The facial motor system is a two-motor neuron system: upper and lower (Figure 175.1). The facial upper motor neuron component arises from cortical neurons in the lower third of the precentral gyrus. The axons of these neurons travel ipsilaterally in the centrum semiovale, genu of the internal capsule, and midbrain. At the level of the upper pons, they cross to the opposite side and make contact with the motor neurons located in the facial nerve motor nucleus. These motor neurons constitute the second component of the two-motor neuron system. The motor neurons in these nuclei destined to innervate the upper facial quadrant also receive ipsilateral innervation (hence these neurons have bilateral cortical innervation), whereas those destined to innervate the lower quadrant facial muscles only receive contralateral innervation (Figure 175.1).
Figure 175.1.—Facial motor system. T: thalamus; AC: internal auditory canal; FC: facial canal; SMO: stylomastoid orifice; BB: buccal branch; MB: mandibular branch; TB: temporal branch; OOM: orbicularis oculi muscle; RM: risorius muscle; DAOM: depressor angularis oris muscle; BM: buccinator muscle; MM: mentoris muscle. Light blue line indicates components of the facial nerve that have ipsilateral (hence bilateral) cortical innervation; dark blue line indicates components of the facial nerve that have contralateral innervation.