The 12 Cranial Nerves at a Glance
Mnemonic: 'Oh Oh Oh To Touch And Feel Very Good Velvet — Absolutely Heavenly' (I–XII: Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal). Sensory only: I, II, VIII. Motor only: III, IV, VI, XI, XII. Mixed: V, VII, IX, X.
The High-Yield Palsies
CN III (oculomotor): 'down and out' eye, ptosis, mydriasis. Posterior communicating artery aneurysm = painful CN III palsy with dilated pupil. Diabetic CN III = pupil-sparing (vasa nervorum ischemia spares outer pupillomotor fibers). CN VI (abducens): medial deviation, can't abduct — most common cranial nerve palsy due to long intracranial course. CN VII (facial): UMN lesion spares forehead (bilateral cortical innervation); LMN lesion (Bell's palsy) affects entire ipsilateral face.
CN V — Trigeminal
Three divisions: V1 (ophthalmic — forehead, cornea), V2 (maxillary — cheek, upper teeth), V3 (mandibular — jaw, lower teeth, also motor to muscles of mastication). Corneal reflex: afferent V1, efferent VII. Jaw jerk: afferent and efferent V3. Absent corneal reflex on one side with intact bilateral jaw jerk suggests a pontine lesion.
CN IX & X — Gag Reflex
Afferent of gag reflex: CN IX (glossopharyngeal). Efferent of gag reflex: CN X (vagus). Vagus also controls voice (recurrent laryngeal nerve — left RLN loops around aortic arch, right around subclavian). Left RLN palsy should prompt evaluation for mediastinal/aortic pathology. CN XI (accessory) innervates SCM and trapezius — test by head-turning against resistance and shoulder shrug.