✅ Summary:
The oculomotor nerve (CN III) arises in the midbrain, passes through the cavernous sinus and superior orbital fissure, and innervates most extraocular muscles, the levator palpebrae, and via parasympathetic fibers, the pupil and lens. Its physiology underlies eye movements, eyelid elevation, pupillary constriction, and accommodation.
Detail
1. Origin
Nuclear origin: Arises from two main groups of nuclei in the midbrain at the level of the superior colliculus:
Oculomotor nucleus (somatic motor): supplies most extraocular muscles.
Edinger–Westphal nucleus (visceral parasympathetic): supplies sphincter pupillae and ciliary muscle.
Emergence from the brainstem: Fibers exit the anterior surface of the midbrain in the interpeduncular fossa.
Subarachnoid course: Travels forward between the posterior cerebral artery and superior cerebellar artery; runs parallel to the posterior communicating artery (aneurysms here can compress CN III).
Cavernous sinus: Runs along the lateral wall of the cavernous sinus, superior to CN IV, V1, and V2.
Entry into the orbit: Enters through the superior orbital fissure inside the common tendinous ring.
Superior division:
Superior rectus
Levator palpebrae superioris
Inferior division:
Medial rectus
Inferior rectus
Inferior oblique
Parasympathetic fibers to the ciliary ganglion → short ciliary nerves → sphincter pupillae & ciliary muscle
Controls 4 of the 6 extraocular muscles plus levator palpebrae:
Superior rectus → elevates, adducts, and medially rotates the eye
Inferior rectus → depresses, adducts, and laterally rotates the eye
Medial rectus → adducts the eye
Inferior oblique → elevates, abducts, and laterally rotates the eye
Levator palpebrae superioris → raises the upper eyelid
Overall role: coordinates most eye movements and eyelid elevation.
Via Edinger–Westphal nucleus → ciliary ganglion → short ciliary nerves:
Sphincter pupillae → constricts the pupil (pupillary light reflex, accommodation reflex)
Ciliary muscle → contracts to thicken lens for near vision (accommodation)
Oculomotor palsy:
“Down and out” eye (unopposed lateral rectus & superior oblique)
Ptosis (loss of levator palpebrae)
Dilated, fixed pupil (loss of parasympathetic to sphincter pupillae)
Loss of accommodation
Compressive lesions (e.g., aneurysm) often affect pupil fibers first
Ischemic lesions (e.g., diabetes) often spare the pupil (central somatic fibers more vulnerable)
Would you like me to also make a diagrammatic summary table (Origin → Course → Branches → Functions → Clinical lesions) for quick review, like what medical students use?