Clinical Neuroscience for Communication Disorders: Neuroanatomy and Neurophysiology by Margaret Lehman Blake;Jerry K. Hoepner;

Author:Margaret Lehman Blake;Jerry K. Hoepner;
Language: eng
Format: epub
Publisher: Plural Publishing Inc.
Published: 2021-02-15T00:00:00+00:00

FIGURE 11–7. Branches of the facial nerve. Yellow, special visceral efferent (muscles of facial expression); red, general visceral efferent (glands); green, special visceral afferent (taste); blue, general somatic afferent (external ear canal).

The SVE component of the facial nerve innervates muscles of facial expression, including the orbicularis oris that forms the lips, all of the muscles that elevate or depress the lips and the corners of the mouth, the orbicularis oculi around the eyes, and the frontalis muscle that elevates the eyebrows. The stapedius muscle in the middle ear also is innervated by CN VII. As with the GVE fibers, the UMN axons extend down to the pons, and most cross over before synapsing in the facial motor nucleus. The innervation of facial muscles is not strictly contralateral. The upper face including around the eyes and the frontalis muscle on the forehead receive bilateral innervation. Some UMNs targeting the upper face cross over in the pons and synapse contralaterally, whereas others remain ipsilateral. This provides bilateral innervation to the muscles in the upper face. LMNs to the lower face receive only contralateral innervation (Figure 11–8). This mixture of bilateral and unilateral innervation leads to specific patterns of weakness that aid in diagnosing the level of damage, as discussed later.

Several reflexes are mediated by the facial nerve, including the corneal, sucking, and stapedial reflex. The first two involve sensory signals carried through CN V (from the cornea and the lips, respectively), with synapses onto LMNs in the facial nuclei. The stapedial reflex is triggered by loud sounds that are carried through CN VIII to the LMNs of the facial nucleus to contract the stapedius muscle, which retracts the stapes from the oval window and limits the movement of the ossicles to protect the cochlea.

Unilateral UMN damage will result in contralateral lower facial weakness, with spared movement of the upper face (forehead). Essentially, the person will not be able to elevate/retract the lips on the contralateral side when asked to smile (lower face weakness) but will be able to raise both eyebrows (preserved upper face movement). This pattern is due to the bilateral innervation of the upper face: When UMNs from one side of the brain are damaged, the upper face still receives ipsilateral innervation (Figure 11–9). If damage occurs to the facial nerve unilaterally, the person will exhibit weakness or paralysis to the entire ipsilateral face: They will not be able to elevate their lips in a smile or raise their eyebrow on the affected side. This is because the damage is to the LMN to the ipsilateral muscles of both the upper and lower face. Because of the clinical relevance of these movements, they are a common component of an oral mechanism exam (OME), as described later in this chapter.

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