Electrical and mechanical activities in the denervated semitendinosus muscle of the frog

Denervated frog's semitendinosus muscles were studied. No differences in resting potential were found between denervated and control fibres. Spontaneous discharges of fibrillation potentials were recorded from random points along the length of denervated fibres. The frequency of the potentials varied between 2 and 30/sec; the duration of the discharges varied between 5 and 120 sec. Each potential originated from a prepotential, reversed the sign of the membrane potential between 10--20 mV, and was followed by a transient hyperpolarization. The time course of the spontaneous potential produced by a fibre was slower than that of a spike elicited by an electrical pulse applied to the same fibre. After the cessation of a train the fibrillation potentials were replaced by small rhythmic potentials 10--15 mV in amplitude; this activity lasted for periods as long as 7 min. Some reduction of the resting potential was usually observed during the discharge of a train of fibrillation potentials, however, repolarization occurred after the end of electrical activity and a second discharge of full size fibrillation potentials could be observed in most of the fibres. Mechanical activity was not always present. When observed, it was reduced to light movements unable to eject the microelectrode. In general, no one-to-one correlation could be observed between fibrillation potentials and mechanical activity. In those fibres in which the correlation was present, the tension developed by a single fibrillation was less than 0.2/kg/cm2. Some denervated fibres developed tetanus-like contractions, the maximal tension developed by these fibres was less than 1.0 kg/cm2. The data are discussed according to the current ideas on generation of fibrillation potentials.