Cranial Nerves: Function & Lesions
A board-focused walkthrough of the 12 cranial nerves — modality, brainstem nucleus, exit foramen, and the signature lesion for each — anchored by the classic reflex arcs, ocular-motor palsies, and deviation rules Step 1 loves to test.
The four things to know per nerve
The 12 cranial nerves (CN I–XII) carry motor, sensory, or mixed traffic to and from the head and neck; only CN X (vagus) extends into the thorax and abdomen. For boards, lock in four facts per nerve: its modality (sensory / motor / both), the brainstem level of its nucleus, the skull foramen it exits, and its signature lesion. Two high-yield rules cut across the whole list: all parasympathetic outflow of the head rides on CN III, VII, IX, and X, and every somatic-motor nerve is ipsilateral except CN IV (trochlear) — the only one that decussates and exits the dorsal brainstem, so each nucleus drives the contralateral superior oblique.
Names (I→XII): On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops — Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Auditory (vestibulocochlear), Glossopharyngeal, Vagus, Spinal accessory, Hypoglossal.
Modality (S = Sensory, M = Motor, B = Both): Some Say Marry Money But My Brother Says Big Brains Matter Most → S · S · M · M · B · M · B · S · B · B · M · M
The 12 cranial nerves at a glance
| CN | Name | Type | Key function |
|---|---|---|---|
| I | Olfactory | Sensory | Smell |
| II | Optic | Sensory | Vision |
| III | Oculomotor | Motor | Most extraocular muscles + levator palpebrae; pupil constriction & accommodation (parasympathetic) |
| IV | Trochlear | Motor | Superior oblique (depresses adducted eye → tested "down-and-in") |
| V | Trigeminal | Both | Facial sensation; muscles of mastication |
| VI | Abducens | Motor | Lateral rectus (abduction) |
| VII | Facial | Both | Facial expression; taste anterior ⅔ tongue; lacrimation/salivation; stapedius |
| VIII | Vestibulocochlear | Sensory | Hearing & balance |
| IX | Glossopharyngeal | Both | Taste posterior ⅓; stylopharyngeus; parotid; carotid body/sinus |
| X | Vagus | Both | Palate/pharynx/larynx; thoraco-abdominal parasympathetic; taste at epiglottis |
| XI | Accessory | Motor | Sternocleidomastoid & trapezius |
| XII | Hypoglossal | Motor | Intrinsic/extrinsic tongue muscles (except palatoglossus) |
- Brainstem level (rostral→caudal): Midbrain = III, IV · Pons = V, VI, VII, VIII · Medulla = IX, X, XII (XI arises from the upper cervical spinal cord, C1–C5/6).
- CN IV is the thinnest nerve, has the longest intracranial course (prone to head trauma), is the only one exiting dorsally, and the only one whose fibers decussate — so each nucleus drives the contralateral SO. Watch the laterality trap: a nuclear lesion weakens the opposite SO, but the far more common peripheral nerve palsy weakens the ipsilateral SO.
- Foramina: cribriform plate (I) · optic canal (II) · superior orbital fissure (III, IV, V1, VI) · foramen rotundum (V2) · foramen ovale (V3) · internal acoustic meatus (VII, VIII) · jugular foramen (IX, X, XI) · hypoglossal canal (XII).
- Trigeminal branch mnemonic: "Standing Room Only" → V1 Superior orbital fissure, V2 Rotundum, V3 Ovale.
A favorite vignette style is "which nerve, which limb?" for a lost reflex:
- Pupillary light: afferent CN II → efferent CN III
- Corneal (blink): afferent CN V1 → efferent CN VII
- Lacrimation: afferent CN V1 → efferent CN VII
- Jaw-jerk: afferent and efferent CN V3
- Gag: afferent CN IX → efferent CN X
- Cough: afferent and efferent CN X
Isolated loss of the sensory limb vs motor limb localizes the nerve — e.g., an absent corneal reflex with intact facial movement points to V1, not VII.
The three ocular-motor palsies (III vs IV vs VI)
| Feature | CN III (Oculomotor) | CN IV (Trochlear) | CN VI (Abducens) |
|---|---|---|---|
| Muscle(s) | All EOM except SO & LR; + LPS, sphincter pupillae | Superior oblique | Lateral rectus |
| Eye at rest | "Down & out," ptosis, dilated pupil | Slightly elevated & extorted | Adducted (medial) |
| Diplopia | Masked when ptosis is complete | Vertical, worse on downgaze (reading, stairs) | Horizontal, worse on lateral gaze |
| Classic clue | PCoM aneurysm (pupil-involving), diabetes (pupil-sparing) | Head tilt away from lesion | ↑ICP false-localizing sign (long course) |
Facial nerve — the central-seven distinction is a boards staple:
- LMN lesion (Bell's palsy): paralysis of the entire ipsilateral face, including the forehead; may add hyperacusis (stapedius), loss of taste (anterior ⅔), and altered lacrimation. Associated with HSV reactivation, Lyme disease, diabetes, sarcoidosis.
- UMN lesion (stroke): weakness of the contralateral lower face only — the forehead is spared because the upper face gets bilateral cortical input.
Cerebellopontine-angle (CPA) tumor — vestibular schwannoma ("acoustic neuroma"): starts as CN VIII signs (sensorineural hearing loss, tinnitus, vertigo), then spreads to CN V (↓ corneal reflex) and CN VII; bilateral schwannomas = NF2.
Trigeminal neuralgia (tic douloureux): paroxysmal, lancinating unilateral V2/V3 pain triggered by chewing or light touch; first-line therapy is carbamazepine.
- Jaw (CN V motor): deviates TOWARD the lesion (weak pterygoid).
- Uvula/palate (CN X): deviates AWAY from the lesion (weak side droops, uvula pulled to the intact side).
- Tongue (CN XII, LMN): deviates TOWARD the lesion ("lick your wounds").
- CN XI: ipsilateral shoulder droop (trapezius) + weakness turning the head away from the lesion (SCM).
- CN III pupil rule: pupil-involving = compressive (PCoM aneurysm, uncal herniation) — parasympathetic fibers run on the outside; pupil-sparing = ischemic/diabetic (spares the peripheral fibers, hits the central somatic core).
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