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The corticospinal projection from the cingulate motor cortex to the lower cervical enlargement (C5-T1) was investigated in four rhesus monkeys. Each received an injection of biotinylated dextran amine involving the arm representation of M3 (area 24c) or M4 (area 23c). In M3 cases, contralateral terminal label occurred in the lateral part of laminae V and VI of the intermediate zone including the reticulated marginal border. Lighter labeling was found in laminae IV, VII and the dorsolateral part of the anterior horn (lamina IX). In marked contrast, M4 cases demonstrated contralateral terminal labeling in the medial part of the dorsal and intermediate zones (laminae III, IV, V and VI). Lighter labeling involved the medial part of laminae VII, X and the dorsolateral anterior horn (lamina IX). Our experiments demonstrate that the corticospinal projection from the arm representations of M3 and M4 innervate distinct and separate parts of the spinal gray. Along with the noted differences in the cortical inputs to M3 and M4, these data suggest that the two cingulospinal systems may mediate independent and specialized forms of information effecting upper limb movement. |
