Cortical Representation of Muscular Action of the Face- left hemisphere

The cortex of the cerebrum has groups of cells in areas that are important for the "voluntary" or conscious, directed activation of muscles. Other areas of the cortex have cell groups that are important for our conscious perception of sensations from areas of the body. These groups of cells are not arranged randomly around the brain, but are laid out in the cortex in an order that approximates the physical relationships of the body areas they represent. The particular place in the cortex that these cells appear is on either side of the central fissure of the cerebrum. Motor areas are anterior (in front) of this fissure, and sensory areas are posterior (in back) to this fissure. The diagram above is old, but illustrates the approximate locations of the motor areas for controlling muscles. The central fissure is marked, and the front part of the brain is on the left. Notice how large the motor representation of the head muscles is in comparison to the rest of the body. Connections of nerve fibers in these areas eventually reach the facial nucleus in the brainstem, but before they do, they cross over to the opposite side of the brain. Thus, if these so-called upper motor neurons are damaged, the opposite side of the body is affected. In the case of the face, in general for muscles below the eyebrows, damage to the upper motor neurons on left side results in paralysis of the right face, and conversely. Damage to the facial nucleus or facial nerve results in paralysis of the same side of the face. Be aware, however, that this discusion of the upper motor neurons concerns only cerebral nerve centers significant in willful, directed control of muscles, and that other nerve centers control unwilled or automatic muscular movements such as those of emotional facial expressions. Both types of upper motor neurons for the face have the final common path of the brainstem nuclei and peripheral nerves.