Electrical Properties of the Myotendon Region of Frog Twitch Muscle Fibers Measured in the Frequency Domain

The electrical properties of the end of a muscle fiber were determined using three microelectrodes, one passing sinusoidal current, the other two recording the resulting voltages. An electrical model was constructed from the morphology of the fiber, including the resistance of the extracellular space between cells; the parameters of this model were determined by fitting the model to the observed voltage responses. Our results, analyzed directly or by curve fits, show that the end of muscle fibers contains a large capacitance resulting from the extensive membrane folds at the myotendon junction. Analysis and simulations show that the extra capacitance at the myotendon junction has substantial effects on measurements of linear properties, in particular on estimates of the capacitance of the membranes. There is little qualitative effect on classical measurements of nonlinear charge movement (provided they were made with one set of electrode locations) if the linear components have been subtracted. Quantitative estimates of nonlinear charge movement and ionic currents are significantly affected, however, because these estimates are customarily normalized with respect to the linear capacitance.