PERSISTENCE OF FUNCTIONAL NEUROMUSCULAR-JUNCTIONS FORMED IN A DENERVATED SKELETAL-MUSCLE OF THE ADULT-RAT BY AXONS THAT HAVE REGROWN FROM THE INJURED SPINAL-CORD THROUGH A PERIPHERAL-NERVE AUTOGRAFT

In previous 'short-term' (2 to 7 months) experiments, we had demonstra ted, in the adult rat, that motoneurons of the injured cervical spinal cord could extend lengthy axons into an autologous peripheral nerve s egment which was connected to a nearby denervated skeletal muscle. In addition, we had shown that new functional motor endplates were formed by these axons both at the original sites of innervation and at ectop ic locations of the denervated muscle. This substitution motor system, although quite functional, was anatomically very different from the o riginal model of innervation in the intact animal, relating to its mot oneuronal pool, the course of its motor axons and the sites of termina l innervation. The present 'long-term' (11 to 21 months) experiments d emonstrate the anatomical and functional permanency of the new motor c ircuitry, despite a lack of strict specificity in the new neuromuscula r connections. However, some minor modifications or adjustments were o bserved with time: (i) the maintenance of functional ectopic endplates could be consistently demonstrated, while functional reinnervated end plates at the initial sites of innervation were rare or even lacking; (ii) there was a definitive withdrawal of all non target-specific rege nerated axons from the vicinity of the muscle. It is now necessary to address the question to what extent this substitution motor system is actually controlled by central and/or peripheral inputs.