large, ~40 mV. Thus, a substantial force tends to drive K+ into the vestibular hair cell across the apical membrane. We will see that the driving force for K+ influx is even higher in the auditory system (see p. 378).

The appropriate stimulus for a hair cell is the bending of its hairs, but not just any deflection will do. Bending of the hair bundle toward the longer stereovilli (Fig. 15-16A) excites the cell and causes a depolarizing receptor potential (see pp. 353–354). Bending of the hair bundle away from the longer stereovilli (see Fig. 15-16B) hyperpolarizes the cell. Only tiny movements are needed. In auditory hair cells, as little as 0.5 nm (which is the diameter of a large atom) gives a detectable response, and the response is saturated at ~150 nm, about the diameter of one stereovillus or 1 degree of angular deflection! In fact, the sensitivity of hair cells is limited only by noise from the brownian motion of surrounding molecules. The cell is also exquisitely selective to direction. If the hairs are bent along the axis 90 degrees to their preferred direction, they are less than one tenth as responsive.

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FIGURE 15-16 Mechanotransduction in the hair cell. Both panels portray a vestibular hair cell, with an endolymph voltage of 0 mV. A, At rest, a small amount of K+ leaks into the cells, driven by the negative membrane potential and high apical [K+]. Mechanical deformation of the hair bundle toward the longer stereovilli increases the opening of nonselective cation channels at the tips of the stereovilli, which allows K+ influx and depolarizes the cell. In all hair cells except the auditory outer hair cells, the depolarization activates voltage-sensitive Ca2+ channels on the basal membrane, causing release of synaptic vesicles and stimulating the postsynaptic membrane of the accompanying sensory neuron. B, Mechanical deformation of the hair bundle away from the longer stereovilli causes the nonselective cation channels to close, which leads to hyperpolarization.

Mechanotransduction in hair cells seems to be accomplished by directly linking the movement of the stereovilli to the gating of apical mechanosensitive cation channels. Electrical measurements, as well as the imaging of intracellular Ca2+, imply that the transduction channels are located near the tips of the stereovilli. How is channel gating connected to movement of the hairs? The latency of channel opening is extremely short, <40 µs. If one deflects the hairs more rapidly, the channels are activated more quickly. This observation suggests a direct, physical coupling inasmuch as diffusion of a second messenger would take much longer. Corey and Hudspeth have suggested a spring-like molecular linkage between the movement of stereovilli and channel gating. The tip links may be the tethers between stereovilli and the channels, with the channels located at the lower ends of the tip links. Tip links themselves are formed from two types of cadherins, cadherin 23 and protocadherin 15, which form strands of high stiffness. Brief exposure to low-Ca2+ solutions destroys the tip links, without otherwise causing obvious harm to the cells, and thereby abolishes transduction.

The mechanosensitive channels at the tips of the stereovilli are nonselective cation channels with relatively large unitary conductances (150 to 300 picosiemens, depending on hair cell type), allowing monovalent and some divalent cations, including Ca2+, to pass easily. Each stereovillus has no more than two channels, which makes their identification elusive. Under physiological conditions, K+ carries most of the current through the transduction channels. When the cell is at rest—hairs straight up—a small but steady leak of depolarizing K+ current flows through the cell. This leak allows the hair cell to respond to both positive and negative deflections of its stereovilli. A positive deflection—toward the tallest stereovilli—further opens the apical channels, leading to influx of K+ and thus depolarization. K+ leaves the cell through mechanoinsensitive K+ channels on the basolateral side (see Fig. 15-16A), along a favorable electrochemical gradient. A negative deflection closes the apical channels and thus leads to hyperpolarization (see Fig. 15-16B).

A hair cell is not a neuron. Hair cells do not project axons of their own, and most do not generate action potentials. Instead—in the case of vestibular hair cells and auditory inner hair cells—the membrane near the presynaptic (i.e., basolateral) face of the cell has voltage-gated Ca2+ channels that are somewhat active at rest but more active during mechanically induced depolarization (i.e., the receptor potential) of the hair cell. The Ca2+ that enters the hair cell through these channels triggers the graded release of glutamate as well as aspartate in the case of vestibular hair cells. These excitatory transmitters stimulate the postsynaptic terminal of sensory neurons that transmit information to the brain. The greater the transmitter release, the greater the rate of action potential firing in the postsynaptic axon.

In mammals, all hair cells—whether part of the vestibular or auditory system—are contained within bilateral sets of interconnected tubes and chambers called, appropriately enough, the membranous labyrinth (Fig. 15-17). The vestibular portion has five sensory structures: two otolithic organs, which detect gravity (i.e., head position) and linear head movements, and three semicircular canals, which detect head rotation. Also contributing to our sense of spatial orientation and motion are proprioceptors (see pp. 383–389) and the visual system (see pp. 359–371). imageN15-5 The auditory portion of the labyrinth is the spiraling cochlea, which detects rapid vibrations (sound) transmitted to it from the surrounding air.

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FIGURE 15-17 The ear, cochlea, and semicircular canals. A, This section through the right ear of a human shows the outer, middle, and inner ear. B, The labyrinth consists of an auditory and a vestibular portion. The auditory portion is the cochlea. The vestibular portion includes the two otolithic organs (the utricle and saccule) and the three semicircular canals.

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Spatial Orientation

Contributed by Philine Wangemann

Proprioceptors in the skin, in tendons and muscles, and in joints provide information about posture, and the visual system provides clues from our surrounding. Although the vestibular system contributes to our conscious perception of motion and body position, vestibular information is mostly processed subconsciously. Through vestibulospinal reflexes, the vestibular system influences body posture, which is essential for balancing our body, preventing it from falling, and, when it falls, lifting our head to prevent it from impact injury. Further, through vestibulo-ocular reflexes,imageN15-4 the vestibular system influences movements of the eyes, which stabilize images on our retinas during head movements.

The ultimate function of each of these sensory structures is to transmit mechanical energy to their hair cells. In each case, transduction occurs in the manner described above. The specificity of the transduction process depends much less on the hair cells than on the structure of the labyrinth organs around them.

The otolithic organs (saccule and utricle) detect the orientation and linear acceleration of the head

The otolithic organs are a pair of relatively large chambers—the saccule and the utricle—near the center of the labyrinth (see Fig. 15-17B). These otolithic organs as well as the semicircular canals are (1) lined by epithelial cells, (2) filled with endolymph, (3) surrounded by perilymph, and (4) encased in the temporal bone. Within the epithelium, specialized vestibular dark cells secrete K+ and are responsible for the high [K+] of the endolymph. The mechanism of K+ secretion is similar to that by the stria vascularis in the auditory system (see p. 378).

The saccule and utricle each have a sensory epithelium called the macula, which contains the hair cells that lie among a bed of supporting cells. The stereovilli project into the gelatinous otolithic membrane, a mass of mucopolysaccharides that is studded with otoliths or otoconia (Fig. 15-18A, B). Otoconia are crystals of calcium carbonate, 1 to 5 µm in diameter, that give the otolithic membrane a higher density than the surrounding endolymph. With either a change in the angle of the head or a linear acceleration, the inertia of the otoconia causes the otolithic membrane to move slightly, deflecting the stereovilli.

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FIGURE 15-18 Vestibular sensory organs. A, In the saccule, the longer stereovilli point toward the reversal line. B, In the utricle, the longer stereovilli point away from the reversal line. C, In the ampulla, all stereovilli point in the same direction. D, The arrows point toward the longer stereovilli (and kinocilia) and thereby indicate the directions of greatest sensitivity for opening the transduction channels. (Data from Philine Wangemann, Kansas State University.)

The macula is vertically oriented (in the sagittal plane) within the saccule and horizontally oriented within the utricle when the head is tilted down by ~25 degrees, as during walking. Recall that hair cells are depolarized or hyperpolarized when stereovilli bend toward or away from the kinocilium, respectively (see Fig. 15-16). In the saccule, the kinocilia point away from a curving reversal line that divides the macula into two regions (see Fig. 15-18D). In the utricle, the kinocilia point toward the reversal line. The hair cells of the saccule and utricle respond well to changes in head angle and to acceleration of the sort that is experienced as a car or an elevator starts or stops. Of course, the head can tilt or experience acceleration in many directions. Indeed, the orientations of hair cells of the saccule and utricle covers a full range of directions. Any tilt or linear acceleration of the head will enhance the stimulation of some hair cells, reduce the stimulation of others, and have no effect on the rest.

Each hair cell synapses on the ending of a primary sensory axon that is part of the vestibular nerve, which in turn is a branch of the vestibulocochlear nerve (CN VIII). The cell bodies of these sensory neurons are located in Scarpa's ganglion within the temporal bone. The dendrites project to multiple hair cells, which increases the signal-to-noise ratio. The axons project to the ipsilateral vestibular nucleus in the brainstem. imageN15-6 Because the saccule and utricle are paired structures (one on each side of the head), the CNS can simultaneously use information encoded by the full population of otolithic hair cells and unambiguously interpret any angle of tilt or linear acceleration. The push-pull arrangement of increased/decreased activity within each macula (for hair cells of opposite orientation) and between maculae on either side of the head enhances the fidelity of the signal.

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Vestibular Innervation

Contributed by Philine Wangemann

The vestibular system is innervated by the vestibular nerve, which is a branch of CN VIII. The vestibular nerve is comprised of afferent and efferent fibers.

Afferent fibers consist of dendrites of nerve cells from Scarpa's ganglion housed within the temporal bone. Afferent dendrites contact multiple hair cells within small regions of a macula or crista. Integration over several sensory cells increases the signal-to-noise ratio of the sensory information. Axons of Scarpa's ganglion cells contact the ipsilateral vestibular nuclei in the brainstem. Vestibular nuclei analyze information from the labyrinths on both sides of the head and control oculomotor and postural reflexes.

Efferent innervation of the vestibular labyrinth originates from cell bodies that are located lateral to the facial genu in the brainstem. Bilateral axons to the left and right vestibular system synapse onto afferent calyces of type I hair cells and onto type II vestibular hair cells. Vestibular efferent innervation has been hypothesized to maintain long-term calibration of afferent activity between the two vestibular labyrinths.

The semicircular canals detect the angular acceleration of the head

Semicircular canals (see Fig. 15-17B) also sense acceleration, but not the linear acceleration that the otolithic organs prefer. Angular acceleration generated by sudden head rotations is the primary stimulus for the semicircular canals. Shake your head side to side or nod it up and down. Each rotation of your head will excite some of your canals and inhibit others.

The semicircular canals stimulate their hair cells differently than the otolithic organs. In each canal, the hair cells are clustered within a sensory epithelium (the crista ampullaris) that is located in a bulge along the canal called the ampulla (see Fig. 15-18C). The hair bundles—all of which have the same orientation—project into a gelatinous, dome-shaped structure called the cupula, which spans the lumen of the ampulla. The cupula contains no otoconia, and its mucopolysaccharides have the same density as the surrounding endolymph. Thus, the cupula is not sensitive to linear acceleration. However, with a sudden rotation of the canal, the endolymph tends to stay behind because of its inertia. imageN15-4 The endolymph exerts a force on the movable cupula, much like wind on a sail. This force bows the cupula, which bends the hairs and (depending on the direction of rotation) either excites or suppresses the release of transmitter from the hair cells onto the sensory axons of the vestibular nerve. This arrangement makes the semicircular canals very sensitive to angular acceleration of the head. imageN15-7

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Sensitivity of Semicircular Canals to Head Rotation

Contributed by Barry Connors

If head rotation is maintained at a constant velocity, the friction between the endolymph and the canal walls eventually makes the two move together, so that the bending of the cupula gradually extinguishes during ~15 seconds. When rotation is then stopped, the inertia of the endolymph causes bending of the cupula in the other direction and thus gives a temporary sensation of counter-rotation.

Each side of the head has three semicircular canals that lie in approximately orthogonal planes. The anterior canal is tilted ~41 degrees anterolaterally from the sagittal plane, the posterior canal is tilted ~56 degrees posterolaterally from the sagittal plane, and the lateral canal is tipped ~25 degrees back from the horizontal plane. Because each canal best senses rotation about a particular axis, the three together give a good representation of all possible angles of head rotation. This complete representation is further ensured because each canal is paired with another on the opposite side of the head. Each member of a pair sits within the same plane and responds to rotation about the same axis. However, whereas rotation excites the hair cells of one canal, it inhibits the canals of its contralateral axis mate. This push-pull arrangement presumably increases the sensitivity of detection.

The outer and middle ears collect and condition air pressure waves for transduction within the inner ear

Sound is a perceptual phenomenon that is produced by periodic longitudinal waves of low pressure (rarefactions) and high pressure (compressions) that propagate through air at a speed of 330 to 340 m/s. Absolute sound depends on the amplitude of the longitudinal wave, measured in pascals (Pa). Intensities of audible sounds are commonly expressed in decibel sound pressure level (dB SPL), which relates the absolute sound intensity (PT) to a reference pressure (Pref) of 20 µPa, close to the average human threshold at 2000 Hz.

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The logarithmic scale compresses the wide extent of sound pressures into a convenient range. An increase of 6 dB SPL corresponds to a doubling of the absolute sound pressure level; an increase of 20 dB SPL corresponds to a 10-fold increase.

Sound can be a pure tone of a single frequency, measured in hertz (Hz). Sounds produced by musical instruments or the human voice consist of a perceived fundamental frequency (pitch) and overtones. Sound that is noise contains no recognizable periodic elements. imageN15-8 Pure tones are used clinically for the determination of hearing thresholds (pure-tone audiogram). Humans do not perceive sounds that have the same sound pressure level but different frequencies as equally loud. The psychoacoustic phon scale accounts for these differences in perception. imageN15-9

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Sound Waveforms

Contributed by Philine Wangemann

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EFIGURE 15-2 Waveforms of a pure tone, a sound, and noise. A pure tone (left panel) consists of a sine wave of only one single frequency. Noise (right panel) does not contain any recognizable periodic elements. Other sounds (middle panel) have a notable periodic pattern and consist of multiple superimposed waves. (Courtesy Philine Wangemann.)

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Phon Scale

Contributed by Philine Wangemann

Sounds that have identical decibel sound pressure levels (dB SPL) are not perceived as equally loud at all frequencies. The phon scale, which accounts for these differences in perception, has been developed by asking a number of human subjects to adjust the intensities of test tones to be equal in loudness to reference tones of 1000 Hz (eFig. 15-3). The normal hearing threshold is ~4 phon, discomfort is perceived at 110 phon, and the pain threshold is at 130 phon. Industrial noise levels are often given in units of A-weighted decibels, or dB(A). The dB(A) scale is weighted to approximate human loudness perception.

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EFIGURE 15-3 Relationship between sound pressure level (dB SPL) and frequency (Hz) of continuous pure tones of equal loudness. Combinations of sound pressure levels and frequencies that are perceived as equally loud are graphed as equal-loudness-level contours. The human ear is most sensitive between 1000 and 5000 Hz. Equal-loudness-level contours for 4 to 80 phon are based on ISO 226:2003 and equal-loudness-level contours for 110 and 130 phon are based on ISO 226:1987. (Courtesy Philine Wangemann.)

Sound waves vary in frequency, amplitude, and direction; our auditory systems are specialized to discriminate all three. We can also interpret the rapid and intricate temporal patterns of sound frequency and amplitude that constitute words and music. Encoding of sound frequency and amplitude begins with mechanisms in the cochlea, followed by further analysis in the CNS. To distinguish the direction of a sound along the horizontal plane, the brain compares signals from the two ears.

All mammalian ears are strikingly similar in structure. The ear is traditionally divided into outer, middle, and inner components (see Fig. 15-17A). We discuss the outer and middle ear here. The inner ear consists of the membranous labyrinth, with both its vestibular and auditory components.

Outer Ear

Proceeding from outside to inside, the most visible part of the ear is the pinna, a skin-covered flap of cartilage, and its small extension, the tragus. Together, they funnel sound waves into the external auditory canal. These structures, which compose the outer ear, focus sound waves on the tympanic membrane. Many animals (e.g., cats) can turn each pinna independently to facilitate hearing without changing head position. The shapes of the pinna and tragus tend to emphasize certain sound frequencies over others, depending on their angle of incidence. The external ear parts in humans are essential for localization of sounds in the vertical plane. Sound enters the auditory canal both directly and after being reflected; the sound that we hear is a combination of the two. Depending on a sound's angle of elevation, it is reflected differently off the pinna and tragus. Thus, we hear a sound coming from above our head slightly differently than a sound coming from straight in front of us.

The external auditory canal is lined with skin and penetrates ~2.5 cm into the temporal bone, where it ends blindly at the eardrum (or tympanic membrane). Sound causes the tympanic membrane to vibrate, much like the head of a drum.

Middle Ear

The air-filled chamber between the tympanic membrane on one side and the oval window on the other is the middle ear (see Fig. 15-17A). The eustachian tube connects the middle ear to the nasopharynx and makes it possible to equalize the air pressure on opposite sides of the tympanic membrane. The eustachian tube can also provide a path for throat infections and epithelial inflammation to invade the middle ear and lead to otitis media. The primary function of the middle ear is to transfer vibrations of the tympanic membrane to the oval window (Fig. 15-19). The key to accomplishing this task is a chain of three delicate bones called ossicles: the malleus (or hammer), incus (anvil), and stapes (stirrup). The ossicles are the smallest bones in the body.

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FIGURE 15-19 The middle ear. Displacement of the stapes and the oval window moves fluid in the scala vestibuli, causing opposite fluid movement in the scala tympani and thus an opposite displacement of the round window. (Data from Philine Wangemann, Kansas State University.)

Vibration transfer is not as simple as it might seem because sound starts as a set of pressure waves in the air (within the ear canal) and ends up as pressure waves in a watery cochlear fluid within the inner ear. Air and water have very different acoustic impedances, which is the tendency of each medium to oppose movement brought about by a pressure wave. imageN15-10 This impedance mismatch means that sound traveling directly from air to water has insufficient pressure to move the dense water molecules. Instead, without some system of compensation, >97% of a sound's energy would be reflected when it met a surface of water. The middle ear serves as an impedance-matching device that saves most of the aforementioned energy by two primary methods. First, the tympanic membrane has an area that is ~20-fold larger than that of the oval window, so a given pressure at the air side (the tympanic membrane) is amplified as it is transferred to the water side (the footplate of the stapes). Second, the malleus and incus act as a lever system, which again amplifies the pressure of the wave. Rather than being reflected, most of the energy is successfully transferred to the liquids of the inner ear.

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Acoustic Impedance

Contributed by Barry Connors

Acoustic impedance is defined as the ratio of sound pressure to volume velocity. Air has a low acoustic impedance. Consider what happens when the membrane of a loudspeaker is displaced in air. Because the air is very compressible, the displacement of the loudspeaker membrane does not increase air pressure very much, but it does impart a high volume velocity to the air. On the other hand, the acoustic impedance of water is about 10,000 times higher than that of air because water is highly incompressible and dense. Consider what happens when we submerge a watertight loudspeaker in water. If we were able to displace the loudspeaker membrane to the same extent just as fast as we did when the loudspeaker was in air, we would find that the resulting pressure wave would be far greater. (The way we set up this thought experiment, the volume velocities would be identical in the two cases.)

In the case of the ear—which takes advantage of (1) the area difference between the tympanic membrane and the oval window, and (2) the lever action of the ossicles—the pressure amplification occurs at the expense of volume velocity, thereby conserving energy.

Two tiny muscles of the middle ear, the tensor tympani and the stapedius, insert onto the malleus and the stapes, respectively. These muscles exert some control over the stiffness of the ossicular chain, and their contraction serves to dampen the transfer of sound to the inner ear. They are reflexively activated when ambient sound levels become high. These reflexes are probably protective and may be particularly important for suppression of self-produced sounds, such as the roar you produce in your head when you speak or chew.

The cochlea is a spiral of three parallel, fluid-filled tubes

The auditory portion of the inner ear is mainly the cochlea, a tubular structure that is ~35 mm long and is coiled 2.5 times into a snail shape about the size of a large pea (Fig. 15-20). Counting its stereovilli, the cochlea has a million moving parts, which makes it the most complex mechanical apparatus in the body.

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FIGURE 15-20 The cochlea. Reissner's membrane and the basilar membrane divide the cochlea into three spiraling fluid-filled compartments: the scala vestibuli, the scala media, and the scala tympani. (Data from Philine Wangemann, Kansas State University.)

The cut through the cochlea in the lower left drawing in Figure 15-20 reveals five cross sections of the spiral. We see that in each cross section, two membranes divide the cochlea into three fluid-filled compartments. On one side is the compartment called the scala vestibuli (see Fig. 15-20, right side), which begins at its large end near the oval window—where vibrations enter the inner ear. Reissner's membrane separates the scala vestibuli from the middle compartment, the scala media. The other boundary of the scala media is the basilar membrane, on which rides the organ of Corti and its hair cells. Below the basilar membrane is the scala tympani, which terminates at its basal or large end at the round window. Both the oval and round windows look into the middle ear (see Fig. 15-19B).

Both the scala vestibuli and scala tympani are lined by a network of fibrocytes and filled with perilymph (see pp. 372–373). Like its counterpart in the vestibular system, this perilymph is akin to CSF (i.e., low [K+], high [Na+]). Along the lengths of scala vestibuli and scala tympani, the two perilymphs communicate through the leaky interstitial fluid spaces between the fibrocytes. At the apex of the cochlea, the two perilymphs communicate through a small opening called the helicotrema. Cochlear perilymph communicates with vestibular perilymph through a wide passage at the base of the scala vestibuli (see Fig. 15-17B), and it communicates with the CSF through the cochlear aqueduct.

The scala media is filled with endolymph. Like its vestibular counterpart—with which it communicates through the ductus reuniens (see Fig. 15-17B)—auditory endolymph is extremely rich in K+. Unlike vestibular endolymph, which has the same voltage as the perilymph, auditory endolymph has a voltage of +80 mV relative to the perilymph (see Fig. 15-20, right side). This endocochlear potential, which is the highest transepithelial voltage in the body, is the main driving force for sensory transduction in both inner and outer hair cells. Moreover, loss of the endocochlear potential is a frequent cause of hearing loss. A highly vascularized tissue called the stria vascularis secretes the K+ into the scala media, and the resulting K+ gradient between endolymph and perilymph generates the strong endocochlear potential.

The stria vascularis is functionally a two-layered epithelium (Fig. 15-21). Marginal cells separate endolymph from a very small intrastrial compartment inside the stria vascularis, and basal cells separate the intrastrial compartment from the interstitial fluid of the spiral ligament, which is contiguous with perilymph. Gap junctions connect one side of the basal cells to intermediate cells and the other side of the basal cells to fibrocytes of the spiral ligament. This architecture is essential for generation of the endocochlear potential.

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FIGURE 15-21 K+ secretion into the endolymph by the stria vascularis. (Data from Philine Wangemann, Kansas State University.)

The fibrocytes are endowed with K+ uptake mechanisms that maintain a high [K+]i in the intermediate cells. The KCNJ10 K+ channel of the intermediate cells generates the endocochlear potential. The K+equilibrium potential of these cells is extremely negative because of the combination of their very high [K+]i and the very low [K+] of the intrastrial fluid. Finally, the marginal cells support the endocochlear potential by mopping up the K+ from the intrastrial fluid—keeping the intrastrial [K+] very low—and depositing the K+ in the endolymph through a KCNQ1 K+ channel.

Inner hair cells transduce sound, whereas the active movements of outer hair cells amplify the signal

The business end of the cochlea is the organ of Corti,imageN15-11 the portion of the basilar membrane that contains the hair cells. The organ of Corti stretches the length of the basilar membrane and has four rows of hair cells: one row of ~3500 inner hair cells and three rows with a total of ~16,000 outer hair cells (Fig. 15-22). In the auditory system, the arrangement of stereovilli is also quite orderly. The hair cells lie within a matrix of supporting cells, with their apical ends facing the endolymph of the scala media (Fig. 15-23A). The stereovilli of inner hair cells (see Fig. 15-23B) are unique in that they float freely in the endolymph. The stereovilli of the outer hair cells (see Fig. 15-23C) project into the gelatinous, collagen-containing tectorial membrane. The tectorial membrane is firmly attached only along one edge, with a sort of hinge, so that it is free to tilt up and down.

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FIGURE 15-22 Hair cells of the cochlea. This scanning electron microscopic image shows the outer and inner hair cells of the organ of Corti of a chinchilla after removal of the tectorial membrane. The three rows of outer hair cells are on the bottom, and the single row of inner hair cells is along the top. (Courtesy of I. Hunter-Duvar, MD, and R. Harrison, PhD, the Hospital for Sick Children, Toronto, Canada.)

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FIGURE 15-23 Organ of Corti. A, Upward movement of the basilar membrane tilts the hair bundles toward the longer stereovilli, opening transduction channels. B, In inner hair cells, depolarization causes enhanced transmitter release. C, In outer hair cells, depolarization causes prestin to contract. D, Downward movement of the basilar membrane tilts the hair bundles away from the longer stereovilli, closing transduction channels. (Data from Philine Wangemann, Kansas State University.)

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Alfonso Giacomo Gaspare Corti

Contributed by Emile Boulpaep, Walter Boron

While working in the laboratory of Albert von Kölliker in Würzburg (Germany), Corti (1822–1876) developed novel histological staining techniques that allowed him to distinguish individual—and previously unidentified—elements within the cochlea. It was he who first identified the sensory organ that now bears his name, the organ of Corti.

How do air pressure waves actually stimulate the auditory hair cells? Movements of the stapes against the oval window create traveling pressure waves within the cochlear fluids. Consider, for example, what happens as sound pressure falls in the outer ear.

Step 1: Stapes moves outward. As a result, the oval window moves outward, causing pressure in the scala vestibuli to decrease. Because the perilymph that fills the scala vestibuli and scala tympani is incompressible and the cochlea is encased in rigid bone, the round window moves inward (see Fig. 15-19B).

Step 2: Scala vestibuli pressure falls below scala tympani pressure.

Step 3: Basilar membrane bows upward. Because Reissner's membrane is very thin and flexible, the low scala vestibuli pressure pulls up the incompressible scala media, which in turn causes the basilar membrane (and the organ of Corti) to bow upward.

Step 4: Organ of Corti shears toward hinge of tectorial membrane. The upward bowing of the basilar membrane creates a shear force between the hair bundle of the outer hair cells and the attached tectorial membrane.

Step 5: Hair bundles of outer hair cells tilt toward their longer stereovilli.

Step 6: Transduction channels open in outer hair cells. Because K+ is the major ion, the result is depolarization of the outer hair cells (see Fig. 15-16A)—mechanical to electrical transduction. The transduction-induced changes in membrane potential are called receptor potentials. The molecular mechanisms of these Vm changes (see p. 373) are basically the same as in vestibular hair cells.

Step 7: Depolarization contracts the motor protein prestin. Outer hair cells of mammals express very high levels of prestin (named for the musical notation presto, or fast). The contraction of myriad prestin molecules—each attached to its neighbors—causes the outer hair cell to contract; this phenomenon is unique to outer hair cells and is called electrical to mechanical transduction or electromotility. Conversely, hyperpolarization (during downward movements of the basilar membrane) causes outer hair cells to elongate. Indeed, imposing changes in Vm causes cell length to change by as much as ~5%. The change in shape is fast, beginning within 100 µs. The mechanical response of the outer hair cell does not depend on ATP, microtubule or actin systems, extracellular Ca2+, or changes in cell volume. Prestin is a member of the SLC26 family of anion transporters (see p. 125), although it is not clear whether prestin also functions as an anion transporter.

Step 8: Contraction of outer hair cells accentuates upward movement of the basilar membrane. Conversely, outer hair cell elongation (during downward movements of the basilar membrane) accentuates the downward movement of the basilar membrane. Thus, outer hair cells act as a cochlear amplifier—sensing and then rapidly accentuating movements of the basilar membrane. The electromotility of outer hair cells is a prerequisite for sensitive hearing and, as we will see (see pp. 380–383), the ability to discriminate frequencies sharply. In the absence of prestin, the cochlear amplifier ceases to function and animals become deaf.

Step 9: Endolymph moves beneath the tectorial membrane. The upward movement of the basilar membrane—accentuated by the cochlear amplifier—forces endolymph to flow out from beneath the tectorial membrane, toward its tip.

Step 10: Inner hair cell hair bundles bend toward longer stereovilli. The flow of endolymph now causes the free-floating hair bundles of the inner hair cells to bend.

Step 11: Transduction channels open in inner hair cells. As in the outer hair cells, the result is a depolarization.

Step 12: Depolarization opens voltage-gated Ca2+ channels. [Ca2+]i rises in the inner hair cells.

Step 13: Synaptic vesicles fuse, releasing glutamate. The neurotransmitter triggers action potentials in afferent neurons, relaying auditory signals to the brainstem. Note that the main response to depolarization is very different in the two types of hair cells. The outer hair cell contracts and thereby amplifies the movement of the basilar membrane. The inner hair cell releases neurotransmitter.

When the stapes reverses direction and moves inward, all of these processes reverse as well. The basilar membrane bows downward. In the outer hair cells, transduction channels close, causing hyperpolarization and cell elongation. The accentuated downward movement of the basilar membrane causes endolymph to move back under the tectorial membrane. In inner hair cells, transduction channels close, causing hyperpolarization and reduced neurotransmitter release.

A fascinating clue to the existence of the cochlear amplifier was the early observation that the ear not only detects sounds, but also generates them! Short click sounds trigger an “echo,” a brief vibration of the tympanic membrane that far outlasts the click. A microphone in the auditory canal can detect the echo, which is called an evoked otoacoustic emission. On occasion, damaged ears may produce spontaneous otoacoustic emissions that can even be loud enough to be heard by a nearby listener. imageN15-12 The source of evoked otoacoustic emissions is the prestin-mediated cochlear amplifier: sounds outside the ear lead to vibrations of the basilar membrane, which trigger active length changes of the outer hair cells, and these in turn accentuate the vibrations of the basilar membrane (step 8). In otoacoustic emission, the system then works in reverse as the basilar membrane causes pressure waves in the cochlear fluids, which vibrate the oval window, ossicles, and tympanic membrane, and finally generates new pressure waves in the air of the auditory canal.

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Otoacoustic Emissions

Contributed by Philine Wangemann

Amplification by the outer hair cells evokes vibrations of the basilar membrane that travel through the middle ear, set the tympanic membrane in motion, and produce a sound that comes out of the ear canal. Clinically most relevant are transient otoacoustic emissions and distortion-product otoacoustic emissions. Transient otoacoustic emissions are sounds that are detected in the ear canal milliseconds after a very brief stimulus. Amplification by the outer hair cells is nonlinear, which means that the cochlea produces and emits distortion products. Distortion products in response to two pure tones at nearby frequencies (f1 and f2) relate to these stimuli by simple math, for example, 2f1 − f2 or 2f2 − f1. Transient otoacoustic emissions and distortion-product otoacoustic emissions provide useful clues for the evaluation of outer hair cell function.

The cochlea receives sensory and motor innervation from the auditory or cochlear nerve, a branch of CN VIII. We discuss the motor innervation below (see p. 382). The cell bodies of the sensory or afferent neurons of the cochlear nerve lie within the spiral ganglion, which corkscrews up around the axis of the cochlea (see Fig. 15-20, lower left). The dendrites of these neurons contact nearby hair cells, whereas the axons project to the cochlear nucleus in the brainstem (see Fig. 16-15). About 95% of the roughly 30,000 sensory neurons (i.e., type I cells) of each cochlear nerve innervate the relatively few inner hair cells—the true auditory sensory cells. The remaining 5% of spiral ganglion neurons (i.e., type II cells) innervate the abundant outer hair cells, which are so poorly innervated that they must contribute very little direct information about sound to the brain.

The frequency sensitivity of auditory hair cells depends on their position along the basilar membrane of the cochlea

The subjective experience of tonal discrimination is called pitch. Young humans can hear sounds with frequencies from ~20 to 20,000 Hz. imageN15-13 This range is modest by the standards of most mammals because many hear up to 50,000 Hz, and some, notably whales and bats, can hear sounds with frequencies >100,000 Hz.

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Auditory Range

Contributed by Philine Wangemann

The auditory frequency range of the human ear is well adapted to the perception of speech, which encompasses frequencies between 60 and 12,000 Hz. We can comfortably hear sounds with amplitudes from 0 to 120 dB SPL. Higher sound pressure levels cause pain and destruction of the ear imageN15-9. Typical sound pressure levels are 20 dB SPL for whispering, 60 dB SPL for normal conversation, 80 dB SPL for loud traffic, and 120 dB SPL for a nearby train horn.

A continuous pure tone (see p. 376) produces a wave that travels along the basilar membrane and has different amplitudes at different points along the base-apex axis (Fig. 15-24A). Increases in sound amplitude cause an increase in the rate of action potentials in auditory nerve axons—rate coding.imageN15-14 The frequency of the sound determines where along the cochlea the cochlear membranes vibrate most—high frequencies at one end and low at the other—and thus which hair cells are stimulated. This selectivity is the basis for place coding in the auditory system; that is, the frequency selectivity of a hair cell depends mainly on its longitudinal position along the cochlear membranes. The cochlea is essentially a spectral analyzer that evaluates a complex sound according to its pure tonal components, with each pure tone stimulating a specific region of the cochlea.

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FIGURE 15-24 Waves along the basilar membrane of the cochlea. A, As a wave generated by a sound of a single frequency travels along the basilar membrane, its amplitude changes. The green and yellow curves represent a sample wave at two different times. The upper and lower broken lines (i.e., the envelope) encompass all maximum amplitudes of all waves, at all points in time. Thus, a wave can never escape the envelope. The figure exaggerates the amplitudes of the traveling waves ~1 million-fold. B, For a pure tone of 10,000 Hz, the envelope is confined to a short region of the basilar membrane near the stapes. For pure tones of 4000 Hz and 200 Hz, the widest part of the envelope moves closer to the helicotrema. C, The cochlea narrows in diameter from base to apex, whereas the basilar membrane tapers in the opposite direction.

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Rate Coding

Contributed by Philine Wangemann

Amplitude information is transmitted by rate coding. Rate coding refers to the principle that increases in sound amplitude result in an increase the rate of action potentials. Cooperation between neurons is required to code the full range of sound pressure levels from 0 to 120 dB SPL.

Using optical methods to study cadaver ears, Georg von Békésy found that sounds of a particular frequency generate relatively localized waves in the basilar membrane and that the envelope of these waves changes position according to the frequency of the sound (see Fig. 15-24B). Low frequencies generate their maximal amplitudes near the apex. As sound frequency increases, the envelope shifts progressively toward the basal end (i.e., near the oval and round windows). For his work, von Békésy imageN15-15 received the 1961 Nobel Prize in Physiology or Medicine.

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Georg von Békésy

For more information about Georg von Békésy and the work that led to his Nobel Prize, visit http://www.nobel.se/medicine/laureates/1961/index.html (accessed December 2014).

Two properties of the basilar membrane underlie the low-apical to high-basal gradient of resonance: taper and stiffness (see Fig. 15-24C). If we could unwind the cochlea and stretch it straight, we would see that it tapers from base to apex. The basilar membrane tapers in the opposite direction—wider at the apex, narrower at the base. More important, the narrow basal end is ~100-fold stiffer than its wide and floppy apical end. Thus, the basilar membrane resembles a harp. At one end—the base, near the oval and round windows—it has short, taut strings that vibrate at high frequencies. At the other end—the apex—it has longer, looser strings that vibrate at low frequencies.

Although von Békésy's experiments were illuminating, they were also paradoxical. A variety of experimental data suggested that the tuning of living hair cells is considerably sharper than the broad envelopes of von Békésy's traveling waves on the basilar membrane could possibly produce. imageN15-16 Recordings from primary auditory nerve cells are also very sharp, implying that this tuning must occur within the cochlea, not in the CNS. Some enhancement of tuning comes from the structure of the inner hair cells themselves. Those near the base have shorter, stiffer stereovilli, which makes them resonate to higher frequencies than possible with the longer, floppier stereovilli on cells near the apex.

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Sharpening of Cochlear Tuning

Contributed by Philine Wangemann

Outer hair cells express the motor protein prestin along the lateral cell wall, which is responsible for electromotility. Transduction-mediated depolarization of outer hair cells during upward movements of the basilar membrane causes prestin to contract; this shortens the hair cell body and increases the upward movement of the basilar membrane (see Fig. 15-23). Conversely, hyperpolarization during downward movements of the basilar membrane expands prestin, elongates the outer hair cells, and enlarges the downward movement of the basilar membrane. This electromotility, which amplifies and sharpens the peak of the sound-induced traveling wave, is a prerequisite for the sensitivity of hearing and the ability to sharply discriminate frequencies (see Fig. 15-25).

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FIGURE 15-25 Peak movement of the basilar membrane. The graph illustrates the displacement of the basilar membrane in response to a pure tone as a function of distance along the base-to-apex axis. The dashed line indicates the displacement threshold for triggering of an electrical response. (Data from Ashmore JF: Mammalian hearing and the cellular mechanisms of the cochlear amplifier. In Corey DP, Roper SD [eds]: Sensory Transduction. New York, Rockefeller University Press, 1992, pp 396–412.)

The blue curve in Figure 15-25 approximates von Békésy's envelope of traveling waves for a passive basilar membrane from cadavers. It is important to note that von Békésy used unnaturally loud sounds. With reasonable sound levels, the maximum passive displacement of the basilar membrane would be slightly more than 0.1 nm. This distance is less than the pore diameter of an ion channel and also less than the threshold (0.3 to 0.4 nm) for an electrical response from a hair cell. However, measurements from the basilar membrane in living animals (the orange curve in Fig. 15-25) by very sensitive methods show that movements of the basilar membrane are much more localized and much larger than predicted by von Békésy. The maximal physiological displacement is ~20-fold greater than threshold and ~40-fold greater than that predicted by the passive von Békésy model. Moreover, the physiological displacement decays sharply on either side of the peak, >100-fold within ~0.5 mm (recall that the human basilar membrane has a total length of >30 mm).

Both the extremely large physiological excursions of the basilar membrane and the exquisitely sharp tuning of the cochlea depend on the cochlear amplifier (see p. 380). Indeed, selectively damaging outer hair cells—with large doses of certain antibiotics, for example—considerably dulls the sharpness of cochlear tuning and dramatically reduces the amplification.

The brain can control the tuning of hair cells. Axons that arise in the superior olivary complex in the brainstem synapse mainly on the outer hair cells and, sparsely, on the afferent axons that innervate the inner hair cells. imageN15-17 Stimulation of these olivocochlear efferent fibers suppresses the responsiveness of the cochlea to sound and is thought to provide auditory focus by suppressing responsiveness to unwanted sounds—allowing us to hear better in noisy environments (Box 15-2). The main efferent neurotransmitter is acetylcholine (ACh), which activates ionotropic ACh receptors (see pp. 206–207)—nonselective cation channels—and triggers an entry of Ca2+. The influx of Ca2+ activates Ca2+-activated K+ channels, causing a hyperpolarization—effectively an inhibitory postsynaptic potential—that suppresses the electromotility of outer hair cells and action potentials in afferent dendrites. Thus, the efferent axons allow the brain to control the gain of the inner ear.

Box 15-2

Cochlear Implants

The most common cause of human deafness is damage to the hair cells of the cochlea. imageN15-18 This damage can be caused by genetic factors, a variety of drugs (e.g., some antibiotics, including quinine), chronic exposure to excessively loud sounds, and other types of disease. Even when all hair cells have been destroyed, if the auditory nerve is intact, it is often possible to restore substantial hearing with a cochlear implant.

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Conductive Hearing Loss

Contributed by Philine Wangemann

Conductive hearing losses are disorders that compromise the conduction of sound through the external ear, tympanic membrane, or middle ear. Pressure differences across the tympanic membrane (eardrum) can rupture it. Accumulations of fluid in the middle ear can lead to conductive hearing losses that are seen particularly often in children with middle ear infections (otitis media). With proper treatment, the hearing loss due to otitis media is usually self-limited. Otosclerosis, which stiffens the ossicular chain, is another common cause of conductive hearing loss.

Treatments for conductive hearing loss encompass a palette of devices including hearing aids and middle ear implants. Hearing aids amplify the sound in the external ear canal. Prosthetic devices can replace the tympanic membrane and the ossicular chain. Middle ear implants are clamped onto the incus and enhance the vibrations of the ossicular chain.

A cochlear implant imageN15-19 is essentially an electronic cochlea. Most of the system resides outside the body. The user wears a headpiece with a microphone, which is connected to a small, battery-powered digital speech processor. This processor sends signals to a miniature radio transmitter next to the scalp, which transmits digitally encoded signals—no wires penetrate the skin—to a receiver/decoder that is surgically implanted in the mastoid bone behind the ear. A very thin and flexible set of wires carries the signals through a tiny hole into the basal end of the cochlea, where an array of 8 to 22 electrodes lies adjacent to the auditory nerve endings (where healthy hair cells would normally be) along the cochlea. Each electrode activates a small portion of the auditory nerve axons.

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Cochlear Implants

Contributed by Emile Boulpaep, Walter Boron

See the following websites for more information on cochlear implants:

http://www.nidcd.nih.gov/health/hearing/pages/coch.aspx

http://ecs.utdallas.edu/loizou/cimplants/tutorial/

The cochlear implant exploits the tonotopic arrangement of auditory nerve fibers. By stimulating near the base of the cochlea, it is possible to trigger a perception of high-frequency sounds; stimulation toward the apex evokes low-frequency sounds. The efficacy of the implant can be extraordinary. Users require training of a few months or longer, and in many cases, they achieve very good comprehension of spoken speech, even as it comes across on a telephone.

As the technology and safety of cochlear implants have improved, so has their popularity. By 2010, >200,000 people were using cochlear implants worldwide, ~80,000 of them infants and children. The best candidates for cochlear implants are young children (optimally as young as 1 year) and older children or adults whose deafness was acquired after they learned some speech. Children older than ~7 years and adults whose deafness preceded any experience with speech generally do not fare as well with cochlear implants. The systems of sensory neurons in the brain, including the auditory system, need to experience normal inputs at a young age to develop properly. When the auditory system is deprived of sounds early in life, it can never develop completely normal function even if sensory inputs are restored during adulthood.

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Central Processing of Auditory Patterns

Contributed by Philine Wangemann

Auditory patterns are analyzed in the medial geniculate and the auditory cortex. Neurons in these areas are often highly specialized and respond only to a specific frequency and intensity pattern. Interpretation of sound elements requires cortical input beyond the auditory cortex.

Central processing is clinically evaluated by auditory brainstem recordings. The coordinated firing of groups of neurons in responses to brief stimuli (clicks or tone pips) produces transient voltage fluctuations that can be detected with surface electrodes. Distinctive voltage fluctuations occur 2 to 12 ms after the stimulus and can be associated with neuronal activity in the auditory pathway including the cochlear nerve, cochlear nucleus, and superior olivary complex.