By MRCSA in MRCS Part A — 04 Jul 2026

12 CRANIAL NERVE ANATOMY

# 13 CRANIAL NERVE ANATOMY

👩‍⚕️ Cranial nerves are an MRCS Part A favourite. The examiner will not expect every nuclear column, but they will expect you to know each nerve by number, function, foramen of exit, and brainstem origin. Add the cavernous sinus and jugular foramen contents, plus the classic CN III palsy, and you have most of the marks.

The twelve cranial nerves at a glance

Memorise this table cold. It answers more single-best-answer questions than any other single block of cranial-nerve knowledge.

#	Name	Function	Foramen	Brainstem origin
I	Olfactory	S — smell	Cribriform plate	Cerebrum
II	Optic	S — vision	Optic canal	Diencephalon
III	Oculomotor	M — most EOM, levator palpebrae, pupil, accommodation	Superior orbital fissure	Midbrain
IV	Trochlear	M — superior oblique	Superior orbital fissure	Midbrain (dorsal exit)
V1	Trigeminal — Ophthalmic	S — forehead, cornea, nose	Superior orbital fissure	Pons
V2	Trigeminal — Maxillary	S — mid-face	Foramen rotundum	Pons
V3	Trigeminal — Mandibular	B — lower face sensation + muscles of mastication	Foramen ovale	Pons
VI	Abducens	M — lateral rectus	Superior orbital fissure	Ponto-medullary junction
VII	Facial	B — facial expression, taste ant 2/3, lacrimation, salivation	Internal acoustic meatus → stylomastoid foramen	Ponto-medullary junction
VIII	Vestibulocochlear	S — hearing, balance	Internal acoustic meatus	Ponto-medullary junction
IX	Glossopharyngeal	B — stylopharyngeus, post 1/3 tongue, parotid	Jugular foramen	Medulla
X	Vagus	B — pharynx, larynx, thoracoabdominal viscera	Jugular foramen	Medulla
XI	Accessory	M — SCM, trapezius	Jugular foramen	Medulla + upper cervical cord
XII	Hypoglossal	M — tongue (except palatoglossus)	Hypoglossal canal	Medulla

👩‍⚕️ Number mnemonic: "Some Say Marry Money But My Brother Says Big Brains Matter More" — S = sensory, M = motor, B = both. In order: I S, II S, III M, IV M, V B, VI M, VII B, VIII S, IX B, X B, XI M, XII M.

👩‍⚕️ Brainstem origin shortcut: Midbrain = III, IV. Pons = V, VI, VII, VIII. Medulla = IX, X, XI, XII. CN IV is the only nerve to exit the brainstem dorsally and the only one to decussate before innervating its muscle.

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Skull base foramina — anterior to posterior

Walk the skull base from front to back:

➡ Cribriform plate (ethmoid) — CN I

➡ Optic canal (sphenoid lesser wing) — CN II + ophthalmic artery

➡ Superior orbital fissure — CN III, IV, V1, VI + superior ophthalmic vein

➡ Foramen rotundum — V2

➡ Foramen ovale — V3 + accessory meningeal artery + lesser petrosal nerve

➡ Foramen spinosum — middle meningeal artery (mnemonic trap: not a cranial nerve)

➡ Internal acoustic meatus — CN VII, VIII + labyrinthine artery

➡ Jugular foramen — CN IX, X, XI + internal jugular vein + sigmoid sinus drainage

➡ Hypoglossal canal — CN XII

➡ Foramen magnum — medulla/spinal cord, vertebral arteries, spinal roots of CN XI

👩‍⚕️ Easy SBA trap: foramen spinosum carries the middle meningeal artery, not a nerve. Students who memorise "ovale, rotundum, spinosum" by sound often misallocate V3.

Cavernous sinus contents

A paired venous sinus lateral to the sphenoid body and pituitary fossa. A single lesion (thrombosis, tumour, carotico-cavernous fistula) produces facial pain plus ophthalmoplegia — explained entirely by its contents.

Lateral wall (superior → inferior): CN III, IV, V1, V2

Inside the sinus: CN VI + internal carotid artery + sympathetic plexus

👩‍⚕️ "O TOM CAT" — Oculomotor, Trochlear, Ophthalmic, Maxillary in the wall; Carotid + Abducens in the middle. CN VI is the most vulnerable nerve because it runs free in the sinus, not embedded in the wall.

Jugular foramen contents

➡ CN IX, X, XI + internal jugular vein + sigmoid sinus drainage

👩‍⚕️ Vernet syndrome: loss of gag (IX), hoarse voice and dysphagia (X), weak SCM/trapezius (XI). Classic cause: glomus jugulare tumour.

Eye movements — CN III, IV, VI in 60 seconds

Covered in Lessons 08–09. Headlines:

➡ CN III — all extraocular muscles except lateral rectus and superior oblique; plus levator palpebrae, sphincter pupillae, ciliary muscle.

➡ CN IV — superior oblique (trouble descending stairs).

➡ CN VI — lateral rectus.

CN III palsy: ptosis, "down and out" eye, fixed dilated pupil, loss of accommodation. Parasympathetic fibres sit on the outside of the nerve — compressive lesions (PCom aneurysm, uncal herniation) blow the pupil early; ischaemic lesions (diabetes) spare it.

Facial nerve (CN VII) — the surgical workhorse

Heavily tested because it is so often injured (parotidectomy, mastoid surgery, mandible fractures).

Five terminal branches inside the parotid — "To Zanzibar By Motor Car": Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical.

Branches given off proximal to the parotid (spared in parotidectomy):

➡ Greater petrosal — lacrimation (via pterygopalatine ganglion)

➡ Nerve to stapedius — lesion → hyperacusis

➡ Chorda tympani — taste anterior 2/3 tongue + parasympathetic to submandibular/sublingual glands

👩‍⚕️ Frey syndrome — gustatory sweating post-parotidectomy from aberrant regrowth of parasympathetic fibres along the auriculotemporal nerve (V3) into skin sweat glands.

Tongue innervation — a perennial exam favourite

Function	Anterior 2/3	Posterior 1/3
General sensation	Lingual nerve (V3)	CN IX
Taste	Chorda tympani (VII)	CN IX
Motor	CN XII (all intrinsic + extrinsic muscles)	CN XII
Exception	—	Palatoglossus = CN X

👩‍⚕️ Hypoglossal lesion → tongue deviates toward the side of the lesion ("the tongue points at the lesion") because the unopposed contralateral genioglossus pushes it across.

Test yourself

![MCQs banner]

A patient sustains head trauma and presents with a right, fixed and dilated pupil. Hours later he dies. What is the likely cause?

((Right extradural haematoma::☑️ Expanding clot → uncal herniation → ipsilateral CN III compression on tentorial edge.))
((Left extradural haematoma::Wrong side — uncal herniation blows the pupil on the same side as the lesion.))
((Acute subdural haematoma::Slower clinical course; lacks the classic lucid interval of an extradural.))
((Subarachnoid haemorrhage::Thunderclap headache and meningism; CN III palsy from PCom aneurysm, not trauma.))
((Posterior communicating artery aneurysm::Commonest surgical cause of isolated CN III palsy but no trauma history.))

👩‍⚕️ A fixed dilated pupil after head trauma is a neurosurgical emergency — surface fibres carry parasympathetics and are crushed first.

A patient develops a down-and-out eye after surgery. Which accompanying feature would you expect?

((Mydriasis::☑️ Loss of parasympathetic to sphincter pupillae via CN III → fixed dilated pupil.))
((Exophthalmos::Proptosis from Graves' orbitopathy; not a feature of CN III palsy.))
((Miosis::Constricted pupil of Horner syndrome (sympathetic loss) — opposite of CN III palsy.))
((Enophthalmos::Sunken eye of orbital floor blow-out or Horner; CN III does not affect orbital volume.))
((Nystagmus::Vestibular or cerebellar disease; CN III palsy gives a fixed deviation.))

👩‍⚕️ Pupil-sparing CN III palsy = diabetic microvascular ischaemia. Pupil-involving = compressive — image urgently.

A man presents with ipsilateral facial pain and ophthalmoplegia. Which structure is most likely involved?

((Cavernous sinus::☑️ Holds CN III, IV, V1, V2 + VI + ICA — single lesion causes pain plus total ophthalmoplegia.))
((Superior orbital fissure::Carries III, IV, V1, VI but spares V2 — pain is limited to forehead.))
((Cerebellopontine angle::Affects V, VII, VIII — facial numbness, weakness, hearing loss, not ophthalmoplegia.))
((Orbital apex::SOF contents + optic nerve → adds visual loss; V2 spared.))
((Pterygopalatine fossa::Contains V2 only; no oculomotor nerves pass through it.))

👩‍⚕️ "O TOM CAT" — Oculomotor, Trochlear, Ophthalmic, Maxillary in the wall; Carotid + Abducens through the middle.

Through which foramen does the mandibular division of CN V3 pass?

((Foramen ovale::☑️ V3 exits here — the only trigeminal division carrying motor fibres.))
((Foramen rotundum::V2 (maxillary) passes here into the pterygopalatine fossa.))
((Foramen spinosum::Middle meningeal artery — not a cranial nerve.))
((Superior orbital fissure::CN III, IV, V1, VI into the orbit.))
((Jugular foramen::CN IX, X, XI + IJV in the posterior cranial fossa.))

👩‍⚕️ Rotundum = V2, Ovale = V3, Spinosum = middle meningeal artery. Easy SBA mark.

During parotidectomy, transection of the facial nerve is a recognised risk. Which of the following would NOT result from this injury?

((Loss of taste to anterior two-thirds of the tongue::☑️ Chorda tympani leaves CN VII in the middle ear, before the parotid — spared.))
((Loss of forehead wrinkling::Temporal branch supplies frontalis; lost in parotid injury.))
((Loss of nasal flaring::Buccal and zygomatic branches supply nasal muscles.))
((Loss of buccinator function::Buccal branch of CN VII supplies buccinator.))
((Loss of eye closure::Zygomatic and temporal branches supply orbicularis oculi — corneal exposure risk.))

👩‍⚕️ Branches lost at the parotid follow the LMN pattern: whole hemiface, including forehead.

A patient has facial nerve injury after left parotid surgery. Which abnormality would you expect?

((Inability to close the left eye::☑️ Zygomatic and temporal branches of VII paralyse orbicularis oculi.))
((Loss of sensation over the left cheek::Cheek sensation = CN V, not VII.))
((Loss of taste to the posterior third of the tongue::Posterior 1/3 taste = CN IX, not VII.))
((Deviation of the tongue to the left::Tongue deviation = CN XII, points toward the lesion.))
((Loss of sensation to anterior two-thirds of the tongue::General sensation = lingual nerve (V3), not VII.))

👩‍⚕️ Test all five branches after parotid surgery. Marginal mandibular is most often injured — droopy lip.

After parotid surgery, a patient notices sweating on the cheek while eating. What nerve is involved?

((Auriculotemporal nerve::☑️ Frey syndrome — parasympathetic fibres regrow along V3 into skin sweat glands.))
((Facial nerve::Motor only to facial muscles; carries no sympathetic fibres to skin.))
((Great auricular nerve::C2/C3 sensory branch — injury causes numbness, not sweating.))
((Buccal nerve::V3 sensory to buccal mucosa; no parasympathetic role.))
((Chorda tympani::Parasympathetic to submandibular/sublingual glands, not parotid bed.))

👩‍⚕️ Frey syndrome occurs in up to 60% post-parotidectomy. Treatment: botulinum toxin to the affected skin.

Revision summary

➡ Sensory/Motor/Both — Some Say Marry Money But My Brother Says Big Brains Matter More.

➡ Brainstem origins — Midbrain: III, IV. Pons: V–VIII. Medulla: IX–XII.

➡ Cribriform I; Optic canal II; SOF III, IV, V1, VI; Rotundum V2; Ovale V3; Spinosum = middle meningeal artery (not a CN); IAM VII, VIII; Jugular IX, X, XI + IJV; Hypoglossal canal XII.

➡ Cavernous sinus — wall: III, IV, V1, V2 (O TOM CAT); inside: VI + ICA. CN VI most vulnerable.

➡ Jugular foramen — IX, X, XI + IJV. Vernet syndrome = loss of gag, hoarse voice, weak SCM/trapezius.

➡ CN III palsy — Ptosis, Down and out, Dilated pupil. Compressive = pupil blown; ischaemic (diabetes) = pupil spared.

➡ CN IV — only nerve to exit brainstem dorsally; only one to decussate.

➡ CN VII in parotid — To Zanzibar By Motor Car (T, Z, B, M, C). Chorda tympani and stapedius branch off before the parotid.

➡ Frey syndrome — gustatory sweating via aberrant auriculotemporal (V3) regeneration.

➡ Tongue — Ant 2/3 sensation V3, taste VII; Post 1/3 both IX; motor XII (except palatoglossus = X). XII lesion → tongue points to lesion.