Responses of the red nucleus neurons to limb stimulation after cerebellar lesions

Two series of experiments concerning the effects induced by unilateral cerebellar lesions on background activity and responses of red nucleus neurons contralateral to lesion sites are reported in this short review. The first series describes the effects of cerebellar cortex ablations, the second reports the results of hemicerebellectomy. The major source of input to the deep cerebellar nuclei, the Purkinje cells of the cerebellar cortex are known to be inhibitory. Removal of this influence by cerebellar cortex lesion results in an increase of the interpositus neurons background activity. This effect strengthens the interpositus excitatory influence upon rubral neurons by evoking exaggerated depolarization and enhancing sensitivity of these cells to somatosensory volleys. This causes a considerable increase in the background firing, eliminates the pauses in discharges occurring in responses generated by somatosensory stimuli, and augments the excitatory rebound. These changes destroy the content and timing of feedback information flowing through the spino-cerebello-rubro-spinal loop, and are, at least in part, responsible for motor deficits of cerebellar dysfunction. Hemicerebellectomy reduces drastically spontaneous firing of rubro-spinal neurons and simplifies response patterns. Due to direct spino-rubral pathways the latency of response is not changed.