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 i s involved in the control of proximal vs. distal musculature, independ ent finger movements, and movements characteristic of different specie s of animals now is unclear. The objective of this study was to examin e the effects of pyramidal tract lesions on skilled forelimb use in ra ts. In addition, cell morphology in motor cortex following lesions was examined. Naive and trained rats received unilateral pyramidal sectio ns just rostral to the pyramidal decussation. Performance was assessed and filmed on two reaching tasks. Measures of reaching consisted of s uccess in obtaining food, kinematic analysis of limb trajectory and ve locity, and qualitative evaluation of 10 movement components comprisin g a reach. Pyramidal tract lesions only impaired reaching for single f ood pellets. Almost all movements comprising a reach, except digit ope ning, were impaired, including lifting, aiming, pronating and supinati ng the limb, and releasing food. Although success in limb use was unch anged over the 180 day observation period, there were significant impr ovements 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 structur es, such as the medial lemniscus and olivary complex, were much more s everely impaired on the reaching tasks, and displayed similar impairme nts as judged by qualitative and kinematic measures. The results demon strate that a number of movements involved in independent limb use are chronically impaired by pyramidal tract lesions in the rat. Neverthel ess, 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.