An overall view
Hearing, a key sense in human communication, commences with the ear's capture of sound. Mechanical energy flows through the middle ear to the cochlea, where it makes the elastic basilar membrane vibrate. An array of 16,000 hair cells detects the frequency component of a stimulus, transduces it into receptor potentials, and encodes it in the firing pattern of eighth-nerve fibers. The complex auditory pathways of the brain stem mediate certain functions, such as the localization of sound sources and forward auditory information to the cerebral cortex. here, several distinct areas analyze sound to detect the complex patterns characteristic of speech.
As our population ages and as our society becomes more concerned about hearing loss, physicians will increasingly confront patients and families who are experiencing social difficulties associated with deafness. This subject is at present politically charged. On the one hand, the rapid technical improvement in cochlear prostheses leads their developers to advocate use of the devices whenever practical, including for young children. Many members of the deaf community, on the other hand, believe that widespread implantation of cochlear prostheses, particularly in children, will foster a generation of individuals whose ability to communicate is dependent upon technological support of as yet unproved durability. The extensive application of prostheses might also diminish the use of ASL and thus reverse the deaf community's remarkable recent advances. Although this debate will not soon subside, it is worth-while to note the most positive aspect of the issue. A few decades ago there were no widely effective ways of coping with profound deafness; now there are two. Moreover, these solutions are not mutually exclusive; a deaf individual can benefit from bilinguality in spoken English, mediated through a cochlear prosthesis and ASL
As our population ages and as our society becomes more concerned about hearing loss, physicians will increasingly confront patients and families who are experiencing social difficulties associated with deafness. This subject is at present politically charged. On the one hand, the rapid technical improvement in cochlear prostheses leads their developers to advocate use of the devices whenever practical, including for young children. Many members of the deaf community, on the other hand, believe that widespread implantation of cochlear prostheses, particularly in children, will foster a generation of individuals whose ability to communicate is dependent upon technological support of as yet unproved durability. The extensive application of prostheses might also diminish the use of ASL and thus reverse the deaf community's remarkable recent advances. Although this debate will not soon subside, it is worth-while to note the most positive aspect of the issue. A few decades ago there were no widely effective ways of coping with profound deafness; now there are two. Moreover, these solutions are not mutually exclusive; a deaf individual can benefit from bilinguality in spoken English, mediated through a cochlear prosthesis and ASL
The Structure of the Human Ear
The external ear, especially the prominent auricle, focuses sound into the external auditory meatus. Alternating increases and decreases in air pressure vibrate the tympanum. These vibrations are conveyed across the air-filled middle ear by three tiny, linked bones: the malleus, the incus, and the stapes. Vibration of the stapes stimulates the cochlea, the hearing organ of the inner ear.
The Cochlea consists of three fluid-filled compartments
A cross section of the cochlea shows the arrangement of the three ducts. The oval window, against which the stapes pushes in response to sound, communicates with the scala vestibuli. The scala tympani is closed at its base by the round window, a thin, flexible membrane. Between these two compartments lies the scala media, an endolymph-filled tube whose epithelial lining includes the 16,000 hair cells surrounding the basilar membrane.
Motion of the basilar membrane
Sound vibrates the tympanum, which sets the three ossicles of the middle ear in motion. The stapes, a piston-like bone set in the elastic oval window produces oscillatory pressure differences that rapidly propagate along the scala vestibuli and scala tympani. Low-frequency pressure differences are shunted (diverted) through the helicotrema.
The increased pressure in the scala tympani is relieved by outward bowing of the round-window membrane.
The basilar membrane's mechanical properties in fact vary continuously along it's length, oscillatory stimulation by sound causes a traveling wave on the basilar membrane.
Each frequency of stimulation excites maximal motion at a particular position along the basilar membrane. Low-frequency sounds, such as a 100 Hz stimulus, excite basilar-membrane motion near the apex (top) where the membrane is relatively broad and flaccid. Mid-frequency sounds excite the membrane in it's middle. the highest frequencies that we can hear excite the basilar membrane at it's base (bottom). The mapping of sound frequency onto the basilar membrane is approximately logarithmic (constructed so that successive points along an axis, or graduations that are an equal distance apart, represent values that are in an equal ratio).
The basilar membrane performs spectral analysis of complex sounds. In this example a sound with three prominent frequencies (such as the three dominant components of human speech) excites basilar-membrane motion in three regions, each of which represents a particular frequency. Hair cells in the corresponding positions transduce the basilar-membrane oscillations into receptor potentials, which in turn excite the nerve fibers that innervate these particular regions.
The increased pressure in the scala tympani is relieved by outward bowing of the round-window membrane.
The basilar membrane's mechanical properties in fact vary continuously along it's length, oscillatory stimulation by sound causes a traveling wave on the basilar membrane.
Each frequency of stimulation excites maximal motion at a particular position along the basilar membrane. Low-frequency sounds, such as a 100 Hz stimulus, excite basilar-membrane motion near the apex (top) where the membrane is relatively broad and flaccid. Mid-frequency sounds excite the membrane in it's middle. the highest frequencies that we can hear excite the basilar membrane at it's base (bottom). The mapping of sound frequency onto the basilar membrane is approximately logarithmic (constructed so that successive points along an axis, or graduations that are an equal distance apart, represent values that are in an equal ratio).
The basilar membrane performs spectral analysis of complex sounds. In this example a sound with three prominent frequencies (such as the three dominant components of human speech) excites basilar-membrane motion in three regions, each of which represents a particular frequency. Hair cells in the corresponding positions transduce the basilar-membrane oscillations into receptor potentials, which in turn excite the nerve fibers that innervate these particular regions.
Cellular architecture of the organ of Corti in the human cochlea
The inner ear's receptor organ is the organ of Corti, an epithelial strip that surmounts the elastic basilar membrane along it's 33mm spiraling course. The organ contains some 16,000 hair cells arrayed in four rows: a single row of inner hair cells and three of outer hair cells. The mechanically sensitive hair bundles of these receptor cells protrude into endolymph, the fluid contents of the scala media. The hair bundles of outer hair cells are attached at their tops to the lower surface of the tectorial membrane, a gelatinous shelf that extends the full length of the basilar membrane.
Detailed structure of the organ of Corti. The hair bundle of each inner cell is a linear arrangement of the cell's stereocilia, while the hair bundle of each outer hair cell is more elaborate., V-shaped palisade (fixed deeply in a close row) of stereocilia. The hair cells are separated and supported by phalangeal and pillar cells. The diameter of an outer hair cell is approximately 7 micrometers. Empty spaces at the bases of outer hair cells are occupied by efferent nerve endings.
Detailed structure of the organ of Corti. The hair bundle of each inner cell is a linear arrangement of the cell's stereocilia, while the hair bundle of each outer hair cell is more elaborate., V-shaped palisade (fixed deeply in a close row) of stereocilia. The hair cells are separated and supported by phalangeal and pillar cells. The diameter of an outer hair cell is approximately 7 micrometers. Empty spaces at the bases of outer hair cells are occupied by efferent nerve endings.
Hair cells in the cochlea are stimulated when the basilar membrane is driven up and down by differences in the fluid pressure between the scala vestibuli and scala tympani
Because this motion is accompanied by shearing motion between the tectorial membrane and organ of Corti, the hair bundles that link the two are deflected. This deflection initiates mechanoelectrical transduction of the stimulus.
When the basilar membrane is driven upward, shear between the hair cells and the tectorial membrane deflects hair bundles in the excitatory direction, toward their tail edge. At the midpoint of an oscillation the hair bundles resume their resting position. When the basilar membrane moves downward, the hair bundles are driven in the inhibitory direction.
When the basilar membrane is driven upward, shear between the hair cells and the tectorial membrane deflects hair bundles in the excitatory direction, toward their tail edge. At the midpoint of an oscillation the hair bundles resume their resting position. When the basilar membrane moves downward, the hair bundles are driven in the inhibitory direction.
Tuning curves for cochlear hair cells
To construct a curve, the experimenter presents sound at each frequency at increasing amplitudes until the cell produces a criterion response. The curve thus reflects the threshold of the cell for stimulation at a range of frequencies. Each cell is most sensitive to a specific frequcny, its characteristic (or best) frequency. The threshold rises briskly (sensitivity falls abruptly) as the stimulus is raised or lowered
Evoked otoacouostical emissions are evidence for a cochlear amplifier and cochlear amplification is mediated by movement of hair cells
Mechanical amplification of vibrations within the cochlea is an active process that enhances the sensitivity of hearing. When an isolated outer hair cell is depolarized (reduced polarity) by the electrode at its base, its cell body shortens. Hyperpolarization (increased polarity), on the other hand, causes the cell to lengthen. The oscillatory motions of outer hair cells may provide the mechanical energy that amplifies basilar-membrane motion and thus enhances the sensitivity of human hearing.
Innervation of the organ of Corti
The great majority of afferent axons end on inner hair cells, each of which constitutes the sole terminus for an average of 10 axons. A few afferent axons of small caliber provide diffuse innervation to the outer hair cells. Efferent axons largely innervate outer hair cells, and do so directly. In contrast, efferent innervation of inner hair cells is sparse and is predominantly axoaxonic, at the ending of afferent nerve fibers.
The Firing Pattern in auditory nerve fibers has phasic and tonic components
An auditory nerve fiber was stimulated with tone bursts at about 5000 kHz (the characteristic frequency of the cell) lasting approximately 250 ms. The stimulus was followed by a quiet period, then was repeated, over a period of 2 minutes. Histograms show the average response patterns of the fiber to tone bursts as a function of stimulus level. The entire sample period is divided into a number of small time units and the number of spikes is displayed. There is an initiial, phasic increase in firing correlated with the onset of the stimulus. following adaptation, discharge continues during the remainder of the stimulus; a decrease in activity follows termination. This pattern is evident when the stimulus is more than 20dB above threshold. There is a gradual return to baseline activity during the interstimulus interval.
The central auditory pathways extend from the cochlear nucleus to the auditory cortex
Postsynaptic neurons in the cochlear nucleus send their axons to other centers in the brain via three main pathways: the dorsal acoustic stria, the intermediate acoustic stria and the trapezoid body. The first binaural interactions occur in the superior olivary nucleus, which receives input via the trapezoid body. In particular, the medial and lateral divisions of the superior olivary nucleus are involved in the localization of sounds in space. Postsynaptic axons from the superior olivary nucleus, along with axons from the cochlear nuclei, project to the inferior colliculus in the midbrain via the lateral lemniscus. Each lateral lemniscus contains axons relaying input from both ears. Cells in the colliculus send their axons to the medial geniculate nucleus of the thalamus. The geniculate axons terminate in the primary auditory cortex (Brodmann's areas 41 and 42), a part of the superior temporal gyrus.
The representation of stimulus frequency in the three divisions of the cochlear nucleus
Stimulation with three frequencies of sound vibrates the basilar membrane at three positions, exciting distinct populations of afferent nerve fibers. the fibers project to the components of the cochlear nucleus in an orderly pattern.