Re‐innervation of fast and slow twitch muscle following nerve crush at birth.

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Abstract
The frequency of miniature end-plate potentials (mepp) was significantly greater in the [rat] fast twitch extensor digitorum longus muscle (extensor) than in the slow twitch soleus muscle even though end-plate surface area was greater for fibers in the latter. Crush of the sciatic nerve at birth did not prevent the appearance of this difference in mepp frequency. The frequency of the potentials in the re-innervated muscles was less than normal, even though the regenerated neuromuscular junction was qualitatively normal in morphology. Though the re-innervated muscles were differentiated with respect to twitch time course, the extensor muscle was more responsive than normal to the contracture-inducing action of caffeine. The Z line of the re-innervated extensor muscle was similar to that of the normal soleus in thickness. Resting potential, passive electrical properties and action potential generating mechanism of the sarcoolemma were normal. Since the re-innervated muscles lacked muscle spindles, a role of sensory feed-back in the function of the neuromuscular junction and the neurotrophic regulation of muscle is discussed.