Bilateral effects of spinal overhemisections on the development of the somatosensory system in rats

Connections of the forepaw regions of somatosensory cortex (S1) were determined in rats reared to maturity after spinal cord overhemisections at cervical level C3 on postnatal day 3. Overhemisections cut all ascending and descending pathways and intervening gray on one side of the spinal cord and the pathways of the dorsal funiculus contralaterally. Bilateral lesions of the dorsal columns reduced the size of the brainstem nuclei by 41%, and the ventroposterior lateral subnucleus (VPL) of the thalamus by 20%. Bilateral lesions also prevented the emergence of the normal cytochrome oxidase barrel pattern in forepaw and hindpaw regions of S1. Injections of wheat germ agglutinin conjugated to horseradish peroxidase were placed in the forepaw region of granular S1 and surrounding dysgranular S1 contralateral to the hemisection. The VPL nucleus was densely labeled, whereas the adjoining ventroposterior medial subnucleus, VPM, representing the head, was unlabeled. Thus, there was no evidence of abnormal connections of VPM to forepaw cortex. Foci of transported label in the ipsilateral hemisphere appeared to be in normal locations and of normal extents, but connections in the opposite hemisphere were broadly and nearly uniformly distributed in sensorimotor cortex in a pattern similar to that in postnatal rats. Rats with incomplete lesions that spared the dorsal column pathway on the left side but not the right demonstrated surprisingly normal distributions of callosal connections in the nondeprived right hemisphere, even though the injected left hemisphere was deprived. Thus, the development of the normal pattern of callosal connections depends on dorsal column input and not on normal interhemsipheric interactions.