Regular steps in bending cilia during the effective stroke.

A ciliary beat cycle consists of an effective stroke in which the extended cilium makes an oar-like movement towards one side, and a recovery stroke in which the cilium moves back by propagating a bend from base to tip (Fig. 1A). In the sliding microtubule model of ciliary and flagellar movement, which is now supported by substantial evidence1-3, the sliding displacement of microtubules in any region of the cilium is related linearly to the angular change in the direction of that region (Fig. 1B). Thus, during the effective stroke, microtubule sliding is not confined to the region near the base of the cilium but involves the whole length of the extended organelle, and the relative speed of sliding can be measured as the angular velocity of the ciliary motion. I report here that in molluscan cilia the effective stroke consists of regularly alternating rapid and slow phases of angular movement. This suggests that the microtubules slide in quantal steps.