NERVE-FIBER REGENERATION ACROSS THE PERIPHERAL-CENTRAL TRANSITIONAL ZONE

Neurons cannot negotiate an elongation across the peripheral (PNS)-cen tral nervous system (CNS) transitional zone and grow into or out of th e spinal cord in the mature mammal. The astrocytic rich CNS part of th e spinal nerve root is most effective in preventing regeneration even of nerve fibres from transplanted embryonic ganglion cells. Regenerati on of severed nerve fibres into the spinal cord occurs when the transi tion zone is absent as in the immature animal. Before the establishmen t of a transition zone there is also new growth of neuronal processes from dorsal horn neurons distally to the injured dorsal root. Thus the experimental strategy to reestablish spinal cord to peripheral nerve connectivity has been to delete the transitional region and implant se vered ventral or dorsal roots into the spinal cord. Dorsal root implan tation resulted in reestablished afferent connectivity by new neuronal processes from secondary sensory neurons in the dorsal horn of the sp inal cord extending into the PNS. The ability for plasticity in these cells allowed for a concurrent retention of their original rostral pro jection. Ventral root implantation into the spinal cord corrected defi cit motor function. In a long series of experiments performed in diffe rent species, the functional restitution was demonstrated to depend on an initial regrowth of motor neuron axons through spinal cord tissue (CNS). These findings have led to the design of a new surgical strateg y in cases of traumatic spinal nerve root injuries.