Denervation Changes in Frog Skeletal Muscle

The effects of denervation on acetylcholine sensitivity and on the electrical properties of frog tonic and phasic muscle fibres were examined by the use of iontophoretic micro‐application of acetylcholine and conventional intracellular techniques for recording and stimulation. In tonic fibres extrajunctional cholinergic sensitivity appeared 4 days after sectioning of the nerve and by 2 weeks the entire membrane had attained a high level of acetylcholine sensitivity. The sensitivity of former end‐plate regions also increased with time. In phasic fibres cholinergic sensitivity appeared about one week later but otherwise followed the same pattern as in tonic fibres. Innervated tonic fibres are incapable of generating action potentials but this property is induced upon denervation. After one week of denervation small spikes were observed in response to depolarizing current pulses and by 2 weeks large action potentials, sometimes followed by repetitive discharges were recorded. Denervation failed to affect the action potential in phasic fibres. Tetrodotoxin completely blocked action potentials in tonic and in phasic fibres. Denervation reduced the effective resistance and the time constant of tonic fibres but had no effect on these properties in phasic fibres. Actinornycin D injected i.p. in a single dose (7.5 or 9.0 μg) failed to prevent the appearance of extrajunctional cholinergic receptors but almost completely blocked the induction of action potentials in denervated tonic fibres.