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 μF/cm 2 , and the value was almost constant over a range of diameter from 55 to 140 μ. 3. In fibres, in which the transverse tubular system was disconnected from the surface by the glycerol treatment, C f was 0·9 μF/cm 2 and was fairly constant over a range of diameter from 60 to 130 μ. The low frequency capacity in glycerol‐treated fibres was 1·9 μF/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 μF/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.