Anatomy of the oculomotor nerve

1. Big picture

The oculomotor nerve, cranial nerve III (CN III), is one of the most clinically important cranial nerves because it controls most eye movements, eyelid elevation, pupillary constriction, and accommodation. In the exam, CN III is not just anatomy: it is a localizing nerve.

A complete CN III lesion gives the classic picture:

Ptosis + “down and out” eye + diplopia + mydriasis

The dangerous exam association is:

Painful third nerve palsy with a dilated pupil = posterior communicating artery aneurysm until proven otherwise.

Another emergency association is:

Coma + ipsilateral dilated fixed pupil = transtentorial/uncal herniation compressing CN III.