Characterization of adaptation motors in saccular hair cells by fluctuation analysis.

The mechanical sensitivity of hair cells, the sensory receptors of the vestibular and auditory systems, is maintained by adaptation, which resets the transducer to cancel the effects of static stimuli. Adaptation motors in hair cells can be experimentally activated by externally applying a transduction channel blocker to the hair bundle, causing the hair bundle to move in the negative direction. We studied the variance in the position of the hair bundle during these displacements and found that it increases as the bundle moves to its new position. Often the variance peaks, and then declines to a steady-state value. We describe both displacement and variance with a model in which a motor acting on the bundle takes approximately 3.6-nm steps whose frequency (approximately 22 s(-1)) declines with the motor's load.