DIFFERENTIAL-DELAY OF REINNERVATING AXONS ALTERS SPECIFICITY IN THE RAT SERRATUS ANTERIOR MUSCLE

Previous studies have shown remarkable rostrocaudal selectivity by reg enerating motoneurons to the rat serratus anterior (SA) muscle after f reezing, crushing, or sectioning the long thoracic (LT) nerve. The LT nerve contains motoneurons from both the sixth and seventh cervical sp inal nerves (C6 and C7), nith C6 motoneurons as the major source of in nervation throughout the muscle, and with C7 motoneurons innervating a larger percentage of muscle fibers caudally than rostrally. To determ ine if synaptic competition can play a role in neuromuscular topograph y, 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, lifter an initial period during which C7 motoneurons reinnervated a larger prop ortion of muscle fibers than normal in all SA muscle sectors, C6 moton eurons regained their original proportion of rostral muscle fibers, Ca udally; however, C7 motoneurons maintained an expanded territory, With this two-site crush method, the number of C6 motoneurons that reinner vate the SA muscle was significantly decreased from normal, whereas th e number of C7 motoneurons remained the same, It is concluded that whe n C7 motoneurons are gi,en a temporal advantage, synaptic specificity can be altered transiently in rostral muscle sectors and permanently i n caudal sectors, and this is correlated with a disproportionate loss of C6 motoneurons, Moreover, this may be an important model for studie s of synaptic competition, a here terminals destined to be eliminated can he identified beforehand. (C) 1995 John Wiley & Sons, Inc.