The primary motor cortex, shown in Figure 1, lies in the first convolution of the frontal lobes anterior to the central sulcus. It begins laterally in the sylvian fissure, spreads superiorly to the uppermost portion of the brain, and then dips deep into the longitudinal fissure. (This area is the same as area 4 in Brodmann’s classification of the brain cortical areas).
Fig1. Motor and somatosensory functional areas of the cerebral cortex. The numbers 4, 5, 6, and 7 are Brodmann’s cortical areas, as explained in Chapter 48.
Figure 1 lists the approximate topographical representations of the different muscle areas of the body in the primary motor cortex, beginning with the face and mouth region near the sylvian fissure; the arm and hand area, in the midportion of the primary motor cortex; the trunk, near the apex of the brain; and the leg and foot areas, in the part of the primary motor cortex that dips into the longitudinal fissure. This topographical organization is demonstrated even more graphically in Figure 2, which shows the degrees of representation of the different muscle areas as mapped by Penfield and Rasmussen. This mapping was done by electrically stimulating the different areas of the motor cortex in human beings who were undergoing neurosurgical operations. Note that more than one half of the entire primary motor cortex is concerned with controlling the muscles of the hands and the muscles of speech. Point stimulation in these hand and speech motor areas on rare occasion causes contraction of a single muscle, but most often, stimulation contracts a group of muscles. To express this in another way, excitation of a single motor cortex neuron usually excites a specific movement rather than one specific muscle. To do this, it excites a “pattern” of separate muscles, each of which contributes its own direction and strength of muscle movement.
Fig2. Degree of representation of the different muscles of the body in the motor cortex. (Modified from Penfield W, Rasmussen T: The Cerebral Cortex of Man: A Clinical Study of Localization of Function. New York: Hafner, 1968.)
