SIMULATION OF PROPAGATION ALONG AN ISOLATED SKELETAL-MUSCLE FIBER IN AN ISOTROPIC VOLUME CONDUCTOR

This paper describes a model of the frog skeletal muscle fiber that in cludes the effects of the transverse tubular system (T system) on prop agation. Uniform propagation on an isolated fiber suspended in Ringer' s solution or in air is simulated by placing the cylindrical fiber mod el in a concentric three-dimensional isotropic volume conductor. The c urrent through the T system outlets at the sarcolemmal surface is comp arable in magnitude to the sarcolemmal current density, but is of oppo site polarity. When it is added to the sarcolemmal current, the result ing triphasic waveform has a 100% increase in the leading positive pea k, a 50% reduction in the negative peak, and more than 60% reduction i n the trailing positive peak. As a result the tubular output current c auses a reduction in the conduction velocity, a decrease in the maximu m rate of rise of the action potential, and an important modification of the extracellular potential. Compared to an isolated fiber in a lar ge volume of Ringer's solution, uniform propagation within a 2-mu m-th ick volume conductor annulus is slowed down from 1.92 to 0.72 m/s, and the extracellular potential is increased from 1 to 108 mV peak to pea k, in agreement with published experimental measurements.