Electromechanical coupling in tubular muscle fibers. II. Resistance and capacitance of one transverse tubule

In tubular muscle fibers of the yellow scorpion the transverse tubules are arranged in a radial symmetry. This particular morphology, enables one to derive values for electrical components of one transverse tubule (TT) by treating the TT as a core conductor rather than a complex network. The electrical properties of tubular muscle fibers were completely characterized and analyzed by measuring two independent functions of frequency, i.e., the characteristic impedance and the propagation function. The impedance of a single tubular muscle fiber was determined with microelectrodes over the frequency range 1 Hz to 1.5 kHz. The results were fitted to a possible equivalent circuit model which is based on morphological evidence. The average component values for this model are: Ri = 209 omega-cm, Rm, and RT = 980 omega-cm2 (referred to unit area of surface membrane), Cm and CT = 0.9 muF/cm2, and RL = 103 omega-cm. Relating the equivalent circuit to ultrastructure shows that the average component values are consistent with the hypothesis that the TT is open to the extracellular medium, the electrical capacity of surface and TT membranes is about 1 muF/cm2, and the spread of surface depolarization into the TT is attenuated by about 25%.