Activities of the primary and supplementary motor areas increase in preparation and execution of voluntary muscle relaxation: An event-related fMRI study

Brain activity associated with voluntary muscle relaxation was examined by applying event-related functional magnetic resonance imaging (fMRI) techniq ue, which enables us to observe change of fMRI signals associated with a si ngle motor trial. The subject voluntarily relaxed or contracted the right u pper limb muscles. Each motor mode had two conditions; one required joint m ovement, and the other did not. Five axial images covering the primary moto r area (M1) and supplementary motor area (SMA) were obtained once every sec ond, using an echo-planar 1.5 tesla MRI scanner. One session consisted of 6 0 dynamic scans (i.e., 60 sec). The subject performed a single motor trial (i.e., relaxation or contraction) during one session in his own time. Ten s essions were done for each task. During fMRI scanning, electromyogram (EMG) was monitored from the right forearm muscles to identify the motor onset. We calculated the correlation between the obtained fMRI signal and the expe cted hemodynamic response. The muscle relaxation showed transient signal in crease time-locked to the EMG offset in the M1 contralateral to the movemen t and bilateral SMAs, where activation was observed also in the muscle cont raction. Activated volume in both the rostral and caudal parts of SMA was s ignificantly larger for the muscle relaxation than for the muscle contracti on (p < 0.05). The results suggest that voluntary muscle relaxation occurs as a consequence of excitation of corticospinal projection neurons or intra cortical inhibitory interneurons, or both, in the M1 and SMA, and both pre- SMA and SMA proper play an important role in motor inhibition.