FIGURE 3.

These images illustrate (A) that the resting membrane potential of Paramecium is negative; the ciliary beat is toward the posterior and cell swims forward. (B) In depolarizing conditions, such as high K or Ba solutions, the cell’s membrane depolarizes and reaches threshold for the action potential, during which Ca²⁺ enters the cilium through Ca_V channels and the Ca²⁺ changes the power stroke toward the anterior, moving the cell backward. The action potential is quickly terminated, returned to resting V_m levels, and the extra Ca²⁺ removed (Kung et al., 1975).