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The water balance of a serpulid polychaete Mercierella enigmatica (Fauvel). IV. The excitability of the longitudinal muscle cells.

1. The electrical activity of the two types of longitudinal muscles of an osmoconforming polychaete worm, Mercierella enigmatica , have been studied in media of widely varying osmotic and ionic composition. Activity persists practically unaltered in both types of muscle cell. 2. The possible effects of osmotically induced changes in cell volume on the ionic gradients across the cell membranes are considered. It is concluded that the normal gradients are unlikely to be maintained as a result of such changes. 3. The involvement of ion pumps in the maintenance of the normal gradients across the muscle cell membranes has been studied using specific and metabolic poisons. It is evident that the persistence of electrical activity in media of altered ionic content does not depend on the sodium-potassium exchange pump. 4. The ionic basis of the overshoot of action potentials recorded from cells of the small resting potential type has been studied. It is concluded that calcium ions but not sodium ions are responsible for the inward current although there is a component of the inward current carried by some other as yet unidentified ion. 5. Alterations in the external concentrations of chloride ions are found to alter both the height of the overshoot and the length of the action potential. 6. Profound alterations in the overshoot height are produced only when the normal ratio of calcium to chloride concentration in the external medium is altered. Possible mechanisms to explain these effects are discussed. 7. It is suggested that the stability of the action potential in the muscle cells of M. enigmatica , despite large fluctuations in the salinity of the external medium, depends on the constancy of the ratios between the concentrations of the ions in the fluids bathing the cells and not on the absolute concentrations of the ions.