SENSORY AFFERENT-PROJECTIONS AND AREA 3B SOMATOTOPY FOLLOWING MEDIAN NERVE CUT AND REPAIR IN MACAQUE MONKEYS

The fidelity of median nerve regeneration and the consequent effects o f regeneration errors on cortical organization were determined in comb ined anatomical and electrophysiological studies. In three adult macaq ue monkeys, the median nerve was cut, sutured, and allowed to regenera te for 7-13 months. After regeneration, distributions of afferents to the dorsal horn of the spinal cord and the cuneate nucleus of the brai nstem were determined by making injections of horseradish peroxidase c onjugates into the distal phalanges of digit 1 or 2. While label from a single digit on the normal hand was confined to the appropriate loca tions in the median nerve territories of the dorsal horn and cuneate n ucleus, label from the reinnervated digits spread out to cover most of the median nerve territories in those structures. These results are c onsistent with the interpretation that some proportion of primary sens ory fibers normally innervating other digits and pads of median nerve skin erroneously reinnervated the skin of the injected digits. In the same monkeys, microelectrodes were used to record from an array of clo sely spaced sites across the representation of the hand in area 3b of somatosensory cortex. The reactivation pattern was abnormal, with neur ons at many recording sites having more than one receptive field, larg er than normal receptive fields, or receptive fields at abnormal skin locations. Thus, there is somatotopic disorder both in the regenerated median nerve and in reactivated cortex, indicating that primary somat osensory cortex does not reorganize to compensate fully for peripheral reinnervation errors in these adult primates. Nevertheless, the organ ization of receptive fields in area 3b suggests the existence of some central selection of synapses.