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The primary motor cortex is a region in the brain that works in tandem with other brain regions to coordinate voluntary movement throughout the body. It is located in the frontal lobe along a bumpy region known as the precentral gyrus. The primary motor cortex contains Betz cells, which are large neurons that communicate through the axons of the spinal cord with alpha motor neurons. Alpha motor neurons are the cells responsible for the actual perceived movement of the body. Signals initiated by this cortex cross the midline of the body when acting, meaning that a stimulation of the right side of the region controls the left side of the body and vice versa.
The primary motor cortex is arranged somatotopically, coinciding with every major part of the body. This means that every part of the body has a subregion in the cortex and that these regions are placed logically next to one another. For example, the region that controls the actions of the foot is next to the region associated with the leg or the region that controls most movement on the trunk of the body is located adjacent to the arm. The size of the subregions that control specific body parts depends on the complexity of the actions of the body part. For example, one of the largest subregions in the primary motor cortex is dedicated to the complicated action of the hands and fingers.
Debate continues among scientists as to the extent and exact mechanism by which this area acts. It is thought that the cortex does not control individual muscles by themselves. Instead, it seems to direct individual movements or sequences of movements from varying muscle groups to perform a voluntary action. The information encoded in the primary motor cortex is more of an abstraction of the actual movement that the alpha neurons carry out upon stimulation.
The primary motor cortex has been shown to help encode the force of a muscle contraction using information that is sent back by the peripheral nervous system. For example, the force that is required to raise the arm differs greatly whether the person is holding a bowling ball or a fork. The capability of the cortex to orchestrate different muscle groups simultaneously helps it to make these types of distinctions. It also helps the body to orient and encode the direction of movement. Lab studies have shown that one cell in the region may fire more strongly than its counterpart when a body part is moved in a single direction.
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