Bioelectric Basis of Behavior in Protozoa

Locomotor responses of ciliate protozoans to external stimuli primarily depend on changes in ciliary motion evoked by the stimuli. Certain regions of the protozoan cell produce a receptor potential in response to stimulation. The receptor potential electrotronically spreads to the entire cell membrane and generates an overall electric response due to the electrical excitability of the general membrane. The ionic mechanisms for electrogensis are basically identical to those in nerves, muscles, and receptors of metazoan organisms. The ionic movements across the membrane associated with the electrogensis modify directly and/or indirectly the concentration of certain cations within the cell. The catonic concentration change brings about a modification of the contractile activity of cilia, which in turn results in a change in the ciliary movements. Cilia on different locations of the cell have intrinsically ionic concentrations. This fact, together with the morphological specialization of cilia in different locations on the cell, contributes to the complexity and adaptiveness of locomotor responses found in the ciliated protozoa.