A role for the cerebellum in the control of limb movement velocity

Accepting, rejecting or modifying the many different theories of the cerebe llum's role in the control of movement requires an understanding of the sig nals encoded in the discharge of cerebellar neurons and how those signals a re transformed by the cerebellar circuitry. Particularly challenging is und erstanding the sensory and motor signals carried by the two types of action potentials generated by cerebellar Purkinje cells, the simple spikes and c omplex spikes. Advances have been made in understanding this signal process ing in the context of voluntary arm movements. Recent evidence suggests tha t mossy fiber afferents to the cerebellar cortex are a source of kinematic signals, providing information about movement direction and speed. In turn, the simple spike discharge of Purkinje cells integrates this mossy fiber i nformation to generate a movement velocity signal. Complex spikes may signa l errors in movement velocity. It is proposed that the cerebellum uses the signals carried by the simple and complex spike discharges to control movem ent velocity for both step and tracking arm movements.