Sensory afferent projections and area 3b somatotopy following median nerve cut and repair in macaque monkeys.

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The fidelity of median nerve regeneration and the consequent effects of regeneration errors on cortical organization were determined 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 spinal cord and the cuneate nucleus of the brainstem were determined by making injections of horseradish peroxidase conjugates into the distal phalanges of digit 1 or 2. While label from a single digit on the normal hand was confined to the appropriate locations in the median nerve territories of the dorsal horn and cuneate nucleus, label from the reinnervated digits spread out to cover most of the median nerve territories in those structures. These results are consistent with the interpretation that some proportion of primary sensory 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 closely spaced sites across the representation of the hand in area 3b of somatosensory cortex. The reactivation pattern was abnormal, with neurons at many recording sites having more than one receptive field, larger 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 somatosensory cortex does not reorganize to compensate fully for peripheral reinnervation errors in these adult primates. Nevertheless, the organization of receptive fields in area 3b suggests the existence of some central selection of synapses.