Figure 2.

(a) These images illustrate that the resting membrane potential of Paramecium is negative (about −25 to −40 mV); the corresponding ciliary beat is toward the posterior of the cell and the cell swims forward. (b) In depolarizing solutions, such as high K⁺ or Ba²⁺, the cell’s plasma membrane depolarizes and reaches threshold for the action potential. During the action potential, Ca²⁺ enters the cilia through voltage-gated channels; the high levels of Ca²⁺ change the power stroke of the cilia, which now beat most strongly toward the anterior and move the cell backward. The action potential is quickly terminated and the Ca²⁺ removed from or sequestered in the cilia, allowing ciliary beat and swimming to return to normal. From [94] Figure 1; used with permission.