Differential delay of reinnervating axons alters specificity in the rat serratus anterior muscle.

Previous studies have shown remarkable rostrocaudal selectivity by regenerating motoneurons to the rat serratus anterior (SA) muscle after freezing, crushing, or sectioning the long thoracic (LT) nerve. The LT nerve contains motoneurons from both the sixth and seventh cervical spinal nerves (C6 and C7), with C6 motoneurons as the major source of innervation throughout the muscle, and with C7 motoneurons innervating a larger percentage of muscle fibers caudally than rostrally. To determine if synaptic competition can play a role in neuromuscular topography, both the LT nerve and the branch carrying C6 (rostral) motoneurons to the LT nerve were crushed in newborn rats. This approach provides a temporal advantage to regenerating C7 (caudal) motoneurons. After an initial period during which C7 motoneurons reinnervated a larger proportion of muscle fibers than normal in all SA muscle sectors, C6 motoneurons regained their original proportion of rostral muscle fibers. Caudally, however, C7 motoneurons maintained an expanded territory. With this two-site crush method, the number of C6 motoneurons that reinnervate the SA muscle was significantly decreased from normal, whereas the number of C7 motoneurons remained the same. It is concluded that when C7 motoneurons are given a temporal advantage, synaptic specificity can be altered transiently in rostral muscle sectors and permanently in caudal sectors, and this is correlated with a disproportionate loss of C6 motoneurons. Moreover, this may be an important model for studies of synaptic competition, where terminals destined to be eliminated can be identified beforehand.