Cerebellar participation in generation of prompt arm movements

Abstract

Participation of the cerebellum in movement generation was investigated in 3 Cebus monkeys that were trained to make prompt elbow flexions and extensions by moving a handle through a horizontal arc in response to a visual GO signal (simple reaction time task). Cooling to 10.degree. C through either 1 of 2 probes stereotaxically implanted lateral or medial to the dentate nucleus, ipsilateral to the operant arm, increased reaction times (RT) for both EMG [electromyogram] and movement onset by up to 100 ms without any slowing of the initial velocity of movement. More severe cerebellar dysfunction produced by cooling both probes simultaneously resulted in a further increase in RT, in part due to a slowing of velocity at movement onset. Single unit recording from motor cortex revealed that the increase in RT was accompanied by a delay in onset of discharge of movement-related neurons. For 43 of 48 neurons the delay could not be explained by a decrease in tonic background activity. Cooling the cerebellar nuclei diminished the well defined phasic nature of the discharge of 16 of 48 neurons. The results indicated that the cerebellum participates in generation of prompt arm movements and that 1 possible mechanism for this is by transmission of a phasic movement instruction from cerebellar nuclei to motor cortex. The results were not compatible with the possibility that the movement instruction travels over additional, but slower, parallel pathways to motor cortex or that the cerebellum may influence motoneurons via subcortical routes.