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Reorganization of the Primate Dorsal Horn in Response to a Deafferentation Lesion Affecting Hand Function

Fisher, KM;Garner, J;Darian-Smith, CJ;

The loss of sensory input following a spinal deafferentation injury can be debilitating, and this is especially true in primates when the hand is involved. While significant recovery of function occurs, little is currently understood about the reorganization of the neuronal circuitry, particularly within the dorsal horn. This region receives primary afferent input from the periphery, and cortical input via the somatosensory subcomponent of the corticospinal tract (S1 CST), and is critically important in modulating sensory transmission, both in normal and lesioned states. To determine how dorsal horn circuitry alters to facilitate recovery post-injury, we used an established deafferentation lesion (DRL/DCL dorsal root/dorsal column) model in monkeys to remove sensory input from just the opposing digits (D1-D3) of one hand. This results in a deficit in fine dexterity that then recovers over several months. Electrophysiological mapping, tract tracing, and immunolabeling techniques were combined to delineate specific changes to dorsal horn input circuitry. Our main findings show that (1) there is complementary sprouting of the primary afferent and S1 CST populations into an overlapping region of the reorganizing dorsal horn, (2) afferent and efferent inputs converge on neuronal cell bodies in the dorsal horn pre- and post-injury, though most inputs are likely to be axodendritic, and (3) there is a loss of larger S1 CST terminal boutons in the affected dorsal horn, but no change in the size profile of the spared/sprouted primary afferent terminal boutons post-lesion. Understanding such changes helps to inform new and targeted therapies that best promote recovery.