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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