Cuneate nucleus reorganization following cervical dorsal rhizotomy in the macaque monkey: Its role in the recovery of manual dexterity

Immediately following a dorsal rhizotomy that removes input from the thumb, index, and middle fingers, the macaque is unable to execute movements that require controlled apposition of these digits. We have previously shown that within the early weeks and months following one of these lesions, there is 1) a re‐emergence of part or all of the cortical hand map; 2) central axonal sprouting of spared primary afferents into the dorsal horn and cuneate nucleus; and 3) substantial although incomplete recovery of hand function (Darian‐Smith [204] J. Comp. Neurol. 470:134–150; Darian‐Smith and Ciferri [2005] J. Comp. Neurol. 491:27–45). In this study we asked: What neuronal reorganization occurs in the cuneate nucleus during this “recovery” period? And, does it contribute to the recovery of manual dexterity? To address these questions, the representation of the hand was electrophysiologically mapped (by unitary receptive field [RF] recordings) in the pars rotunda of the cuneate nucleus at either 1–2 weeks (short term) or 16–32 weeks (long term) post‐rhizotomy. In short‐term monkeys, the region deprived of input from the thumb, index, and middle finger was found to be unresponsive to cutaneous stimulation. However, at 16–32 weeks later, when dexterity had largely recovered, RFs of cuneate neurons could again be mapped within the cuneate nucleus, primarily in a region bordering the deprived zone. We conclude that the cuneate pre‐ and postsynaptic reorganization that occurs following dorsal rhizotomy plays a key role in the recovery of hand function. J. Comp. Neurol. 498:552–565, 2006. © 2006 Wiley‐Liss, Inc.