Barnacle Muscle Fibers

Constant current pulses have been applied to single muscle fibers of the barnacle, Balanus nubilus Darwin, with an axial metal electrode. The membrane potential change, which took place over a large part of the muscle fiber, was measured with a similar electrode. Depolarizing pulses, if the voltage was greater than threshold, produced tension. The size of the tension was a function of the magnitude and the duration of the depolarizing pulses. The latency between the onset of depolarization and tension can be only in part attributable to mechanical factors. AC stimulation produced tension, but 5 to 10 seconds were required for the steady-state level of the tension to be reached. Muscles were depolarized in elevated K and studied after the contracture had terminated. If not too depolarized, further depolarization produced tension. Termination of hyperpolarizing pulses also produced tension, which decayed quite slowly. Hyperpolarizing pulses reduced, or abolished, any preexisting tension. Thus, it appears that at certain values of the membrane potential tension is set up, but there is also a slow process of accommodation present. The relation between muscle tension and membrane potential in solutions with various potassium concentrations has been measured by Hodgkin and Horowicz (1960). In single muscle fibers of the frog, the tension was found to be related by an S-shaped curve to the external potassium concentration or the membrane potential. No tension was developed until the potential reached about -50 my. A similar relation between tension and potential has recently been described for single muscle fibers of the barnacle, Balanus nubilus Darwin (Hoyle and Smyth, 1963). These muscle fibers are large enough to permit the insertion of wire electrodes for passing current and recording changes in membrane po-