Head & Neck Neurology · High Yield

Cranial Nerves

The twelve cranial nerves arise from the brain and brainstem. Their number, name, and fibre type form the basis of every neurological examination.

✦ The Mnemonic

"On Our Odyssey, Three Travellers Always Found Vital Ground; Various Ascending Heights"

I–XII in sequence — S=Sensory, M=Motor, B=Both

I Olfactory Sensory — smell; cribriform plate; anosmia after head injury

II Optic Sensory — vision; optic canal; decussates at chiasm

III Oculomotor Motor — levator, SR, MR, IR, IO; pupil constriction + accommodation

IV Trochlear Motor — superior oblique; only CN exiting dorsal brainstem

V Trigeminal Both — facial sensation (V1/V2/V3) + muscles of mastication (V3)

VI Abducens Motor — lateral rectus; longest intracranial course → first palsy with raised ICP

VII Facial Both — facial expression, taste ant. 2/3 tongue, lacrimation, salivation

VIII Vestibulocochlear Sensory — hearing (cochlear) and balance (vestibular)

IX Glossopharyngeal Both — taste post. 1/3 tongue, parotid secretion, gag reflex afferent

X Vagus Both — pharynx, larynx, thoracoabdominal parasympathetics

XI Accessory Motor — sternocleidomastoid and trapezius

XII Hypoglossal Motor — tongue muscles; deviates toward the side of a LMN lesion

📚 Clinical Breakdown

The functional types follow "Some Say Marry Money But My Brother Says Bad Business Marry Money" (I–XII: S, S, M, M, B, M, B, S, B, B, M, M). Pure motor: III, IV, VI, XI, XII. Pure sensory: I, II, VIII. Mixed: V, VII, IX, X.

The oculomotor nerve (CN III) parasympathetic fibres run on the outside of the nerve. Surgical or compressive CN III palsies compress these outer fibres first — dilating the pupil before causing ptosis or ophthalmoplegia. Diabetic ischaemic CN III palsies destroy the vasa nervorum centrally, sparing the outer pupillary fibres.

The vagus nerve (CN X) provides parasympathetics to the thorax and abdomen as far as the splenic flexure. The left recurrent laryngeal nerve loops under the aortic arch — vulnerable in aortic aneurysm, lung cancer, and mediastinal masses. Unexplained hoarseness mandates chest imaging.

Longest intracranial course CN VI — first to palsy with raised ICP

Dorsal brainstem exit CN IV (trochlear)

CN III surgical vs diabetic Surgical = dilated pupil; Diabetic = pupil spared

Tongue deviation Toward side of CN XII lesion

⭐ Clinical Pearl

UMN vs LMN facial nerve: UMN lesion (stroke) spares the forehead — bilateral cortical representation. LMN lesion (Bell's palsy, parotid tumour) affects the entire ipsilateral face including forehead. One of the most commonly tested neurological distinctions.

⚠ Exam Trap

Blown pupil + sudden headache = posterior communicating artery aneurysm until proven otherwise. Parasympathetic fibres on CN III surface are compressed first — fixed dilated pupil may precede motor deficit. Urgent CT then CTA.

Next: CN Foramina →