Chronic Schwann cell denervation and the presence of a sensory nerve reduce motor axonal regeneration.

Motor axonal regeneration is compromised by chronic distal nerve stump denervation, induced by delayed repair or prolonged regeneration distance, suggesting that the pathway for regeneration is progressively impaired with time and/or distance. In the present experiments, we tested the impacts of (i) chronic distal sensory nerve stump denervation on axonal regeneration and (ii) sensory or motor innervation of a nerve graft on the ability of motoneurons to regenerate their axons from the opposite end of the graft. Using the motor and sensory branches of rat femoral nerve and application of neuroanatomical tracers, we evaluated the numbers of regenerated femoral motoneurons and nerve fibers when motoneurons regenerated (i) into freshly cut and 2-month chronically denervated distal sensory nerve stump, (ii) alone into a 4-cm-long distally ligated sensory autograft (MGL) and, (iii) concurrently as sensory (MGS) or motor (MGM) nerves regenerated into the same autograft from the opposite end. We found that all (315 +/- 24: mean +/- SE) the femoral motoneurons regenerated into a freshly cut distal sensory nerve stump as compared to 254 +/- 20 after 2 months of chronic denervation. Under the MGL condition, 151 +/- 5 motoneurons regenerated, which was not significantly different from the MGM group (134 +/- 13) but was significantly reduced to 99 +/- 2 in the MGS group (P < 0.05). The number of regenerated nerve fibers was 1522 +/- 81 in the MGL group, 888 +/- 18 in the MGM group, and 516 +/- 44 in the MGS group, although the high number of nerve fibers in the MGL group was due partly to the elaboration of multiple sprouts. Nerve fiber number and myelination were reduced in the MGS group and increased in the MGM group. These results demonstrate that both chronic denervation and the presence of sensory nerve axons reduced desired motor axonal regeneration into sensory pathways. A common mechanism may involve reduced responsiveness of sensory Schwann cells within the nerve graft or chronically denervated distal nerve stump to regenerating motor axons. The findings confirm that motor regeneration is optimized by avoiding even short-term denervation. They also imply that repairing pure motor nerves (without their cutaneous sensory components) to distal nerve stumps should be considered clinically when motor recovery is the main desired outcome.