The Twelve Cranial Nerves

From smell to swallowing: the twelve pairs of nerves that leave the brain directly, what each one does, and how doctors test them at the bedside.

The cranial nerves are the twelve pairs of nerves that connect directly to the brain, numbered with Roman numerals roughly from front to back. Two (I and II) connect to the forebrain; the other ten to the brainstem.

The twelve

I — Olfactory: smell. II — Optic: vision. III — Oculomotor: most eye movements, lifting the eyelid, constricting the pupil. IV — Trochlear: one eye muscle that turns the eye down and inward. V — Trigeminal: sensation of the face and the muscles of chewing; it's the nerve involved in the severe facial pain of trigeminal neuralgia and much dental pain. VI — Abducens: turns the eye outward.

VII — Facial: muscles of facial expression, taste from the front of the tongue, tears and saliva; inflammation of this nerve causes Bell's palsy, a sudden drooping of one side of the face. VIII — Vestibulocochlear: hearing and balance. IX — Glossopharyngeal: taste from the back of the tongue, swallowing and sensors in the carotid artery. X — Vagus: the great wanderer — heart, lungs and gut, the throat and voice box. XI — Accessory: muscles that shrug the shoulders and turn the head. XII — Hypoglossal: movements of the tongue.

Generations of medical students have learned them with rude and not-so-rude mnemonics; one polite version is "Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven".

The bedside exam

Because each nerve has a specific job and leaves the brain at a specific point, a neurological examination tests them one by one: following a finger with the eyes (III, IV, VI), checking the pupils' response to light (II and III), touching the face (V), smiling and puffing the cheeks (VII), whispering in each ear (VIII), saying "ah" (IX and X), shrugging (XI) and sticking out the tongue (XII). A pattern of problems can pinpoint damage in the brainstem.