Transient reinnervation of antagonistic muscles by the same motoneuron

When reinnervation is allowed after a sciatic nerve cut in the adult rat, motoneuron axons may branch to innervate antagonistic muscles. This multiple innervation is widespread, but transient. Fourteen weeks after denervation isometric muscle contraction experiments and studies with anterograde transport of the fluorescent tracer Fast Blue showed that branches from the same motoneurons reached the distal part of the tibial nerve and either the soleus or the extensor digitorum longus muscles or both muscles. Retrogradely double-labeled motoneurons were found after injections of different fluorescent tracers into these muscles. Sixty-four to 88 weeks after the nerve cut, similar experiments showed that selective innervation was reestablished. The findings suggest a selective mechanism for axon withdrawal in an adult mammal.