Occulomotor Nerve (CNIII)

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Movement of the eye is important for pursuit movements, and is coordinated by connections between the cranial nerve nuclei i.e. the medial longitudinal fasciculus. The oculomotor nerve is the third cranial nerve, and originates from the midbrain, inferior to the mammillary bodies. It innervates 4 of the 6 extraocular eye muscles and also constricts the pupils. 

Gross Anatomy

The oculomotor nerve is also known as cranial nerve 3. It arises from the oculomotor nerve of the brainstem and passes forwards to leave the skull via the superior orbital fissure. Its name suggests its function, which is to move the eyeball. It divides into a superior branch and an inferior branch once it enters the orbit. The superior branch passes across the optic nerve, and supplies the superior rectus (depresses the eye), and the levator palpebrae superioris. The inferior branch is larger, and supplies the medial rectus (adducts the eye), inferior rectus (elevates the eye), inferior oblique (elevates and adducts the eye) and the lower part of the ciliary ganglion. These branches of the inferior division all enter the muscles on their ocular surface, except inferior oblique, where the nerve enters the inferior oblique at its posterior border. It also, which is the muscle that elevates the upper eyelid. It also innervates constrictor pupillae, via the short ciliary nerves that run on its outer surface, with the motor component of the nerve in the centre.


The Edinger-Westphal nucleus is the other oculomotor nuclei, and sends parasympathetic innervation via the ciliary ganglion. The short ciliary nerves innervate the sphincter pupillae, which constricts the pupil. The long ciliary nerves also arise from the ciliary ganglion, and provide sensation to the surface of the eye and cornea. They also provide sympathetic innervation to the dilator pupillae muscle. The medial longitudinal fasciculus connects the oculomotor nucleus of the midbrain (superior to the superior colliculus) to the trochlear and abducens nucleus on the contralateral side. Hence, our eyes are able to move in the same direction when we look at something.


The light reflex involves an afferent and efferent pathway. When light is shone onto one eye it follows the afferent pathway i.e. he optic nerve, optic chiasm, optic tract or Pretectal nucleus (which in the dorsal midbrain). The efferent pathway consists of the Edinger-Westphal nucleus, third cranial nerve and ciliary ganglion (which constricts the pupil).

Clinical Anatomy

Third Nerve palsy- Palsy of the nerve affects the superior, medial and inferior rectus, as well as the inferior oblique and levator palpebrae superioris. The constrictor pupillae muscle (innervated by the parasympathetic component of the nerve) also is affected. The loss of innervation to the muscles results in a down and out position of the eye. The lack of innervation to the levator palpebrae superioris results in ptosis of the upper eyelid, and lack of innervation to constrictor pupillae results in a dilated pupil.


Relative afferent pupillary defect/Marcus Gunn pupil- This is elicited by the swinging light test, where a torch is swung between each eye. Normally, when one eye constricts (due to light), so does the other eye. In RAPD this occurs, but to a lesser extent in one eye when compared to the other. RAPD results in the other eye (the consensual reflex) not constricting as much as the eye being lit by the torch. The condition is characterised by a reduced amount of neural information being sent from one optic pathway to another i.e. a failure to transmit fully the light information to the other eye. The condition is due to optic nerve pathology.

Quick Anatomy

Key Facts

Developmental precursor- Basal plate of the embryonic midbrain

Muscles- Superior rectus, medial rectus, inferior rectus, inferior oblique, levator palpebrae superioris.



Superior Oblique innervated by the fourth cranial nerve (trochlear nerve)

Lateral rectus innervated by the 6th cranial nerve (abducens)

The others are supplied by the oculomotor nerve.


The oculomotor nerve is the third cranial nerve, and originates from the midbrain, inferior to the mammillary bodies. It innervates 4 of the 6 extraocular eye muscles and also constricts the pupils. 


1.     Frank H.Netter MD: Atlas of Human Anatomy, 5th Edition, Elsevier Saunders, Chapter 1 Head and Neck


2.     Chummy S.Sinnatamby: Last’s Anatomy Regional and Applied, 12th Edition, Churchill Livingstone Elsevier


3.     Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray’s Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier


4.     Elliiot L.Manchell: Gray's Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience


5.     The Definitive Neurological Surgery Board Review

By Shawn P. Moore, 2005


6.     Human Neuroanatomy

By James R. Augustine, 2008