Displacement-clamp measurement of the forces exerted by gating springs in the hair bundle.

Mechanical stimuli applied to the hair bundle of a hair cell are communicated to the transduction channels by gating springs, elastic elements that are stretched when the bundle is displaced toward its tall edge. To quantify the magnitude and time dependence of the forces exerted by gating springs, we have developed a displacement-clamp system that constrains a bundle's motion while measuring the forces that the bundle produces during adaptation to mechanical stimuli, in response to channel blockage, and upon destruction of the gating springs. Our results suggest that each gating spring exerts a tension of approximately 8 pN in the resting bundle and can sustain at least 4-13 pN of additional tension. The experiments provide further evidence that the gating springs account for at least one-third of the hair bundle's dynamic stiffness and that a force of approximately 100 fN is sufficient to open a single transduction channel.