To establish the magnitude of the electromechanical force generated by the outer hair cell from the mammalian cochlea, we used a cylindrical membrane model characterized by area and shear moduli for a passive elastic element, and a membrane potential-dependent, active tension-generating element. We measured pressure-strain relations to determine the elastic moduli, and found the area modulus was both close to a lipid bilayer and an order of magnitude larger than the shear modulus. We also determined that the active tension element is nearly isotropic, with an amplitude sensitivity of about 2x10~2 Nm-1V-1. At 73 dB of acoustical stimulation, the active force generated per outer hair cell is about 0.6 nN--close to the force applied to a corresponding area of the basilar membrane by acoustic pressure. This finding supports the hypothesis that the outer hair cell acts as a feedback motor in the fine-tuning mechanism of the mammalian ear. A model has been developed to study outer hair-cell feedback in waves on the cochlear partitions and the resulting sharpening mechanism.
