Response phase: a view from the inner hair cell.

Inner hair cell (IHC) responses are recorded from the apical three turns of the guinea pig cochlea in order to define the relationship between hair cell depolarization and position of the basilar membrane. At low frequencies, inner hair cell depolarization is generally observed near basilar membrane velocity to scala vestibuli, reflecting the putative freestanding nature of the IHC's stereocilia. While this is consistent with previous IHC results, independent of location, and with neural responses for fibers with low best frequencies, it is inconsistent with single-unit results from the base of the cochlea, where response phase is associated with basilar membrane velocity to scala tympani. Results suggest that the temporal disparity between IHC and neural data from the base of the cochlea may relate to several factors that influence transmembrane voltage in IHCs. First, extracellular voltages (Ingvarsson, 1981; Sellick et al., 1982; Russell and Sellick, 1983) can potentially affect low- and high-frequency regions differently because electrical interactions are more likely in the base of the cochlea than in the apex (Dallos, 1983, 1985). Second, waveform distortion and kinetic properties associated with voltage-dependent ion channels in the IHC's basolateral membrane can both influence response phase by adding harmonic components and lagging the receptor potential by as much as 90 deg. Third, the velocity dependence of IHCs in the apex appears to extend to higher frequencies than the velocity dependence demonstrated for IHCs in the base of the cochlea. These features, which influence the timing of discharges in the auditory nerve, are compared and evaluated.