CorpusHuman Anatomy
NervousNerve

Oculomotor nerve (CN III)

Nervus oculomotorius

Also known as: CN III, third cranial nerve

A motor nerve (GSE + GVE) supplying four of the six extraocular muscles and levator palpebrae superioris. It also carries the parasympathetic fibres that constrict the pupil and allow accommodation.

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Overview

NucleiOculomotor + Edinger–Westphal (midbrain, superior colliculus level)
ExitSuperior orbital fissure (within common tendinous ring)
Parasympathetic ganglionCiliary ganglion
Muscles supplied5 skeletal (SR, IR, MR, IO, LPS) + 2 smooth

CN III arises from two midbrain nuclei at the level of the superior colliculus:
• the oculomotor nucleus (GSE), a complex of subnuclei for the individual muscles;
• the Edinger–Westphal nucleus (GVE, preganglionic parasympathetic).

Its fibres run ventrally through the red nucleus and emerge into the interpeduncular fossa. The nerve then passes between the posterior cerebral and superior cerebellar arteries, runs alongside the posterior communicating artery, and enters the lateral wall of the cavernous sinus. It enters the orbit through the superior orbital fissure within the common tendinous ring and divides into superior and inferior divisions.

The superior division supplies the superior rectus and levator palpebrae superioris. The inferior division supplies the medial rectus, inferior rectus and inferior oblique, and gives the parasympathetic root to the ciliary ganglion. Postganglionic fibres from the ciliary ganglion travel in the short ciliary nerves to the sphincter pupillae (miosis) and ciliary muscle (accommodation). The parasympathetic fibres run in the superficial, dorsomedial part of the nerve. This is the anatomical basis for the difference between compressive and ischaemic third nerve palsies.

Location

Midbrain (interpeduncular fossa) → between the PCA and SCA → lateral wall of the cavernous sinus → superior orbital fissure → orbit.

Parts & subdivisions

  1. Functional components
    Motor (GSE) to the extraocular muscles and levator palpebrae superioris. Parasympathetic (GVE) from the Edinger–Westphal nucleus via the ciliary ganglion.
  2. Clinical testing
    Inspect for ptosis and the resting eye position. Test smooth pursuit in an "H" pattern, asking about diplopia. Test the pupils (size, direct and consensual light reflex, near reflex). Medial rectus weakness gives horizontal diplopia; superior and inferior rectus weakness gives vertical diplopia.
  3. Lesion signs (complete palsy)
    - "Down and out" eye, from the unopposed lateral rectus (VI) and superior oblique (IV). • Ptosis, often complete. • Fixed dilated pupil (mydriasis) and loss of accommodation. • Diplopia, which may be masked by the complete ptosis.

Anatomy

Innervation

- Skeletal muscle: superior, medial and inferior recti, inferior oblique and levator palpebrae superioris.
• Smooth muscle (parasympathetic via the ciliary ganglion): sphincter pupillae and ciliary muscle.
• The superior tarsal (Müller) muscle is sympathetic, not CN III.

Course

Oculomotor and Edinger–Westphal nuclei (periaqueductal midbrain, superior colliculus level) → fascicles through the tegmentum and red nucleus → interpeduncular fossa → subarachnoid space between the PCA and SCA, lateral to the PComm → over the free edge of the tentorium near the uncus → roof, then lateral wall, of the cavernous sinus (highest nerve in the wall) → superior orbital fissure inside the common tendinous ring → superior and inferior divisions.

Branches

Superior division: superior rectus and levator palpebrae superioris. Inferior division: medial rectus, inferior rectus, inferior oblique, and the parasympathetic (motor) root to the ciliary ganglion. From the ciliary ganglion, short ciliary nerves run to the sphincter pupillae and ciliary muscle.

Function

  • Elevation (superior rectus, inferior oblique), depression (inferior rectus) and adduction (medial rectus) of the eye
  • Elevation of the upper eyelid (levator palpebrae superioris)
  • Pupillary constriction (efferent limb of the light reflex)
  • Accommodation and convergence for near vision (near triad)

Clinical significance

Compressive vs ischaemic CN III palsy

A posterior communicating artery aneurysm or uncal herniation compresses the nerve from outside. The superficial parasympathetic fibres are affected early, giving a "surgical" third nerve palsy with a dilated, unreactive pupil. This is a neurosurgical emergency and needs urgent CT/CT angiography. In diabetic or hypertensive microvascular (ischaemic) palsy, the infarct is in the core of the nerve, so the pupil is usually spared. It is often painful and recovers within about 3 months.

Uncal (transtentorial) herniation

A supratentorial mass, such as an extradural haematoma, pushes the medial temporal lobe (uncus) over the tentorial edge, where it stretches CN III. An ipsilateral fixed, dilated ("blown") pupil in a patient with falling consciousness is the classic warning sign of herniation.

Weber syndrome

An infarct of the ventral midbrain (paramedian branches of the PCA) involves the CN III fascicles and the cerebral peduncle. The result is an ipsilateral CN III palsy with contralateral hemiplegia.

Mnemonics

Extraocular innervation

"LR6 SO4, the rest 3." The lateral rectus is supplied by CN VI, the superior oblique by CN IV, and all the others (including LPS) by CN III.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.