Cochlear Receptors

The cochlea receives dual innervation: afferent fibers, which originate from cell bodies in the adjacent spiral ganglion efferent fibers, which originate in the brainstem. Both types of fibers form synapses with sensory hair cells in the spiral organ of Corti (experimental studies have shown that activity in the efferent fibers can inhibit the discharge of cochlear afferent fibers). At the center of the organ of Corti is the tunnel of Corti, flanked by two sets of supporting rods of Corti (pillar cells). When hair cells are activated, impulse transmission is triggered in fibers of the spiral ganglion. The fibers then enter the brainstem as the cochlear nerve.

Hearing. The stapes ossicle bone transmits vibrations to the oval window on the outside of the cochlea. The perilymph vibrates in the scala vestibuli toward the helicotrema. Within the scala media is the receptor organ, the organ of Corti, which rests on top of the basilar membrane. The vibrations spread through the cochlea and induce vibrations in the basilar membrane, which are then transduced into afferent nerve excitation by the hair cells. The hair cells are arranged in inner and outer groups, and each cell is capped with 50 to 100 hairlike stereocilia that are imbedded in the tectorial membrane. The inner hair cells, about 3,500 in number, are arranged in a single row on the inner side of the inner rods of Corti; the 12,000 outer hair cells are longer and are arranged in three rows in the basal coil of the cochlea, and in four or five rows in the apical coil. Physiologic studies suggest that cochlear hair cells behave like their vestibular counterparts; bending of stereocilia in one direction leads to a depolarization of hair cells and an accelerated rate of nerve discharge, while bending in the opposite direction produces hyperpolarization and a slowing of discharge.

Another type of frequency analysis is based on the differences in the shape and stiffness of the basilar membrane located between the base and the apex of the cochlea. The basilar membrane vibrates to high frequencies at the base of the cochlea, where the basilar membrane is thinner and narrower, and to low frequencies at the apex. The dimensions of the basilar membrane gradually changes, so that for each vibration frequency between the two extremes, a somewhat restricted region of the membrane, and hence a certain group of afferent fibers, responds most vigorously. The cochlea is therefore said to be tonotopically organized: each afferent fiber will respond to some extent to a range of frequencies, while within the range is one frequency to which it will respond most readily.

Deafness. The cochlea is often the source of deafness, either to a specific pitch or to a broad range of frequencies. Head trauma can produce transient deafness, but severe injury involving a fracture of the petrous part of the temporal bone can cause permanent spiral ganglion or cochlear damage. Intense noise can cause temporary deafness; if it is sustained, permanent cochlear damage will result. The most common cause of deafness in adult life is otosclerosis, a non-neural process that results in the fixation of the stapes to the oval window.

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