Oculomotor nerve (CN III)
The oculomotor nerve (third cranial nerve, CN III, latin: nervus oculomotorius) is a mixed cranial nerve containing motor and parasympathetic fibers. The general somatic efferent fibers of the oculomotor nerve supply the levator palpebrae superioris and four of the six extraocular muscles: the medial, superior, and inferior recti, and the inferior oblique muscles. The oculomotor nerve also conveys preganglionic parasympathetic fibers to synapse in the ciliary ganglion, which then provides postganglionic parasympathetic fibers to intraocular muscles enabling pupillary constriction and accommodation.
The oculomotor nerve fibers originate from neurons in two nuclei located in the midbrain:
- oculomotor nucleus,
- accessory nuclei of oculomotor nerve.
The oculomotor nucleus is a somatic motor nucleus which provides the oculomotor nerve with general somatic efferent fibers.
The accessory nuclei of the oculomotor nerve are parasympathetic nuclei, among which the clinically most important one is the Edinger-Westphal nucleus. The accessory nuclei give rise to preganglionic parasympathetic (general visceral efferent) fibers of the oculomotor nerve.
The oculomotor nerve appear outside of the midbrain emerging into the interpeduncular fossa between the cerebral peduncles. The oculomotor nerve then travels forward below the posterior cerebral artery and above the superior cerebral artery, then pierces the dura mater and continues in ventral direction inside the lateral wall of the cavernous sinus. The oculomotor nerve leaves cranial cavity and enter the orbit through the superior orbital fissure dividing into superior and inferior divisions that run beneath the trochlear (CN IV) and ophthalmic (CN V1) nerves.
The superior division of the oculomotor nerve passes above the optic nerve (CN II) to supply the superior rectus muscle, also giving off a branch that innervates the levator palpebrae superioris muscle.
The inferior division of the oculomotor nerve separates into medial, central and lateral branches. The medial branch enters the ocular surface of the medial rectus muscle innervating it. The central branch enters the ocular surface of the inferior rectus muscle. And the lateral branch of the inferior division of the oculomotor nerve enters the orbital surface of the inferior oblique muscle and communicates with the ciliary ganglion to distribute parasympathetic fibers to the sphincter pupillae and to the ciliary muscle.