Reinnervation of partially denervated rat soleus muscle.

The reinnervation of partially denervated rat soleus muscles by their interrupted, regenerating motor axons has been examined in adult white rats. If reinnervation occurred after the remaining, intact motor axons had sprouted to their full, maximal extent, then the regenerating axons formed synapses preferentially with denervated muscle fibers and not with fibers innervated by sprouts. The sprouted motor units retained their size as if no reinnervation had occurred. On the other hand, if reinnervation occurred early during the sprouting process, the sprouting motor units were never able to attain their maximal size. Further, some muscle fibers became innervated by both sprouted and regenerating axons. These "hyperinnervated" fibers lost their dual innervation within a few weeks. The sprouted axons seemed to be the nerve fibers preferentially eliminated from these hyperinnervated fibers, since during the loss of hyperinnervation the sprouted motor units decreased in size while the motor units formed by the regenerating axons did not change in size. It is proposed that the occurrence of hyperinnervation is influenced by the amount of time sprouting axons have to consolidate their synapses with muscle fibers. Further, it is proposed that on muscle fibers which can become hyperinnervated, the sprouted motor neurons are at a disadvantage in the competition for maintenance because of their larger unit sizes.