An analysis of the transverse electrical impedance of striated muscle

Abstract

Transverse impedance measurements on vertebrate striated muscle over the frequency range 1$\cdot $5 c/s to 130 kc/s show two dispersions at widely separated frequencies, indicative of two time-dependent impedance elements. The measurements are analyzed on the assumption that the properties of the muscle fibre, relevant to the current produced by a transverse electric field, are included in two parallel paths, each involving a resistance in the bulk of the fibre and a capacitance across the surface; the properties of the path effective in producing the low-frequency dispersion are designated C$_{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$_{o}$), 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 NaCl is replaced by sucrose, so that R$_{o}$ = 440 $\Omega $cm, the properties of the fibres are found to be C$_{x}$ = 54 $\mu $F/cm$^{2}$, R$_{x}$ = 24 000 $\Omega $cm, C$_{m}$ = 2$\cdot $6 $\mu $F/cm$^{2}$, R$_{i}$ = 200 $\Omega $cm (average results from 9 muscles). Graded replacement of NaCl by sucrose indicates R$_{x}$ to vary approximately as the squareroot of R$_{o}$, 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 X$_{m}$ 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.