Asymmetry in voltage-dependent movements of isolated outer hair cells from the organ of Corti

The electrically induced movements of outer hair cells (OHC) were studied using the whole-cell voltage-clamp technique and video analysis. Cell shortening occurs during depolarization and elongation occurs during hyperpolarization from holding potentials near -70 mV. However, a marked asymmetry in response magnitude exists such that depolarization produces larger cell length changes than do comparable levels of hyperpolarization. The response is such that at normal resting potentials in vivo, displacements are about 2 nm/mV, but increase to about 15 nm/mV as the cell is depolarized. This mechanical rectification in the depolarizing direction manifests itself during symmetrical sinusoidal voltage stimulation as a "DC" reduction in cell length superimposed upon "AC" length changes. The observed OHC mechanical rectification may be involved in the reported production of "DC" basilar membrane displacements during suprathreshold acoustic stimulation (LePage, 1987). Estimates of the magnitude of OHC movements at acoustic threshold levels induced by receptor potentials in the high-frequency region of the cochlea indicate a disparity between basilar membrane and OHC movements on the order of 21 dB. Thus, it appears questionable whether OHC mechanical movements solely underlie the "active process" thought to be responsible for the high degree of neural tuning at sound pressures near 0 dB.