Electrical Mechanisms Controlling Locomotion in the Ciliated Protozoa

Abstract

The direction of swimming (forward or backward) in the ciliated protozoa depends on the orientation of the polarized cycle of ciliary movement. This orientation is coupled to membrane potential by membrane-limited movements of Ca²⁺ into the cell cortex and cilia. Stimuli which depolarize the cell cause the conductance of the membrane to Ca²⁺ to rise. As a consequence Ca²⁺, driven by its electrochemical gradient, shows a net influx and accumulates in the cilia and cell cortex. The increasing Ca²⁺ concentration activates the mechanism for ciliary reversal. As depolarization subsides and Ca²⁺ diffuses away from the cortex and/or is actively transported out of the cell, the cilia resume their forward-swimming orientation.