Adaptation in hair cells.

Hair cells adapt to sustained deflections of the hair bundle via Ca(2+)-dependent negative feedback on the open probability of the mechanosensitive transduction channels. A model posits that adaptation relieves the input to the transduction channels--force applied by elastic tip links between stereocilia--by repositioning the insertions of the links in the stereocilium. The tip link insertion and transduction channel are dragged by myosins moving on the stereocilium's actin core. This model accounts for many aspects of adaptation in hair cells of the frog saccule, where adaptation time constants are tens of milliseconds. Adaptation in hair cells of the turtle cochlea is much faster, possibly reflecting a more direct mechanism such as Ca2+ binding to the transduction channel. Adaptation mechanisms attenuate the transduction current at low frequencies and may be tuned to different corner frequencies according to the stimulus demands of the inner ear organ. Other sites of adaptation in the inner ear include accessory structures, voltage-dependent properties of hair cells, and afferent transmitter release. A remaining challenge is to understand how these processes work together to shape the output of the inner ear to natural stimuli.