Nuclei and fibre types
CN III carries two fibre types from two midbrain nuclei at the level of the superior colliculus, lying ventral to the cerebral aqueduct.
| Fibre type | Nucleus | Supplies |
|---|---|---|
| General somatic efferent (GSE): motor to skeletal muscle | Oculomotor nucleus | Superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae superioris |
| General visceral efferent (GVE): preganglionic parasympathetic | Edinger–Westphal nucleus | Sphincter pupillae and ciliary muscle, via the ciliary ganglion |
The oculomotor nucleus is a group of subnuclei, one per muscle. Two quirks matter clinically. A single midline subnucleus supplies both levator muscles, so a nuclear lesion causes bilateral ptosis. The superior rectus subnucleus supplies the opposite eye, so a nuclear lesion also weakens the contralateral superior rectus.
Emergence and skull foramen
CN III emerges from the anterior surface of the midbrain, on the medial side of the cerebral peduncle, into the interpeduncular fossa. It leaves the cranial cavity through the superior orbital fissure, passing within the common tendinous ring, the fibrous ring from which the four recti arise.
Course and branches
From the interpeduncular fossa CN III runs forward through the subarachnoid space, through the cavernous sinus and into the orbit.
- Subarachnoid space: it passes between the posterior cerebral artery above and the superior cerebellar artery below, then runs forward alongside the posterior communicating artery, close to the free edge of the tentorium cerebelli and the uncus of the temporal lobe.
- Cavernous sinus: it pierces the dura lateral to the posterior clinoid process and runs in the lateral wall of the sinus, highest of the nerves there, above the trochlear nerve and the ophthalmic and maxillary divisions of the trigeminal nerve.
- Superior orbital fissure: it divides into superior and inferior divisions, which enter the orbit within the common tendinous ring, with the nasociliary and abducens nerves.
The superior division supplies superior rectus and levator palpebrae superioris. The inferior division supplies medial rectus, inferior rectus and inferior oblique. The branch to inferior oblique gives the parasympathetic root to the ciliary ganglion, which lies near the orbital apex between the optic nerve and lateral rectus. Postganglionic fibres run in the short ciliary nerves to sphincter pupillae (pupil constriction) and ciliary muscle (accommodation).
The smooth-muscle part of levator, the superior tarsal muscle, is supplied by sympathetic fibres, not by CN III. Within the nerve, the parasympathetic fibres run superficially, a detail that explains the pupil findings in different palsies.
Clinical testing
CN III is tested by inspecting the lids and pupils, then tracking eye movements.
- Inspection: look for ptosis and for the resting position of each eye.
- Eye movements: ask the patient to follow a target in an H pattern and report double vision. Medial rectus adducts; superior rectus elevates best in abduction; inferior rectus depresses best in abduction; inferior oblique elevates best in adduction.
- Pupils: compare size, then test the direct and consensual light reflexes. CN III is the efferent limb of both.
- Accommodation: ask the patient to look at a near target; the eyes converge and the pupils constrict.
Lesions and palsies
A complete CN III palsy leaves the eye down and out, with complete ptosis and a fixed dilated pupil. Lateral rectus (CN VI) and superior oblique (CN IV) act unopposed, so the eye abducts and depresses. Loss of levator produces the ptosis, and loss of parasympathetic supply gives mydriasis (a dilated pupil) and absent accommodation.
| Cause | Features |
|---|---|
| Posterior communicating artery aneurysm | Painful palsy with pupil involvement, the surgical third nerve palsy; needs urgent imaging |
| Uncal herniation | Raised intracranial pressure pushes the uncus over the tentorial edge; an ipsilateral dilated pupil appears first |
| Microvascular (diabetes, hypertension) | Pupil usually spared; recovers over about three months |
| Cavernous sinus lesion | With CN IV, V1, V2 and VI involvement |
| Midbrain infarct | Weber syndrome (CN III with contralateral hemiplegia) or Benedikt syndrome (with contralateral tremor and ataxia) |
The pupil rule follows from the anatomy. Compression from outside, by an aneurysm or herniation, hits the superficial parasympathetic fibres first and dilates the pupil. Ischaemia of the vasa nervorum damages the core of the nerve and tends to spare the pupil.