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* Department of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts; and Department of Neurobiology and Howard Hughes Medical Institute, Harvard Medical School, Boston, Massachusetts
Correspondence: Address reprint requests to David P. Corey, Dept. of Neurobiology, Harvard Medical School, 220 Longwood Ave., Boston, MA 02115. Tel.: 617-432-2506; E-mail: dcorey{at}hms.harvard.edu.
The mechanically gated transduction channels of vertebrate hair cells tend to close in 
1 ms after their activation by hair bundle deflection. This fast adaptation is correlated with a quick negative movement of the bundle (a "twitch"), which can exert force and may mediate an active mechanical amplification of sound stimuli in hearing organs. We used an optical trap to deflect bullfrog hair bundles and to measure bundle movement while controlling Ca2+ entry with a voltage clamp. The twitch elicited by repolarization of the cell varied with force applied to the bundle, going to zero where channels were all open or closed. The force dependence is quantitatively consistent with a model in which a Ca2+-bound channel requires 3 pN more force to open, and rules out other models for the site of Ca2+ action. In addition, we characterized a faster, voltage-dependent "flick", which requires intact tip links but not current through transduction channels.
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