Nucleus of oculomotor nerve

At a glance

English

Nucleus of oculomotor nerve

Latin (TA2)

Nucleus nervi oculomotorius

System

Nerve

Region

Head

Side

Paired, left and right

About the nucleus of oculomotor nerve

The oculomotor nucleus is the somatic motor nucleus of the oculomotor nerve. It lies in the upper midbrain at the level of the superior colliculus, in the grey matter in front of the cerebral aqueduct, close to the midline, and its upper end reaches a short way forwards beneath the third ventricle. The medial longitudinal fasciculus runs along its front and side, linking it with the other eye-movement nuclei.

The nucleus is not a single uniform mass but a complex of subnuclei, each serving one muscle. Classical descriptions divided it into an anterior and a posterior group of cell clusters, some paired and one in the midline shared by both sides. In modern terms there are paired subnuclei for the medial rectus, inferior rectus and inferior oblique, which supply the eye on the same side, and for the superior rectus, which sends its fibres to the eye of the opposite side. A single midline group, the central caudal nucleus, supplies levator palpebrae superioris of both eyes. The parasympathetic accessory (Edinger-Westphal) nucleus lies just dorsal to the complex and handles constriction of the pupil and focusing of the lens.

Axons from the subnuclei run forwards through the tegmentum, passing through and beside the red nucleus and across the medial edge of the substantia nigra in a set of curved bundles that bow outwards. They come out as several rootlets in the oculomotor sulcus, on the inner side of the cerebral peduncle, and gather into the oculomotor nerve in the interpeduncular fossa. The nucleus receives commands for vertical gaze from the rostral interstitial nucleus of the medial longitudinal fasciculus and for horizontal gaze from the opposite abducens nucleus.

The internal plan of the complex shows up in disease. A lesion of the whole nucleus causes droop of both upper lids, because the levator nucleus is shared, and weakness of upward gaze in both eyes, because each superior rectus subnucleus serves the opposite eye. A lesion of the nerve, by contrast, affects one side only. Strokes of the paramedian midbrain, which often reach the red nucleus or cerebral peduncle too, are the usual cause, giving tremor or limb weakness on the opposite side with the eye palsy.

Course and supply

Supplies

Holds the motor neurons for the medial, superior and inferior recti, the inferior oblique and levator palpebrae superioris, with the parasympathetic accessory nucleus beside it for the pupil and lens.

Blood supply

Paramedian (interpeduncular) branches from the top of the basilar artery and the first part of the posterior cerebral arteries.