Central Facial Palsy - The Motor System and Facial Patterns

The Motor System and Facial Patterns

In contemporary perspectives, the motor cortex is composed of two distinct areas; however, this viewpoint is incorrect. The motor cortex is located in the posterior frontal lobe, and has multiple areas with anatomical and functional regions. Each area is involved in the circuitry of various inputs of sensory information. The motor and parietal areas are reciprocally intertwined and form a group of specialized circuits that work parallel to one another. These circuits transform sensory information into an action or movement.

The parieto-frontal circuits are the basic compositions of the main elements of the cortical motor system. These circuits are dependent on the motor area in order to receive afferent information from the parietal areas. The input in one area is predominant, containing full amounts of information. The other input area is known as moderate or weak. When the input is moderate or weak, it contains additional secondary information. Each parietal area is connected to several motor areas. However, it only makes privileged contact with one motor area. There are exceptions to this, which include the prefrontal gyrus where the parietal area sends an equal amount of fibers to many motor areas. This interaction is vital because the activity in the facial muscles is due to voluntary control of the direct and indirect pathways that are corticobulbar pathways. Facial muscles will often respond to emotional influences by these pathways also. Most of our emotions are expressed more intensely on the left side than the right side of the face. The reason for the asymmetry however, remains unclear, a commonly concluded theory is that the right side of the hemisphere has an advantage in emotional processing than the left hemisphere. In order to examine facial muscle movement often, transcranial magnetic stimulation (TMS) is used.

Upper motoneuron lesions to the face often cause paralysis. The lesions will cause weakness in various areas of the face while other areas of the face are not impacted. This pattern of weakness due to the input of the motor neurons of the lower facial muscles is often maintained contralateral. The strength of the muscles in the upper region of the face are preserved better than the muscles in the lower face. It was found that in many anataomical studies that cortical input from both hemispheres could reach motoneurons that supply muscles of all aspects of the face. Through the combination of anterograde and retrograde tracing techniques in monkeys it was found that the facial nucleus, which supplies muscles of the lower face are innervated bilaterally. Using TMS has shown the activation of both hemispheres during facial expression and emotion. However, there have been some discrepancies with the use of this method including differences in observations when using single and multiple needles as well as the areas of where the needles are placed. Using electrical cortical mapping bilateral movements were observed in the lower facial muscles compared to unilateral movements. From anatomic studies on patients with unilateral infarction, motoneurons in the lower facial area were innervated bilaterally; however, there was predominance in contralateral areas of the lower face.