The supplementary motor area (SMA) is on the inner, midline face of each frontal lobe, just in front of the leg region of the primary motor cortex. It is the medial part of Brodmann area 6, which reaches onto the rear of the superior frontal gyrus. [1][2][3]
The 3D brain needs JavaScript. The text below still says where the supplementary motor area is and what it does.
Opens cut by a side (sagittal) slice through the superior frontal gyrus, which holds the supplementary motor area. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
Lit: the whole superior frontal gyrus, on both sides. The area is part of this gyrus; its exact border is not drawn. The SMA is the medial extension of Brodmann area 6 onto the midline surface, and area 6 reaches onto the rear part of the superior frontal gyrus, whose medial portion forms that surface. The model draws the superior frontal gyrus as one shape with no separate medial face, so the whole gyrus is lit. [2][3][4][5]
Takes part in: movement — see every part of each lit together.
Where it is
It lies on the midline surface of the hemisphere, just in front of the leg representation of the primary motor cortex. [1]
It is the medial extension of Brodmann area 6 onto the midline surface of the hemisphere; the part of area 6 on the outer surface is the premotor cortex. [2]
The medial portion of the superior frontal gyrus is also called the medial frontal gyrus. [4]
At least six areas are now recognised in the region once defined as the SMA, mostly studied in monkeys: the frontmost is the pre-SMA, and the part in the cingulate sulcus can be split into three cingulate motor areas. [1]
What it does
Neurons in the SMA project directly to the spinal cord and may take part in the direct control of movement. [1]
Proposed roles are keeping the body stable in stance or walking, coordinating the two sides of the body as in two-handed actions, movements started from within rather than triggered by the senses, and sequences of movements; all remain hypotheses, and its precise role is not yet known. [1]
In monkeys it holds a rough map of the body, the legs at the back and the face at the front, with heavy overlap; in humans that body map is not apparent. [1]
In humans it generates the early part of the readiness potential (Bereitschaftspotential), the slow brain signal that builds up before a voluntary movement; SMA proper and the pre-SMA are active before a volitional action. [1]
Connections
Resting-state imaging shows the SMA strongly connected with the thalamus and epithalamus, the back of the pre-SMA with the putamen, globus pallidus and subthalamic nucleus, and the front of the pre-SMA with the caudate nucleus. [1]
SMA proper projects directly to the spinal cord, while the pre-SMA has sparse or no connections to the spinal cord or the primary motor cortex and many with prefrontal areas. [1]
The premotor cortex beside it has strong input and output links with it. [2]
Other names
Supplementary motor area, SMA: the medial part of Brodmann area 6. [1][2]
Its subdivisions include SMA proper, the pre-SMA, the supplementary eye field and the cingulate motor areas. [1]
Size
The model has no shape for the supplementary motor area itself, so it is not measured here. The whole left superior frontal gyrus that holds it measures about 116 mm front to back, 78 mm top to bottom and 32 mm side to side on this model (the box it fits in, not its volume). [5]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.