Somatotopic Nucleocortical Projections to the Multiple Somatosensory Cerebellar Maps

Abstract

The cerebellum is organized in a series of parasagittal compartments: in C1–C3 and C2 compartments Purkinje cells receive climbing fibre afferents from the rostral part of the accessory olives, and project their axon to the nucleus interpositus anterior and posterior, respectively. Within these compartments electrophysiological studies have shown that the cutaneous input carried by climbing fibre afferents is topographically organized so as to design a map of peripheral body districts. The body map is replicated over the anterior lobe-pars intermedia and the paramedian lobule, and anatomical studies have indicated that the replication is partly due to the axonal branching of olivocerebellar neurons. The aim of this study was to analyse the presence of a somatotopic organization and of a branching pattern in the nucleocortical projections, in relation to the replicated body maps within C1–C3 and C2 compartments. By using double retrograde neuronal tracing we explored, in the cat, the topographic distribution of single- and double-labelled cells in the interposed nuclear subdivisions, after tracer injections into forelimb or hindlimb regions of the anterior lobe-pars intermedia, paramedian lobule and hemisphere (medial crus II). Most of the nucleocortical neurons were found in ipsilateral nucleus interpositus posterior, with smaller numbers in the ipsilateral nucleus interpositus anterior. Nucleocortical neurons projecting to forelimb- or hindlimb-related areas are completely segregated, the forelimb neurons being located laterally and the hindlimb neurons medially in the nucleus interpositus posterior. Within their respective domains both the forelimb and hindlimb populations projecting to the anterior lobe-pars intermedia are partly segregated from those projecting to the paramedian lobule, in that the two populations are slightly shifted along the dorsoventral axis of the nucleus. Although mostly different, some of the cells are common to the two forelimb populations, since they send axonal branches to the homologous areas of the anterior lobe and paramedian lobule. Contralateral fastigial or interposed nucleocortical projections are restricted to the anterior lobe-pars intermedia, and their neurons of origin are different from those that project to the ipsilateral cerebellar cortex: i.e. they are not a bilateral, but a separate contralateral component.