FIGURE 4.

Simulation of hair cell mechanotransduction: single channel activations. A hair cell with 42 transduction channels was subjected to a step force of 50 pN. The bundle tip was displaced by 30 nm. (Right top) Top view of the bundle shows the channel identification numbers in the circles indicating stereocilia. Analogous to the glass fiber stimulation, the force is applied at the taller edge of the bundle. (The three arrows indicate the location of applied force acting on the tips of the cilia.) (Left top) Each of 42 channels opened and closed individually according to the prescribed gating criteria. The channels in the tall stereocilia opened first, and the activation propagated toward the short stereocilia. Channels 1 and 2 opened at the very beginning of stimulation and stayed open. Channel 3 opened and closed around 0.5 ms. Only four channels were not activated during the simulation, and they were mostly peripheral channels (Channels 4, 17, 26, and 35). (Right center) The hair cell activation level, analogous to the whole cell current, I, is shown. Even though most channels were activated once during the simulation period of 2 ms, the maximum number of channels open at the same time was 20 (I = 20/42 = 0.48). Five channels remained open after the fast adaptation, and those unclosed channels were near the taller edge where the force was applied. (Bottom) Sequential plots of channel activations are shown with a 0.1-ms interval. Darker circles indicate the stereocilia with the open transduction channels. The stereocilia marked with ‘N’ are those without a transduction channel. The channel activations propagate from the taller edge toward the shorter edge.