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

Abstract

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.

Encoding of forelimb forces by corticospinal tract activity in the rat

“…Micro stimulation of the primary motor cortex in rhesus macaques generated both facilitatory and suppression effects in both flexor and extensor muscles of the distal and proximal forearm (Park et al, 2004); which is a response presumably mediated through the CST. Transcranial magnetic stimulation of the supplementary and primary motor areas during isometric static hand force task produced EMG effects with similar amplitude and latencies recorded from the intrinsic hand muscles, suggesting that both cortical areas effectively control the spinal cord excitability (Entakli et al, 2013). Indeed, many areas of the neocortex send projections to the spinal cord through the CST.…”

The assessment of interhemispheric imbalance using functional near-infrared spectroscopic and transcranial magnetic stimulation for predicting motor outcome after stroke

“…The SMA is a secondary motor area that plays a pivotal role in complex hand movements ( Shirota et al, 2012 ). SMA and M1 both have a direct influence on force production during fine manual motor tasks ( Entakli et al, 2014 ). A previous study using fMRI also found a correlation between task-related brain activation patterns and final motor status.…”

Connecting the dots: Harnessing dual-site transcranial magnetic stimulation to assess the causal influence of medial frontal areas on the motor cortex

“…Importantly though, other observations question the validity of this assumption. Indeed, like M1, SMA also presents pyramidal cells that project to the spine (Dum and Strick 1996) and in specific contexts, a unique stimulation of SMA with single-pulse TMS can evoke MEPs (Entakli et al 2014;Spieser et al 2013), indicating that these pyramidal cells can also recruit motoneurons. Thus, it is possible that the MEP potentiation reported in ppTMS studies using short intervals reflects the summation of volleys descending from SMA and M1 and converging at close times on motoneurons.…”