In neurological practice, it is well known that patients suffering from certain cerebrovascular accidents interrupting the fibers from the face region of the primary motor cortex to the caudal brainstem are not able to show their teeth or move other mouth muscles on the contralateral side of the lesion. However, in the same patient, a funny story or other positive emotional event produces a smile, making use of the same facial muscles that could not be moved voluntarily. Apparently, in the central nervous motor system, there is a difference in the voluntary control of the mouth muscle motoneurons and the emotionally driven control of these motoneurons producing motor activities such as smiling. 1 Moreover, the same clinical observations prove that these different control systems do not use the same pathways to reach the mouth muscle motoneurons. In 1992, Holstege 2 brought forward the concept of two motor systems, a somatic and an emotional one, both making use of the same basic "hardware," i.e., the motoneurons themselves (somatic and autonomic) and their premotor interneurons.
Motoneurons and Premotor Interneurons of the Facial Muscles