Re‐innervation of twitch and slow muscle fibres of the frog after crushing the motor nerves.

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Abstract
The conduction velocities of individual motor axons innervating twitch and slow muscle fibers of the frog [Rana temporaria] were determined by intracellular recording of junctional potentials elicited by stimulating the motor nerves at 2 different points. In normal pyriformis muscles twitch and slow fibers were innervated by 2 distinct populations of motor axons. Twitch fiber axons conducted at 10-18.7 m/s, while the conduction velocities of slow fiber axons ranged from 0.5-5 m/s (at 7-9.degree. C). The thresholds for electrical stimulation were significantly lower in the fast than in the slow axon population. Following denervation by crushing the sciatic nerve, fast axons which re-innervated the muscle had lower conduction velocities than normal but could still be identified. These lower conduction velocities were measured proximal to the site of the crush and did not recover over a period of 446 days. Fast motor axons regenerated more quickly than slow axons and re-innervated twitch and slow muscle fibers non-selectively. About 1 mo. later slow axons re-established synaptic contacts with slow (and some twitch) muscle fibers. Simultaneous re-innervation by fast and slow motor axons was occasionally observed in slow muscle fibers. The slow muscle fibers were innervated by slow axons only, while synapses of fast axons could no longer be found in this type of muscle fiber. Action potentials were observed in denervated and in re-innervated slow muscle fibers; they disappeared as re-innervation progressed. Non-selective re-innervation of slow muscle fibers is apparently present in the frog; it is a transient phenomenon followed by restoration of the original innervation pattern.