ANATOMO-FUNCTIONAL ORGANIZATION OF THE NE OCEREBELLAR CONTROL PATHWAYS TO THE CEREBRAL MOTOR CORTEX

In a first phase of this study, the topographic arrangement of the den tato-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 injecte d cortical site was defined. As a consequence of the precise topograph ic arrangement of the dentato-thalamocortical pathways, we infer a som atotopic motor organization within the three cytoarchitectonic subdivi sions of the cerebellar nucleus lateralis (NL), which corresponds to t he dentate nucleus of the primate cerebellum : the hindlimb would be p oorly represented, rostrally, whereas the head and forelimb would be m ore 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 anatomofunctiona l arrangement deduced from the observations made in the first part of the study. The principles of the functional organization of the NL wer e also defined: i) the synergistic character of the movements induced by dentate stimulation is reflected by the activation of agonist as we ll as antagonist muscles that combine to move a particular segment of the body; ii) some body segments (e.g. those of the hindlimb) are repr esented within only one subdivision of the NL (single representation); others, such as the axio-proximal segments of the body, can be activa ted in a similar way from sites located in two or three subdivisions ( multiple representation); finally, distal segments (digits or vibrissa e) are multiply represented but activated in different ways by their v arious representation sites (specific representation): they can be mov ed independently from the parvocellular subdivision and only synchrono usly from their other representation site The parvocellular subdivisio n of the rat NL is proposed as an equivalent to the [neodentatum] of t he primate cerebellum.