Factors contributing to preferential motor reinnervation in the primate peripheral nervous system

Functional recovery after nerve lesions in the peripheral nervous system requires the accurate regeneration of axons to their original target end organs. This paper examines axonal regeneration of the primate median nerve lesioned at the wrist over nerve gap distances of up to 50 mm. Nerve gaps were bridged by either a sural nerve graft or a biodegradable collagen nerve guide tube, and recovery was followed for up to 1100 d. Nondestructive physiological methods were used to serially examine the number of regenerated motor units, and binomial statistics were used to compare the observed number of regenerated motor units with that expected if axonal regeneration of motor neurons were random. We found up to twice the number of motor units expected by random regeneration in direct suture and sural cable graft groups but not in nerve guide repairs of 20 or 50 mm. In all repaired nerves, aberrant motor axon collaterals were detected in digital sensory nerve territory. The results support the contention that the aberrant fibers represent collaterals of an alpha-motor axon, which also innervates muscle. Although the aberrant motor axon collaterals remained in digital sensory nerve territory for long periods, they remained relatively immature compared with their sibling collateral projecting to muscle, or sensory axons within the digital nerve. The number of such aberrant motor axon collaterals decreased over time in some repair groups, suggesting a selective pruning of the inappropriate collateral under certain conditions.