An analysis of the transverse electrical impedance of striated muscle

Transverse impedance measurements on vertebrate striated muscle over the frequency range 1.5 c/s to 130 kc/s show two dispersions at widely separated frequencies, indicative of two time-dependent impedance elements. The measurements are analyzed on the assumption that the properties of the muscle fibre, relevant to the current produced by a transverse electric field, are included in two parallel paths, each involving a resistance in the bulk of the fibre and a capacitance across the surface; the properties of the path effective in producing the low-frequency dispersion are designated Cx and Rx, and in the high-frequency dispersion Cm and Ri. The relative extent of the low-frequency dispersion is increased by increasing the extracellular resistivity (R0), and by this means accurate determinations can be made of Cx and Rx. In the frog sartorius muscle, equilibrated with a solution in which most of the NaCI is replaced by sucrose, so that R0 = 440 Ωcm, the properties of the fibres are found to be Cx = 54 μF/cm2, Rx = 24000 Ωcm, Cm = 2.6 μF/cm2, Ri = 200 Ωcm (average results from 9 muscles). Graded replacement of NaCl by sucrose indicates Rx to vary approximately as the square-root of R0, the other properties of the fibres being unchanged. Changes in the specific ionic composition of the bathing solution (involving Na, Cl, K or Ca) are found to have no significant effect on the properties of the fibre under examination. Changes of tonicity show, apart from the expected effect on Ri, a decrease in Cx and an increase in Rx produced by hypotonic solution. Treatment of the muscle with a solution of sufficiently low tonicity to cause an irreversible loss of osmotic properties (referred to as ‘bursting’) results in the disappearance of the low-frequency dispersion. The quantities Ri and Cm are considered to indicate the resistivity of the myoplasm and the capacitance of the surface membrane of the fibre. It is suggested that Rx and Cx indicate the effective resistivity in the fibre of a system of ramifying tubules (the sarcotubular system) and the effective impedance at the surface of the fibre of a barrier through which current passes between these tubules and the extracellular fluid.