Two channels in the cerebellothalamocortical system.

Two channels of the cerebellothalamocortical system were investigated in cats by using cerebellar-evoked synaptic responses and cortical-evoked antidromic invasion of single thalamic cells. One channel arises in interpositus and dentate cerebellar nuclei and mainly projects through ventroanterior-ventrolateral (VA-VL) thalamic nuclei to cortical motor areas 4 and 6; the other channel arises in cerebellar fastigial nuclei and projects through ventromedial (VM) thalamic nuclei to more widespread cortical areas. The antidromic response latencies of VM neurons to stimuli applied to cortical areas 4 and 6 were longer (medians 2.8 and 3.0 msec, respectively) than the antidromic response latencies of VA-VL neurons to stimulation of the same cortical areas (1.8 and 2.3 msec). This was a statistically significant difference, and it matched the longer latencies of fastigial-evoked synaptic responses of VM cells (2.9 msec) compared to the response latencies of VA-VL cells elicited by stimulation of interpositus or dentate nuclei (1.7 and 2.4 msec). These differences among thalamic nuclei relaying cerebellocortical impulses were corroborated by dissimilar effects exerted on the electroencephalogram (EEG) during high-frequency (300 Hz) pulse trains applied to different deep cerebellar nuclei. The distribution of activated EEG patterns over the cortex depended on the stimulated site. Fastigial stimulation elicited the blockage of slow EEG rhythms and the appearance of fast oscillations (20-40 Hz) over widespread cortical areas in the proreus, pericruciate, and suprasylvian gyri. At variance, the activating influence of interpositus or dentate nuclei was restricted to the motor cortex. It is proposed that, besides their role in controlling the postural axial and proximal musculature, fastigial nuclei are part of diffusely activating systems.