[The thalamus: motor functions].

Thalamic nuclei constitute the diencephalic relay system between different afferent systems and the telencephalon. The "motor" thalamus links cerebellar and pallidal afferents with the motor and pre-motor cortical regions. Separation of pallido-thalamo-cortical and cerebello-thalamo-cortical pathways is relatively complete: afferents of pallidal origin are relayed by thalamic nuclei VA, VLo and VLm and are projected onto prefrontal and supplementary motor areas whereas afferents of cerebellar origin are relayed in the VL and VPLo and then projected onto motor areas 4.6 and parietal areas 5 and 7. This report is concerned mainly with the analysis of the motor thalamus as a relay system for afferents of cerebellar origin. The anatomofunctional organization of the cerebello-thalamo-cortical tract possesses a topographic arrangement and also wide convergences and appears able to participate in the realization, through the motor cortex, of motor synergies and of co-ordinating postures and movements. The cerebello-cortical tract apparently plays a different role during learning of a movement and executing an automatized movement. For the latter, it is involved in their onset and in their completion. Its contribution to fine control of temporal parameters of motor commands or control of peripheral afferent messages is not decisive. During learning of a movement the cerebello-cortical tract plays a predominant role. According to Ito, the cerebellum is involved in the process of automatization of a movement, organized initially entirely by the cortex and then becoming progressively subcortical. Plasticity of cerebellar microcircuits could be the basis for these automatization processes expressed through the cerebello-thalamo-cortical tract.