HRP studies on thalamocortical neurons related to the cerebellocerebral projection in the monkey.

In monkeys (Macaca fuscata and mulatta), horseradish peroxidase (HRP) was injected in the cerebral cortices, areas 4 (medial and lateral parts), 5, 6 and 9 (a part just rostral to area 6), in which stimulation of cerebellar nuclei was known to elicit superficial or deep thalamocortical responses. Retrogradely labelled thalamic neurons consisted of two separate clusters of neurons, as judged by the continuity of labelled thalamic cells; lateral (VApc, VLo, VLm, VPLo, VPLc, LP) and medial (VAmc, X, Pcn, Cl, Pulo, Pulm, Pull, MD) ones. In each cluster, neurons in the medial portion tend to project on more rostral cerebral cortices than those in the lateral portion. In the lateral cluster, medially and laterally located thalamic neurons tend to project, respectively, on lateral and medial parts of a cortical area. Retrograde and anterograde labelling revealed that thalamocortical and corticothalamic projections are mostly reciprocal except CM which receives massive terminals from area 4 but projects scarcely on to the area. According to the references which have demonstrated terminations of cerebellothalamic neurons, thalamocortical neurons relaying the cerebellocerbral projection are presumed to be included in both the medial and lateral thalamic cell clusters mentioned above. The organization of cerebellothalamocortical projections appears more complicated and elaborated than that reported before.