Ciliary Movement and Its Control in Paramecium

A cilium of Paramecium exhibits movement in three dimensions. In the effective stroke phase of the beat cycle, ending of the cilium occurs near the basal region, while the rest of the cilium is more or less straight. Thus, the cilium, which stands straight upon the cell surface, leaning slightly toward the back at the beginning of the effective stroke, moves much like a pole, first standing straight upon the ground, then falling down. The movement continues until the cilium comes to lie nearly parallel to the cell surface. In the subsequent recovery stroke phase the cilium gyrates counterclockwise (as seen from above) while standing up, until it returns to the starting position for the beginning of the next effective stroke. The viscous counterforce experienced by a moving cilium of whip-like shape is smaller than that experienced by a straight cilium. The surrounding medium is consequently driven by the cilium in the approximate direction toward which the cilium points at the end of the effective stroke. This is called "the direction of the effective stroke" or simply "the beat direction of the cilium".