The Occulomotor, Trochlear & Abducent Cranial Nerves Dr. Nimir Dr. Safaa

Objectives Discuss the central connections of the Occulomotor(III) nerve. Discuss the central connections of the trochlear(IV) nerve. Discuss the central connections of the abducent(III,IV,VI) nerve.

Oculomotor Nerve (Cranial Nerve III) Oculomotor nerve is entirely motor in function. Has two motor nuclei: Main motor nucleus. Accessory parasympathetic nucleus. Main nucleus is anterior to cerebral aqueduct at level of superior colliculus. It supplies all extrinsic muscles of eye except superior oblique and lateral rectus.

Main nucleus receives: Corticonuclear fibers from both cerebral hemispheres. Tectobulbar fibers from superior colliculus (information from visual cortex). It is connected to nuclei of fourth, sixth, and eighth cranial nerves by medial longitudinal fasciculus.

Accessory parasympathetic nucleus (Edinger-Westphal nucleus) is posterior to main nucleus. Preganglionic fibers accompany other oculomotor fibers to orbit & synapse in ciliary ganglion. Postganglionic fibers pass through short ciliary nerves to constrictor pupillae of iris and ciliary muscles. Accessory nucleus receives corticonuclear fibers for accommodation reflex and from pretectal nucleus for the direct and consensual light reflexes.

Course of Oculomotor Nerve Oculomotor fibers passes through red nuleus and emerge between cerebral peduncles & pass forward between posterior cerebral and superior cerebellar arteries. It then continues into the middle cranial fossa in lateral wall of cavernous sinus. Here, it divides into a superior and an inferior branches, which enter the orbital cavity through superior orbital fissure.

Oculomotor nerve supplies levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique. It also supplies, through parasympathetic fibers, constrictor pupillae of iris and ciliary muscles. It is responsible for lifting upper eyelid, turning eye upward, downward, and medially. Constricting pupil and accommodating the eye.

Trochlear Nerve Trochlear nerve is entirely motor in function. Trochlear Nerve Nucleus: The trochlear nucleus is situated anterior to cerebral aqueduct. It lies inferior to oculomotor nucleus at level of inferior colliculus. The nerve fibers, after leaving nucleus, pass posteriorly to reach posterior surface of midbrain.

The trochlear nucleus receives Corticonuclear fibers from both cerebral hemispheres. It receives: Tectobulbar fibers, which connect it to visual cortex through superior colliculus. Fibers from medial longitudinal fasciculus, by which it is connected to nuclei of third, sixth, and eighth cranial nerves.

Course of the Trochlear Nerve Trochlear, most slender cranial nerves and the only one to leave posterior surface of brainstem. Emerges from midbrain and immediately decussates with that of opposite side.

Trochlear nerve passes forward through middle cranial fossa in lateral wall of cavernous sinus and enters orbit through superior orbital fissure. Trochlear nerve supplies superior oblique muscle of eyeball. Trochlear nerve assists in turning eye downward and laterally.

Abducent Nerve (Cranial Nerve VI): It is entirely motor in function. Abducent has small motor nucleus situated beneath floor of upper part of fourth ventricle, close to midline and beneath facial colliculus. The nucleus receives: Afferent corticonuclear fibers from both cerebral hemispheres. Tectobulbar tract from superior colliculus, by which it is connected to visual cortex. Fibers from medial longitudinal fasciculus, by which it is connected to the nuclei of third, fourth, and eighth cranial nerves.

Course of the Abducent Nerve Abducent fibers pass anteriorly through pons and emerge in groove between lower border of pons and medulla oblongata. It passes forward through cavernous sinus, lying below and lateral to internal carotid artery.

The nerve then enters orbit through superior orbital fissure. Abducent supplies lateral rectus muscle and, therefore, is responsible for turning eye laterally.