Central neural mechanisms contributing to cerebellar tremor produced by limb perturbations

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Abstract
Central mechanisms contributing to cerebellar tremor were studied in 3 Cebus monkeys trained to return their arm rapidly to a target region after it was displaced by a perturbation applied to a handle. Cooling through cryoprobe sheaths implanted alongside the dentate and interpositus nuclei resulted in a series of oscillations (tremor) following the perturbation. During progressive cooling from control conditions there was a progressive increase in the number and amplitude of the oscillations and a progressive decrease in their frequency (from 6-8 to 3-5 Hz). The instability of these oscillations and their amplitude and frequency were related to the degree of cerebellar dysfunction. Analysis of EMG [electromyogram] activity in biceps and triceps confirmed that during cooling no consistent changes occurred in the segmental (20 ms) and suprasegmental (35-100 ms) reflex responses in the agonist (stretched) muscle, which contributed to the return of the arm following the perturbation. Responses of 74 precentral neurons responding to the torque pulse were studied under control conditions and during cerebellar cooling. Of these, 24 were closely related to activity of either the biceps or triceps muscle in that they had reciprocal responses: inhibition (20-50 ms) followed by excitation (50-100 ms) for 1 direction of perturbation, and excitation followed by inhibition for the other direction. Following a limb perturbation, return of the limb is brought about, partly by mechanical factors and partly by segmental and suprasegmental stretch reflexes. During cerebellar dysfunction the motor cortex does not receive predictive information, but receives only delayed information resulting for stretch of the antagonist muscle. This results in a descending command, which arrives too late to stop the corrective return and allows it to overshoot the target. It also prolongs the antagonist EMG activity, initiating a 2nd cycle of oscillation and subsequent cerebellar tremor. Cerebellar tremor is probably a series of alternating stretch reflexes mediated in part through motor cortex, and during normal cerebellar function, the cerebellum breaks this driving by peripheral afferent stretch responses by providing to the motor cortex a predictive signal for the antagonist muscle, which is phase advanced.