Corticospinal sprouting occurs selectively following dorsal rhizotomy in the macaque monkey

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The corticospinal tract in the macaque and human forms the major descending pathway involved in volitional hand movements. Following a unilateral cervical dorsal root lesion, by which sensory input to the first three digits (D1– D3) is removed, monkeys are initially unable to perform a grasp retrieval task requiring sensory feedback. Over several months, however, they recover much of this capability. Past studies in our laboratory have identified a number of changes in the afferent circuitry that occur as function returns, but do changes to the efferent pathways also contribute to compensatory recovery? In this study we examined the role of the corticospinal tract in pathway reorganization following a unilateral cervical dorsal rhizotomy. Several months after animals received a lesion, the corticospinal pathways originating in the primary somatosensory and motor cortex were labeled, and terminal distribution patterns on the two sides of the cervical cord were compared. Tracers were injected only into the region of D1– D3 representation (identified electrophysiologically). We observed a strikingly different terminal labeling pattern post lesion for projections originating in the somatosensory versus motor cortex. The terminal territory from the somatosensory cortex was significantly smaller compared with the contralateral side (area mean = 0.30 vs. 0.55 mm2), indicating retraction or atrophy of terminals. In contrast, the terminal territory from the motor cortex did not shrink, and in three of four animals, aberrant terminal label was observed in the dorsal horn ipsilateral to the lesion, indicating sprouting. These differences suggest that cortical regions play a different role in post- injury recovery. J. Comp. Neurol. 521:2359– 2372, 2013. © 2012 Wiley Periodicals, Inc.The corticospinal tract in the macaque and human originates from at least nine separate cortical regions, and is the major descending pathway mediating volitional hand movements. Here we compared primary motor and somatosensory corticospinal subcomponents in the recovery process several months following a unilateral cervical dorsal rhizotomy in macaques. Our findings show a very different response from each cortical region, indicating that each plays a very different role post injury. Our findings also indicate that the corticospinal tract response to spinal cord injury is considerably more complex than is generally recognized.

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