Ionic Mechanisms of Excitation in Paramecium

Abstract

The first intracellular electrical measurements made with capillary glass electrodes from animal cells were made on Paramecium and were reported in 1934 by T. Kamada (27). More recently, this ciliate has come into use in interdisciplinary studies of the biophysics, genetics, and cell biology of membrane excitation and of sensory-motor behavior in a single-celled organism. The surface membrane exhibits properties of excitability similar in fundamental respects to those exhibited by metazoan nerve and muscle cells. A prominent feature is a graded electrical response to depolarization (similar to an action potential except that it is not all-or-none) produced by an inward regenerative current. The inward current, carried by Ca, is followed by a delayed outward K current. The electrically excitable channels that carry the Ca current reside almost exclusively in those por­ tions of the surface membrane covering the cilia. Removal of the cilia eliminates the inward Ca current, which returns gradually as the ciliary membrane regenerates over a period of several hours. The entry of Ca ions carrying the regenerative current into the interior space of the cilium produces a transient rise in intraciliary [Ca] that mediates a reorganization of axone mal activity seen as reversed ciliary beating. The ionic mechanisms of excitation and recovery in Paramecium, as we presently understand them, are summarized in Figure 1 , which serves also as an overview of the bioelectric organization that the reader may wish to refer back to in the discussions that follow.