Distribution of cerebellar terminations and their relation to other afferent terminations in the ventral lateral thalamic region of the monkey.

The efferent projections of the deep cerebellar nuclei were studied and their fiber trajectories and thalamic termination zones described. The thalamic termination zones for the dentate, interposed and fastigial nuclei are identical and coincide with the cytoarchitectonically unique cell-sparse region of the ventral lateral complex. This region includes nuclei VPLo, VLc, VLps, X and extensions of these between the cell clusters of nucleus VLo. The inputs from dentate and interpositus are contralateral, dense, and their termination patterns extend continuously throughout all nuclear components of the cell-sparse zone. Interdigitation of these two inputs within the cell-sparse region is directly demonstrated. The fastigial input is more restricted but bilateral. Each of the deep cerebellar nuclei also projects to the central lateral nucleus of the intralaminar complex. The strong interconnection of the cell-sparse zone with cortical area 4 is confirmed. The patterns of retrogradely labeled thalamocortical cells and of anterogradely labeled corticothalamic terminations following cortical injections of horseradish peroxidase and of tritiated amino acids, extend continuously through the VPLo-VLc region and its extensions, but do not invade the posteriorly situated VPLc nucleus. Thalamic inputs from the dorsal column nuclei terminate independently within the morphologically distinct VPLc nucleus adjacent to the cell-sparse cerebellar terminal zone. The dorsal column-lemniscal terminations do not overlap the cerebellar terminations. The clear segregation of the two sets of terminations is demonstrated directly using an anterograde double labeling method. Spinothalamic terminations end in VPLc but extend into the cerebellar terminal zone. Another ascending input, from the vestibular nuclei, is also shown to terminate within the cell-sparse zone. Comparison with other studies implies that cerebellar, pallidal and substantia nigral inputs do not converge in the monkey thalamus and that the nuclei in which they terminate project to different cortical areas. The relation of these and of sensory influences ascending to motor cortex from the periphery are discussed.