Quantitative investigation of the neuromuscular junction of rat skeletal muscle fibres after double innervation.

In normal (untreated) rats the mean length ratio of postsynaptic to presynaptic membrane was 2.7 +/- 0.8 for neuromuscular junctions of slow-twitch soleus muscle fibres and 4.2 +/- 1.0 for neuromuscular junctions of fast-twitch extensor digitorum longus muscle fibres; this difference was significant (P less than 0.001). After experimental double innervation by fast and slow muscle nerves for four months, the ratio was (1) 2.9 +/- 0.8 for the original slow-twitch fibre end-plate and 2.8 +/- 0.8 for the newly established one, both not significantly different from that of the normal slow-twitch fibres; and (2) 2.2 +/- 0.5 for the original fast-twitch fibre end-plate and 2.2 +/- 0.7 for the newly established one, both significantly smaller than that of the normal fast-twitch fibres (P less than 0.001). This means that the double innervated slow-twitch muscle fibres retained their original neuromuscular junction type, whereas the doubly-innervated fast-twitch muscle fibres underwent a dramatic transformation of their neuromuscular junction from the fast-muscle to the slow-muscle type. In both doubly innervated fibres, the ultrastructural characteristics of neuromuscular junctions, whether altered or not, were identical at both end-plate regions.