Facial nerve

Special sensory function

Carries taste fibers from the anterior 2/3 of the tongue coming from lingual branch of mandibular nerve (trigeminus branch).

Sensory function

Sensory innervation to the skin around the auricle (through nervus intermedius).

Sensory innervation of a small region of pharynx caudal to the palatine tonsil.

Motor function

Innervation of the muscles of facial expression:

  • Occipitofrontalis
  • Temporoparietalis
  • Procerus
  • Nasalis
  • Depressor septi nasi
  • Orbicularis oculi
  • Corrugator supercilii
  • Depressor supercilii
  • Auricular muscles (anterior, superior and posterior)
  • Orbicularis oris
  • Depressor anguli oris
  • Risorius
  • Zygomaticus major
  • Zygomaticus minor
  • Levator labii superioris
  • Levator labii superioris alaeque nasi
  • Depressor labii inferioris
  • Levator anguli oris
  • Buccinator
  • Mentalis
  • Posterior auricular
  • Occipitalis

Visceral motor function

Supplies preganglionic parasympathetic fibers to head and neck ganglia.

Anatomy

1. Intra-axial segment

There are 3 nuclear complexes (2 sensory, 1 motor) associated with the facial nerve.

The motor nucleus of the facial nerve is located in the ventrolateral pontine tegmentum. Efferent fibers from the motor nucleus create a loop around the abducens nucleus forming the facial colliculus at the floor of the 4th ventricle. The fibers exit the pons at the pontomedullary junction.

The superior salivatory nucleus is located laterally to the motor nucleus in pons. Efferent parasympathetic fibers from this nucleus exit the pons posterior to the facial nerve asnervus intermedius which supplies the submandibular, sublingual and lacrimal glands.

The nucleus of tractus solitarius is the destination of taste signals from the anterior 2/3 of the tongue. The cell bodies for the axons transmitting these signals to tractus solitarius are in the geniculate ganglion and the fibers travel within nervus intermedius.

2. Cisternal segment

The facial nerve consists of a larger motor root exiting anteriorly and the smaller nervus intermedius entering posteriorly. The nerve emerges from the pontomedullary junction laterally and enters the cerebellopontine angle cistern.

The facial nerve exits the brainstem anterior to the vestibulocochlear (CN VIII) nerve.

3. Intratemporal segment

This segment can be subdivided into:

Meatal segment: runs from porus acusticus to internal auditory canal (IAC) fundus.

Labyrinthine segment: runs from IAC fundus to the anterior genu of the geniculate ganglion.

Tympanic segment: connects anterior and posterior genu of geniculate ganglion. Passes under the lateral semicircular canal and medial to the incus.

Mastoid segment: Runs from posterior genu of geniculate ganglion to the stylomastoid foramen.

4. Extracranial segment

The facial nerve exits the skull through the stylomastoid foramen and gives off the posterior auricular nerve to occipitalis and posterior auricular muscles. It then enters the parotid space and gives 5 major motor branches: temporal, zygomatic, buccal, marginal mandibular, and cervical.

Intracranial branches

1. Greater petrosal nerve

Carries parasympathetic fibers from the geniculate ganglion to the nasal, palatine, pharyngeal and lacrimal glands as well as the nasal cavity, sphenoid, frontal and maxillary sinus and ethmoidal cells.

It also carries taste afferents from the palate (via lesser and greater palatine nerves).

2. Parasympathetic fibers to otic ganglion

These fibers arise in the geniculate ganglion and via the lesser petrosal nerve reach the otic ganglion.

3. Nerve to stapedius

Carries motor efferents to the stapedius muscle in the middle ear.

4. Chorda tympani

It carries taste afferents from the anterior 2/3 of the tongue. It also carries presynaptic parasympathetic efferents to the submandibular ganglion which innervates the submandibular and sublingual glands. Emerges from the lingual nerve, enters the petrotympanic fissure, courses across the tympanic membrane and passes between the malleus and the incus. It enters the cranial cavity through the internal acoustic meatus along with the facial nerve.

Extracranial branches (distal to stylomastoid foramen)

1. Posterior auricular nerve

Innervates occipitalis and posterior auricular muscles.

2. Motor branch to digastric and stylohyoid muscle

Innervates the posterior belly of the digastric muscle and the stylohyoid muscle.

3. Motor branches

  • The temporal branch innervates frontalis, orbicularis oculi and corrugator supercilii muscles. It is the efferent limb of the corneal reflex.
  • The zygomatic branch innervates orbicularis oculi.
  • The buccal branch innervates risorius, buccinator, levator labii superioris, levator labii superioris alaeque nasi, levator anguli oris, nasalis, orbicularis oris, depressor septi nasi and procerus muscles.
  • The marginal mandibular branch innervates the depressor labii inferioris, depressor anguli oris and mentalis muscles.
  • The cervical branch innervates the platysma muscle.

Clinical implications

  • Upper motor neuron type of facial palsy (supranuclear injury) results in contralateral paralysis of facial expression with sparing of the forehead because orbicularis oculi and frontalis muscles receive bilateral innervation.
  • Lower motor neuron type of facial palsy (nuclear or infranuclear injury) results in ipsilateral paralysis of all facial muscles.
  • Loss of taste and lacrimation and sound dampening occurs if a peripheral lesion is proximal to the geniculate ganglion. If vestibulochochlear nerve is also affected, a cerebellopontine angle lesion should be suspected. If abducens nerve is also affected, a pons lesion should be suspected.
  • The most common causes of facial nerve palsy is Bell’s palsy. Other causes include infection, trauma, compression by tumor, stroke, autoimmune (eg in Guillain-Barré syndrome) and congenital (eg Möbius syndrome).