Anatomy Atlas

Oculomotor nerve (CN III)

Nervus oculomotorius
Also: CN III · Cranial nerve III · Third nerve palsy
NerveHigh-yielddraftNeuroEENT

Supplies four extraocular muscles, levator palpebrae and the pupillary constrictor. A palsy gives a "down and out" eye with ptosis; a blown pupil signals compression (aneurysm or herniation) rather than diabetic ischemia.

Core anatomy

Origin

Oculomotor nucleus (somatic motor) and Edinger–Westphal nucleus (parasympathetic) in the midbrain at the level of the superior colliculus. The nerve emerges in the interpeduncular fossa, medial to the cerebral peduncle.

Course

Passes between the posterior cerebral and superior cerebellar arteries, runs forward beside the posterior communicating artery, crosses the edge of the tentorium near the uncus, then runs in the lateral wall of the cavernous sinus (see dural venous sinuses) and enters the orbit through the superior orbital fissure within the common tendinous ring, dividing into superior and inferior divisions.

Branches
  • Superior division: superior rectus, levator palpebrae superioris
  • Inferior division: medial rectus, inferior rectus, inferior oblique, and the parasympathetic root to the ciliary ganglion
Motor

Extraocular muscles — superior, inferior and medial recti and inferior oblique; levator palpebrae superioris. Parasympathetic (via the ciliary ganglion and short ciliary nerves): sphincter pupillae (miosis) and ciliary muscle (accommodation).

Relations

The parasympathetic fibers lie superficially (dorsomedial) on the nerve, so compression affects the pupil early, while ischemia (vasa nervorum, core of the nerve) tends to spare it.

Other structures innervated

Found in

Mentioned in

Generated automatically from links in other entries.

Where the cranial nerves leave the brainstem (ventral view)The front of the brainstem. The optic nerves meet at the chiasm above. The oculomotor nerve emerges from the interpeduncular fossa between the cerebral peduncles of the midbrain. The trochlear nerve is the only one to leave from the back, just below the inferior colliculus, and winds forward around the peduncle. The trigeminal nerve leaves the lateral pons. The abducens leaves at the pontomedullary junction near the midline; the facial and vestibulocochlear nerves leave at the cerebellopontine angle. The glossopharyngeal and vagus nerves leave the postolivary sulcus of the medulla, the hypoglossal leaves the preolivary sulcus between the pyramid and the olive, and the accessory nerve rises from the upper cervical cord.Optic (II)Oculomotor (III)Trochlear (IV) — from the dorsal sideTrigeminal (V)Abducens (VI)Facial (VII)Vestibulocochlear (VIII)Glossopharyngeal (IX)Vagus (X)Accessory (XI) — from C1–C5Hypoglossal (XII)Midbrain (cerebral peduncle)PonsPyramidOliveSpinal cordMedullaVentral view · dashed IV leaves from the back

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Where the cranial nerves leave the brainstem. Ventral view. III leaves the interpeduncular fossa of the midbrain; IV is the only nerve to leave from the back (dashed); V leaves the lateral pons; VI the pontomedullary junction near the midline; VII and VIII the cerebellopontine angle; IX and X the postolivary sulcus; XII the preolivary sulcus between pyramid and olive; and XI rises from the upper cervical cord. Nuclei follow the same levels: III–IV midbrain, V–VIII pons, IX–XII medulla.
Original schematic, created for this atlas.
Lateral view of the orbit showing the oculomotor nerve divisions, the ciliary ganglion, the abducent nerve to the lateral rectus and the trigeminal ganglion
The oculomotor nerve dividing into upper and lower divisions, with the ciliary ganglion.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 777. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Section of the cavernous sinus showing the internal carotid artery and abducent nerve within the sinus, and the oculomotor, trochlear, ophthalmic and maxillary nerves in its lateral wall, beside the sphenoidal sinus
The oculomotor nerve in the lateral wall of the cavernous sinus.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 786. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Third nerve palsy PANCE

Mechanism / lesion

Compressive: posterior communicating artery aneurysm, uncal (transtentorial) herniation, tumor. Ischemic (microvascular): diabetes, hypertension. Also midbrain stroke, cavernous sinus lesions, trauma.

Classic presentation

Eye "down and out" (unopposed lateral rectus and superior oblique), ptosis, and — if the parasympathetic fibers are involved — a fixed dilated pupil and loss of accommodation; diplopia. Pupil-involving palsy (especially painful, sudden) = aneurysm until proven otherwise → emergency CTA/MRA. Pupil-sparing palsy in an older diabetic = usually microvascular ischemia, recovers over about 3 months.

Uncal herniation PANCE

Mechanism / lesion

A supratentorial mass (e.g. epidural or subdural hematoma) pushes the medial temporal lobe (uncus) over the tentorium, compressing CN III.

Classic presentation

Ipsilateral blown pupil followed by third nerve palsy, decreasing consciousness, and contralateral hemiparesis (or ipsilateral from Kernohan notch compression of the opposite peduncle). Neurosurgical emergency.

Weber syndrome

Mechanism / lesion

Paramedian midbrain infarct (branches of the PCA) involving the CN III fascicles and the cerebral peduncle.

Classic presentation

Ipsilateral CN III palsy with contralateral hemiparesis.

High-yield PANCE pearls

  • "Down and out" eye + ptosis → CN III palsy.
  • CN III palsy with a dilated pupil → PComm aneurysm (or herniation) → emergency CTA.
  • Pupil-sparing CN III palsy in a diabetic → microvascular ischemia.
  • Unilateral blown pupil in a head-injured patient → uncal herniation.
  • Ipsilateral CN III palsy + contralateral hemiparesis → Weber syndrome (midbrain).

Imaging anatomy

CT

Noncontrast CT head for trauma/herniation; CT angiography for suspected PComm aneurysm.

MRI

MRI with MRA to evaluate the midbrain, cavernous sinus and aneurysm; catheter angiography if noninvasive imaging is negative but suspicion is high.

Physical exam

Strength / function testing

Inspect for ptosis and eye position at rest; H-pattern eye movements (adduction, elevation, depression); pupil size, direct and consensual light reflex and accommodation (near response). See the H-test diagram on extraocular muscles.

Self-quiz

Q1.

A 52-year-old woman has a sudden severe headache and drooping of the left eyelid. The left eye is deviated down and out, and the left pupil is 7 mm and unreactive. What is the most likely cause?

Show answer

B

A pupil-involving third nerve palsy with sudden headache indicates compression of the superficial parasympathetic fibers, classically by a posterior communicating artery aneurysm — obtain emergency CTA.

Q2.

A 68-year-old man with poorly controlled diabetes has double vision and right ptosis. The right eye is down and out; the pupils are equal and reactive. What is the most likely mechanism?

Show answer

B

A pupil-sparing CN III palsy in a patient with vascular risk factors is usually microvascular ischemia, which affects the core of the nerve and spares the superficial pupillary fibers. It typically recovers within about 3 months.

Q3.

Why does a compressive lesion of CN III affect the pupil early, while ischemic lesions often spare it?

Show answer

C

The parasympathetic fibers lie on the surface of the nerve, so external compression (aneurysm, herniation) affects the pupil early, while microvascular ischemia of the nerve's core tends to spare it.

Q4.

Between which two arteries does the oculomotor nerve emerge?

Show answer

E

CN III emerges from the interpeduncular fossa and passes between the posterior cerebral and superior cerebellar arteries, then runs alongside the posterior communicating artery.

Q5.

A patient with a large left subdural hematoma has a blown left pupil but weakness on the LEFT side. What explains the ipsilateral weakness?

Show answer

B

In uncal herniation, the midbrain can be pushed so that the opposite cerebral peduncle is compressed against the tentorial edge (Kernohan notch), causing weakness on the same side as the lesion, a false localizing sign.

Flashcards

See also