EXPEHIMENTAL NEUROLOGY 43, 207-215 (1974) Responses of Normally Innervated Soleus Muscle to Supplementary Nerve Implantation J. L. CROCKETT AND V. R. EDGERTON Department of Kinetiology, University of California, Los Angeles, California 90024 Received May 7,1973; revision received October 24,1973 It has been reported that when the proximal end of a transected nerve to a fast-twitch muscle is implanted into a normally innervated rat soleus muscle, changes are induced in the soleus indicative of muscle hypertrophy and speeding of its dynamic properties. To test these findings, in nine rats (group I) the nerve to the flexor digitorum longus was severed and implanted into the normally innervated soleus muscle of one leg. In the contralateral leg, the flexor digitorum longus nerve was implanted into the medial gastrocnemius to prevent reinnervation of the flexor digitorum longus. Two additional groups of rats also underwent flexor digitorum longus nerve implantation into the normal soleus. In group II the implanted nerve was sutured to the soleus, and in group III it was “glued” to the muscle using a thrombin-fibrinogen clot. The contralateral control legs of these latter two groups were left intact. With the three groups of animals it was possible to compare both nerve implantation and synergist denervation with unoperated muscle. An 8% (P < 0.05) greater wet weight of the implanted soleus was found in group II but not I and III. This study does not support earlier reports of enhanced tension-generating capabilities in the implanted solei induced by a supplemental nerve trunk anchored to a normally innervated muscle. Histochemical analysis of cholinesterase-stained muscle sections revealed no motor endplates at the site of nerve implantation. We found no change in contractile speed of either the implanted or the synergist-denervated muscle, nor was there any change in the percentage of fast-twitch fibers as determined histochemically by the myofibrillar adenosine triphosphatase (ATPase) stain. In conclusion, we have found no physio!ogical, biochemical, or histochemical differences in nerve-implanted, synergist-denervated, or unoperated solei. No evidence was found for the earlier proposed trophic influence of an implanted nerve on a normally innervated muscle. INTRODUCTION A close relationship has been observed between contractile properties and energy metabolism of skeletal muscle and the nature of the moto- 207 Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reservrd