Facial nerve

The facial nerve is seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and taste to the anterior two-thirds of the tongue.

Its main function is motor control of most of the facial muscles and the stapedius muscle of the middle ear. It also supplies parasympathetic fibers to the submandibular gland and sublingual glands via chorda tympani and the submandibular ganglion, and to the lacrimal gland via the pterygopalatine ganglion. In addition, it receives taste sensations from the anterior two-thirds of the tongue and sends them to the nucleus of solitary tract. There is also a small amount of cutaneous sensation carried by the nervus intermedius from the skin in and around the auricle (earlobe).

Anatomy of the facial nerve

The motor part of the facial nerve arises from the facial nerve nucleus in the pons while the sensory part of the facial nerve arises from the nervus intermedius.

The motor part of the facial nerve enters the petrous temporal bone into the internal auditory meatus (intimately close to the inner ear) then runs a tortuous course (including two tight turns) through the facial canal, emerges from the stylomastoid foramen and passes through the parotid gland, where it divides into five major branches. Though it passes through the parotid gland, it does not innervate the gland. This action is the responsibility of cranial nerve IX, the glossopharyngeal nerve.

No other nerve in the body travels such a long distance through a bony canal.

Branches

Inside the facial canal:

Greater petrosal nerve - provides parasympathetic innervation to lacrimal gland
Nerve to stapedius - provides motor innervation for stapedius muscle in middle ear
Chorda tympani - provides parasympathetic innervation to submandibular and sublingual glands

Outside stylomastoid foramen:

Posterior auricular nerve - controls movements of some of the scalp muscles around the ear
Five major facial branches (from top to bottom):

A helpful mnemonic device for remembering the major branches is the phrase, "To Zanzibar By Motor Car".

Two divisions occur in the parotid gland - temporofacial and cervicofacial divisions. The temporofacial division is made up of: temporal branch, zygomatic branch and buccal branch. Cervicofacial branch is made up of cervical branch, marginal mandibular branch and buccal branch. The two buccal branches later join.

Facial nerve nucleus

See Facial nucleus.

Testing the facial nerve

Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly, pursing the lips and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.

In an upper motor neuron lesion, only the lower part of the face will be affected, due to the bilateral control to the upper facial muscles.

Taste can be tested on the anterior of the tongue, this can be tested with a swab dipped in a flavoured solution, or with electronic stimulation (similar to putting your tongue on a battery).

Facial nerve pathology

People may suffer from acute facial nerve paralysis, which is usually manifested by facial paralysis. Bell's palsy is one type of idiopathic acute facial nerve paralysis, which is more accurately described as a multiple cranial nerve ganglionitis that involves the facial nerve, and most likely results from viral infection.

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