TMS reveals a direct influence of spinal projections from human SMAp on precise force production

• Published in: The European journal of neuroscience 2014-01, Vol.39 (1), p.132-140
• Main Authors: Entakli, Jonathan, Bonnard, Mireille, Chen, Sophie, Berton, Eric, De Graaf, Jozina B.
• Format: Article
• Language: English
• Subjects: Adult; Biological and medical sciences; Cortex (motor); corticospinal tract; Evoked Potentials, Motor; Female; Fundamental and applied biological sciences. Psychology; Hand Strength; Humans; Life Sciences; Male; Middle Aged; Motor Cortex - physiology; Motor Skills; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Neurobiology; Neurons and Cognition; precision grip; Pyramidal Tracts - physiology; Reaction Time; silent period; Transcranial Magnetic Stimulation; Vertebrates: nervous system and sense organs
• ISSN: 0953-816X; 1460-9568
• DOI: 10.1111/ejn.12392

The corticospinal (CS) system plays an important role in fine motor control, especially in precision grip tasks. Although the primary motor cortex (M1) is the main source of the CS projections, other projections have been found, especially from the supplementary motor area proper (SMAp). To study the characteristics of these CS projections from SMAp, we compared muscle responses of an intrinsic hand muscle (FDI) evoked by stimulation of human M1 and SMAp during an isometric static low‐force control task. Subjects were instructed to maintain a small cursor on a target force curve by applying a pressure with their right precision grip on a force sensor. Neuronavigated transcranial magnetic stimulation was used to stimulate either left M1 or left SMAp with equal induced electric field values at the defined cortical targets. The results show that the SMAp stimulation evokes reproducible muscle responses with similar latencies and amplitudes as M1 stimulation, and with a clear and significant shorter silent period. These results suggest that (i) CS projections from human SMAp are as rapid and efficient as those from M1, (ii) CS projections from SMAp are directly involved in control of the excitability of spinal motoneurons and (iii) SMAp has a different intracortical inhibitory circuitry. We conclude that human SMAp and M1 both have direct influence on force production during fine manual motor tasks. We studied the muscle responses evoked by TMS of either SMAp or M1 during a precision grip task. TMS of SMAp evoked muscle responses with similar latencies and amplitudes as TMS of M1, but with a shorter silent period. This suggests that SMAp has a different intracortical inhibitory circuitry and that corticospinal projections from SMAp are directly involved in the control of excitability of spinal motoneurons. We conclude that SMAp and M1 both have direct influence on precise force production.