Supplementary Motor Cortex

The SMA was first described in the cortex of the medial wall of the frontal lobe of humans more than 50 years ago. In monkeys, SMA extends onto the dorsal surface of the medial frontal lobe just rostral to M1 and medial to PMD (Fig. 1). The area is also known as the medial premotor cortex (MPC). Although SMA is usually considered to be a single area, it has been divided into medial (SMAm) and dorsal (SMAd) subdivisions in monkeys (Fig. 2). SMAm appears to be more densely connected to M1, whereas SMAd is somewhat more myelinated. More consistently, the SMA region has been divided into a SMA-proper (SMAd plus SMAm), located immediately rostral to the mesial sector of M1 representing foot, and a pre-SMA, extending toward prefrontal cortex just anterior to a SMA-proper. Both regions border the agranular cingulate cortex in the cingulate sulcus.

SMA was initially recognized when electrical stimulation of the region evoked movements in humans.

SMA has subsequently been described in monkeys and carnivores. The region known as M2 in rats is likely to be SMA. Thus, SMA may be an area that exists in many mammals. Stimulation of posterior SMA evokes movements of the contralateral leg, the middle portion is related to movements of the arm and hand, and the most anterior portion is devoted to the face. Current thresholds for evoking movements are generally higher in SMA than in M1. The internal organization of SMA is like a smaller version of M1. Like M1, similar movements may be elicited from more than one site in SMA. Matching bilateral movements are occasionally evoked from sites in SMA.

SMA is within the medial part of Brodmann's area 6. The cortex is agranular, without an obvious layer 4 of granule cells. The layer 5 pyramidal neurons are generally smaller than in adjoining M1. SMA has dense interconnections with M1 and dense projections to the motor neuron pools in the spinal cord. Inputs include those from visual, somatosensory, and auditory association and multimodal areas of the temporal and parietal lobes. SMA receives major inputs from regions of the thalamus with inputs from the internal segment of the globus pallidus, and a minor input from that part of the thalamus receiving inputs from the cerebellum.

Neurons in SMA respond to visual, auditory, and tactile stimuli when these stimuli are used as signals to start a movement or series of movements. When the signal indicates that a movement must be delayed for a short period of time, SMA neurons respond strongly during the delay period. This is partial evidence that SMA has a role in initiating and planning movements.

SMA was originally thought to have a significant role in coordinating movements of the two hands, partly because electrical stimulation of SMA neurons was once thought to commonly evoke ipsilateral movements. This has proven to be rare, however. During movements of one side of the body, only contralateral SMA shows significant activity. Although connections are strong between the SMA of each cerebral hemisphere, lesions of SMA do not produce deficits that suggest a major role in bimanual coordination. Instead, SMA appears to be important in the initiation of contralateral movements, motor programming, motor planning, and motor learning. After lesions, voluntary movements can be elicited by sensory cues and skilled movements can be executed. However, lesions impair the ability to self-initiate learned movements and sequences of movements when no sensory cue indicates the time for movement onset.

Pre-SMA is a small region of cortex just rostral to SMA. Movements are usually not evoked by electrical stimulation of pre-SMA unless much higher current levels are used and a series of current pulses of longer duration are employed. Pre-SMA lacks direct connections with M1, and it does not project or projects very weakly to motor neurons in the spinal cord. Pre-SMA has strong connections with prefrontal cortex, SMA, and cingulate motor areas. Neurons in pre-SMA are preferentially active prior to movement, and they respond less frequently to somatosensory stimuli but more frequently to visual stimuli. Pre-SMA is thought to be involved in the more cognitive aspects of motor behavior, possibly in updating motor plans and in the learning of new motor sequences.

In addition to somatomotor areas, a separate oculomotor area, the supplementary eye field (SEF), is found in the dorsomedial frontal cortex. SEF lies rostrolateral to the SMA and pre-SMA, and it is distinguishable from these two areas by its close relation to eye movements and its anatomical connections with cortical and subcortical oculomotor centers.

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