The natural discharges of Purkinje cells in paravermal regions of lobules V and VI of the monkey's cerebellum

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Abstract
Conscious monkeys [Macaca fascicularis] were trained with food rewards to perform movement tasks with the left hand and to accept manipulation of the joints and muscles and natural non-noxious stimulation of the skin of both forelimbs. Recordings were made from 230 Purkinje cells situated in the paravermal region of lobules V and VI or immediately adjacent folia of the left cerebellum in a region from 2-7 mm from the mid line. These neurons were all in a zone demonstrated to received inputs from the ipsilateral hand and known to receive projections, via the pontine nuclei from the arm area of motor cortex in the right hemisphere. Modulation of the natural activity of 182 of these 230 Purkinje cells (79%) occurred in a reproducible manner in temporal association, each with a particular phase of the self-paced movement tasks performed by the animal using the ipsilateral arm and hand. The patterns of modulation of Purkinje cell firing in this limited zone of cerebellar cortex could be classified into 1 of 4 groups, and each cell''s discharge was associated with a particular aspect of movement such as general arm flexion, shoulder retraction, elbow extension or elbow flexion whenever it occurred. The cells were spontaneously active at rest. Most commonly, marked accelerations of the discharge were related to 1 direction of the particular aspect of movement and a reduction of activity or even total silence accompanied movement in the opposite direction. Variation of the amount of discharge demonstrated during a movement performance with which this discharge was characteristically associated could be related to the range of the movement or its duration, more activity being characteristic of more prolonged movement performance through larger angles of joint displacement. Both simple spikes and complex spikes of some cells showed characteristic modulation of their activity during the monkey''s self-initiated movements. Cells whose simple spikes did not change in frequency during the movement task, also showed no modification of complex spike discharge. Of the 182 neurons whose discharges changed during active movement performance, 105 (roughly 60%) received an input from peripheral receptors in the hand which could be activated by brisk tapping of the skin or brushing of hairs. None of the Purkinje cells whose discharges were unchanged during arm movements received such an input. Movement of joints through their full range and prodding of muscles were completely ineffective stimuli for causing changes in Purkinje cell firing in this zone of the cerebellar cortex while the animal was passive and relaxed. Imposed perturbations of movement performance injected unexpectedly during the execution of a movement task were also ineffective in modifying the discharge of these Purkinje cells in relation to the task.