Anatomy of the Facial Nerve (CN VII)

I. Introduction

II. Fiber Components (5 Types)

Key point: The superior part of the facial motor nucleus (innervating frontalis, orbicularis oculi) receives bilateral supranuclear input; the inferior part receives only contralateral cortical input. This explains why UMN lesions spare the forehead.

III. Course of the Facial Nerve — Six Segments

1. Intracranial (Cisternal) Segment — 17-24 mm

• Arises from the pontomedullary junction, between the pons and olive
• Traverses the cerebellopontine angle (CPA) alongside CN VIII and the nervus intermedius
• Vulnerable to: acoustic neuromas, meningiomas, CPA tumors

2. Meatal (Intracanalicular) Segment — 8-10 mm

• Enters the internal auditory meatus (IAM) at the porus acusticus
• Lies in the anterosuperior quadrant of the IAC (above the cochlear nerve anteroinferiorly; above the superior vestibular nerve posterosuperiorly - Bill's bar separates it from the superior vestibular nerve)
• Exits the IAC at the meatal foramen (narrowest point of the intratemporal canal)
• Vulnerable to: acoustic neuromas, labyrinthitis

3. Labyrinthine Segment — 4 mm (Shortest and Narrowest Segment)

• Runs from the meatal foramen to the geniculate ganglion
• Passes superior to the cochlea, between the cochlea and vestibule
• The nerve lacks epineurium here; surrounded only by thin arachnoid → most susceptible to compression from edema (Bell's palsy mechanism)
• Ends at the geniculate ganglion (the only sensory ganglion of CN VII; contains cell bodies of taste and somatic sensory neurons)
• First genu occurs at the geniculate ganglion - the nerve makes an acute posterior turn
• First branch given off: Greater (Superficial) Petrosal Nerve (GSPN) - exits via the facial hiatus

4. Tympanic (Horizontal) Segment — 11-13 mm

• From the geniculate ganglion (first genu) to the second genu (at the level of the lateral semicircular canal)
• Runs in the medial wall of the middle ear (tympanic cavity), forming part of the medial wall anterior attic
• Passes superior to the cochleariform process and the oval window niche posteriorly
• This segment is the most common site of congenital bony dehiscence (especially above the oval window; ~66% of dehiscences occur here; 75% are bilateral)
• A dehiscent nerve may prolapse into the middle ear, presenting as a middle ear mass
• Vulnerable to: acute suppurative otitis media, cholesteatoma erosion

5. Mastoid (Vertical) Segment — 13-20 mm

• From the second genu (at the pyramidal eminence/lateral SCC) to the stylomastoid foramen
• Second genu is just anteroinferior to the lateral semicircular canal, anterior to a line through the short process of the incus
• Passes posterior to the external auditory canal
• The space between the mastoid segment and the chorda tympani = facial recess (used in posterior tympanotomy / intact canal wall mastoidectomy)
• Most variable in course; especially abnormal in congenital atresia
• In the newborn, the underdeveloped mastoid tip places this segment superficially - at risk during postauricular incisions and forceps delivery

6. Extratemporal Segment

• Exits at the stylomastoid foramen
• Immediately surrounded by aponeurosis of the posterior belly of digastric (preserving this during rerouting improves post-op nerve function)
• Gives off branches before entering the parotid:
  • Posterior auricular nerve: occipital belly of occipitofrontalis + auricular muscles
  • Branch to posterior belly of digastric and stylohyoid
• Enters the parotid gland, divides into upper (temporofacial) and lower (cervicofacial) divisions, then fans out into 5 terminal branches

IV. Summary of Surgical Landmarks by Segment

V. Branches and Their Functions

A. Within the Temporal Bone

B. Extratemporal Branches (after stylomastoid foramen)

1. Posterior auricular nerve - auricular muscles, occipital belly of occipitofrontalis
2. Digastric branch - posterior belly of digastric
3. Stylohyoid branch - stylohyoid muscle
4. Terminal motor branches (5, through parotid substance):
   • Temporal (T) - frontalis, corrugator supercilii, orbicularis oculi
   • Zygomatic (Z) - orbicularis oculi (lower), zygomaticus
   • Buccal (B) - buccinator, muscles of upper lip and nose
   • Marginal mandibular (M) - depressor anguli oris, depressor labii, mentalis
   • Cervical (C) - platysma

Mnemonic: "To Zanzibar By Motor Car" (Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical)

VI. Nervus Intermedius

• The nerve of Wrisberg; carries all non-motor fibers of CN VII
• Runs as a separate trunk between the main motor root and CN VIII within the IAC (anterosuperiorly)
• Merges with the main facial nerve at the geniculate ganglion
• Carries: parasympathetic (GVE) + taste (SVA) + somatic sensation (GSA)
• The chorda tympani is the peripheral expression of the nervus intermedius at the mastoid segment

VII. Blood Supply

VIII. Applied / Clinical Anatomy (Exam-High-Yield)

Localization of Lesions by Testing

Forehead Sparing (UMN vs LMN)

• UMN lesion (e.g., stroke): Contralateral lower face palsy; forehead spared (bilateral cortical representation of upper face)
• LMN lesion (e.g., Bell's palsy): Complete ipsilateral hemifacial palsy including forehead

Dehiscence of Fallopian Canal

• Most common at tympanic segment over oval window (~55-66% of cases)
• ~25% dehiscence at geniculate ganglion fossa
• Predisposes to otitis media-related facial palsy; surgical injury

Vulnerability Points Summary

Facial Recess

• Triangular space bounded by: facial nerve (medially), chorda tympani (laterally), incudal fossa (superiorly)
• Surgical corridor used in posterior tympanotomy for cochlear implant surgery and intact canal wall mastoidectomy

IX. Diagram

Figure: Complete anatomy of the facial nerve showing its motor root, nervus intermedius, segments, ganglion, and all branches (Scott-Brown's Otorhinolaryngology Vol. 2)

X. Quick Revision Summary

• Nerve of 2nd branchial arch
• 5 fiber types: SVE (motor), GVE (parasympathetic), SVA (taste), GSA (somatic sensation), visceral afferent
• 6 segments: Intracranial → Meatal → Labyrinthine (shortest, 4 mm) → Tympanic (horizontal, 11 mm, most dehiscent) → Mastoid (vertical, 13 mm) → Extratemporal
• Geniculate ganglion: only sensory ganglion; first branch = GSPN; first genu here
• Narrowest point: meatal foramen (between meatal and labyrinthine segments)
• Terminal 5 branches through parotid: T-Z-B-M-C (To Zanzibar By Motor Car)
• Facial recess: between facial nerve and chorda tympani - key surgical landmark
• Forehead sparing in UMN lesions due to bilateral cortical representation

• Cummings Otolaryngology Head and Neck Surgery, Chapter 126
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 2, Chapter 111-112
• Shambaugh's Surgery of the Ear, Chapter 2
• KJ Lee's Essential Otolaryngology, Chapter 21
• Gray's Anatomy for Students, Chapter 8