The outer hair cell is a critical factor for fine tuning and for the wide dynamic range of the mammalian ear. We have demonstrated earlier that this cell has an unusual membrane motor which directly uses electrical energy available at the plasma membrane. Charges transferable across the membrane enable the motor to obtain electrical energy, and coupling between charge transfers and changes in the membrane area enables conversion into mechanical energy. Such charge transfers are observed as an excess membrane capacitance. A direct energy conversion mechanism requires that the membrane capacitance be dependent on both the membrane potential and membrane tension. That indeed is the case. We are studying electrical and mechanical properties of the outer hair cell to clarify details of this novel motile mechanism. We recently found that turgor pressure of the cell does not affect the amplitude of the voltage-dependent motility. This observation indicates that the motile mechanism is indeed based on membrane area changes as we had assumed earlier. Deformations of the cell can be explained by membrane elasticity for the time scale that corresponds to the auditory frequencies. We have been trying to incorporate the motor and the mechanical characteristics of the outer hair cell into theoretical models. Our updated model predicts force generation of 0.1 nN/mV for a single cell, agreeing with our experiment. Although we recently found that the basal part of the cell is softer than the rest of the cell, we found that such inhomogeneity does not affect our earlier force and stiffness measurements. In addition to the unique energy source, the high speed of the response, which apparently exceeds 10 kHz, characterizes this motility. We recently formulated a theory predicting that the intrinsic speed of the membrane motor can be determined by the frequency dependence of the membrane capacitance and spectral strength of membrane noise due to flipping of motor charges. We are currently examining these features experimentally. These efforts are aimed at further clarification of the mechanism of outer hair cell motility and its biological role.
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