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The facial nerve, also known as the seventh or VII cranial 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 the muscles 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.
Nuclei and brainstem tracts
Four cranial nerve nuclei contribute to the facial nerve: one motor, one secretomotor, and two sensory:
- facial nucleus
- superior salivary (salivatory) nucleus
- nucleus of tractus solitarius (solitary nucleus)
- spinal nucleus of the trigeminal nerve
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, 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 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. 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 intracranial 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.
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).
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.
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
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 enters 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 mandibular division of trigeminal nerve)
- zygomatic (do not confuse with zygomatic nerve of maxillary division of trigeminal nerve)
- buccal (do not confuse with buccal nerve of mandibular division of trigeminal nerve)
- mandibular (marginal) (do not confuse with mandibular division of trigeminal nerve)
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.
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)).
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
The muscles of facial expression are supplied by the branches of the terminal facial nerve.
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 essentially being end arteries.
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.
Disruption or compromise of the nerve leads to facial palsy.
- Melkersson-Rosenthal syndrome
- Möbius syndrome
- Guillain-Barré syndrome
- HIV infection-related facial nerve palsy: may precede seroconversion
- congenital facial palsy
- Bell palsy
- sarcoidosis and other granulomatous disorders
- neoplasm(s) and masses
- trauma: especially temporal bone fractures
- cardiofacial syndrome: typically lower lip or complete facial palsy
- familial facial palsy
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