Determination of Muscle Cable Parameters from a Single Membrane Voltage Response

Computer techniques have been developed to achieve a least-squares fit of the Hodgkin and Rushton one time-constant cable equation to the recorded responses of muscle cells to the intracellular injection of square current pulses. In the rat diaphragm the response of the muscle fiber to square current pulses is well fit by a one time-constant model. This makes possible the estimation of the passive electrical properties of muscle sarcolemma using the response of the fiber to a single stimulus. A comparison of the results of this method and the older method of spatial decay in the rat diaphragm shows no significant differences. Average values obtained using the one-point method for estimating membrane resistance and capacitance are 520 omega cm2 and 4.6 micronF/cm2, respectively. An estimation of the average specific resistivity of the cytoplasm was made using this method, and it was found to be 213 omega cm. At levels of injected currents producing large membrane hyperpolarizations where delayed conductance changes make the spatial decay method useless, the one-point method continues to give consistent results since it utilizes only the early portion of the membrane response. Work with the frog sartorius muscle, which is known to display the characteristics of a two time-constant system, shows that this method is capable of estimating the slower components of this system by using only the later portion of the response curve. The immunity of this method to delayed conductance changes and its experimental facility make it useful in estimating the passive electrical properties of muscle fibers, particularly when working with tissues that are small and delicate or that have poor in vitro viability.