The outer hair cell is a critical factor for the fine tuning capabilities and the wide dynamic range of the mammalian ear. This cell has an unusual motility which is dependent on the membrane potential and not on adenosine triphosphate (ATP) or calcium ion. To clarify the motile mechanism, both motile and passive properties of the cell were studied. Deformations of the cell due to pressure are explained by a simple membrane model based on mechanical isotropy. Our recent results on axial stiffness show that the cell membrane is indeed not too far from a mechanical isotropy. The membrane has a viscoelastic relaxation with a characteristic time constant of about 40 seconds. The motility is associated with the membrane capacitance of the cell that is dependent on membrane tension as well as membrane potential. The source of this capacitance is the transfer of a charge associated with the motor across the membrane. These observations reveal that this motility is based on a membrane motor which directly utilizes electric energy. It was shown that the mechanical change in the motor is primarily in its membrane area and not in its elasticity . While the basic feature of the motor is explained by two states, finer details of capacitance changes require three states. These observations on the motor and the mechanical characteristics of the outer hair cell are incorporated into a theoretical model, which predicts force generation of 0.1 nN/mV for a single cell. Our recent data support this prediction and this value is also consistent with estimates obtained from in vivo experiments. All of these results indicate that the essential elements of the mechanism for this unique cellular motility are being captured. Further efforts are important for clarifying this motility and identifying the role of the outer hair cell in hearing.
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