CRANIAL NERVES — EXAM IMPRINT

Only the 5 that earn marks. One image per nerve.

THE MAP — WHERE THEY LIVE

MIDBRAIN III, IV PONS V, VI, VII, VIII MEDULLA IX, X, XI, XII III → Eye V → Face VII → Face M IX,X → Throat XII → Tongue Eye movement + pupil Face sensation (3 divisions) Face movement + taste Swallowing + voice + gag Tongue movement 2 – 4 – 4 rule

CN III — OCULOMOTOR MOTOR +PARA

What it does: ALL eye muscles EXCEPT lateral rectus (VI) and superior oblique (IV). Plus: levator palpebrae (eyelid) + parasympathetic pupil constriction.

Memory: CN III does everything. VI and IV are the exceptions. LR₆SO₄ — Lateral Rectus = VI, Superior Oblique = IV, rest = III.

SURGICAL (Compressive) — CN III

Pupil affected FIRST

→ Parasympathetic fibers run on the OUTSIDE of the nerve

→ Compression hits outer fibers first

Causes: Aneurysm (PComm), uncal herniation, tumor

⚠️ Dilated pupil = EMERGENCY

MEDICAL (Ischaemic) — CN III

Pupil SPARED

→ Ischaemia hits the INSIDE (vasa nervorum)

→ Outer parasympathetic fibers still get blood from outside

Causes: Diabetes, hypertension, vasculitis

Painful but pupil is normal

THE ANCHOR: Parasympathetic fibers ride on the OUTSIDE of CN III like a passenger on a bus roof.
Compression → roof passenger falls first → pupil dilates (surgical = emergency)
Ischaemia → engine fails inside → pupil spared (medical = diabetes)

CN V — TRIGEMINAL MIXED

Three divisions — one image:

V1 Ophthalmic Forehead + cornea Exit: Superior orbital fissure V2 Maxillary Cheek + upper lip + teeth Exit: Foramen rotundum V3 Mandibular Jaw + lower lip + tongue sens. Exit: Foramen ovale + MOTOR

Motor component: V3 ONLY — muscles of mastication (masseter, temporalis, pterygoids). "V3 = jaw power."

Key clinical tests:

• Corneal reflex: Afferent = V1 (ophthalmic), Efferent = VII (facial). Lost in Wallenberg (spinal V nucleus hit).

• Jaw jerk: V3. Brisk = UMN lesion above pons (pseudobulbar palsy).

• Trigeminal neuralgia: Electric shock pain in V2/V3 distribution. India: carbamazepine first-line.

Foramen exit mnemonic: Superior orbital fissure, Foramen Rotundum, Foramen Ovale = S-R-O = "SiR, One" (V1, V2, V3)

Only V3 has motor. Think: you need your jaw (V3) to chew = motor.

CN VII — FACIAL MIXED

THE most tested CN distinction in medicine:

UMN LESION (Stroke)

Forehead SPARED ✓

Only lower face weak (contralateral)

→ Upper face gets bilateral cortical supply

→ One cortex gone? Other still feeds forehead.

Look for: associated arm/leg weakness

LMN LESION (Bell's Palsy)

ENTIRE half face paralyzed

Forehead INCLUDED — can't raise eyebrow

→ The nerve itself is damaged below the nucleus

→ No alternate supply possible

Look for: hyperacusis, loss of taste ant 2/3, dry eye

UMN (Stroke) SPARED ✓ Lower face WEAK LMN (Bell's) FLAT — can't wrinkle ✗ ENTIRE face WEAK

Bell's palsy extras: Most common CN mononeuropathy. Viral (HSV-1). Steroids within 72h. India: must rule out Hansen's disease (leprosy) — thickened nerve, patch near ear.

Bell's = Below (LMN) = Both (upper AND lower face).
Stroke = Superior (UMN) = Spares forehead.

CN IX (Glossopharyngeal) + X (Vagus) MIXED +PARA

Why together: They travel together through jugular foramen, share nucleus ambiguus, and are tested together clinically.

Division of labor:

• IX: Sensation posterior 1/3 tongue + taste post 1/3 + afferent gag reflex + carotid sinus/body

• X: Motor to palate, pharynx, larynx (recurrent laryngeal nerve) + efferent gag + parasympathetic to thorax/abdomen to splenic flexure

Bedside test — "say AAH":

Unilateral X lesion → uvula deviates AWAY from lesion (weak side can't pull) → palate drags toward normal side.

Recurrent laryngeal nerve (branch of X):

Left RLN loops under aortic arch — longer course — more vulnerable.

Causes of left RLN palsy: aortic aneurysm, mitral stenosis (large LA), lung cancer (L hilum), post-thyroidectomy

→ Hoarseness of voice. India: think thyroid surgery complication first.

Uvula points AWAY from the bad side. (Opposite of tongue rule!)
Tongue (XII) → deviates TO lesion. Uvula (X) → deviates AWAY from lesion.

Left RLN = "Left loops Low" (under aortic arch) = Left is more vulnerable.

CN XII — HYPOGLOSSAL MOTOR

Pure motor to tongue.

LMN Lesion (nucleus/nerve)

Tongue deviates TO the lesion

→ Weak side can't push, strong side pushes tongue toward weak

+ Ipsilateral atrophy + fasciculations

Seen in: Dejerine, tumors, MND

UMN Lesion (cortex/tract)

Tongue deviates AWAY from lesion

→ CST crosses → cortical damage affects opposite genioglossus

No atrophy, no fasciculations

Seen in: Stroke (with hemiplegia)

LMN tongue: "Licks its wound" — deviates TO the lesion.
UMN tongue: Away from cortical lesion (fibers cross).

Bulbar vs Pseudobulbar:
Bulbar (LMN — IX,X,XII nuclei) = fasciculations, wasting, quiet nasal speech, absent jaw jerk
Pseudobulbar (UMN — bilateral cortex) = spastic tongue, brisk jaw jerk, emotional lability (pathological crying/laughing)

⚠️ DIRECTION RULES — YOUR KNOWN TRAP

Structure LMN lesion → deviates UMN lesion → deviates Anchor
Tongue (XII) TO lesion AWAY from lesion LMN "licks its wound"
Uvula (X) AWAY from lesion (both UMN & LMN) Dragged by strong side
Face (VII) Entire half (LMN) Lower half only (UMN) Bell's = Both halves

5 RETENTION ANCHORS

  1. CN III pupil: outside = surgical (dilated, emergency), inside = medical (spared, diabetes). Passenger on roof of bus falls first when bus is crushed.
  2. CN VII: Bell's = Below = Both halves. Stroke = Superior = Spares forehead. This one distinction earns 2 marks alone.
  3. Tongue TO the lesion (LMN), Uvula AWAY from lesion. They are OPPOSITES. Tongue licks its wound. Uvula is dragged by the strong side.
  4. Left RLN loops low under aortic arch. Left vocal cord palsy → think aortic aneurysm, mitral stenosis, lung cancer, thyroid surgery.
  5. Corneal reflex: V1 in, VII out. Lost in Wallenberg (V nucleus hit). Tests two nerves in one — examiners love it.