Mathematical analysis of the changes in the parameters of the action potentials, membrane and ionic currents of frog muscle fibre during the recovery cycle.

The method of mathematical modelling was used to study the excitability changes of the membrane of a frog skeletal muscle fibre and the parameters of the action potentials, membrane and ionic currents during the first 30 ms of the recovery cycle. The threshold current for a fibre at rest was found to be 0.32 microA and the durations of the absolute and relative refractory periods were respectively 4 ms and 5.2 ms. With increasing interpulse interval, the subnormality of the membrane excitability is followed by supernormality. Under the same condition the supernormality in the velocity recovery cycle is not obtained. In the recovery cycle, the shape (polarity, sequence and number of phases) of the action potentials, of the membrane and ionic currents and their conductances, are unchanged. Only the time and amplitude parameters of the quantities listed above are known to vary. With increasing the interpulse interval, the amplitudes of the quantities increase and their durations are shortened attaining the values of the corresponding quantities of the initial action potential. The membrane properties are recovered 30 ms after application of the initial pulse, but the supernormality of the excitability is still preserved.