Do ipsilateral corticospinal fibers participate in the functional recovery following unilateral pyramidal lesions in monkeys?

In monkeys with a unilateral subtotal lesion of the bulbar pyramid, a partial recovery of manual dexterity occurred within 4-6 weeks. This recovery may have been due to sprouting of fibers from the intact pyramid into the partly denervated areas of the spinal gray matter. In order to test this hypothesis, small injections of radioamino acids were made in the precentral hand area of two experimental animals (3 and 18 months after pyramidotomy) and one normal control animal. The pattern of spinal labeling was analyzed in autoradiographs from the cervicothoracic segments. No statistically significant additional labeling was found in laminae VI-VIII where the majority of the ipsilateral corticospinal fibers terminate. However, in the monkey with a long survival time, many large perikarya, situated exclusively within lamina IX in segments C8 and T1, were labeled with aggregations of silver grains. On the basis of the position and size of these neurons we conclude that a postsynaptic labeling occurred selectively in those motoneurons which control hand movements and which were deprived from their contralateral pyramidal control. We tentatively suggest that such a labeling may reflect an influence of the (injected) ipsilateral cortex on these distal motoneurons via direct connections which are normally not observed in the intact monkey.