The interaction between foreign and original motor nerves innervating the soleus muscle of rats.

1. The fibular nerve was transplanted on to the soleus muscle of the rats. Interruption of the original soleus nerve then permitted cross-innervation, and subsequently, over a period of weeks, re-innervation by the original nerve. 2. Individual muscle fibres were often innervated by both the original and the foreign nerve. The original and foreign end-plates were located in separate regions of the muscle. There were no indications that the original nerve could displace or repress the foreign innervation. 3. The extent of re-innervation by the original nerve depended upon the method of denervation. A single crush of the nerve was followed by virtually complete re-innervation, even of muscle fibres already innervated by the foreign nerve. When re-innervation was delayed by resection of a segment of the nerve only muscle fibres without foreign nerve innervation were re-innervated. Denervation by a simple nerve cut gave an intermediate result. 4. Re-innervation by the original nerve can take place without measurable extrajunctional sensitivity to ACh. 5. The original end-plate region could retain high and localized sensitivity to ACh for several months despite degeneration of its motor nerve terminal and activity of the muscle fibre. 6. Established foreign end-plates were re-innervated by the foreign nerve on muscle fibres with intact original innervation. 7. The factors controlling synapse formation in skeletal muscles are discussed.