Oculomotor Nerve (CN III)
The oculomotor nerve is the third cranial nerve controlling most eye movements and pupillary constriction. It innervates four of the six extraocular muscles and carries parasympathetic fibers to the pupil and ciliary muscle. Its name means "eye mover" in Latin.
Origin
Oculomotor nucleus in midbrain (at level of superior colliculus); Edinger-Westphal nucleus for parasympathetics
Foramina
Superior orbital fissure
Course
Exits midbrain → passes between posterior cerebral and superior cerebellar arteries → traverses cavernous sinus → enters orbit through superior orbital fissure
Functions
- •Eye elevation, depression, adduction
- •Eyelid elevation
- •Pupillary constriction (miosis)
- •Lens accommodation
Branches
Structures Innervated
- →Superior rectus
- →Inferior rectus
- →Medial rectus
- →Inferior oblique
- →Levator palpebrae superioris
- →Sphincter pupillae (parasympathetic)
- →Ciliary muscle (parasympathetic)
Clinical Testing
Test eye movements in H-pattern, check for ptosis, assess pupil size and reactivity, test accommodation reflex. Compare both sides.
Clinical Relevance
CN III palsy causes ptosis (drooping eyelid), "down and out" eye position, and mydriasis (dilated pupil). Pupil-sparing CN III palsy suggests microvascular ischemia (diabetes). Pupil-involving palsy suggests compression (aneurysm, herniation) - a neurosurgical emergency.
Study Tips
- ✓LR6SO4 rest3 - CN III innervates all extraocular muscles except lateral rectus (VI) and superior oblique (IV)
- ✓Parasympathetic fibers run superficially - compression affects pupil first
- ✓Diabetic CN III palsy spares pupil because ischemia affects interior fibers
- ✓Uncal herniation compresses CN III against tentorial edge
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Common questions about this cranial nerve
With CN III paralyzed, only lateral rectus (CN VI) and superior oblique (CN IV) function. The lateral rectus abducts the eye (out) and superior oblique depresses it when abducted (down), resulting in the characteristic position.
Parasympathetic fibers run superficially in CN III. Compression (aneurysm, herniation) affects these first, causing mydriasis. Ischemia (diabetes) affects central fibers, sparing the pupil. Pupil involvement suggests surgical emergency.
The Edinger-Westphal nucleus is the parasympathetic nucleus associated with CN III. It sends preganglionic fibers to the ciliary ganglion, which then innervates the sphincter pupillae and ciliary muscle for pupil constriction and accommodation.