The inner ear, located in the temporal bone of the skull, is a complex structure serving both hearing and balance. Figures 2B-2D illustrate in detail the structures that make up that portion involved in hearing known as the cochlea (Fig. 2A). The cochlea is a coiled, tapered, and fluid-filled chamber divided along almost its entire length by a membranous partition. The two spaces thus formed, the scala vestibuli and scala tympani, are filled with a fluid called perilymph. The two scalae communicate with each other through an opening at the top (apex) of the cochlea called the helicotrema. At the base of the cochlea each scala terminates at a membrane that faces the middle ear cavity. The scala vestibuli ends at the oval window into which the footplate of the stapes rocks when the ear drum moves; the scala tympani ends at the round window, a structure that provides a pressure relief for movement of the cochlear fluid. The partition that divides the cochlea lengthwise is a fluid-filled tube called the scala media or cochlear duct. Its fluid, endolymph, is chemically different from perilymph. The cochlear duct is bounded on three sides by a bed of capillaries and secretory cells (the stria vascu-laris), a layer of simple squamous epithelial cells (Reissner's membrane), and the basilar membrane, on which rests the receptor organ for hearing—the organ of Corti.
The organ of Corti contains the auditory receptor cells (hair cells) and a system of supporting cells that hold them in place. Hair cells are modified epithelial cells with hairs (stereocilia) protruding from their apical ends (Fig. 2D). Figure 3 is a drawing of a stereotypic ear hair cell. A kinocilium, which seems to play no active role in the transduction process, is seen throughout life in the vestibular epithelium but is no longer present in the adult cochlea. Within the organ of Corti two kinds of hair cell—inner (IHC) and outer (OHC)—are distinguishable by location, morphology, and connections with the auditory nerve. Approximately 3500 IHCs form a single linear array, from base to apex (Fig. 2D). About 12,000 OHCs are arranged in three or four rows, parallel to the IHCs (Fig. 2D). Approximately 100-150 stereocilia form a V or W pattern on each OHC, whereas approximately 40-50 stereocilia, arranged in a U shape, adorn each IHC. Displacement of stereocilia is the adequate stimulus for generating receptor currents in hair cells that eventually lead to action potentials in auditory nerve axons.
Spiral ganglion cells, located in the bony core of the cochlea, are the bipolar first-order neurons of the auditory pathway. Their distal processes make synap-tic contact with the base of hair cells. The central axons, which in the human number approximately 35,000, form each auditory nerve bundle. The majority (~95%) of the central processes of spiral ganglion neurons originate at the base of IHCs. Thus, axons connected to IHCs transmit to the brain trains ofnerve impulses that encode essentially all the acoustic information eventually perceived by a listener. The innervation is highly focused: Each auditory nerve is connected to one IHC, whereas each IHC is innervated
Afferent Nerve Fiber
Afferent Nerve Fiber
Efferent Nerve Fiber
Kinocilium i=> Excitation
Efferent Nerve Fiber
Figure 3 Stylized hair cell of the vertebrate inner ear (left). Deflection of stereocilia toward the kinocilium and basal body results in hair cell depolarization and excitation of auditory nerve fibers, whereas movement in the opposite direction leads to hyperpolarization and inhibition. Cross section of hair bundle is shown at the right (adapted with permission from Geisler, C.D, From Sound to Synapse. Oxford Univ. Press, New York, 1998).
by no more than approximately 10-20 spiral ganglion cells. The remaining 5% of axons of the auditory nerve are efferent fibers arising from cells in and around the superior olivary complex of the brain stem. Upon entering the organ of Corti they branch profusely, with each axon reaching many OHCs over considerable distance.
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