Effects of changes in mechanical state of limb on cerebellar intention tremor.

1. An animal model for the study of cerebellar intention tremor was produced by implanting cryoprobe sheaths alongside the dentate and interpositus nuclei of the cerebellum in three Cc>bris monkeys. These monkeys were trained to make fast flexion and extension elbow movements by moving a handle in a horizontal arc and to resist perturbations applied to the handle as it was held within target regions. 2. Cooling through both sheaths simultaneously (to 10°C sheath reference temperature) resulted in a 3to ~-HZ tremor in the ipsilateral forelimb, which was especially evident during rapid movements and following perturbations applied to the handle. 3. The character of the tremor was altered by mechanical loads applied to the limb: the frequency was increased by addition of spring stiffness and by an increase in constant torque and decreased by addition of mass. The amplitude of the tremor was decreased by an increase in viscous resistance. In addition, randomly timed perturbations applied to the forearm resynchronized the phase of the tremor. 4. The frequency of the tremor was also dependent on the position of the forelimb, being higher in flexion than in extension. 5. During cerebellar nuclear cooling the EM G responses to limb perturbations sometimes showed an increase in the segmental response (20 ms), while later responses (35400 ms), which may reflect activity in suprasegmental pathways, exhibited little change. There was no apparent relationship between changes in these early EM G responses and the tendency toward oscillation. 6. The fact that changes in the mechanical state of the limb can affect the character of cerebellar intention tremor supports the view that this tremor originates from instability in one or more reflex pathways. However, the lack of increase of the late EM G responses during cerebellar cooling suggests that the tremor is not due simply to an increase in gain in these reflex