Facial nerve anatomy with definition to terms from starand ent text bo

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Facial nerve (CN VII): definition

The facial nerve, or cranial nerve VII, is a mixed cranial nerve associated developmentally with the second pharyngeal arch. It carries:

Branchial motor (special visceral efferent) fibers to muscles of facial expression, stapedius, stylohyoid, and posterior belly of digastric.
Parasympathetic (general visceral efferent) fibers to the lacrimal, submandibular, sublingual, nasal, and palatal glands.
Taste (special visceral afferent) fibers from the anterior two-thirds of the tongue.
A small amount of general sensory supply from the concha and external acoustic canal.
Cummings Otolaryngology Head and Neck Surgery, p. 2450.

Components and nuclei

1. Motor root

The larger root of CN VII supplies muscles derived from the second pharyngeal arch. Its motor fibers arise from the facial motor nucleus in the caudal pons.

2. Nervus intermedius

The nervus intermedius is the smaller sensory-parasympathetic root that accompanies the motor root. It contains:

Taste fibers
Preganglionic parasympathetic fibers
Small general sensory fibers

3. Important nuclei

Nucleus	Main function
Facial motor nucleus	Motor supply to facial-expression muscles, stapedius, stylohyoid, posterior belly of digastric
Superior salivatory nucleus	Preganglionic parasympathetic fibers for lacrimal and salivary glands
Nucleus of solitary tract	Central reception of taste fibers
Spinal trigeminal nucleus	Receives small general sensory component from auricle/external canal

Cummings Otolaryngology Head and Neck Surgery, p. 2450.

Course of the facial nerve

A useful surgical division is into intracranial, intratemporal, and extratemporal parts.

A. Intracranial course

Fibers arise in the pons. The motor fibers loop around the abducens nucleus, producing the internal genu of the facial nerve.
The motor root and nervus intermedius emerge at the cerebellopontine angle, close to CN VIII.
They enter the internal acoustic meatus (IAM). CN VII is usually located in the anterosuperior quadrant of the meatus.

Internal genu: The bend made by facial motor fibers around the abducens nucleus inside the pons. A lesion here may produce ipsilateral facial weakness with abducens palsy.

B. Intratemporal course: facial canal or Fallopian canal

The facial nerve enters the bony facial canal, also called the Fallopian canal, which extends from the fundus of the internal acoustic meatus to the stylomastoid foramen.

1. Labyrinthine segment

Runs from the IAM to the geniculate ganglion.
It is the shortest and narrowest segment of the facial canal.
It lies superior to the cochlea.

2. Geniculate ganglion and first genu

The geniculate ganglion is the sensory ganglion of CN VII and contains cell bodies of taste and other sensory neurons.
At this point, the nerve bends sharply posteriorly. This bend is the first genu.
The greater superficial petrosal nerve arises here.

3. Tympanic or horizontal segment

Runs posteriorly along the medial wall of the middle ear.
It lies superior to the oval window and near the cochleariform process.

4. Mastoid or vertical segment

At the pyramidal eminence, the nerve turns inferiorly, forming the second genu.
The nerve to stapedius and chorda tympani arise from this region.
The facial nerve exits the skull through the stylomastoid foramen.

The temporal-bone course and middle-ear relations make CN VII particularly important during mastoid and middle-ear surgery. Cummings Otolaryngology Head and Neck Surgery, pp. 2450-2451.

Branches within the temporal bone

Greater superficial petrosal nerve

Origin: geniculate ganglion region.
Fiber type: preganglionic parasympathetic fibers.
Course: joins the deep petrosal nerve to form the nerve of the pterygoid canal, then synapses in the pterygopalatine ganglion.
Function: secretomotor supply to the lacrimal gland and glands of nasal cavity and palate.

Nerve to stapedius

Motor branch to the stapedius muscle.
Paralysis causes hyperacusis, meaning abnormal sensitivity to sound, because stapedius normally dampens stapes movement.

Chorda tympani

Arises from the mastoid segment.
Crosses the middle ear and exits through the petrotympanic fissure.
Joins the lingual nerve in the infratemporal fossa.
Carries:
- Taste from anterior two-thirds of tongue.
- Preganglionic parasympathetic fibers to submandibular ganglion, then to submandibular and sublingual glands.

Extratemporal facial nerve

After leaving the stylomastoid foramen, the nerve gives early branches before entering the parotid gland.

Branches before the parotid gland

Branch	Supply
Posterior auricular nerve	Occipital belly of occipitofrontalis, auricular muscles
Nerve to posterior belly of digastric	Posterior belly of digastric
Nerve to stylohyoid	Stylohyoid muscle

The main trunk then passes into the parotid gland and usually divides into:

Temporofacial division: upper division
Cervicofacial division: lower division

Within the parotid, it forms the parotid plexus or pes anserinus.

The facial nerve traverses the parotid gland but does not provide secretomotor innervation to it. Parotid secretomotor fibers arise from CN IX via the otic ganglion.

Gray's Anatomy for Students, p. 1051.

Terminal branches in the face

The five terminal branch groups are remembered as:

T Z B M C
Temporal - Zygomatic - Buccal - Marginal mandibular - Cervical

Terminal branch	Main muscles supplied
Temporal (frontal)	Frontalis, superior orbicularis oculi, corrugator supercilii
Zygomatic	Orbicularis oculi and upper facial muscles
Buccal	Buccinator, orbicularis oris, muscles of upper lip and cheek
Marginal mandibular	Depressor anguli oris, depressor labii inferioris, mentalis
Cervical	Platysma

The branches emerge from the upper, anterior, and lower margins of the parotid gland. Gray's Anatomy for Students, p. 1051.

Applied anatomy

Upper motor neuron versus lower motor neuron facial palsy

Upper motor neuron lesion: contralateral weakness mainly of the lower face, with relative forehead sparing. This occurs because the upper facial nucleus has bilateral corticobulbar input.
Lower motor neuron lesion: ipsilateral weakness of the entire face, including inability to wrinkle the forehead or close the eye fully.

Localization in facial nerve lesions

Site of lesion	Likely findings
Proximal to greater petrosal nerve	Facial paralysis, reduced lacrimation, hyperacusis, loss of taste anterior two-thirds of tongue, reduced salivation
Between greater petrosal nerve and nerve to stapedius	Facial paralysis, hyperacusis, taste/salivary disturbance, lacrimation retained
Between nerve to stapedius and chorda tympani	Facial paralysis with taste and salivary disturbance, but no hyperacusis
Distal to chorda tympani or after stylomastoid foramen	Predominantly facial-expression muscle weakness

Anatomical variation is relevant during parotid, submandibular, facial, and neck surgery. Recent systematic reviews describe variation in the cervical and marginal mandibular branches, and facial-canal dimensions/dehiscence. This does not alter the standard course described above but reinforces the need for careful nerve identification. See recent reviews indexed as PMID 42862379 and PMID 38997588.

References

Cummings Otolaryngology Head and Neck Surgery, p. 2450.
Cummings Otolaryngology Head and Neck Surgery, p. 2451.
Gray's Anatomy for Students, p. 1051.
Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 2, facial nerve and temporal bone sections.
NCBI overview of CN VII.

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