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Abstract

The mammalian auditory organs have a dual sensory system (inner vs. outer hair cells) with distinctly different cellular organizations and innervation patterns. However, the inner (IHCs) and outer (OHCs) hair cells are mechanoreceptors sharing similar general characteristics such as organization of stereocilia (including linkage system) and a gradation of stereociliary height along the length of the cochlea. This gradation of stereociliary height may be the single most important anatomic feature in the tuning capability of the sensory cell. Several lines of evidence suggest that the stereociliary stiffness may be modulated by the sensory cells themselves, most likely via the cuticular plate-rootlet complex.The stereociliary bundles of both types of hair cell are organized in a ‘W’ formation with a steplike arrangment. In the OHCs, the ‘W’ formation is sharply angulated and slanted toward the apex, coinciding with the slanted fiber arrangment of the overlying tectorial membrane, which is firmly coupled to the tips of the tallest row of the stereociliary bundles. However, in the IHCs, the ‘W’ formation is wide and its long axis is linear and arranged at a right angle to the radial axis of the organ of Corti; also, the ciliary bundles are freestanding (with a few exceptions in the basal turn). This arrangement in the IHCs would be best suited for deflection by the radial flow of the endolymph. Present evidence suggests that the subtectorial fluid space exists, is filled with endolymph, and freely communicates with endolymph.Because of the discovery of the phenomenon of ‘cochlear emission’, the possible motility of the sensory cells, particularly of the OHCs, has drawn intense interest in recent years. Recent investigations with dissociated sensory cells (OHCs) indicate some motile capability under various experimental conditions, although it has not been established that this motility is present in vivo. For this reason, the specialized cellular organization for motility and localization of contractile and cytoskeletal proteins have been investigated. These results support the possibility that the OHCs may have cellular facilities for this function.The most striking cellular features of the OHCs that distinguish them from the IHCs are the cell shape (cylindrical in OHCs vs. wine-bottle shaped in IHCs), endoplasmic reticular organization (well developed in OHCs vs. poorly developed in IHCs), extra cell wall membrane (well developed in the OHCs vs. poorly developed or absent in IHCs), mitochondrial organization (closely associated with ER in OHCs vs. dispersed in IHCs), and specialization of postsynaptic apparatus (well-developed subsynaptic cistern in OHCs vs. poorly developed subsynaptic cistern in IHCs). The well-developed ER system in the OHCs resembles the sarcoplasmic reticulum of the muscles. The ER system of the OHCs includes apical cistern, subsurface cistern, Hensen's body, and subsynaptic cistern, and these structures are interconnected. Owing to this connection, and because the subsynaptic cistern directly apposes the efferent nerve endings, the ER system of the OHCs is under efferent neural influence. The nerve ending sides of both cells have accumulations of neurotransmitter vesicles that are being released and recycled via exo- and endocytosis. This finding implies that both sensory cells are capable of transmitting afferent neural impulses via neurotransmitter release.The current morphological evidence is compatible with the concept that the IHCs are passive mechanoreceptors and the OHCs are bidirectional mechanoreceptors that can be passive as well as active.