Regeneration by skeletomotor axons in neonatal rats is topographically selective at an early stage of reinnervation.

The seven sectors of the rat's serratus anterior (SA) muscle are innervated topographically by motor neurons of spinal cord segments C6 and C7 whose axons travel in the long thoracic (LT) nerve. That pattern is roughly mapped early in development and gains its final precision postnatally. The segmentotopic pattern is reestablished better in neonates than adults after cutting the LT nerve. We examined the process of reinnervation to see whether segmental selectivity is reestablished at the outset or whether it arises by rearrangement of the regenerated axons. Recordings were made from muscle fibers 1 to 70 days following a cryogenic lesion of the LT nerve done within 48 h of birth, as well as from sham-operated and unoperated control rats. Reinnervation of all sectors of SA occurred within a week after freezing the nerve. Reinnervation by C6 and C7 motor neurons was topographically selective though not quite to the degree found in controls. The precision observed during the first week of reinnervation did not improve over the next 9 weeks. Thus, selectivity exists from the start rather than being a more random reinnervation subsequently sharpened by elimination of inappropriate connections. The number of muscle fibers innervated by both C6 plus C7 motor neurons was greater after reinnervation than in controls. There was a significant decrease in the percentage of these dually innervated fibers over the initial few weeks of reinnervation but there was no difference among the reinnervated sectors of SA. Reinnervation of SA under optimal conditions resembles normal development in that there is a degree of topographic selectivity of (re)innervation that is present even at the earliest time periods studied. Unlike normal development the topographic selectivity after neonatal reinnervation does not improve over time, and fibers receiving a dual segmental innervation are not preferentially located in sectors where there is the most overlap in segmental projection.