10 FACIAL CN VII
The facial nerve (CN VII) is the most heavily examined cranial nerve in MRCS Part A. It has four modalities — motor, parasympathetic, taste and a patch of general sensory — and its long bony course means lesions at different levels produce predictable, dissociable deficits. Mnemonic: "Face, Ear, Taste, Tear."
Functional components — what CN VII actually does
👩⚕️ M ➡ Branchial motor (the main job)
- Muscles of facial expression — frontalis, orbicularis oculi, orbicularis oris, buccinator, platysma and the rest. These are derivatives of the second pharyngeal arch, hence their CN VII supply.
- Stapedius in the middle ear — dampens stapes vibration. Loss causes hyperacusis (sounds painfully loud).
- Posterior belly of digastric and stylohyoid — elevate the hyoid.
👩⚕️ Pa ➡ Parasympathetic (secretomotor) — two pathways, each tested:
- Greater petrosal → pterygopalatine ganglion → lacrimal gland. Postganglionic fibres hitchhike on V2 (zygomatic) then V1 (lacrimal) to reach the gland.
- Chorda tympani → submandibular ganglion → submandibular and sublingual glands. Chorda tympani joins the lingual nerve (V3).
The parotid is NOT supplied by CN VII — that is CN IX via the otic ganglion. CN VII merely passes through. Classic trap.
👩⚕️ S ➡ Special sensory (taste)
- Taste from the anterior two-thirds of the tongue via the chorda tympani. Cell bodies sit in the geniculate ganglion. (Posterior third taste = CN IX; epiglottis/vallecula = CN X.)
👩⚕️ General sensory ➡ small patch of skin around the external auditory meatus, tympanic membrane and concha. Site of vesicles in Ramsay Hunt syndrome.
Anatomical course of the facial CN VII
👩⚕️ The level of the lesion dictates which modalities are lost — learn the order of branches.
Intracranial / intratemporal:
1. Two roots (motor + nervus intermedius) emerge at the cerebellopontine angle alongside CN VIII.
2. Enter the internal acoustic meatus.
3. Fuse in the Z-shaped facial canal, forming the geniculate ganglion at the bend.
4. Three intratemporal branches, in this order: greater petrosal → nerve to stapedius → chorda tympani.
5. Exits skull at the stylomastoid foramen.
Extracranial:
6. Three small motor branches immediately given off: posterior auricular, nerve to posterior belly of digastric, nerve to stylohyoid.
7. Main trunk enters the parotid (does NOT supply it) and splits into five terminal branches = the parotid plexus.
The five terminal branches — "To Zanzibar By Motor Car":
| Branch | Supplies |
|---|---|
| Temporal | Frontalis, orbicularis oculi (upper), corrugator supercilii |
| Zygomatic | Orbicularis oculi (lower), zygomaticus major/minor |
| Buccal | Buccinator, orbicularis oris, levator labii superioris |
| Marginal mandibular | Depressor anguli oris, depressor labii inferioris, mentalis |
| Cervical | Platysma |
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Lesion localisation — the high-yield skill
The exam tests this constantly. Work proximal-to-distal and note what is lost vs preserved:
| Lesion site | Motor face | Hyperacusis | Taste ant. 2/3 | Lacrimation |
|---|---|---|---|---|
| Cerebellopontine angle / IAM | Yes | Yes | Yes | Yes (+ CN VIII signs) |
| Facial canal, proximal to greater petrosal | Yes | Yes | Yes | Yes |
| Distal to greater petrosal, proximal to stapedius | Yes | Yes | Yes | No |
| Distal to stapedius, proximal to chorda tympani | Yes | No | Yes | No |
| Stylomastoid foramen | Yes | No | No | No |
| Within parotid | Patchy (one or more terminal branches) | No | No | No |
UMN vs LMN facial palsy
A frequent SBA. The upper face (frontalis, orbicularis oculi) receives bilateral cortical input; the lower face receives only contralateral input.
| UMN lesion (e.g. stroke) | LMN lesion (e.g. Bell's, Ramsay Hunt) | |
|---|---|---|
| Forehead | Spared (can still wrinkle) | Involved (cannot wrinkle) |
| Lower face | Drooping, contralateral to lesion | Drooping, ipsilateral to lesion |
| Eye closure | Preserved | Weak (risk of exposure keratitis) |
| Other features | Limb signs | Hyperacusis, taste loss, post-auricular pain |
Bell's palsy — idiopathic LMN palsy, presumed HSV-1 inflammation in the facial canal. Acute, complete unilateral facial weakness including the forehead. Oral steroids within 72 hours; protect the eye.
Ramsay Hunt syndrome — VZV reactivation in the geniculate ganglion. LMN palsy + vesicles in the ear canal + ipsilateral hearing loss/vertigo. Steroids and aciclovir.
Surgical relevance — the parotid
The facial nerve runs through the parotid, dividing it into artificial superficial and deep lobes. It is the structure most at risk during parotidectomy. The main trunk is identified just deep to the tragal pointer, between the mastoid and the posterior belly of digastric.
The marginal mandibular branch runs along the lower border of the mandible, very superficial — at high risk during submandibular gland surgery and upper neck dissections.
> Pearl: Frey's syndrome (gustatory sweating over the parotid bed after parotidectomy) is aberrant regeneration — CN IX parasympathetic fibres reinnervate cutaneous sweat glands.
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Test yourself
A 50-year-old man falls at work and is briefly unconscious. In the emergency department he complains only of hyperacusis. Which cranial nerve is most likely injured?
- ((CN V::Mastication and facial sensation — would not cause hyperacusis in isolation.))
- ((CN VIII::Hearing/balance — causes deafness or vertigo, not hyperacusis.))
- ((CN IX::Posterior 1/3 taste and parotid secretion.))
- ((CN VII::☑️ Supplies stapedius — loss removes sound dampening, causing hyperacusis.))
- ((CN X::Parasympathetic to thoracoabdominal viscera.))
👩⚕️ Hyperacusis = think stapedius = think CN VII proximal to the nerve to stapedius.
Which of the following muscles is NOT innervated by the facial nerve?
- ((Buccinator::Buccal branch of CN VII.))
- ((Temporalis::☑️ Muscle of mastication — supplied by deep temporal nerves of V3.))
- ((Orbicularis oculi::Temporal/zygomatic branches of CN VII.))
- ((Mentalis::Marginal mandibular branch of CN VII.))
- ((Frontalis::Temporal branch of CN VII.))
👩⚕️ Classic trap: muscles of facial expression = CN VII; muscles of mastication = V3.
A patient has a facial nerve injury following left parotid surgery. Which abnormality is most likely?
- ((Ptosis of the left eyelid::Levator palpebrae superioris is CN III.))
- ((Drooping of the left lower lip::☑️ Marginal mandibular branch (very superficial) is the most commonly injured during parotid/submandibular surgery.))
- ((Numbness on the whole face::Facial sensation is CN V.))
- ((Numbness of the ipsilateral cheek::Cheek sensation is V2.))
A patient developed an uneven smile after an upper deep cervical lymph node biopsy. Which nerve was most likely injured?
- ((Buccal n.::Supplies buccinator and upper lip — would not selectively affect smile asymmetry from neck surgery.))
- ((Mental n.::Sensory branch of V3 — no motor function.))
- ((Inferior alveolar n.::Sensory to lower teeth via V3.))
- ((Marginal mandibular n.::☑️ Runs along lower mandible just under platysma — easily injured in upper neck surgery, causing lower lip droop.))
A man with a skull injury develops ipsilateral facial muscle weakness but no hyperacusis or taste disturbance. Where is the most likely site of facial nerve injury?
- ((Internal acoustic meatus::Would also cause hyperacusis, taste and lacrimal loss, plus CN VIII signs.))
- ((Facial canal proximal to nerve to stapedius::Would cause hyperacusis.))
- ((Stylomastoid foramen::☑️ Distal to all three intratemporal branches — motor loss only, modalities preserved.))
- ((Chorda tympani::Carries no motor fibres to the face.))
- ((Parotid gland::Causes patchy weakness of one or more terminal branches, not complete unilateral weakness.))
👩⚕️ Pure motor loss with preserved taste and hearing localises the lesion at or distal to the stylomastoid foramen.
A patient presents with loss of lacrimation. Which ganglion is involved?
- ((Otic::CN IX parasympathetics to parotid.))
- ((Submandibular::CN VII parasympathetics to submandibular/sublingual glands.))
- ((Pterygopalatine::☑️ Relay for greater petrosal → lacrimal gland pathway.))
- ((Ciliary::CN III parasympathetics to pupil and ciliary muscle.))
A patient underwent parotidectomy followed by radiation therapy, and later presents with dryness of the eyes. Which ganglion is most likely involved?
- ((Pterygopalatine::☑️ Parasympathetic relay to the lacrimal gland — radiation fibrosis can disrupt the pathway.))
- ((Otic::Parotid parasympathetics (CN IX).))
- ((Ciliary::CN III parasympathetics — pupil/accommodation.))
- ((Submandibular::Submandibular/sublingual salivation.))
The chorda tympani exits through which foramen?
- ((Stylomastoid foramen::CN VII main trunk exit.))
- ((Foramen ovale::V3 exit.))
- ((Petrotympanic fissure::☑️ Chorda tympani passes from middle ear into the infratemporal fossa here, then joins the lingual nerve.))
- ((Internal acoustic meatus::Entry of CN VII and VIII.))
- ((Jugular foramen::CN IX, X, XI exit.))
Which of the following is the anatomical cause of hyperacusis following facial nerve injury?
- ((Tensor tympani::Also dampens hearing, but innervated by V3.))
- ((Cochlear nerve::Sensory hearing — loss causes deafness, not hyperacusis.))
- ((Nerve to stapedius::☑️ CN VII branch in the facial canal; loss removes dampening of stapes.))
- ((Chorda tympani::Taste + submandibular/sublingual secretion.))
- ((Levator veli palatini::Soft palate (CN X).))
A patient suffers an orbital floor fracture and now has loss of lacrimation. Which nerve is most likely affected?
- ((Lacrimal nerve::Only a conduit for postganglionic parasympathetics — isolated injury rarely abolishes lacrimation.))
- ((Facial nerve::Whole-trunk injury would cause facial paralysis too.))
- ((Greater petrosal nerve::☑️ Carries preganglionic parasympathetics from CN VII to pterygopalatine ganglion → lacrimal gland.))
- ((Auriculotemporal nerve::Parasympathetics to parotid (CN IX).))
Revision summary
- CN VII = Face, Ear, Taste, Tear.
- 4 modalities: motor (facial expression + stapedius + stylohyoid + posterior belly of digastric); parasympathetic (lacrimal via greater petrosal/pterygopalatine; submandibular + sublingual via chorda tympani/submandibular ganglion); taste anterior 2/3 tongue (chorda tympani); small patch of sensation around the ear.
- Parotid = CN IX, NOT CN VII. CN VII only passes through.
- Course: CP angle → internal acoustic meatus → facial canal (geniculate ganglion → greater petrosal → stapedius → chorda tympani) → stylomastoid foramen → posterior auricular + digastric + stylohyoid → parotid plexus → Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical (To Zanzibar By Motor Car).
- Lesion localisation: hyperacusis = proximal to stapedius branch; taste loss = proximal to chorda tympani; loss of lacrimation = proximal to greater petrosal; pure motor = at/beyond stylomastoid foramen.
- UMN palsy spares the forehead (bilateral cortical input). LMN palsy involves the whole half of the face.
- Bell's: idiopathic LMN palsy, steroids within 72 h, protect the eye.
- Ramsay Hunt: VZV at geniculate ganglion — LMN palsy + ear vesicles + hearing loss; steroids + aciclovir.
- Surgery: identify CN VII at the tragal pointer during parotidectomy; marginal mandibular branch most vulnerable in submandibular/upper neck surgery.