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Friction from Ion Channels’ Gating on Vibrating Hair-Cell Bundles from the Inner Ear
Hearing starts when sound-evoked mechanical vibrations of the hair-cell bundle activate mechanosensitive ion channels, giving birth to an electrical signal As for any mechanical system, friction impedes movements of the hair bundle and thus constrains the sensitivity and frequency selectivity of auditory transduction. We characterized friction by analyzing hysteresis in the force-displacement relation of single hair-cell bundles in response to periodic triangular stimuli. We found that the opening and closing of the transduction channels produce internal frictional forces that can dominate viscous drag on the micron-sized hair bundle. A theoretical analysis reveals that channel friction arises from coupling the dynamics of the conformational change associated with channel gating to tension in the tip links that interconnect neighboring stereocilia of the hair bundle. As a result, varying channel properties affects friction, with faster channels producing smaller friction. Friction originating from gating of ion channels is a general concept that is relevant to all mechanosensitive channels. In the context of hearing, this intrinsic source of friction may contribute to the process that sets the hair cell’s characteristic frequency of responsiveness.