Supplementary motor area
The supplementary motor area (SMA) is involved in preparing for voluntary movements carried out by the primary motor area (precentral gyrus). It is located on the medial surface of the frontal lobe, contained posteriorly within the medial surface superior frontal gyrus (sometimes referred to as the medial frontal gyrus) just anterior to the paracentral lobule .
Its posterior border is the primary motor area, whereas its anterior border is defined fairly arbitrary by a line passing tangentially to the most rostral part of the genu of the corpus callosum, perpendicular to the AC-PC line . Inferiorly it is bounded by the cingulate sulcus.
The supplementary motor area is sometimes divided into two parts, 1) SMA proper and 2) pre-SMA by a second line perpendicular to the AC-PC line, running through the anterior commissure .