| Abstract: | SYNOPSIS. Locomotor behavior in the ciliate protozoa is controlled by the cell membrane through electrophysiological principles already familiar in receptor, nerve, and effector cells of the metazoa. This is illustrated by the avoiding reaction (15). When the membrane of the anterior part of the ciliate receives a mechanical stimulus, as during collision, it permits a local influx of Ca++. This constitutes a receptor current which depolarizes the remaining cell membrane by electrotonic spread. Depolarization causes a secondary transient increase in the calcium conductance of the entire cell membrane, and a general influx of Ca++ occurs. The resulting increase in concentration of intracellular Ca++ activates a reorientation (“reversal”) of the ciliary power stroke, causing the organism to swim backward. Forward locomotion is restored as the resting concentration of intracellular Ca++ in the cell cortex is restored by diffusion, active extrusion, or intracellular sequestering. The control and coordination of locomotion in ciliates depend on several factors in addition to the excitable properties of the membrane. These include the sensitivities of the ciliary apparatus to intracellular concentrations of calcium and other regulating substances, the anatomical distribution of sensory receptor properties of the cell membrane, and the cable properties of the cell which permit electrotonic spread of graded potential signals without need of all-or-none conducted signals. |
