Central consequences of peripheral nerve injuries.

Experiments in primates suggest that the skin senses are served by distinct classes of sensory receptors that relay their messages to the cortex on separate and parallel paths. When a peripheral nerve is transected and repaired, each sensory receptor that becomes reinnervated is innervated by a member of the class of axons that originally served that receptor. This specificity of reconnection allows the appropriate classes of sensations to be transmitted again from the skin to the cortex. However, sensory experiences are not completely normal because the regenerated axons have lost their previous spatial relationships and convey a spatially disordered message. Recent experiments suggest that the maps in the somatosensory cortex can respond to alterations of the somatosensory input in a manner that may compensate for some of the deficits produced by nerve injuries. There are, however, severe constraints on this reorganization, including restrictions to a cytoarchitectonic area, restrictions by certain boundaries within the somatotopic maps, and a requirement that a minimal topological order exist in the regenerated peripheral nerve.