Abstract
1. Cross-innervation of the slow soleus and fast flexor hallucis longus or flexor digitorum longus muscles has been performed in new-born kittens and rabbits and in adult cats.2. The effects on the histochemical and structural properties of the muscle have been studied and compared with the changes in the contractile properties.3. Cross-innervation has produced a dramatic change in histochemical pattern in the fast muscles, with the development of areas of muscle fibres indistinguishable from normal soleus muscle. The converse change from the histochemical pattern of slow soleus to that of fast muscle has also occurred, but has been less consistent.4. It is concluded that the neural influence determining the contractile properties of fast and slow muscle also has a profound controlling influence on the structure and metabolic activity of the muscle fibres.5. No significant changes could be demonstrated biochemically in the ATPase activities of the fast and slow muscles following cross-innervation.