Control of ACh sensitivity in temporarily unconnected ("decentralized") segments of diaphragm-muscle fibres of the rat.

1. After a local lesion of the diaphragm muscle, which produced a segment of innervated muscle fibres connected with an intact nerve-free segment by a region of crushed muscle fibres, the sensitivity to ACh, the presence of tetrodotoxin resistant action potentials (AP) and the transmission of AP along the muscle fibres were studied. 2. Three days after local injury of the diaphragm muscle ACh sensitivity and TTX resistance appeared in the crushed and nerve-free segments between the place of injury and the tendineous attachment. 5-7 days after injury transmission of action potentials through the damaged to the undamaged ("decentralized"), nerve-free part of the fibres is restored. High ACh sensitivity and TTX resistance of the latter segment, however, are completely lost only 20 days after the local injury. During this period contractility of the muscle remains practically unchanged. Enzymatic activities (SDH, ATPase and phosphorylase) of the damaged part were lost 3 days after crushing and recovered slowly between 7-10 days of regeneration of the diaphragm muscle fibres. 3. The experiments suggest that during regeneration of damaged muscle fibres supersensitivity to ACh remains high inspite of normal AP activity and that intracellular mechanisms may be involved in the induction and disappearance of ACh hypersensitivity.