Cerebellar Modulation of Cortically Evoked Complex Movements in Rats.

Intracortical microstimulation (ICMS) delivered to the motor cortex (M1) via long- or short-train duration (long- or short-duration ICMS) can evoke coordinated complex movements or muscle twitches, respectively. The role of subcortical cerebellar input in M1 output, in terms of long- and short-duration ICMS-evoked movement and motor skill performance, was evaluated in rats with bilateral lesion of the deep cerebellar nuclei. After the lesion, distal forelimb movements were seldom observed, and almost 30% of proximal forelimb movements failed to match criteria defining the movement class observed under control conditions. The classifiable movements could be evoked in different cortical regions with respect to control and many kinematic variables were strongly affected. Furthermore, movement endpoints within the rat's workspace shrunk closer to the body, while performance in the reaching/grasping task worsened. Surprisingly, neither the threshold current values for evoking movements nor the overall size of forelimb movement representation changed with respect to controls in either long- or short-duration ICMS. We therefore conclude that cerebellar input via the motor thalamus is crucial for expressing the basic functional features of the motor cortex.