Effects of cerebellar nuclei stimulation on the electrical activity of the neurons in the centrum medianum nucleus in cats

The responses evoked in the neurons of the centrum medianum nucleus of the thalamus by stimulation of the deep cerebellar nuclei were studied. A marked predominance of activation effects during single shocks and lowfrequency stimulation was revealed. This was most striking with stimulation of the dentate nucleus. Parallel with increases in stimulation repetition rates, there was a noticeable rise both in unit responsiveness and in depressive responses. In general the percentage of unidirectional responses (either activation or depression) was considerably greater than that of responses which changed in direction with increases in stimulus rate. A predominance of tonic post-stimulation effects was also revealed. This was more marked after dentate or fastigial stimulation. In some centrum medianum neurons single shocks to the cerebellar nuclei evoked phasic responses. One type of these responses had a constant latency of 2- to 12-ms duration. A second type showed somewhat longer, although inconstant, latencies (≤20 ms). Single shocks to the dentate nucleus produced two temporal classes of latency (2 to 6 and 13 to 17 ms). In another type of centrum medianum neuron single shocks to the cerebellar nuclei, especially dentate and interpose, elicited depression of spontaneous firing for periods of 50 to 100 ms. A clear synchronization of spontaneous neuronal activity was evoked by low-frequency stimulation as well. Nearly 50% of the responsive neurons could be driven by excitation of the cerebellar nuclei. Convergent effects occurred more often with higher frequencies than with single-shock stimulation.