Nerve
Oculomotor nerve (CN III)
Motor and parasympathetic pathways to the eye
High-yield
A substantial external third-nerve palsy can cause ptosis and an eye resting outward and downward. Pupil dilation suggests parasympathetic involvement. Although compression often affects the pupil, pupil sparing does not reliably exclude an aneurysm or another compressive lesion.
Overview
CN III brings midbrain motor output to four extraocular muscles and the upper-eyelid elevator. It also carries preganglionic parasympathetic fibers toward the ciliary ganglion. After emerging between the cerebral peduncles, it passes near the posterior communicating artery, through the cavernous-sinus wall and into the orbit through the superior orbital fissure.
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Reference facts
- Origin
- Somatic motor neurons and nearby preganglionic parasympathetic neurons in the rostral midbrain. Axons emerge ventrally in the interpeduncular region.
- Skull exit
- Superior orbital fissure, within the common tendinous ring.
- Course
- Passes between PCA and SCA, close to the posterior communicating artery, then through the upper lateral wall of the cavernous sinus. Superior and inferior divisions enter the orbit.
- Motor
- The superior division supplies superior rectus and levator palpebrae; the inferior division supplies medial rectus, inferior rectus and inferior oblique. Parasympathetic fibers reach the ciliary ganglion through the inferior division; short ciliary nerves carry postganglionic output to sphincter pupillae and ciliary muscle.
- Sensory
- No cutaneous sensory territory. Its principal functions are eye and eyelid movement, pupil constriction and accommodation.
Mnemonic
Eye-muscle nerve numbers: lateral rectus - VI; superior oblique - IV; the other four extraocular muscles - III.
Clinical correlation
A newly acquired third-nerve palsy needs prompt clinical assessment, particularly with headache or pupil change. An aneurysm is one possible cause even if the pupil is spared. In the midbrain, accompanying damage to descending motor fibers can add opposite-sided weakness (Weber syndrome).
Frequently asked questions
Where does the oculomotor nerve (CN III) arise?
Somatic motor neurons and nearby preganglionic parasympathetic neurons in the rostral midbrain. Axons emerge ventrally in the interpeduncular region.
What muscles does the oculomotor nerve (CN III) supply?
The superior division supplies superior rectus and levator palpebrae; the inferior division supplies medial rectus, inferior rectus and inferior oblique. Parasympathetic fibers reach the ciliary ganglion through the inferior division; short ciliary nerves carry postganglionic output to sphincter pupillae and ciliary muscle.
What is the sensory distribution of the oculomotor nerve (CN III)?
No cutaneous sensory territory. Its principal functions are eye and eyelid movement, pupil constriction and accommodation.
Where does the oculomotor nerve (CN III) leave the skull?
Superior orbital fissure, within the common tendinous ring.