The Journal of Neurophysiology Vol. 87 No. 6 June 2002, pp. 3006-3017
Copyright ©2002 by the American Physiological Society

Integrated Motor Cortical Control of Task-Related Muscles During Pointing in Humans

 ¹Médecine Physique et Réadaptation, CHRU Lille, 59037 Lille Cedex, France and Université du Littoral-Côte d'Opale, BP699, 62228 Calais Cedex, France;  ²Human Cortical Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20982-1430;  ³Laboratoire de Neurophysique et Physiologie du Système Moteur, FRE Centre National de la Recherche Scientifique, 75006 Paris, France; and  ⁴Department of Anatomy and Physiology, Centre de Recherche Université Laval-Robert Gifford, Brain and Movement Laboratory, Université Laval, Quebec City, Quebec G1J 2G3, Canada

Devanne, Hervé, Leonardo G. Cohen, Nezha Kouchtir-Devanne, and Charles Capaday. Integrated Motor Cortical Control of Task-Related Muscles During Pointing in Humans. J. Neurophysiol. 87: 3006-3017, 2002. A large body of compelling but indirect evidence suggests that the motor cortex controls the different forelimb segments as a whole rather than individually. The purpose of this study was to obtain physiological evidence in behaving human subjects on the mode of operation of the primary motor cortex during coordinated movements of the forelimb. We approached this problem by studying a pointing movement involving the shoulder, elbow, wrist, and index finger as follows. Focal transcranial magnetic stimulation (TMS) was used to measure the input-output (I/O) curvesa measure of the corticospinal pathway excitabilityof proximal (anterior deltoid, AD, and triceps brachii, TB) and distal muscles (extensor carpi radialis, ECR, and first dorsal interosseus, 1DI) during isolated contraction of one of these muscles or during selective co-activation with other muscles involved in pointing. Compared to an isolated contraction of the ECR, the plateau-level of the ECR sigmoid I/O curve increased markedly during co-activation with the AD while pointing. In contrast, the I/O curve of AD was not influenced by activation of the more distal muscles involved in pointing. Moreover, the 1DI I/O curve was not influenced by activation of the more proximal muscles. Three arguments argue for a cortical site of facilitation of ECR motor potentials. First, ECR motor potentials evoked by a near threshold TMS stimulus were facilitated when the AD and ECR were co-activated during pointing but not those in response to a near threshold anodal electrical stimulus. Second, the ECR H reflex was not found to be task dependent, indicating that the recruitment gain of the ECR -motoneuron pool did not differ between tasks. Finally, in comparison with an isolated ECR contraction, intracortical inhibition tested at the ECR cortical site was decreased during pointing. These results suggest that activation of shoulder, elbow, and wrist muscles involved in pointing appear to involve, at least in part, common motor cortical circuits. In contrast, at least in the pointing task, the motor cortical circuits involved in activation of the 1DI appear to act independently.