Recovery of skilled reaching following motor cortex stroke: do residual corticofugal fibers mediate compensatory recovery?

Motor cortex (MC) injury impairs skilled reaching in rats, but success scores are eventually restored to approximate preoperative levels. The improvement is attributed to compensatory strategies, such as substituting trunk rotations for the chronically lost rotatory movement of the forelimb, that occur during transport and withdrawal. The present study examined the contributions of the rostral motor cortex (RMC) and the caudal motor cortex (CMC) to skilled reaching performance. The study also examined the role of the ipsilateral and the contralateral hemispheres in supporting the spontaneous recovery. Rats were trained to reach for single food pellets, and their recovery from partial or complete MC injury was documented with quantitative scores and movement element measures in three experiments: (1) devascularization of the CMC, or the RMC, or both, in the hemisphere contralateral to the reaching paw; (2) additional lesions to the CMC and RMC injuries such that the conjoint damage amounted to an MC lesion; and (3) MC lesion followed by damage in the neocortex lateral to the injury or in the opposite MC. The results showed that the CMC made the main contribution to skilled reaching performance, and that there was a lesser contribution by the RMC. MC damage was exacerbated by additional damage to the ipsilateral neocortex as compared to the contralateral neocortex. The results are discussed in relation to the idea that the involvement of the neocortical areas in skilled reaching performance and its recovery is proportional to the region from which corticospinal projections originate.