An analysis of the transverse electrical impedance of striated muscle

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Abstract
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 C x and R x , and in the high-frequency dispersion C m and R i . The relative extent of the low-frequency dispersion is increased by increasing the extracellular resistivity ( R 0 ), and by this means accurate determinations can be made of C x and R x . In the frog sartorius muscle, equilibrated with a solution in which most of the NaCI is replaced by sucrose, so that R 0 = 440 Ωcm, the properties of the fibres are found to be C x = 54 μF/cm 2 , R x = 24000 Ωcm, C m = 2.6 μF/cm 2 , R i = 200 Ωcm (average results from 9 muscles). Graded replacement of NaCl by sucrose indicates R x to vary approximately as the square-root of R 0 , 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 R i , a decrease in C x and an increase in R x 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 R i and C m are considered to indicate the resistivity of the myoplasm and the capacitance of the surface membrane of the fibre. It is suggested that R x and C x 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.

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