Motor Cortex - The Supplementary Motor Cortex

The Supplementary Motor Cortex

Penfield described a cortical motor area, the supplementary motor area (SMA), on the top or dorsal part of the cortex. Each neuron in the SMA may influence many muscles, many body parts, and both sides of the body. The map of the body in SMA is therefore extensively overlapping. SMA projects directly to the spinal cord and may play some direct role in the control of movement.

Based on early work using brain imaging techniques in the human brain, Roland suggested that the SMA was especially active during the internally generated plan to make a sequence of movements. In the monkey brain, neurons in the SMA are active in association with specific learned sequences of movement.

Others have suggested that, because the SMA appears to control movement bilaterally, it may play a role in inter-manual coordination.

Yet others have suggested that, because of the direct projection of SMA to the spinal cord and because of its activity during simple movements, it may play a direct role in motor control rather than solely a high level role in planning sequences.

On the basis of the movements evoked during electrical stimulation, it has been suggested that the SMA may have evolved in primates as a specialist in the part of the motor repertoire involving climbing and other complex locomotion.

Based on the pattern of projections to the spinal cord, it has been suggested that another set of motor areas may lie next to the supplementary motor area, on the medial (or midline) wall of the hemisphere. These medial areas are termed the cingulate motor areas. Their functions are not yet understood.