The goal of this study was to assess the motor output capabilities of the forelimb representation of the supplementary motor area (SMA) in terms of the sign, latency and strength of effects on electromyographic (EMG) activity. Stimulus triggered averages of EMG activity from 24 muscles of the forelimb were computed in SMA during a reach-to-grasp task. Poststimulus facilitation (PStF) from SMA had two distinct peaks (15.2 and 55.2 ms) and one poststimulus suppression (PStS) peak (32.4 ms). The short onset latency PStF and PStS of SMA were 5.5 and 16.8 ms longer than those of the primary motor cortex (M1). The average magnitudes (peak increase or decrease above baseline) of the short and long latency PStF and PStS from SMA at 60 μA were 13.8, 11.3 and -11.9% respectively. In comparison, M1 PStF and PStS magnitudes at 15 μA were 50.2 and -23.8%. Extrapolating M1 PStF magnitude to 60 μA yields a mean effect that is nearly 15 times greater than the mean PStF from SMA. Moreover, unlike M1, the facilitation of distal muscles from SMA was not significantly greater than the facilitation of proximal muscles. We conclude that the output from SMA to motoneurons is markedly weaker compared with M1 raising doubts about the role of SMA corticospinal neurons in the direct control of muscle activity.
Bibliographical noteFunding Information:
We thank Drs Mehmet Bilgen and William Brooks in the Hoglund Brain Imaging Center and Dr John Overman in the Department of Neurosurgery at KUMC for their expert assistance with our imaging procedures. We also thank Dr Matthew Mayo for help with the statistical analysis and Phil Shafer for help with illustrations. This research was supported by NINDS Grant NS39023, NICHD Center Grant HD02528 and the FRSQ scholarship 12014-907.
- Motor cortex
- Stimulus-triggered averaging