HAIR-BUNDLE STIFFNESS DOMINATES THE ELASTIC REACTANCE TO OTOLITHIC-MEMBRANE SHEAR

Efficient transduction by acousticolateralis organs requires that a st imulus force principally deflect hair bundles, rather than flex other structural elements. Hair bundles might therefore be expected to provi de a large fraction of the impedence to shear motions of otolithic mem branes and other accessory structures. We measured the stiffness for s hear motions of the bullfrog's saccular otolithic membrane, and determ ined the stiffness due to a single hair bundle and its associated extr acellular filaments; this component is termed the elemental stiffness. Stiffness measurements were made by displacing the base of a flexible probe whose tip was coupled to the otolithic membrane, and simultaneo usly measuring the flexion of the probe and the displacement of the me mbrane. The average elemental stiffness, about 1350 muN . m-1, only mo destly exceeded the stiffness of individual hair bundles. The hair bun dles therefore provide the dominant component of stiffness in the bull frog's sacculus, and thus account for a significant component of imped ance to otolithic-membrane shear. As a corollary, stiffness changes or active movements in hair bundles should influence the mechanical resp onses of this and other receptor organs.