Almost all of behavior involves motor function, from talking to gesturing to walking. But even a simple movement like reaching out to pick up a glass of water can be a complex motor task to study. Not only does your brain have to figure out which muscles to contract and in which order to steer your hand to the glass, it also has to estimate the force needed to pick up the glass. Other factors, like how much water is in the glass and what material the glass is made from, also influence the brains calculations. Not surprisingly, there are many anatomical regions which are involved in motor function.

The primary motor cortex (M1) lies along the precentral gyrus, and generates the signals that control the execution of movement. Secondary motor areas are involved in motor planning. The plane of section is elaborated in figure 1b.

The primary motor cortex, or M1, is one of the principal brain areas involved in motor function. M1 is located in the frontal lobe of the brain, along a bump called the precentral gyrus (figure 1a). The role of the primary motor cortex is to generate neural impulses that control the execution of movement. Signals from M1 cross the bodys midline to activate skeletal muscles on the opposite side of the body, meaning that the left hemisphere of the brain controls the right side of the body, and the right hemisphere controls the left side of the body. Every part of the body is represented in the primary motor cortex, and these representations are arranged somatotopically -- the foot is next to the leg which is next to the trunk which is next to the arm and the hand. The amount of brain matter devoted to any particular body part represents the amount of control that the primary motor cortex has over that body part. For example, a lot of cortical space is required to control the complex movements of the hand and fingers, and these body parts have larger representations in M1 than the trunk or legs, whose muscle patterns are relatively simple. This disproportionate map of the body in the motor cortex is called the motor homunculus (figure 1b).

Figure 1b: The motor homunculus in primary motor cortex.

A figurative representation of the body map encoded in primary motor cortex. The section corresponds to the plane indicated in figure 1a. Body parts with complex repertories of fine movement, like the hand, require more cortical space in M1, while body parts with relatively simpler movements, like the hip, require less cortical space.

Other regions of the cortex involved in motor function are called the secondary motor cortices. These regions include the posterior parietal cortex, the premotor cortex, and the supplementary motor area (SMA). The posterior parietal cortex is involved in transforming visual information into motor commands. For example, the posterior parietal cortex would be involved in determining how to steer the arm to a glass of water based on where the glass is located in space. The posterior parietal areas send this information on to the premotor cortex and the supplementary motor area. The premotor cortex lies just in front of (anterior to) the primary motor cortex. It is involved in the sensory guidance of movement, and controls the more proximal muscles and trunk muscles of the body. In our example, the premotor cortex would help to orient the body before reaching for the glass of water. The supplementary motor area lies above, or medial to, the premotor area, also in front of the primary motor cortex. It is involved in the planning of complex movements and in coordinating two-handed movements. The supplementary motor area and the premotor regions both send information to the primary motor cortex as well as to brainstem motor regions.

Neurons in M1, SMA and premotor cortex give rise to the fibers of the corticospinal tract. The corticospinal tract is the only direct pathway from the cortex to the spine and is composed of over a million fibers. These fibers descend through the brainstem where the majority of them cross over to the opposite side of the body. After crossing, the fibers continue to descend through the spine, terminating at the appropriate spinal levels. The corticospinal tract is the main pathway for control of voluntary movement in humans. There are other motor pathways which originate from subcortical groups of motor neurons (nuclei). These pathways control posture and balance, coarse movements of the proximal muscles, and coordinate head, neck and eye movements in response to visual targets. Subcortical pathways can modify voluntary movement through interneuronal circuits in the spine and through projections to cortical motor regions.