AXIAL AND TRANSVERSE STIFFNESS MEASURES OF COCHLEAR OUTER HAIR-CELLS SUGGEST A COMMON MECHANICAL BASIS

The function of the hearing organ is based on mechanical processes occ urring at the cellular level. The mechanical properties of guinea-pig isolated sensory cells were investigated using two different technique s. The stiffness of the outer hair cells along the longitudinal axis w as measured by compressing the cell body using stiffness-calibrated qu artz fibres. For cells with a mean length of 69 mu m, the mean axial c ompression stiffness was 1.1+/-0.8 mN/m (+/-SD). There was an inverse relation between stiffness and cell length. The stiffness of the cell membrane perpendicular to the longitudinal axis of the sensory cell wa s measured by indenting the cell membrane with a known force. The mean lateral indentation stiffness was 3.3+/-1.5 mN/m (+/-SD) for cells wi th a mean length of 64 mu m. Longer cells were less stiff than short c ells. Modelling the hair cell as a shell with bending resistance, fini te element calculations demonstrated that the axial compression stiffn ess correlated well with the lateral indentation stiffness, and that a simple isotropic model is sufficient to explain the experimental obse rvations despite the different stress strain states produced by the tw o techniques. The results imply that the two different stiffness prope rties may originate from the same cytoskeletal structures. It is sugge sted that the mechanical properties of the outer hair cells are design ed to influence the sound-induced motion of the reticular lamina. In s uch a system, stiffness changes of the outer hair cell bodies could ac tively control the efficiency of the mechanical coupling between the b asilar membrane and the important mechanoelectrical transduction sites at the surface of the hearing organ.