Subregions
At least six areas are now recognized within the larger region once defined as the SMA. These subdivisions have been studied most extensively in the monkey brain. The most anterior portion is now commonly termed pre-SMA. It has sparse or no connections to the spinal cord or the primary motor cortex and has extensive connectivity with prefrontal areas.
The supplementary eye field (SEF) is a relatively anterior portion of the SMA that, when stimulated, evokes head and eye movements and perhaps movements of the limbs and torso.
Dum and Strick hypothesized on the basis of cytoarchitecture and connections to the spinal cord that the portion of SMA in the cingulate sulcus, on the medial part of the hemisphere, can be split into three separate areas, the cingulate motor areas. The functions of the cingulate motor areas have not yet been systematically studied.
SMA proper in monkeys has now been confined to a region on the crown of the hemisphere and extending partly onto the medial wall, just anterior to the primary motor leg representation. SMA proper projects directly to the spinal cord and therefore is one of the primary output areas of the cortical motor system.
Recently, Zhang et al. investigated the functional subdivisions of the medial SFC on the basis of whole-brain connectivity characterized from a large resting-state fMRI data set. Other than replicating the boundaries between SMA and preSMA, the current results support a functional difference between the posterior and anterior preSMA. In contrast to the posterior preSMA, the anterior preSMA is connected with most of the prefrontal but not somatomotor areas. Overall, the SMA is strongly connected to the thalamus and epithalamus, the posterior preSMA to putamen, pallidum, and STN and anterior preSMA to the caudate, with the caudate showing significant hemispheric asymmetry.
Read more about this topic: Supplementary Motor Area