Mechanical relaxation of the hair bundle mediates adaptation in mechanoelectrical transduction by the bullfrog's saccular hair cell.

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Abstract
Mechanoelectrical transduction by hair cells of the frog''s internal ear displays adaptation: the electrical response to a maintained deflection of the hair bundle declines over a period of tens of milliseconds. We investigated the role of mechanics in adaptation by measuring changes in hair-bundle stiffness following the application of force stimuli. Following step stimulation with a glass fiber, the hair bundle of a saccular hair cell intially had a stiffness of .apprxeq. 1 mN .cntdot. m-1. The stiffness then declined to a steady-state level near 0.6 mN .cntdot. m-1 with a time course comparable to that of adaptation in the receptor current. The hair bundle may be modeled as the parallel combination of a spring, which represents the rotational stiffness of the stereocilia, and a series spring and dashpot, which respectively, represent the elastic element responsible for channel gating and the apparatus for adaptation.