It has been known for over 100 years that when the protoplasm of protozoan cells respond to electrical stimulation by contraction, contraction always starts at the anodal end (e.g., Spirostomum, Jones, Jahn and Fonseca, 1965). It also is known that the cell membrane of protozoa does not have a self-propagating depolarizing action potential of the type found in axons and muscle cells.
If we assume that contraction of the cytoplasmic protein is caused by an increased association of calcium with the protein, as known for muscle, these antithetical facts can easily be explained. Another necessary assumption is that all of the anionic sites on the protein have associated counterions, in accordance with the theories of Debye-Hückel, Ling, and Eisenman.
On the basis of known ionic electrical mobilities it can be predicted for cells with non-excitable membranes that calcium (1) is removed from both the inside and the outside of the membrane at the cathodal end and is added to both sides at the anodal end, (2) is added to cytoplasmic protein throughout the cell, but is added more rapidly at the anodal end, and (3) is made available for addition to cytoplasmic protein at the cathodal end, as it is released from the membrane. In a cell with an excitable membrane the above will be true, and furthermore (4) the action current will release larger quantities of calcium than those resulting directly from the stimulus. For these reasons it is predictable that in cells with a non-excitable membrane, contraction should start at the anodal end, whereas in cells with excitable membranes, it should start at the cathodal end, and that is exactly what occurs.