Plasticity in the spinal cord: the dorsal root connection.

There is considerable evidence supporting a naturally occuring increase in the density of projections by intact systems induced by partial denervation of targets within the spinal cord. The phenomenon of sprouting in the spinal cord appears to be quite similar to the denervation evoked sprouting shown elsewhere in the CNS. Sprouting appears to be regulated rather than random since not all systems projecting to a denervated region may show demonstrable sprouting. In adults, the extent of sprouting is usually limited to the normal target zone of the intact systems and thus reasonably rigorous quantitative methods may be required to provide a clear demonstration of the increased projection. Greater plasticity, including regenerative like changes can be elicited in the undamaged processes of DRGs within the CNS by lesion to the peripheral process. Expansion of terminal fields into novel territories in adults has been demonstrated most convincingly in cases where additional manipulations are made, e.g. addition of peripheral nerve injury to the lesion paradigm. This suggests that the limitation on sprouting in adults can be overridden by increasing the metabolic drive of the sprouting neurons. In contrast there appear to be fewer limitations on sprouting in the neonate, since extensive sprouting into aberrant targets occurs if denervation occurs before development has ceased. Changing patterns of synaptic activity can also evoke quite dramatic modifications of the synthetic activity of neurons. Some of these changes appear to be long lasting and may lead to modifications in the activity of spinal neurons. Naturally occurring plasticity thus represents a major compensatory response to CNS lesions which needs to be taken into account in considering the consequences of CNS lesions, the extent of recovery of function and ways of improving the extent of recovery of function.