The supplementary motor area in the cerebral cortex

Abstract

The supplementary motor area (SMA) occupies an expanse of frontal agranular cortex rostral to the primary motor cortex (MI), largely in the mesial surface of the hemisphere. It is basically organized topographically, although the topography is not as apparent as in the MI. The traditionally defined SMA is now regarded as including two separate areas. The caudal part (SMA proper or F3) projects directly to the MI and to the spinal cord. The rostral part (pre-SMA or F6) is more remote from MI and receive projections from the prefrontal cortex and the cingulate motor areas. The supplementary eye field (SEF) is a small area separate from either the SMA or pre-SMA. The SEF is connected to cortical and subcortical areas related to oculomotor control. The SMA is active when subjects perform distal as well as proximal limb movement. Although the SMA is active in relation to relatively simple motor tasks, the functional significance of this relation to ‘simple’ movement is debatable. The SMA activity is subject to functional plasticity. The SMA is more active than the primary motor cortex if motor tasks are demanding in certain respects. Similarities of lesion effects of the SMA and basal ganglia suggests their intimate relation linked anatomically by the cortico-basal ganglia loops. Studies in both human subjects and in subhuman primates indicate the importance of the SMA in motor tasks that demand retrieval of motor memory. The SMA appears also crucial in temporal organization of movements, especially in sequential performance of multiple movements.