Facial nerve

Dr Craig Hacking ◉ ◈ and A.Prof Frank Gaillard ◉ ◈ et al.

The facial nerve is one of the key cranial nerves with a complex and broad range of functions.

Although at first glance it is the motor nerve of facial expression which begins as a trunk and emerges from the parotid gland as five branches (see facial nerve branches mnemonic), it has taste and parasympathetic fibers that relay in a complex manner.

The facial nerve is the only cranial nerve that may show normal post-contrast enhancement, although this applies only to the labyrinthine segment up to the stylomastoid foramen. Enhancement of the other segments of the facial nerve and other cranial nerves is considered pathologic. Refer to normal facial nerve enhancement on MRI for more information.

facial nerve nucleus in the pons; as the motor fibers of the facial nerve loop posteriorly over the abducens nerve nucleus, they form the facial colliculus in the floor of fourth ventricle
superior salivary nucleus, next to the facial nucleus supplies secretomotor parasympathetic fibers
nucleus of tractus solitarius, lateral to the dorsal nucleus of the vagus nerve, supplies taste fibers that eventually end up in the chorda tympani

General course

The facial nerve has six named segments (facial nerve segments mnemonic):

intracranial (cisternal) segment - zero branches
meatal (canalicular) segment (internal auditory canal): 8 mm long, zero branches
labyrinthine segment (IAC to geniculate ganglion): 3-4 mm long, 3 branches (from geniculate ganglion)
tympanic segment (from geniculate ganglion to pyramidal eminence): 8-11 mm long, zero branches
mastoid segment (from pyramidal eminence to stylomastoid foramen): 8-14 mm long, 3 branches
extratemporal segment (from stylomastoid foramen to division into major branches): 15-20 mm, 9 branches

Intracranial (cisternal) segment

The nerve emerges from lower lateral pons, lateral and rostral to the abducens nerve, and medial and caudal to the vestibulocochlear nerve (CN VIII) ⁹. It is joined by the nervus intermedius, which emerges lateral to the main trunk. Together the two travel laterally through the cerebellopontine angle to the internal acoustic meatus. The cisternal segment has no branches.

Meatal (canalicular) segment

Having been joined by the nervus intermedius, they are located in the anterior superior quadrant of the internal auditory canal, above the falciform crest and anterior to Bill bar. The meatal segment has no branches.

Labyrinthine segment

As the facial nerve and nervus intermedius pass through the anterior superior quadrant of the internal acoustic meatus it enters the Fallopian canal, passing anterolaterally between and superior to the cochlea (anterior) and vestibule (posterior), and then bends posteriorly (the anterior or first genu) at the geniculate ganglion (where the nervus intermedius joins the facial nerve and where fibers for taste synapse - see function below). It is here that three branches originate:

The labyrinthine segment is the shortest only measuring 3-4 mm. It is also the narrowest and the most susceptible to vascular compromise (see blood supply below).

Tympanic segment

As the nerve passes posteriorly from the geniculate ganglion it becomes the tympanic segment (8-11 mm in length) and is immediately beneath the lateral semicircular canal in the medial wall of the middle ear cavity. The bone of the Fallopian canal is often dehiscent in the area of the oval window in 25-55% of postmortem specimens, having mucosa in direct contact with the nerve. The nerve passes posterior to the cochleariform process, tensor tympani and oval window. Just distal to the pyramidal eminence the nerve makes a second turn (posterior or second genu) passing vertically downwards as the mastoid segment. The tympanic segment has no branches.

Mastoid segment

The mastoid segment, measuring 8-14 mm in length, extends from the posterior genu to the stylomastoid foramen, through what is confusingly referred to as the Fallopian canal. It runs in the medial wall of the aditus ad antrum of the mastoid posterior to the facial recess of the mesotympanum. It gives off three branches:

nerve to stapedius
chorda tympani: terminal branch of the nervus intermedius carrying both secretomotor fibers to the submandibular gland and sublingual gland and taste to the anterior two thirds of the tongue
nerve from the auricular branch of the vagus nerve (CN X): pain fibers to the posterior part of the external acoustic meatus hitchhike from the jugular foramen

Extratemporal segment

As the nerve exits the stylomastoid foramen, it gives off the posterior auricular nerve that supplies part of the external acoustic meatus, tympanic membrane, auricular muscles and occipital part of occipitofrontalis. It then passes between the posterior belly of the digastric muscle and the stylohyoid muscle, supplying both and the enters the parotid gland. Lying between the deep and superficial lobes of the gland the nerve divides into two main branches at the pes anserinus (Latin: duck foot) - superior temporofacial and inferior cervicofacial branches. From the anterior border of the gland, five branches emerge (from superior to inferior):

temporal (which should not be confused with the deep temporal nerves of the mandibular division of the trigeminal nerve)
zygomatic (which should not be confused with the zygomatic nerve of the maxillary division of the trigeminal nerve)
buccal (which should not be confused with the buccal nerve of the mandibular division of the trigeminal nerve)
mandibular (marginal) (which should not be confused with the mandibular division of the trigeminal nerve)
cervical

See facial nerve branches mnemonic here.

The temporal branch runs with the superficial musculoaponeurotic system (SMAS) over the zygomatic arch. This branch is at risk during surgery in this region. To avoid damage procedures should be deep to the SMAS (e.g. zygomatic fracture repairs).

The mandibular branch, in 80% of cases, runs along the lower border of the mandible (thus also referred to as marginal branch). In 20% of cases however it can be up to 2 cm below the margin of the mandible. It is crucial to be aware of this if surgery in the submandibular region is being performed. Injury to this branch will result in paralysis of mouth depressors.

Function

Taste

Taste fibers to the anterior two thirds of the tongue originate in the nucleus of the tractus solitarius (NTS), travel in the nervus intermedius (preganglionic) where they join the facial nerve at the geniculate ganglion and synapse. Postganglionic fibers travel with the facial nerve and are given off as the chorda tympani, which eventually joins the lingual nerve (branch of the trigeminal nerve (CN V)).

Parasympathetic

Preganglionic fibers originate in the superior salivary nucleus and join the facial nerve at the geniculate ganglion having traveled with the nervus intermedius. They do not synapse in the ganglion, but rather pass through to be distributed between:

greater superficial petrosal nerve anastomosing in the pterygopalatine ganglion and supplying the lacrimal gland
lesser petrosal nerve anastomosing in the otic ganglion and supplying the parotid gland (along with fibers from the inferior salivary nucleus which arrive via the glossopharyngeal nerve (CN IX) supply to the tympanic plexus)
chorda tympani anastomosing in the submandibular ganglion and supplying the submandibular gland and sublingual gland

Somatic motor

The muscles of facial expression are supplied by the branches of the terminal facial nerve.

Blood supply

The facial nerve receives its arterial supply from three main sources:

labyrinthine artery: a branch of the anterior inferior cerebellar artery (AICA), supplies the meatal segment. It is supplemented by direct twigs in the cisternal position, directly from the AICA.
superficial petrosal artery: a branch of the middle meningeal artery which passes retrogradely along the greater superficial petrosal nerve
stylomastoid artery: a branch of the occipital artery, which passes retrogradely into the stylomastoid foramen

The labyrinthine segment is the most vulnerable to ischemia as the connections between the labyrinthine artery and superficial petrosal artery are very tenuous, with each artery essentially being end arteries.