Increased primary motor cortical excitability by a single-pulse transcranial magnetic stimulation over the supplementary motor area

Abstract

The supplementary motor area (SMA) is a secondary motor area that is involved in various complex hand movements. In animal studies, short latency and probably direct excitatory inputs from SMA to the primary motor cortex (M1) have been established. Although human imaging studies revealed functional connectivity between SMA and M1, its electrophysiological nature has been less studied. This study explored the connection between SMA and M1 in humans using a single-pulse transcranial magnetic stimulation (TMS) over SMA. First, TMS over SMA did not alter the corticospinal tract excitability measured by the size of motor evoked potential elicited by single-pulse TMS over M1. Next, we measured short-interval intracortical facilitation (SICF), which reflects the function of a facilitatory circuit within M1, with or without a single-pulse TMS over SMA. When the intensity of the second pulse in the SICF paradigm (S2) was as weak as 1.0 active motor threshold for a hand muscle, SMA stimulation significantly enhanced the SICF. Furthermore, this enhancement by SMA stimulation was spatially confined and had a limited time window. On the other hand, SMA stimulation did not alter short-interval intracortical inhibition or contralateral silent period duration, which reflects the function of an inhibitory circuit mediated by gamma-aminobutyric acid A (GABA(A)) or GABA(B) receptors, respectively. We conclude that a single-pulse TMS over SMA modulates a facilitatory circuit within M1.