Receptor potential of outer hair cells isolated from base to apex of the adult guinea-pig cochlea: Implications for cochlear tuning mechanisms

If the electromotility of outer hair cells (OHC) is to reduce the mechanical impedance of the cochlear partition, it must not only deliver a force of sufficient magnitude, but the force must be exerted at the correct moment in the stimulus cycle. The amplitude and phase of the receptor potential of OHCs in response to direct mechanical stimulation of the stereocilia bundle were measured for cells isolated from along the entire length of the adult guinea-pig cochlea. Irrespective of their place of origin, the frequency response of the receptor potential, for 20-nm stereociliary displacement per spectral point, was governed by the OHC electrical input impedance measured near the resting potential. The response was a low-pass filter with amplitude that decreased by 6 dB/oct and phase that lagged stereocilia displacement by 90 degrees. The corner frequency of the response decreased exponentially from 546 Hz for the shortest OHC (20 mu m) with an exponential length constant of 25 mu m, or equivalently 0.58 oct per 10 mu m increase of cell length. The tonotopy was achieved by an exponential decrease in the total specific conductance with increasing cell length, beginning at 66 pS/mu m(2) for the shortest OHC; the specific capacitance was constant (2.0 mu F/cm(2)). The corner frequency was 3-6 oct below the presumed place-frequency.