Reinnervation of denervated skeletal muscle by central neurons regenerating via ventral roots implanted into the spinal cord

The reinnervation of denervated skeletal muscle by central axons regenerating via a ventral root implanted into the spinal cord was examined in rats. The 8th thoracic ventral root was severed and its distal end implanted into the ventro-lateral column of the spinal cord via a stab incision. In control animals the root was severed, but was not implanted into the stab incision. After 12-14 months the animals were examined electrophysiologically to determine the presence or absence of motor units in the 8th intercostal muscle which were reinnervated by centrally derived axons regenerating via the implant. Such units were found in implanted animals, but in none of the controls. Evidence that the motor units were reinnervated by central axons included the facts that the units could be activated either, (1) reflexly (i.e. trans-synaptically) by electrical stimulation of the dorsal roots or spinal cord, or (2) pharmacologically by either the intraspinal injection of glutamate or acetycholine, or by the systemic administration of strychnine. Great care was taken to ensure that the only feasible connection between the spinal cord and the 8th intercostal muscle was via the site of implantation. The EMG signals from the motor units were of large amplitude, typical of reinnervated muscle, and their individual activation resulted in discernible contractions of regions of the T8 intercostal muscle. We conclude that regenerating CNS neurons can be guided to innervate denervated skeletal muscle by the implantation of severed ventral roots into the spinal cord. The neuromuscular synapses formed are functional and persistent. The findings may be relevant to the restoration of function after nervous injuries, such as the avulsion of ventral roots.