Projections of the cerebellar and dorsal column nuclei upon the thalamus of the rhesus monkey.

Projections from the cerebellar and dorsal column nuclei to the midbrain and thalamus of the rhesus monkey were traced with anterograde autoradiographic techniques, or, in a few cases, with the Fink-Heimer method. The cerebellar nuclei give rise to a massive projection to the contralateral midbrain and thalamus via the ascending limb of the superior cerebellar peduncle. Cerebellar efferent fibers terminate contralaterally in both divisions of the red nucleus, and bilaterally in the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the oculomotor nucleus, and the central gray. All the deep cerebellar nuclei project upon a broad area of the contralateral ventral thalamus as well as certain intralaminar nuclei. Corresponding ipsilateral thalamic terminations are sparse. The topographic organization of cerebellothalamic fibers does not correspond to individual cerebellar nuclei or to cytoarchitectonic divisions of the ventral thalamic nuclei. Rather there are longitudinally oriented strips of terminal labeling which extend through all divisions of the ventral lateral nucleus, i.e., the VLps, the VLc, the VLo, as well as nucleus X, the oral division of the ventral posterolateral nucleus (VPLo), the central lateral nucleus (CL), and the most caudal region of the ventral anterior nucleus (VA). The topography of the cerebellothalamic fibers is arranged in a mediolateral pattern with fibers originating from anterior zones of the dentate and interpositus ending most laterally and those from posterior dentate and interpositus terminating most medially. The fastigial contribution is relatively sparse. The longitudinal strips of terminal labeling in the ventral thalamic nuclei are made up of still smaller terminal units consisting of disk-like aggregates of silver grains separated from one another by grain-free spaces. The dorsal column nuclei terminate primarily in the contralateral caudal division of the VPL (VPLc) and never extend rostrally into VPLo. These results demonstrate a segregation of cerebellar and dorsal columnar inputs to motor and sensory regions of the thalamus, respectively. Since these regions are separate and discrete in their cortical associations as well (Kalil, '76), it seems unlikely that fast afferent pathways relaying to motor cortex (Lemon and Porter, '76) could arise from the dorsal column nuclei.