Electrophysiologic identification of projections from the midbrain to the paraflocculus and midvermis in the rat.

Regions of the midbrain in the rat were stimulated electrically with bipolar electrodes to identify responsive, single neurons in the parafloccular lobule of the cerebellum. Eighty four percent (44/52) of the cells recorded in the paraflocculus showed evidence of a modulation in simple spike discharge activity (mossy fiber activations) following stimulation with a bipolar electrode whose tip was placed in the ventral layers of the contralateral superior colliculus. Mossy fiber (MF) evoked responses were indicated by the presence of an excitation followed by an inhibition of simple spike frequency at latencies of 5-16 msec and by the demonstration of responsiveness to stimulus frequencies up to 50 Hz. Ten percent (4/41) of identified Purkinje cells in the paraflocculus demonstrated activation of complex spike potentials following stimulation of regions in the ventral superior colliculus. Experiments involving stimulation of the midbrain and visual cortices indicated that 70% of the parafloccular neurons are responsive to inputs from both the cortex and deep regions of the colliculus. Electrophysiologic evidence also is presented that demonstrates the existence of a midbrain projection to midvermal lobules VI and VII of the cerebellum.