Linear Electrical Properties of the Transverse Tubules and Surface Membrane of Skeletal Muscle Fibers

With the use of two intracellular microelectrodes and a circuit designed to compensate for the effects of stray capacitances around the electrodes, transfer impedance measurements were made at frequencies from 0.5 to 1000 c/s on frog sartorius muscle fibers bathed in 7.5 mM K Ringer solution. Complete AC cable analyses performed at 46, 100, 215, 464, and 1000 c/s showed that the fibers behaved as ideal one-dimensional cables having purely resistive internal impedances (R_i = 102 ± 11 Ω cm). Two circuits were considered for fiber inside-outside impedance, a four lumped parameter circuit and a parallel resistance and capacitance shunted by the input impedance of a lattice model for the T-system. Least squares fits to fiber input impedance phase angles were better with the latter circuit than with the former. With the use of the lattice model the specific capacitance of both the surface and transverse tubule membranes was found to be 1 µF/cm² and the internal resistivity of the tubules to be about 300 Ω cm.