The SMA (supplementary motor area) is divided into two regions: the posterior part known as the SMA proper, and the anterior part called the pre-SMA. The SMA proper shares similarities with other motor areas in terms of having a somatotopic map, contributing to the corticospinal tract, and being interconnected with other motor areas. On the other hand, the pre-SMA has weak connections with other motor areas and the spinal cord, but is connected to the prefrontal cortex.

Stimulation studies show that the SMA, like the motor cortex, has a complete somatotopic map. Stimulation of the SMA can produce isolated movements of specific body parts. The musculotopic organization of the motor cortex is represented in a classic view called the "muscular homunculus," but a modern view suggests that each body part is represented multiple times across several discrete regions.

The dynamic nature of the motor cortex musculotopic map is influenced by inhibitory GABAergic interneurons. These interneurons play a crucial role in shaping motor responses to stimulation of each region of the motor cortex. In a rat brain study, electrical stimulation of the Vib region of the motor cortex evoked movements, while application of bicuculline to block GABAergic synapses in the FL region resulted in forelimb movement. However, stimulation of the SMA can also lead to vocalization or complex postural movements, as well as temporary arrest of movement or speech.

The supplementary motor cortex, located rostral to the primary motor cortex in Brodmann area 6, requires higher stimulus intensities to evoke movement compared to the primary motor cortex. It can be divided into dorsal and ventral subdivisions. Both subdivisions are somatotopically organized and contribute to the corticospinal tract. The dorsal division (PMd) has a complete map representing various body parts, while the ventral division (PMv) primarily represents the arm and face with a limited leg representation. The PMv is specialized for the control of upper limb and head movement. Another distinction between the subdivisions is that the PMd has a significant representation of proximal muscles, whereas the PMv has a significant representation of distal muscles.
Fig. 9.14). It is subdivided into two regions: the more posterior part is referred to as the SMA proper (or just SMA), and the anterior portion is called the pre-SMA. The SMA proper is similar to the other motor areas already listed: it contains a complete somatotopic map, it contributes to the corticospinal tract, and it is interconnected with the other motor areas. In contrast, the pre-SMA is not strongly connected with the other motor areas and spinal cord but rather is connected to the prefrontal cortex.
The results of stimulation studies show that as in the motor cortex, there is a complete somatotopic map in the SMA. Stimulation of the SMA can evoke isolated movement
Rostral wall of central sulcus •Fig. 9.15 Traditional and Modern Views of Motor Cortex Musculotopic Organization. A, Lateralviewofthecerebrum,showingaplaneofsectionthroughtheprecentralgyrus(primarymotorcortex)thatcorrespondstothesectionshowninB. B, Classicviewofmotorcortexmusculotopy(“muscularhomunculus”).C, Modernviewofmotorcortexorganizationinwhicheachbodypartisrepresentedmultipletimesacrossseveraldiscreteregions.
CHAPTER 9 Organization of Motor Function •Fig. 9.16 Dynamic Nature of a Motor Cortex Musculotopic Map. InhibitoryGABAergicinterneuronsplayanimportantroleinshapingmotorresponsestostimulationofeachregionofthemotorcortex.A, Schematicviewofexcitatoryconnectionsbetweentworegionsofprimarymotorcortexandlocalinhibitoryneuronswithinasingleregion.B, Schematicviewofaratbrain,indicatingmotorcortex(MC)regionswhereelectricalstimuliwereappliedtoevokemovements(Vibregion)andbicucullinewasappliedtoblockGABAergicsynapses(intheFLregion).FL,forelimb;Vib,vibrissa.C, ForelimbEMGrecordsshowingresponsetostimulationoftheVibregionbeforeandduringtheapplicationofbicucullineandafteritwaswashedout.NotethatVibstimulationevokedvibrissaemovementinallconditionsbutevokedforelimbmovementonlywheninhibitoryinterneuronsinFLwereblocked.(DatafromJacobsK,DonoghueJ.Science. 1991;251:944.) about single joints, similar to that after stimulation of the motor cortex, but stimulation must be of higher intensity and longer duration; moreover, the evoked movements are often more complex than those evoked by stimulation of the motor cortex. However, longer duration stimulation of the primary motor cortex can also evoke complex, apparently purposeful movement sequences, and so the distinction is not absolute. In addition, stimulation of the SMA can produce vocalization or complex postural movements, but it can also have the opposite result: namely, a temporary arrest of movement or speech. Removal of the supplementary motor cortex retards movement of the opposite extremities and may result in forced grasping movements with the contralateral hand.
This area lies rostral to the primary motor cortex and is contained in Brodmann area 6 on the lateral surface of the brain (see
Fig. 9.14 ). It can be distinguished from the primary motor cortex by the higher stimulus intensities needed to evoke movement. The premotor area has been divided into two functionally distinct subdivisions: dorsal and ventral. Like the motor cortex, both subdivisions are somatotopically organized and both contribute to the corticospinal tract. The dorsal division (PMd) contains a relatively complete map representing the leg, trunk, arm, and face. In contrast, the somatotopic map of the ventral division (PMv) is mostly limited to the arm and face, with only a small leg representation. Thus the PMv appears to be specialized for control of upper limb and head movement. A second difference between the subdivisions is that the PMd contains a large representation of the proximal muscles, whereas the PMv has a large representation of the distal muscles.