Cerebellar input to the posterior parietal cortex in the rat

The cerebello-thalamo-parietal projections were investigated in rats by means of a multiple anterograde-retrograde tracing technique. Retrograde fluorescent tracers were injected in different loci of the parietal cortex. Injected areas were verified cytoarchitectonically and confirmed by analyzing the retrograde thalamic labeling pattern obtained. Anterograde fluorescent tracers were placed in the intermediate and lateral deep cerebellar nuclei. The topographical overlap between cerebellar terminals and parietal-projecting thalamic neurons was analyzed. In the central lateral (CL) and ventrolateral (VL) thalamic nuclei, cells projecting to anterior somatosensory (S1) and posterior parietal (PPC) cortices were demonstrated to receive direct cerebellar input. In particular, two patterns of organization were revealed. In CL, the PPC- and S1-projecting neurons, both receiving cerebellar fibers, were intermingled. In VL, PPC, and S1-projecting neurons were instead segregated, and the areas containing labeled neurons received separate contingents of cerebellar fibers. These patterns suggest that in CL axons originating from the same or neighboring cerebellar neurons can terminate on both PPC- and S1-projecting neurons, while in VL cerebellar information is funneled to S1 and PPC through two segregated parallel pathways. The significance of the observed organization is discussed comparing findings in other species and in relation with electrophysiological and functional studies on cerebelloparietal interrelationships.