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It is generally accepted that the acute sensitivity and frequency discrimination of mammalian hearing requires active mechanical amplification of the sound stimulus within the cochlea1. The prevailing hypothesis is that this amplification stems from somatic electromotility of the outer hair cells attributable to the motor protein prestin2,3. Thus outer hair cells contract and elongate in synchrony with the sound-evoked receptor potential4,5. But problems arise with this mechanism at high frequencies, where the periodic component of the receptor potential will be attenuated by the membrane time constant. On the basis of work in non-mammalian vertebrates, force generation by the hair bundles has been proposed as an alternative means of boosting the mechanical stimulus6,7. Here we show that hair bundles of mammalian outer hair cells can also produce force on a submillisecond timescale linked to adaptation of the mechanotransducer channels. Because the bundle motor may ultimately be limited by the deactivation rate of the channels, it could theoretically operate at high frequencies. Our results show the existence of another force generator in outer hair cells that may participate in cochlear amplification.