Article révisé par les pairs
Résumé : Slightly suprathreshold magnetic stimuli were delivered over the left scalp while a normal human subject attended a sensory signal which called for a motor response of right fingers in a reaction time paradigm. The cortico-motoneuronal systems of the two muscles producing finger extension disclosed a remarkable physiological differentiation. Brain magnetic stimulation delivered just before the subject's motor response revealed cortico-motoneuronal facilitation for one extensor muscle and concomitant inhibition for the other, depending on the verbal instructions that had been given to the subject before the experimental trial. Thus, during the planning of specific finger manipulatory activities, the motor brain systems can achieve significant contrast in the membrane excitatory states of the motor cortical pyramidal neurones, even for synergic muscles.