Proximal and distal impairments in rat forelimb use in reaching follow unilateral pyramidal tract lesions

Although it was once thought that the corticospinal (pyramidal) tract was the main substrate of voluntary movement, the extent to which it is involved in the control of proximal vs. distal musculature, independent finger movements, and movements characteristic of different species of animals now is unclear. The objective of this study was to examine the effects of pyramidal tract lesions on skilled forelimb use in rats. In addition, cell morphology in motor cortex following lesions was examined. Naive and trained rats received unilateral pyramidal sections just rostral to the pyramidal decussation. Performance was assessed and filmed on two reaching tasks. Measures of reaching consisted of success in obtaining food, kinematic analysis of limb trajectory and velocity, and qualitative evaluation of 10 movement components comprising a reach. Pyramidal tract lesions only impaired reaching for single food pellets. Almost all movements comprising a reach, except digit opening, were impaired, including lifting, aiming, pronating and supinating the limb, and releasing food. Although success in limb use was unchanged over the 180 day observation period, there were significant improvements in the qualitative features of limb use. Histologically, the morphology of pyramidal cells in the forelimb area ipsilateral to the lesion seemed normal. Rats with additional damage to adjacent structures, such as the medial lemniscus and olivary complex, were much more severely impaired on the reaching tasks, and displayed similar impairments as judged by qualitative and kinematic measures. The results demonstrate that a number of movements involved in independent limb use are chronically impaired by pyramidal tract lesions in the rat. Nevertheless, significant use of the limb is possible, due perhaps to both the contribution of extrapyramidal motor systems and the influence of the remaining pyramidal system through its extrapyramidal connections. The results not only show that the rat pyramidal tract supports functions very similar to those of primates and thus might provide a good model for some aspects of pyramidal tract dysfunctions, but also they argue that the pyramidal tract is involved in both proximal and distal limb movements.