Efficiency of reinnervation of neonatal rat muscle of original and foreign nerves

Formation of neuromuscular connections in mammals may involve a hierarchy of efficiency of synapse formation at a stage when motor nerves have already contacted muscle fibers and during the transitional period of multiple innervation. In an attempt to test for such a hierarchy, we examined, in neonatal rats, the relative efficiency of reinnervation by foreign or original nerves implanted simultaneously in a large muscle so that competition for muscle fibers was minimized. The tibial nerve, containing gastrocnemius nerve fibers, and the “foreign” peroneal nerve were implanted into the denervated lateral gastrocnemius muscle. One to five months later, indirect tetanic tensions obtained upon stimulating the implanted nerves were measured by isometric techniques and were compared to contralateral control muscles. When both nerves were implanted side by side at the end-plate region, approximately equal tetanic tensions were obtained at the time of testing. The same result was also obtained when the tibial and common peroneal nerves were implanted into non-end-plate and end-plate regions, respectively. However, in the reverse experiment, the tibial nerve implanted at the end-plate region produced significantly higher tetanic tension than the peroneal nerve at the non-end-plate site in the same muscle. Thus, the original nerve, compared to a foreign nerve, appeared to reinnervate neonatal muscle more effectively, but this was only revealed under conditions where access to former end-plate regions was unequal.