Facial nerve (CN) VII
The motor fibers of the facial nerve supply predominantly the muscles of facial expression. Taste fibers to the anterior two-thirds of the tongue and parasympathetic fibers to the submandibular ganglion, pass via the chorda tympani, to be distributed with the lingual nerve. The facial nerve has a long course through the skull base, lying adjacent to the inner and middle ear and, on leaving the skull, passes through the parotid gland: it thus may be involved in disease of any of these structures.
The muscles of the upper face are bilaterally innervated and, in upper motor neuron lesions, only the lower part of the face is affected (unless there are bilateral lesions that produce a pseudobulbar palsy). Complete paralysis occurs in lower motor neuron lesions, the commonest cause is that of a Bell’s palsy. Upper and lower motor neuron lesions may compromise eyelid closure, leaving the cornea exposed and prone to injury. Immediate damage of the facial nerve in a skull base fracture is usually permanent, but delayed onset of paralysis is due to edema and undergoes a slow recovery (in spite of the initial severe, usually bilateral, facial paralysis).
Severe infection of the middle ear commonly extends to the mastoid air cells and bone involvement may damage the facial nerve, as can surgical drainage of an abscess or other surgical procedures in the region. Malignant tumors of the parotid gland may invade the facial nerve and the nerve is easily damaged in any surgery of the gland. The mandibular branch on the nerve, supplying the muscles of the lower lip, can be damaged in surgery of the submandibular gland.
Taste in the anterior two-thirds of the tongue (figure 25) is carried to the seventh nerve by the chorda tympani from the mandibular division of the trigeminal nerve. Dampen three cotton wool buds in tap water and, after dipping them respectively into salt, sugar and vinegar, assess recognition of each modality on each side of the tongue. Loss of taste is termed ageusia.
Figure 26 Facial muscles
2. Supraorbital artery
3. Orbicularis oculi
4. Superficial temporal artery
5. Levator labii superioris alaeque nasi
7. Levator labii superioris
8. Levator anguli oris (lying deep to
9. Zygomaticus minor
10. Zygomaticus major
13. Orbicularis oris
14. Facial artery
15. Depressor anguli oris 16. Depressor labii inferioris 17. Mentalis
The facial (seventh) nerve innervates the muscles of facial expression (figure 26; table 2). Note facial expression, and any asymmetry at rest, during talking or when smiling. In particular note the symmetry of the nasolabial folds and the angles of the mouth. Ask the subject to close their eyes, show their teeth and to whistle (figure 27a-d).
Assess the power of the facial muscles by trying to open tightly screwed up eyes and the resistance to squashing blown out cheeks (figure 28a,b).
Damage to the nerve or its nucleus produces paralysis of the whole face (figure 29).