The Ocular Nerves – Oculomotor, Trochlear and Abducens Nerve

Ocular nerves are the oculomotor, the trochlear and the abducens nerves. Since they function together in the regulation of eye movements, they are considered as ocular nerves and are examined together.

The oculomotor nerve (Cranial Nerve III)

The Oculomotor nuclei consist of several paired groups of nerve cells, adjacent to the midline, ventral to the aqueduct of sylvius at the level of the superior colliculi. A centrally located group of nerve cells, the Edinger-Westphal nucleus, innervate the pupillary sphincters and ciliary bodies. It is situated more dorsally, and constitutes the parasympathetic portion of the oculomotor nerve. The nerve cells that mediate the action of the ocular muscles show a definite dorso-ventral topographic representation. The nerve fibers course anteriorly through the mesencephalon, medial tot he red nucleus, the substantia nigra and the cerebral peduncle. The nerve emerges from the anterior aspect of the mid-brain just above the pons, between the superior cerebellar and the posterior cerebral arteries.

It penetrates the dura lateral and anterior to the posterior clinoid process and enters the lateral wall of cavernous sinus. From there, it enters the Orbit through the superior orbital fissure and supplied the levator palpaebrae superioris, the inferior oblique and the superior, medial and inferior recti muscles. The preganglionic parasympathetic fibers from the Edinger-Westphal nucleus pass up to the ciliary ganglion, from where the postganglionic fibers supply the pupillary constrictors and the ciliary muscles which function in accommodation.

The trochlear nerve (Cranial Nerve IV)

this is the smallest of all cranial nerves and is situated just anterior to the aqueduct in the mesencephalon immediately above the pons. The fibers curve posteriorly and caudally around the aqueduct and decussate in the anterior medullary velum. It penetrates the dura posterolateral to the posterior clinoid process to enter the cavernous sinus where it is lateral and inferior to the 3rd nerve. Through the superior Orbital fissure, it enters the Orbit to supply the superior oblique muscle. Paralysis of this nerve causes weakness of downward and outward movement of the eye and extorsion (rotation of the eyeball outwards).

The abducens nerve (Cranial Nerve VI)(Abducent nerve)

This nerve arises from the lower part of the pons in the floor of the fourth ventricle. The nerve emerges from the brain stem at the pontomedullary junction. It has the longest intracranial course among all the cranial nerves and lies between the pons and the clivus. It pierces the dura at the dorsum sellae, between the posterior clinoid and apex of the petrous bone to enter the cavernous sinus, inferomedial to the 3rd nerve. It enters the Orbit through the superior Orbital fissure to supply the external (lateral) rectus muscle.

The medial longitudinal fasciculus: This fiber tract unites the nuclei of 3rd, 4th, 5th, 6th, 7th, 8th, 11th and the 12th cranial nerves and the motor nuclei of upper cervical nerves. Owing to this interconnection isolated eye movments are not possible. Visual, auditory, sensory, vestibular, and other stimuli produces conjugate deviation of the eyes and head.

Sympathetic innervation: The sympathetic fibers which control oculopupillary action arise from the 8th cervical and first thoracic spinal segments. The preganglionic fibers go to the inferior, middle, and superior sympathetic ganglia in the neck. The postganglionic fibers follow the course of the internal carotid artery and travel along the Ophthalmic division of the fifth nerve into the orbit. Through the long ciliary nerves, they supply the pupillary dilators. The tarsal muscles, and the orbital muscle of Muller.

Cortical control: The posterior portion of the second and third frontal convolutions constitute the evolutional cortical control of conjugate ocular movements. A pontine center for lateral gaze has also been described in the vicinity of the abducens nerve- the parapontine gaze center.