The motor cortex, which is located on the precentral gyrus of the brain, plays a vital role in controlling movement. It is organized in a way that is similar to the somatosensory cortex, with different body parts represented on different areas of the cortex. This organization is often depicted in the form of a motor homunculus, which shows the amount of cortex dedicated to controlling each body part.

Early mapping studies of the motor cortex were done using surface stimulation, and they provided a basic understanding of its organization. However, more detailed mapping was later achieved through the use of microelectrodes inserted into the deep layers of the cortex. This technique, called intracortical microstimulation, allowed for higher resolution mapping and revealed a more complex topography than previously known.

It was discovered that movement of each joint is controlled by multiple noncontiguous columns of cells scattered throughout the motor cortex. This means that the columns responsible for controlling movement of a specific joint are mixed with columns controlling movement of other joints within the same area. This mixing of cell columns makes functional sense, as coordinated movement often requires the involvement of muscles throughout a limb.

The connectivity within the motor cortex is primarily localized, with axon collaterals connecting different cell columns within a small region. This allows for the generation of multi-joint movement as a whole. Additionally, the motor cortex is not a fixed map but rather a dynamic one. Axon collaterals link different cell columns, enabling activity in one column to potentially lead to movement of multiple joints. These intercolumnar connections are modulated by inhibitory interneurons that transmit gamma-aminobutyric acid (GABA), which can inhibit or facilitate movement depending on their state.

Inhibition within the motor cortex can be temporarily blocked, leading to contractions in muscles controlled by the region that was previously inhibited. This shows that functional connections between cell columns can be controlled on a millisecond time scale, resulting in changes in the motor cortex map. There are also longer-term plastic changes that can occur in the motor cortex, such as changes in the size of the somatotopic representation of a body part based on its use or disuse.

The supplementary motor area (SMA), located on the medial surface of the hemisphere, is an important region adjacent to the primary motor cortex. It corresponds to the medial portion of Brodmann area 6.
Fig. 10.3). In humans it is located on the parts of the precentral gyrus that form the rostral wall of the central sulcus and the caudal half of the apex of the gyrus. On the basis of initial mapping studies, which were done with surface stimulation, the motor cortex was described as having a topographic organization that parallels that of the somatosensory cortex. The face, body, and upper limb were represented on the lateral surface with the face located inferiorly, near the lateral fissure, the torso most superiorly, and the lower extremity mostly on the medial aspect of the hemisphere. This somatotopic organization is often represented as a figurine or in a graphic form called a motor homunculus (
Fig. 9.15B ). The distortion of the various body parts in the homunculus indicates approximately how much of the cortex is devoted to their motor control. This simple homunculus was likened to a piano keyboard and fit well with traditional conceptions of the motor cortex being the final cortical stage and acting as a relay for sending motor commands to the spinal cord.
Beginning in the 1960s and 1970s, investigators in mapping studies began using microelectrodes inserted to the deep, or output, layers of the cortex to apply stimuli. With this technique, called intracortical microstimulation, much lower stimulus intensities could be used to evoke movements and thus allowed higher resolution mapping of the motor cortex, which revealed a much more complex topography than was previously imagined (see
CHAPTER 9 Organization of Motor Function
Fig. 9.15C ). Movement about each joint was found to be evoked by many noncontiguous columns throughout wide regions of the motor cortex. Thus cell columns related to movement about a particular joint are actually interspersed among columns that control movement about many other joints. In sum, although the motor cortex may have large subdivisions corresponding to a limb or the head, within each such area there is a complex intermingling of cell columns that control the muscles within that body part.
Such mixing of cell columns makes functional sense because most movement requires the coordinated action of muscles throughout a limb and most connectivity in the cortex is localized (i.e., axon collaterals that connect different cell columns are primarily confined to a 1to 3-mm region surrounding the column from which they originate). Thus when multiple cell columns that control movement about a joint are present and intermixed with columns controlling movement about other joints, multijoint movement can be generated as a whole.
Although the topographic map of the motor cortex is in part anatomically determined by the topography of the corticospinal pathway, it is also a dynamic map. Axon collaterals link the different cell columns, so that activity in one column could potentially lead to movement about multiple joints. In fact, this can happen, but these intercolumnar connections are modulated by inhibitory interneurons that transmit or secrete gamma-aminobutyric acid (GABA). This was shown by locally blocking GABA in one region of the motor cortex and then stimulating the neighboring region. Before the block, stimuli evoked contractions of one set of muscles, but once inhibition was blocked, contractions were also evoked in muscles controlled by the region that was no longer inhibited (
Fig. 9.16 ). Functional connections between cell columns can be controlled on a millisecond time scale, and depending on their state, the motor cortex map can be radically changed. Longer term plastic changes are also known to occur; for example, the use (or disuse) of a body part can affect the size of its somatotopic representation.
The supplementary motor area (SMA) is located mainly on the medial surface of the hemisphere, just anterior to the primary motor cortex, and corresponds to the medial portion of Brodmann area 6 (see