[Contribution of cerebellar efferents to the organization of motor synergy].

The lack of coordination in cerebellar patients is due to deficits affecting both the programming and the execution of motor activity. The aspects of motor performance which are most severely affected are the chronology, the time course and the distribution of the activatory inputs to the muscles. The cerebellar control of motor synergies partly depends on the intrinsic organization of the cerebellar cortex, but interconnections between the cerebellum and the cortex, as well as the organization of the cerebello-spinal outputs are also involved. All these elements take part in different ways in the various aspects of motor synergy control. Depending on the type of sensory signals involved in the triggering of coordinations, different cortico-cerebello-cortical connections take part in their programming. Those synergies which are initiated by external sensory signals are triggered by inputs from the dentate and interposed nuclei and transmitted to parietal associative areas 5 and 7. The programming of the synergies induced by internal signals or motivational states involves projections from the dentate nucleus to prefrontal area 9 as well as to the supplementary motor (SMA) and premotor (PM) areas. The distribution of the cerebellar outputs to the muscles participating in motor synergies depends on the organization of the cerebellar circuits projecting to PM and M1 neurones at the origin of the descending pathways. The fastigial and dentate nuclei give off projections to the PM whereby they activate both axial and proximal muscles via bilateral reticulo-spinal pathways, and these nuclei are therefore responsible for synergies which provide for the necessary postural adjustments when limb movements are performed. When voluntary movements are to be performed, the cerebellar efferents from the three cerebellar nuclei to the motor cortex (M1) are able to trigger all the necessary synergies. Since the motor cortex can be subdivided into several representation areas dealing with elementary unidirectional movements performed by a single joint, activating the requisite coordinations requires particular patterns of cerebello-thalamocortical activation. With these networks, which can be said to constitute the morphological basis of motor synergies, several body representation cortical areas can be activated simultaneously. These networks seem to be partly set up before birth; but they may also be shaped during the early stages of life. They are still adaptable during adult-hood, and they can undergo synaptic and functional changes in response to the sensory constraints imposed by the environment.