The facial nerve is one of those cranial nerves that gives a headache just trying to think about it.
Although at first glance it is a motor nerve to facial expression, which begins as a trunk and emerges from the parotid gland as five branches (mnemonic here), it has taste and parasympathetic fibres which relay in complicated ways. It is therefore best to take it one step at a time.
Nuclei and brainstem tracts
Three brainstem nuclei contribute to the facial nerve; one motor, one secretomotor, and one sensory.
- facial nerve nucleus in the pons. As the motor fibres of the facial nerve loop posteriorly over the abducens nerve nucleus, they form the facial colliculus in the floor of fourth ventricle.
- superficial salivary nucleus, next to the facial nucleus supplies secretomotor parasympathetic fibres.
- nucleus of tractus solitarius, lateral to the dorsal nucleus of the vagus nerve, supplies taste fibres that eventually end up in the chorda tympani.
The facial nerve has six named segments:
- intracranial (cisternal) segment
- meatal segment (internal auditory canal) - 8 mm - zero branches
- labyrinthine segment (IAC to geniculate ganglion) - 3 - 4mm - 3 branches (from geniculate ganglion)
- tympanic segment (from geniculate ganglion to pyramidal eminence) - 8 - 11mm - zero branches
- mastoid segment (from pyramidal eminence to stylomastoid foramen) - 8 - 14mm - 3 branches
- extratemporal segment (from stylomastoid foramen to division into major branches) 15 - 20mm - 9 branches
The nerve emerges immediately beneath the pons, lateral to the abducens nerve and medial to the vestibulocochlear nerve, and is joined by the nervus intermedius which has emerged lateral to the main trunk. Together the two travel through the cerebellopontine angle to the internal acoustic meatus
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 runs back posteriorly at the geniculate ganglion (where the nervus intermedius joins the facial nerve, and where fibers for taste synapse - see below). It is here that three branches originate: the greater superfical petrosal nerve, the lesser petrosal nerve and the external petrosal nerve.
The labyrinthine segment is the shortest only measuring 3 to 4 mm. It is also the narrowest and the most susceptible to vascular compromise (see 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 pass posterior to the cochleariform process, tensor tympani and oval window. Just distal to the pyramidal eminence the nerve makes a second turn (second genu) passing vertically downwards as the mastoid segment.
The tympanic segment has no branches.
The mastoid segment, measuring 8 to 14mm in length, extends from the second genu to the stylomastoid foramen, through what is confusingly referred to as the fallopian canal. It gives off three branches:
- nerve to stapedius
- chorda tympani - terminal branch of the nervus intermedius carrying both secretomotor fibres 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 a sensor branch which supplies part of the external acoustic meatus and tympanic membrane. It then passes between the posterior belly of the digastric muscle and the stylohyoid muscle and enters the parotid gland. Lying between the deep and superfical lobes of the gland the nerve divides into to main branches at the pes anserinus (duck foot) - a superior temporofacial and and inferior cervicofacial branches. From the anterior border of the gland, five branches emerge; temporal, zygomatic, buccal, mandibular (marginal) and cervical.
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 2cm 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 synpase. 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 fibres 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 anastamosing in the pterygopalatine ganglion and supplying the lacrimal gland
- lesser petrosal nerve anastamosing 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 anastamosing in the submandibular ganglion and supplying the submandibular gland and sublingual gland
The facial nerve receives its arterial supply from three main sources:
- labyrinthine artery - a branch of the AICA, supplies the meatal segment. It is supplemented by direct twigs in the cisternal postion, 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 pases retrogradely into the stylomastoid foramen.
The labyrinthine segment is the most vulnerable to ischaemia as the connections between the labyrinthine artery and superficial petrosal artery are very tenuous, with each artery essentially being end arteries.
- Melkersson syndrome
- Möbius syndrome
- Guillain Barré syndrome
- HIV infection related facial nerve palsy: may precede seroconversion
- congenital facial palsy
- Bell's palsy
- sarcoidosis & 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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