Contributions des voies vestibulospinale et corticospinale au contrôle des mouvements du bras

The control of arm movement involves many pathways originating from the brain. This thesis, composed essentially of two studies, aims to clarify the contribution of pathways originating from the vestibular system and the motor cortex. The goal of the first study (Raptis et al 2007) was to better define the role of descending pathways originating from the vestibular system in motor equivalence during goal-directed movements, i.e. the ability of the system to reach the same motor goal regardless of the number of degrees of freedom involved. We hypothesized that the vestibular system plays an essential role in motor equivalence. We compared the ability of providing motor equivalence in arm-trunk reaching movements in healthy and chronic vestibulo-deficient subjects. While their vision was temporarily blocked, subjects were asked to maintain a position of the index finger or to reach a remembered target (within arm reach) while simultaneously flexing the trunk. In randomly selected trials, trunk movement was unexpectedly blocked by an electromagnetic device at the ‘‘go’’ signal. Healthy subjects were able to preserve the required finger position and trajectory in the two trunk conditions (trunk-free, trunk-blocked) by short-latency (60-180 ms) adjustments of shoulder and elbow joint rotations. In comparison, six of the seven chronic vestibulo-deficient patients showed deficits in the compensatory angular modifications, so that 30-100% of the trunk displacement was not compensated and thus influenced the finger position or trajectory. These results imply that vestibular influences evoked by head movement during trunk flexion play a major role in providing motor equivalence in these motor tasks when the number of degrees of freedom changes. These findings also suggest that the long-term plasticity following complete unilateral vestibular lesion may not be sufficient to allow the CNS to restore a normal degree of motor equivalence in arm-trunk movements. Arm-trunk coordination tasks constitute a new and sensitive approach in the clinical evaluation of vestibular deficits, targeting a functional dimension of vestibular influence which is not accounted for by standard clinical tests, which often are not sensitive enough to detect vestibular deficits after a 6 months recovery period.The first study thus addressed the question of how the brain and descending systems integrate additional degrees of freedom in the control of arm motion. In the second study (Raptis et al 2010), we aimed to clarify the nature of variables specified by descending systems for the control of arm motor actions. We tested the hypothesis that corticospinal pathways control the upper limb position and movement by modulating the threshold position (reference position from which muscles begin to be activated in response to a deviation from this reference). This control principle implies that descending pathways are not directly involved in the specification of EMG patterns, which would be reflected in a dissociation between the EMG and the corticospinal excitability for different threshold positions. In this study, participants (n=16) moved their wrist placed in a manipulandum from a flexed (45°) to an extended (-25°) position, and vice-versa. The passive elastic muscle forces were compensated by a torque motor, allowing subjects to equalize their background EMG activity at the two positions. The motoneuronal excitability at these positions was compared based on EMG responses to brief stretches. It appeared that not only EMG levels but also motoneuronal excitability were similar at the two positions. Moreover, the same tests showed that wrist repositioning was associated with resetting of the threshold wrist position. Although motoneuronal excitability was similar at these positions, corticospinal excitability of wrist muscles was significantly different: the same TMS pulse (1.2 MT over the M1 wrist area) produced motor-evoked potentials (MEPs) that were bigger for flexors in flexion than in extension and vice-versa for extensors (p<0.005 for all muscles, group data). When the same positions were passively established after profound muscle relaxation, stretch responses and MEP amplitudes drastically diminished. Also, in the majority of subjects, no position-related changes in the MEP amplitude were observed after relaxation. This second study thus suggests that the corticospinal system, possibly together with other descending systems, controls threshold position, a process that determines the spatial frame of reference in which EMG activity emerges. Such "referent" control constitutes a common principle for control of muscle forces, position, movement and muscle relaxation. This study also emphasizes the necessity, in future clinical research and clinical applications using TMS on motor areas, to take into account the threshold configuration of articulations involved in a motor task in order to adequately interpret muscles responses (or their modulation), especially in cases of muscular or cognitive activity, learning or neurological lesions causing motor deficits like spasticity or muscle weakness. Taken together, these two studies also shed light on fundamental principles of motor control, showing a global role of vestibular system in reaching tasks involving a coordination between the arm and its “support” (the trunk) and clarifying the involvement of corticomotor pathways in the specification of elemental parameters of the arm motor control. More studies are needed to better understand how sensory and descending systems (e.g. vestibulo- , reticulo-, rubro- and propriospinal) participate and interact with the corticofugal signals in order to set neuromuscular thresholds in posture and movement control.

Keyphrases: motor control, arm-trunk coordination, reaching, trajectory invariance, redundancy problem, vestibular deficiency, threshold position control, Equilibrium-point hypothesis, posture-movement problem, primary motor cortex, transcranial magnetic stimulation