Effects of neonatal hemispherectomy on location and number of corticospinal neurons in the rat.

Corticospinal neurons in the rat normally project to the contralateral spinal cord only. After neonatal hemispherectomy, the remaining single hemisphere forms both ipsilateral and contralateral projections. The distribution and number of corticospinal neurons projecting to the contralateral and to the ipsilateral spinal cord were studied in the hemispherectomized rat. The method used was that of retrograde labelling of corticospinal neurons by unilateral spinal cord injection of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP). The establishment of both crossed and uncrossed corticospinal projections by one cerebral hemisphere is accomplished in two ways. (1) Some neurons located within the confines of normal motor cortex send axons to ipsilateral spinal cord, where normally only contralateral projections occur. (2) There is expansion of the areas of cerebral cortex that contain corticospinal neurons. This expansion is most pronounced near the frontal pole, but it occurs at all borders of motor cortex except at its posterior limit, where the border with visual cortex persists unchanged. Only corticospinal neurons forming uncrossed projections occur in aberrant sites, while both cells with crossed and those with uncrossed spinal axons are intermingled in cortex that normally contains corticospinal neurons. The findings suggest that plasticity of the corticospinal motor system in the rat occurs to some extent at the expense of impaired localization of functions in cerebral cortex, and that following focal brain injury there may be an increase in areas that subserve multiple functions.