MECHANICAL-PROPERTIES OF THE LATERAL CORTEX OF MAMMALIAN AUDITORY OUTER HAIR-CELLS

Mammalian auditory outer hair cells generate high-frequency mechanical forces that enhance sound-induced displacements of the basilar membra ne within the inner ear. It has been proposed that the resulting cell deformation is directed along the longitudinal axis of the cell by the cortical cytoskeleton. We have tested this proposal by making direct mechanical measurements on outer hair cells. The resultant stiffness m odulus along the axis of whole dissociated cells was 3 x 10(-3) N/m, c onsistent with previously published values. The resultant axial and ci rcumferential stiffness moduli for the cortical lattice were 5 x 10(-4 ) N/m and 3 x 10(-3) N/m, respectively. Thus the cortical lattice is a highly orthotropic structure. Its axial stiffness is small compared w ith that of the intact cell, but its circumferential stiffness is with in the same order of magnitude. These measurements support the theory that the cortical cytoskeleton directs electrically driven length chan ges along the longitudinal axis of the cell. The Young's modulus of th e circumferential filamentous components of the lattice were calculate d to be 1 x 10(7) N/m(2). The axial cross-links, believed to be a form of spectrin, were calculated to have a Young's modulus of 3 x 10(6) N /m(2). Based on the measured values for the lattice and intact cell co rtex, an estimate for the resultant stiffness modulus of the plasma me mbrane was estimated to be on the order of 10(-3) N/m. Thus, the plasm a membrane appears to be relatively stiff and may be the dominant cont ributor to the axial stiffness of the intact cell.