Supplementary motor area

The supplementary motor area (SMA) is a part of the sensorimotor cerebral cortex (perirolandic, i.e. on each side of the Rolando or central sulcus). It was included, on purely cytoarchitectonic arguments, in area 6 of Brodmann and the Vogts. It is located on the medial face of the hemisphere, just in front of primary motor cortex. This is an element that appeared late in evolution, in monkeys, linked to the appearance of a true medial pallidum.

Structure
It has been found that the SMA is likely made up of two anatomically and functionally distinct parts, and was divided into the SMA proper (or: caudal SMA) and the pre-SMA (or: rostral SMA). In primates, the SMA proper is analogous to area F3, whereas the pre-SMA is analogous to area F6.

In monkeys it is a part of the dysgranular cortex. This means an intermediate differentiation between the more posterior agranular motor cortex and the more anterior granular eulaminate frontal cortex.

Function
The SMA is implicated in the planning of motor actions and bimanual control. In contrast to the premotor cortex, the SMA has been implicated in actions that are under internal control, such as the performance of a sequence of movements from memory (as opposed to movements guided by a visual cue).

Pre-SMA is involved in acquiring new sequences. There is more activity in these neurons when the sequence is new, compared to when it has been already learned. In contrast, SMA neurons are more active when performing a sequence already learned than one still being learned. This suggests that the SMA may be more involved in retrieving the sequence. SMA neurons are more active when the task requires the arrangement of multiple movements in the correct sequence and correct temporal order. For example, some SMA neurons "prefer" a specific order of movements to be performed. Other SMA neurons fire more for the preparation of a specific rank order. For example, a neuron can fire more when a monkey is preparing to initiate the third movement, irrespective of the sequence of the three movements.

SMA and Pre-SMA can be distinguished by various physiological techniques that delineate two different areas rostrocaudally. Field and unitary responses to electrical stimulation of the primary motor cortex were distinct in the caudal part, but minimal or absent in the rostral part. Intracortical microstimulation readily evoked limb or orofacial movements in the caudal part, but only infrequently in the rostral part. Neuronal responses to visual stimuli prevailed in the rostral part, but somatosensory responses were rare. The opposite was true in the caudal part. The rostral part, roughly corresponding to area 6a beta, was operationally defined as the presupplementary motor area (pre-SMA). The caudal part was redefined as the SMA proper. Single-cell activity in the pre-SMA was quantitatively compared with that in the SMA proper in relation to a trained motor task. Phasic responses to visual cue signals indicating the direction of forthcoming arm-reaching movement were more abundant in the pre-SMA. Activity changes during the preparatory period, which lasted until the occurrence of the trigger signal for the reaching movement, were more frequent in the pre-SMA. Phasic, movement-related activity was more frequent in the SMA, and its onset was often time locked to the movement onset. In the pre-SMA, the occurrences of response time locked to the movement-trigger signal were more frequent than in the SMA. Among neurons in both areas, directional selectivity was found in all the cue, preparatory, and movement-related responses.

Recent considerations of the diverse activities in which the SMA and pre-SMA play a role suggest existing theories may not fully capture the fundamental functions of these regions.