The orbicularis oculi muscle is innervated by the temporal branch. It has bilateral cortical innervation. The buccinator is innervated by the buccal branch. It has contralateral cortical innervation. The depressor angularis oris muscle is innervated by two branches: the buccal branch and the mandibular branch. The depressor angularis oris muscle has contralateral cortical innervation. The mandibular branch also innervates the depressor labii inferioris muscle and the mentoris muscle. These muscles have contralateral cortical innervation (Figure 177.1).
Figure 177.1.— Anatomical localizations of injuries in the facial motor system. T: thalamus; AC: internal auditory canal; FC: facial canal; SMO: styloidmastoid 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. A: cerebral lesion above the thalamus; B: cerebral lesion below the thalamus and above the pons; C: pontine lesion; D: facial nerve lesion; E: mandibular branch lesion; F: depressor angularis oris muscle.
Facial motor system lesions may involve the facial motor pathway above the facial motor nucleus (upper motor facial asymmetry) or the facial motor nucleus and the structures below it (lower motor facial symmetry).