Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochl ea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated a nd spirally wound basilar membrane (BM). As they travel, waves grow in ampl itude, reaching a maximum and then dying out. The location of maximum BM mo tion is a function of stimulus frequency, with high-frequency waves being l ocalized to the "base" of the cochlea (near the stapes) and low-frequency w aves approaching the "apex" of the cochlea. Thus each cochlear site has a c haracteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduc tion channels leading to the generation of hair cell receptor potentials an d the excitation of afferent auditory nerve fibers. At the base of the coch lea, BM motion exhibits a CF-specific and level-dependent compressive nonli nearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive a nd poorly tuned. The high sensitivity and sharp-frequency tuning, as well a s compression and other nonlinearities (two-tone suppression and intermodul ation distortion), are highly labile, indicating the presence in normal coc hleae of a positive feedback from the organ of Corti, the "cochlear amplifi er." This mechanism involves forces generated by the outer hair cells and c ontrolled, directly or indirectly, by their transduction currents. At the a pex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in d etermining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency -specific properties of the responses to sound of auditory nerve fibers.