Innervation by Nerve Implants of “Fast” and “Slow” Skeletal Muscles of the Rat

The time course of innervation by nerve implants of the fast contracting flexor digitorum longus and the slow contracting soleus muscles were studied by the recorcling of isometric twitch tensions in response to stimulation of the implanted nerve and by histochemical visualization of newly-formed end-plates. When a foreign motor nerve was implanted into a muscle 30 days prior to sectioning of the original nerve supply, synaptic contacts were established 4–8 days after the section of the nerve, while, with simultaneous implantation and denervation, 16 days were required for the implanted nerve to innervate the muscle. Implantation of the nerve which normally innervates the flexor digitorum longus muscle into the soleus altered the contraction and relaxation speeds of the soleus muscle towards those of a fast contracting muscle, the change being apparent as soon as 4–8 days following sectioning of the original nerve. In muscles with nerve implants and intact normal innervation hypertrophy occurred, amounting to 20–30 per cent within one month.