Neurotization of the rat soleus muscle: a quantitative analysis of reinnervation.

Neurotization--reinnervation of muscle by direct nerve implantation--has been the subject of several reports. The underlying neurobiology, however, has not been adequately studied. The use of a combined silver-acetylcholinesterase stain was used in this study to identify reinnervated motor endplates and to quantify motor endplates reinnervated by the neurotization process. This study examined the effect of distance between nerve implantation and native motor endplate zone on the formation of ectopic motor endplates and on the total number of motor endplates reinnervated. Experiments were performed on the rat soleus muscle. The transected tibial nerve was implanted directly into the motor endplate zone (near, n = 10) or distally, far from the motor endplate zone (far, n = 10). After a reinnervation interval, frozen sections were processed to demonstrate both axons and motor endplates. In the near group, a mean of 566 motor endplates were reinnervated in the native motor endplate zone and a mean of only 13 in distant locations. In the far group, a mean of 362 motor endplates were reinnervated in the native zone, while a mean of 477 were reinnervated in distant locations. Significantly more ectopic motor endplates were generated by far implantation, and native motor endplates were increased by near implantation. The total number of motor endplates was independent of implant location. These experiments demonstrate that the distance between implanted nerve and the native motor endplate zone influences the morphology of reinnervation.