Supplementary motor area
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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.
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[edit] Structure
Recently, it was found that the SMA is likely made up of two functionally distinct parts, and was divided into the SMA proper and the pre-SMA. Additionally, the SMA proper is also subdivided into the caudal SMA and the 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. In monkeys after the discovery of this area a "preSMA" was found. It is thus important to check in literature if preSMA was in fact included or not.
[edit] Function
The SMA is implicated in the planning of motor actions and is associated with bimanual control. One could say that the SMA sends a "plan" of the motor action to the primary motor cortex, which executes the action. In contrast to the premotor cortex, the SMA is 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).[1]
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.
[edit] References
- ^ Shima K., Tanji J. (1998) Both supplementary and presupplementary motor areas are crucial for the temporal organization of multiple movements. J. Neurophysiol. 80:3247-3260.
[edit] Further Reading
- Principles of Neural Science (2000), 4th ed., Kandel et al.
- Debaere,-F; Wenderoth,-N; Sunaert,-S; Van-Hecke,-P; Swinnen,-S-P (2003). Internal vs external generation of movements: differential neural pathways involved in bimanual coordination performed in the presence or absence of augmented visual feedback. Neuroimage. 2003 Jul; 19(3): 764-76
- Vorobiev et al. (1998) Parcellation of human mesial area 6: cytoarchitectonic evidence for three separate areas. Eur J Neurosci. 10(6):2199-203.
