Facial Nerve Anatomy

Overview of Facial Nerve Anatomy

The facial nerve (cranial nerve VII) is a motor and sensory nerve that has 3 distinct intracranial, intratemporal and extratemporal anatomical sections.

1. Intracranial: arises from the precentral gyrus & travels via dorsolateral pons.
2. Intratemporal: fallopian canal's labyrinthine, tympanic & mastoid segments.
3. Extra-temporal: from stylomastoid foramen to 5 branches via the parotid.

These three sections of the facial nerve are visualised in the illustration below.

Intracranial Facial Nerve Anatomy

Origin

The facial nerve is formed by the facial nerve proper and the nervous intermedius.

1. Facial Nerve Proper: motor fibres exiting from the cerebellopontine angle.
2. Nervus Intermedius: parasympathetic and sensory fibres.

Intracranial Course

The intracranial section has 3 important anatomical landmarks:

1. Arises from the medullopontine angle of the brain stem.
2. Travels with cranial nerve VIII at the internal auditory meatus.  
3. Enters the petrous temporal bone

This intracranial course of the facial nerve is visualised in the illustration below.

Intratemporal Facial Nerve Anatomy

Intratemporal Course  

• Enters the internal auditory canal with the acoustic and vestibular nerves.
• Travels the fallopian canal by itself through the segments.
• Segments: cisternal, intracanlicular, labyrinthine, tympanic, mastoid

Intratemporal Branches

Three clinically significant branches from this segment are:

Greater Petrosal:

• Innervates lacrimal gland & soft palate taste via geniculate ganglion.
• Arises in the tympanic segment.

Nerve to Stapedius:

• Dampens loud noises (unaffected in Möbius Syndrome)
• Arises from the mastoid segment

Chorda Tympani:

• Innervates salivary glands, anterior 2/3 tongue taste via lingual.
• Arises from the mastoid segment

Extracranial Facial Nerve Anatomy

Extracranial Course

The facial nerve becomes extracranial after the stylomastoid foramen, where it is protected by the mastoid tip, tympanic ring, and mandibular ramus.  

It then travels past bone, muscle and cartilage before reaching the parotid gland.

• Anterior to the posterior belly of the digastric muscle
• Lateral to the styloid process of the temporal bone.
• Inferomedially (~1cm³) to the tip of the tragal pointer of the ear
• Posteriomedially enters the parotid gland
• Forms temprozygomatic and cervicofacial trunks in the pes anserinus.
• Divides into terminal branches that innervate facial muscles.

Extracranial Branches

The facial nerve provides motor branches to the structures it passes by on its way to the parotid gland.

• Postauricular nerve: occipitalis and posterior auricular muscles
• Branch to the posterior belly of digastric and stylohyoid
• Terminal branches: this is discussed in more detail below.  

Terminal Branches of the Facial Nerve

Terminal Branches of Facial Nerve

Facial nerve branches can be remembered by the mnemonic: To Zanzibar By Motor Car: Temporal, Zygomatic, Buccal, Marginal Mandibular, Cervical.

• Temporal: innervates the frontalis muscle.
• Zygomatic: innervates the orbicularis oculi and upper lip elevators
• Buccal: innervates buccinator and upper lip muscles
• Marginal: innervates lower lip mouths
• Cervical branch: platysma

Course of Branches

The terminal branches of the facial nerve exit the parotid gland deep to the subcutaneous musculoaponeurotic system (SMAS) layer.

Some branches of the facial nerve have vital landmarks:

• Temporal nerve: Pitanguy's Line (0.5cm below tragus to 1.5cm above lateral brow) in temporoparietal fascia near the superficial temporal artery
• Buccal branch: travels with the parotid duct (Stenson's duct), Zucker's point.
• Marginal mandibular: inferior border of the mandible, deep to the platysma and superficial to the facial artery and anterior facial vein.

Variations in Facial Nerve

Six distinctive anastomotic patterns were initially classified by Davis et al. in 1956^4, but there is significant variation in the facial nerve due to:

• No consistent spatial or topographic orientation  
• Variations in branching patterns and anastomoses between branches^5,6
• The temporal branch and marginal mandibular nerve have less arborization than other facial nerve branches.

These branching pattern variations and cross-innervation clinically correlate to unexplained facial muscle strength in the presence of a known injury to the facial nerve.

Flashcards