High-frequency motility of outer hair cells and the cochlear amplifier

Outer hair cells undergo somatic elongation-contraction cycles in vitro when electrically stimulated. This "electromotile" response is assumed to underlie the high sensitivity and frequency selectivity of amplification in the mammalian cochlea. This process, presumably operating on a cycle-by-cycle basis at the frequency of the stimulus, is believed to provide mechanical feedback in vivo. However, if driven by the receptor potential of the cell, the mechanical feedback is expected to be severely attenuated at high frequencies because of electrical low-pass filtering by the outer hair cell basolateral membrane. It is proposed that electromotility at high frequencies is driven instead by extracellular potential gradients across the hair cell, and it is shown that this driving voltage is not subject to low-pass filtering and is sufficiently large. It is further shown that if the filtering properties of the cell membrane are canceled, taking advantage of the electrical characteristics of isolated outer hair cells in a partitioning glass microchamber, then the lower bound of the motor's bandwidth is approximately 22 kilohertz, a number determined only by the limitations of our instrumentation.