Facial Nerve (CN VII) - Complete Anatomy

Overview

1. Fiber Types - "What Does It Carry?"

Source: Shambaugh Surgery of the Ear

2. Nuclei - "Where Does It Come From?"

1. Motor nucleus - in the caudal pons. Drives all facial muscle movement.
   • Upper part (forehead/eye muscles) gets input from both sides of the motor cortex (bilateral cortical input). This is clinically important!
   • Lower part (mouth/cheek) gets input from the opposite side only (contralateral only).
   • This is why in a stroke (UMN lesion), only the lower face is weak - the forehead is spared because it has bilateral representation.
2. Superior salivatory nucleus - dorsal to motor nucleus in the pons. Controls the parasympathetic secretomotor fibers (tears, saliva).
3. Nucleus of the solitary tract - in the medulla. Receives taste (gustatory) input.

3. Course of the Facial Nerve - The 5 Segments

Segment 1: Intracranial (Cisternal) - 24 mm

• Exits the pons (at the pontomedullary junction, between pons and olive)
• Travels through the cerebellopontine angle (CPA) cistern
• Travels alongside CN VIII (vestibulocochlear nerve) and the nervus intermedius (see below)
• Enters the internal auditory meatus (IAM) via the porus acusticus

Segment 2: Intracanalicular (IAC) - variable

• Inside the internal auditory canal (IAC), CN VII sits in the anterosuperior quadrant
• CN VIII is posteriorly located
• At the lateral end (fundus) of the IAC, the nervus intermedius joins the main facial nerve trunk

Segment 3: Labyrinthine - 4 mm (shortest segment)

• Runs from the beginning of the fallopian canal (the bony canal for the facial nerve) to the geniculate ganglion
• Passes between the cochlea and vestibule
• Has the narrowest diameter of any part of the facial canal - this is why Bell's palsy edema here causes the most nerve compression

Segment 4: Tympanic (Horizontal) - ~13 mm

• At the geniculate ganglion, the nerve makes a sharp backward turn (the 1st genu)
• Runs horizontally in the medial wall of the middle ear
• Passes above the oval window and the cochleariform process
• Curves down at the 2nd genu (at the level of the lateral semicircular canal)

Segment 5: Mastoid (Vertical) - ~20 mm

• Runs vertically downward from the 2nd genu to the stylomastoid foramen
• Passes behind the external auditory canal
• Finally exits the skull through the stylomastoid foramen

4. Intratemporal Branches - Inside the Skull

a) Greater (Superficial) Petrosal Nerve (GSPN)

• Arises from the geniculate ganglion (1st genu)
• Carries preganglionic parasympathetic fibers to the lacrimal gland (via the pterygopalatine ganglion)
• Also carries taste from the soft palate
• Emerges through the facial hiatus onto the floor of the middle cranial fossa
• Surgical traction on this nerve can cause facial palsy via hemorrhage or edema

b) Nerve to Stapedius

• Branches off the mastoid segment near the pyramidal eminence
• Supplies the stapedius muscle (dampens loud sounds)
• Damage = hyperacusis (sounds are painfully loud)

c) Chorda Tympani

• Leaves the facial nerve ~4 mm above the stylomastoid foramen
• Carries taste from anterior 2/3 of tongue
• Carries preganglionic parasympathetics to the submandibular and sublingual glands (for saliva)
• Ascends in its own canal, enters the middle ear through the iter chordae posterius
• Crosses the tympanic cavity: lateral to the long process of incus, medial to the malleus
• Exits via the iter chordae anterius (canal of Huguier) through the petrotympanic fissure
• Joins the lingual nerve (CN V3) in the infratemporal fossa

5. Extracranial Course - Outside the Skull

1. Gives off the posterior auricular nerve - supplies occipital belly of occipitofrontalis and auricular muscles
2. Gives a muscular branch - supplies posterior belly of digastric and stylohyoid
3. Enters the parotid gland - divides into upper (temporofacial) and lower (cervicofacial) divisions
4. Passes through the parotid substance, branching further, then emerges as 5 terminal branches

6. Five Terminal (Peripheral) Branches

All terminal branches lie deep to the SMAS and enter muscles from their deep surface - so surgical procedures that remain superficial to fat are relatively safe. - Fitzpatrick's Dermatology

7. The Nervus Intermedius - "The Hidden Nerve"

• Also called the nerve of Wrisberg
• A separate small nerve bundle running between CN VII and the superior vestibular nerve in the IAC
• Carries all the non-motor fibers: taste, parasympathetic secretomotor, and sensory fibers
• Joins the main facial nerve trunk within the temporal bone
• The chorda tympani is essentially the "separation" of nervus intermedius sensory fibers at the inferior mastoid segment

8. Geniculate Ganglion - Key Relay Station

• Located at the 1st genu (bend) of the facial nerve at the end of the labyrinthine segment
• Contains the cell bodies of sensory neurons (taste and somatic sensation)
• Key branches arise here (GSPN)
• Viral reactivation here (Herpes zoster) = Ramsay Hunt Syndrome - causes painful ear vesicles, ipsilateral facial palsy, hearing loss, and vertigo

9. Fallopian Canal (Facial Canal)

• The bony canal the facial nerve travels through from the IAC to the stylomastoid foramen
• Has natural dehiscences (gaps) in many people, especially over the oval window area (~50% have some dehiscence)
• These gaps make the nerve vulnerable to infection, trauma, and cholesteatoma erosion

10. Clinical Correlations - Why This Anatomy Matters

Simple Summary Diagram (Text-Based)

BRAINSTEM (pons) → Cerebellopontine Angle → IAC → Labyrinthine segment
                                                              ↓
                                              GENICULATE GANGLION (1st genu)
                                              ↓ gives off: Greater Petrosal N. (to lacrimal gland)
                                              ↓
                                    TYMPANIC segment (horizontal, over oval window)
                                              ↓
                                           2nd genu (at lat. semicircular canal)
                                           ↓ gives off: Nerve to Stapedius
                                              ↓
                                    MASTOID segment (vertical)
                                           ↓ gives off: Chorda Tympani
                                              ↓
                                    STYLOMASTOID FORAMEN (exits skull)
                                           ↓ gives off: Posterior Auricular N.
                                              ↓
                                    PAROTID GLAND (divides here)
                         ___________↙                     ↘___________
                    Temporofacial division             Cervicofacial division
                    ↓                ↓                  ↓             ↓
               Temporal         Zygomatic            Buccal    Marginal Mandibular
                                                                   + Cervical

• Shambaugh Surgery of the Ear, pp. 67-69 (segments, fiber types, intratemporal branches)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 2 (cisternal course, extracranial division)
• Gray's Anatomy for Students (parotid relationships, terminal branches)
• Fitzpatrick's Dermatology (mnemonic, danger zones, surgical relevance)
• Cummings Otolaryngology Head and Neck Surgery (clinical imaging correlations)