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.