STIFFNESS, COMPLIANCE, ELASTICITY AND FORCE GENERATION OF OUTER HAIR-CELLS

Isolated outer hair cells (OHCs) were partially sucked into especially designed cell capillaries allowing an experimental reconstitution of the cells' electroanatomy. The experimental approach separated the apical from the basolateral parts of the cells thus forming an artificial scala media and scala tympani. Resistance between both was 121 +/- 42 M-OMEGA. A sequence of negative and positive pressures was applied to the basal cell pole allowing "pulling" or "pushing" of the sensory cell investigated. The resulting length changes together with the known pressures allowed the estimation of an actual longitudinal compliance of 354 +/- 35 m/N. Following "pulling" OHCs tended to resume their initial shape after the force had ceased to be effective indicating elastic distortions. The calculated elasticity modulus of OHCs amounted to 6.1 +/- 3.4 kN/m2. From this data an actual longitudinal whole cell stiffness of OHCs of 3 x 10(-3) N/m was calculated. Ultrasound scanning of immobilized OHCs identified the cuticular plate (CP) and a central core between CP and basal cell pole as structures contributing to the cells' acoustic stiffness. Changes ot' the potential differences between the artificial scala media and scala tympani resulted in active length changes following the command voltage with a slope of DELTA-1/(1 x U) = 0.055 V-1. Assuming the validity of Hooke's law, the force generation associated with the active length changes can be calculated since the compliance is known. We calculated a force of 14 pN, an energy stored in the elastic cell of 3 x 10(-20) J. Assuming that 50-60 jointly innervated OHCs act as a motor unit. the resulting force amounts to 700 pN/nm. Interestingly, Gummer et al. (1981 ) measured a BM stiffness of 480 pN/nm. Thus, the force exerted by 50 OHCs seems to be sufficiently effective to induce an in situ movement.