[Anatomo-functional organization of the neocerebellar control pathways on the cerebral motor cortex]

In a first phase of this study, the topographic arrangement of the dentato-thalamocortical projections was analyzed anatomically in albinos rats, by using anterograde and retrograde tracing methods. By applying intracortical microstimuli through the cerebral motor cortex prior to each microinjection of the tracer, the motor function of each injected cortical site was defined. As a consequence of the precise topographic arrangement of the dentato-thalamocortical pathways, we infer a somatotopic motor organization within the three cytoarchitectonic subdivisions of the cerebellar nucleus lateralis (NL), which corresponds to the dentate nucleus of the primate cerebellum: the hindlimb would be poorly represented, rostrally, whereas the head and forelimb would be more widely represented, within the central and caudal parts of the NL. In the second phase, the motor responses to direct microstimulations through the NL were mapped. The results confirmed the anatomofunctional arrangement deduced from the observations made in the first part of the study. The principles of the functional organization of the NL were also defined: i) the synergistic character of the movements induced by dentate stimulation is reflected by the activation of agonist as well as antagonist muscles that combine to move a particular segment of the body; ii) some body segments (e.g. those of the hindlimb) are represented within only one subdivision of the NL (single representation); others, such as the axio-proximal segments of the body, can be activated in a similar way from sites located in two or three subdivisions (multiple representation); finally, distal segments (digits or vibrissae) are multiply represented but activated in different ways by their various representation sites (specific representation): they can be moved independently from the parvocellular subdivision and only synchronously from their other representation site. The parvocellular subdivision of the rat NL is proposed as an equivalent to the "neodentatum" of the primate cerebellum.