The facial nerve is the seventh (CN VII) cranial nerve and comprises two roots, a motor root and a smaller mixed sensory, taste and parasympathetic root, known as nervus intermedius, which join together within the temporal bone (TA: nervus facialis or nervus cranialis VII).

The facial nerve has a complex and broad range of functions. Its primary function is as the motor nerve to the muscles of facial expression, however, it also carries taste and parasympathetic fibers that relay in a complex manner with adjacent nerves and ganglia.

Both the motor root and nervus intermedius emerge from the pontomedullary sulcus and pass into the internal acoustic meatus merging at the geniculate ganglion.

From here it performs a number of turns as it passes through the temporal bone closely related to the inner ear and middle ear, giving off a number of branches, before exiting the skull via the stylomastoid foramen

It then enters the parotid gland where it divides into the five branches that diverge across the facial region.

Just like the trigeminal nerve, the facial nerve has sensory and motor roots that travel together to the skull where they join at a ganglion. However, unlike the trigeminal nerve, the sensory root of the facial nerve has a special name; nervus intermedius although it is often forgotten. When the term facial nerve is used without qualification, it generally refers to both the motor root and the adjacent nervus intermedius.

Gross anatomy

Nuclei and brainstem tracts

Four cranial nerve nuclei contribute to the facial nerve: motor, secretomotor (parasympathetic), somatic sensory and special sensory (taste).

facial nucleus (motor)
- in the ventrolateral pontine tegmentum, giving off fibers that loop posteriorly over the abducens nerve nucleus, which together form the facial colliculus in the floor of the fourth ventricle
- supplies motor fibers to the stapedius and muscles of facial expression
superior salivary nucleus (parasympathetic)
- in the pons, lateral to the facial nucleus
- supplies secretomotor parasympathetic fibers to lacrimal, submandibular, and sublingual glands
solitary nucleus (taste)
- in the medulla and lower pons, lateral to the dorsal nucleus of the vagus nerve
- receives taste fibers from the anterior tongue via the chorda tympani
spinal nucleus of the trigeminal nerve (somatic sensory)
- in the upper cervical spinal cord, medulla, and pons, ventral to the vestibular and cochlear nuclei
- receives somatosensory fibers from the posterior external auditory canal

General course

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

intracranial (cisternal) segment - zero branches
meatal (canalicular) segment (internal auditory canal (IAC)): 8 mm long, zero branches
labyrinthine segment (IAC to geniculate ganglion): 3-4 mm long, three 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, three branches
extratemporal segment (from stylomastoid foramen to division into major branches): 15-20 mm, nine branches

Intracranial (cisternal) segment

The facial nerve emerges from the lower lateral pons, lateral and rostral to the abducens nerve, and medial and caudal to the vestibulocochlear nerve (CN VIII) ⁹. Nervus intermedius emerges immediately lateral to the main trunk and the two travel laterally through the cerebellopontine angle to the internal acoustic meatus. The intracranial segment has no branches.

Meatal (canalicular) segment

Both the facial nerve (motor root) and nervus intermedius 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 two branches originate:

Some fibers contributing to the lesser petrosal nerve of the glossopharyngeal nerve also arise from the geniculate ganglion, but the nerve is described as a branch of the glossopharyngeal nerve rather than the facial nerve.

The labyrinthine segment is the shortest, measuring only 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 entering the parotid gland. Lying between the deep and superficial lobes of the gland the nerve divides into two main branches superior temporofacial and inferior cervicofacial branches.

The collection of nerve branches emanating from these two trunks, between the superficial and the deep parts of the gland, comprise the parotid plexus, also called the pes anserinus (Latin: goose's foot). Additional rami between the five named branches are common ¹². The Davis classification of the terminal facial nerve branching in the plexus was devised in 1956, with modification by Katz and Catalano in 1987. Details of this classification exceed the scope of this article and are detailed in the referenced citation ¹².

From the anterior border of the gland, five branches emerge (from superior to inferior):

temporal (do not confuse with deep temporal nerves of the mandibular division of trigeminal nerve)
zygomatic (do not confuse with the zygomatic nerve of the maxillary division of trigeminal nerve)
buccal (do not confuse with buccal nerve of mandibular division of trigeminal nerve)
mandibular (marginal) (do not confuse with the mandibular division of trigeminal nerve) - for more details see mandibular branch of the facial 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

Motor

The facial nerve supplies all the muscles of facial expression along with three other muscles, all arising embryologically from the second branchial arch:

stylohyoid muscle
posterior belly of the diagastric muscle
stapedius muscle

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

Somatosensory

The facial nerve (via nervus intermedius) carries cutaneous sensory fibers that innervate the posterior aspect of the external auditory auditory canal, mastoid, and lateral pinna.

Arterial 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 portion, 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 posterior auricular 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 for all practical purposes being an end artery.

Radiographic features

The facial nerve can be imaged with a variety of modalities at different points in its course from the brainstem to facial muscles.

MRI

MRI can image the facial colliculus, most easily seen on axial images as a small 'bump' on the posterior pons bulging into the fourth ventricle. The cisternal and meatal segment of the nerve can be easily seen on high-resolution T2 weighted imaging (e.g. CISS/FIESTA) although nervus intermedius is difficult to identify as a separate structure.

On post-contrast T1 weighted images, the facial nerve is the only cranial nerve that may show normal post-contrast enhancement. Refer to normal facial nerve enhancement on MRI for more information.

The labyrinthine, tympanic and mastoid segments can be seen on high-resolution T1 or T2 weighted images, however, dedicated neurographic sequences are required to visualize the extracranial branches.

CT

CT is able to elegantly define the facial nerve canal through the temporal bone and its relationship to adjacent structures, but cannot see the nerve itself.