Physiology, Motor Cortical

Excerpt

The primary function of the motor cortex is to send signals to direct the body's movement. The motor cortex is part of the frontal lobe and is anterior to the central sulcus. This region consists of the primary motor cortex, premotor cortex, and supplementary motor area. Not all parts of the motor cortex have the granular cell layer. The primary motor cortex in Brodmann area 4 sends most electrical impulses from the motor cortex. These fibers synapse with spinal cord motor neurons (see Image. Motor Cortex). Corticospinal fibers originate both from the frontal cortex and also the parietal cortex. Anterior to the primary motor cortex, the premotor cortex is situated in Brodmann area 6. The function of the premotor cortex is to prepare for movement, especially proximal musculature. The supplementary motor area is in the medial surface of the longitudinal fissure, anterior to the area represented by the leg in the primary motor cortex. Although not fully understood, proposed functions include body postural stabilization and coordination.

Significant differences exist in the projections from the cortical motor neurons. Fibers from the primary motor cortex (area 4) terminate mainly in the spinal cord, synapsing directly on motor neurons. The rostral frontal motor areas do not terminate directly at the spinal cord. They send fibers that terminate in various areas of the brainstem. Therefore, they control movements indirectly through the brainstem, such as the tectospinal and reticulospinal tracts. Accordingly, the projections control the proximal and axial muscles.

Cortical afferents to the frontal motor cortex arise from 3 sources: the parietal somatosensory cortex, the prefrontal cortex, and the cingulate cortex. The parietal-frontal connections represent different sensory-motor circuits. The cingulate cortex sends fibers to the premotor and supplemental motor areas for limbic information, motivation, and decision. Finally, the prefrontal cortex sends fibers to the motor cortex, leading to motor planning and execution.