Activation of the Supplementary Motor Areas Enhances Spinal Reciprocal Inhibition in Healthy Individuals

The supplementary motor area (SMA) may modulate spinal reciprocal inhibition (RI) because the descending input from the SMA is coupled to interneurons in the spinal cord via the reticulospinal tract. Our study aimed to verify whether the anodal transcranial direct current stimulation (anodal-tDCS) of the SMA enhances RI. Two tDCS conditions were used: the anodal stimulation (anodal-tDCS) and sham stimulation (sham-tDCS) conditions. To measure RI, there were two conditions: one with the test stimulus (alone) and the other with the conditioning-test stimulation intervals (CTIs), including 2 ms and 20 ms. RI was calculated at multiple time points: before the tDCS intervention (Pre); at 5 (Int 5) and 10 min; and immediately after (Post 0); and at 5, 10 (Post 10), 15, and 20 min after the intervention. In anodal-tDCS, the amplitude values of H-reflex were significantly reduced for a CTI of 2 ms at Int 5 to Post 0, and a CTI of 20 ms at Int 5 to Pot 10 compared with Pre. Stimulation of the SMA with anodal-tDCS for 15 min activated inhibitory interneurons in RIs by descending input from the reticulospinal tract via cortico–reticulospinal projections. The results showed that 15 min of anodal-tDCS in the SMA enhanced and sustained RI in healthy individuals.