Analysis of the membrane capacity in frog muscle.

1. The membrane capacity (C(f)) was determined from the conduction velocity and the time constant of the foot of the action potential in frog's skeletal muscle.2. In normal fibres C(f) was 2.6 muF/cm(2), and the value was almost constant over a range of diameter from 55 to 140 mu.3. In fibres, in which the transverse tubular system was disconnected from the surface by the glycerol treatment, C(f) was 0.9 muF/cm(2) and was fairly constant over a range of diameter from 60 to 130 mu. The low frequency capacity in glycerol-treated fibres was 1.9 muF/cm(2).4. These results as well as those obtained at low frequencies were consistent with the electrical model proposed by Adrian, Chandler & Hodgkin (1969).5. Analysis in terms of the model and of Peachey's (1965) data on tubular dimensions led to the following quantitative conclusions. The capacities of the surface membrane (C(S)) and of the tubular wall (C(W)) are both about 1 muF/cm(2). The conductances of the surface membrane (G(S)) and tubular wall (G(W)) are ca. 0.11 and 0.03 mmho/cm(2), respectively. The conductivity of the luminal fluid in the tubules is ca. 6 mmho/cm.