Physiologic and morphologic aspects of nerve regeneration after end-to-end or end-to-side coaptation in a rat model of brachial plexus injury.

The results of repairing a transected rat musculocutaneous nerve by suturing the distal stump, end to side or end to end, to the ipsilateral ulnar nerve were assessed at 3 months by retrograde labeling and morphologic and physiologic analysis. Unlike most other models of end-to-side repair in which the injured recipient and donor reinnervating nerves have overlapping neuron pools in the spinal cord, in this model the neurons of the injured musculocutaneous and the reinnervating ulnar nerves are located in mutually exclusive segments of the spinal cord. Using retrograde labeling we show that the reinnervating fibers are derived solely from the ulnar nerve pool. Both end-to-side and end-to-end coaptation resulted in reinnervation of the distal musculocutaneous nerve and significant functional reinnervation of its dependent biceps brachii muscle. Although end-to-end coaptation resulted in better axon morphology and muscle function, it resulted in total loss of donor nerve function. By contrast, end-to-side coaptation resulted in good recovery with only minimal donor nerve deficit. These results show that significant functional reinnervation of biceps brachii muscle can occur solely on the basis of collateral sprouting of intact axons from the adjacent ulnar nerve.