The Strain Ratio of the Outer Hair Cell Motor Protein

The isolated cochlea outer hair cell (OHC), when depolarized, contracts along its length and dilates diametrically. This behavior is consistent with an increase in internal pressure under the constraint of constant volume. In contrast, when an isolated OHC in the microchamber is electrically stimulated, one segment contracts and the other extends. Further, the diameters dilate and shrink respectively, which suggests antiphasic segmental pressure changes, an untenable proposition. An alternative explanation is that the motility motor has a dual action. On depolarization, it generates negative axial strain and positive radial strain. But what is the relationship between the strains? Concurrent length and diameter change measurements were made in isolated OHCs in the microchamber for cells of a range of lengths and at varying extrusion factors (q). The ratios of the radial to axial strains in each segment were calculated as an estimate of the motor strain. The results suggest that the ratio of the radial and axial motor strains is -0.5. A model of OHC mechanics implies that this ratio is close to ideal for maintaining constant volume in each segment, independent of events in the other segment. The result is reasonably independent of the passive membrane properties of OHCs for a range of values around previous estimates.