Accessory nucleus of oculomotor nerve

At a glance

English

Accessory nucleus of oculomotor nerve

Latin (TA2)

Nucleus accessorius nervi oculomotorii

System

Nerve

Region

Head

Side

Paired, left and right

About the accessory nucleus of oculomotor nerve

The accessory nucleus of the oculomotor nerve, usually called the Edinger-Westphal nucleus, is the parasympathetic part of the third cranial nerve's origin. It lies in the upper midbrain at the level of the superior colliculus, in the grey matter around the cerebral aqueduct, tucked dorsal to the main motor nucleus that runs the eye muscles. The two accessory nuclei, one on each side, sit close to the midline and are often described as a single paired column.

Its cells are preganglionic parasympathetic neurons. Their axons leave with the other oculomotor fibres, curving forwards across the midbrain tegmentum, through the red nucleus and the inner edge of the substantia nigra, and come out of the brainstem in the oculomotor sulcus on the inner side of the cerebral peduncle. Inside the nerve they travel near its surface, then pass along the branch to inferior oblique and leave it to synapse in the ciliary ganglion behind the eye. Short ciliary nerves carry the postganglionic fibres on to the eyeball.

Two smooth muscles are controlled this way. The sphincter pupillae narrows the pupil, and the ciliary muscle relaxes the tension on the lens so it can round up for near vision. The nucleus therefore forms the outgoing limb of two reflexes. In the pupillary light reflex, light in one eye reaches the pretectal area, which projects to the accessory nuclei on both sides, so both pupils constrict. In the near response, looking at a close object brings convergence, accommodation and pupil narrowing together.

Because the pupil fibres run on the outside of the oculomotor nerve, pressure on the nerve, for example from an aneurysm of the posterior communicating artery or a swelling brain pushing the uncus down, tends to cause a large unreactive pupil early. Disease of the vessels inside the nerve, as in diabetes, often spares the pupil because these surface fibres keep their supply.

Course and supply

Supplies

Sends preganglionic parasympathetic fibres via the oculomotor nerve to the ciliary ganglion, controlling the sphincter pupillae (pupil constriction) and ciliary muscle (accommodation for near vision).

Blood supply

Paramedian branches of the posterior cerebral artery and the top of the basilar artery, which supply the midline midbrain.