Impact of the brainstem in normal and impaired human movement control.

The corticospinal tract represents a highly developed neural system that dominates motor control in humans. This allows the performance of skilled finger movements, such as playing the piano. In contrast, according to preclinical studies, automatically performed complex movements, such as locomotion or cooperative forelimb movements, appear to be controlled mainly by brainstem motor centres. Only in recent years, with the advancement of new methodologies, has evidence emerged that limb coordination in humans is also based on neural limb coupling mediated by brainstem motor centres. This neural limb coupling is reflected in the observation that, during a movement, unilateral electrical nerve stimulation elicits muscle responses not only in the stimulated but also in the contralateral limb(s), involved in the respective motor task. The finding that activation of the reticulospinal system by loud acoustic stimuli evokes muscle responses that closely resemble the configuration of 'coupling responses' induced by electrical nerve stimulation suggests a contribution of the brainstem to the neural coupling mechanism. In addition to its role in interlimb coordination during stepping or cooperative hand movements, the reticulospinal system seems to be involved in the control of other motor tasks, including posture, reaching and force development. Furthermore, recent evidence indicates that brainstem motor centres contribute to functional recovery after stroke and spinal cord injury. These observations suggest that the brainstem plays a major role in motor control not only in animals but also in humans.