Sensory Afferent Projections and Area 3b Somatotopy following Median Nerve Cut and Repair in Macaque Monkeys

The fidelity of median nerve regeneration and the consequenteffects of regeneration errors on cortical organization weredetermined in combined anatomical and electrophysiological studies.In three adult macaque monkeys, the median nerve was cut, sutured,and allowed to regenerate for 7–13 months. After regeneration,distributions of afferents to the dorsal horn of the spinalcord and the cuneate nucleus of the brainstem were determinedby making injections of horseradish peroxidase conjugates intothe distal phalanges of digit 1 or 2. While label from a singledigit on the normal hand was confined to the appropriate locationsin the median nerve territories of the dorsal horn and cuneatenucleus, label from the reinnervated digits spread out to covermost of the median nerve territories in those structures. Theseresults are consistent with the interpretation that some proportionof primary sensory fibers normally innervating other digitsand pads of median nerve skin erroneously reinnervated the skinof the injacted digits. In the same monkeys, microelectrodeswere used to record from an array of closely spaced sites acrossthe representation of the hand in area 3b of somatosensory cortex.The reactivation pattern was abnormal, with neurons at manyrecording sites having more than one receptive field, largerthan normal receptive fields, or receptive fields at abnormalskin locations. Thus, there is somatotopic disorder both inthe regenerated median nerve and in reactivated cortex, indicatingthat primary somatosensory cortex does not reorganize to compansatefully for peripheral reinnervation errors in these adult primates.Nevertheless, the organization of receptive fields in area 3bsuggests the existence of some central selection of synapses.