Oculomotor Nerve

Cranial nerve III innervates four of the six extraocular muscles of the eye as well as the levator palpebrae superioris muscle of the eyelid and, via projections to the ciliary ganglion, the small intraocular muscles that control the constriction of the pupil and the shape of the lens. Unlike the other two oculomotor nerves—the abducens nerve, which innervates the lateral rectus muscle, and the trochlear nerve, which innervates the superior oblique muscle—the oculomotor nerve has more than one functional component. It contains a GSE component for innervation of the extraocular and levator palpebrae superioris muscles and a GVE

parasympathetic component for innervation of the ganglion that in turn innervates the intraocular muscles. The oculomotor nerve fibers traverse the tegmentum and exit the brain in the interpeduncular fossa medial to the crus cerebri and then pass rostrally to the orbit with the other oculomotor nerves.

The GSE component of cranial nerve III arises from motor neurons in the oculomotor nucleus proper, which lies in the dorsomedial part of the rostral half of the midbrain tegmentum. This component innervates four muscles that insert on the globe of the eye: the inferior oblique muscle, which extorts, elevates, and abducts the eye; the medial rectus muscle, which adducts the eye; the inferior rectus muscle, which depresses, extorts, and adducts the eye; and the superior rectus muscle, which elevates, intorts, and adducts the eye. (Extorsion is rotation of the eyeball around a horizontal, anteroposterior axis through the pupil such that the bottom moves medially; intorsion is rotation in the opposite direction.)

The oculomotor nuclear complex comprises several nuclei in addition to the GSE main nucleus, including the Edinger-Westphal nucleus and several accessory oculomotor nuclei—the nucleus of Darkschewitsch, the interstitial nucleus of Cajal, and the nuclei of the posterior commissure. These accessory nuclei receive inputs from a variety of visual- and vestibular-related sources and contribute to vertical and smooth-pursuit movements of the eyes directed by the GSE component. The Edinger-Westphal nucleus and a more rostral anterior median nucleus contain preganglionic parasympathetic neurons (GVE) that project via the oculomotor nerve to the ipsilateral ciliary ganglion, which lies deep to the posterior boundary of the eye. Postganglionic parasympathetic fibers from the ciliary ganglion supply the ciliary muscle, which affects the shape of the lens for focusing on near objects, and the sphincter (or constrictor) pupillae muscle. The consensual pupillary light reflex of evoking bilateral constriction of the pupils when light is shined in one eye depends on a pathway from the retina to the olivary pretectal nucleus, which in turn projects bilaterally via the posterior commissure to the anterior median and Edinger-Westphal parasympa-thetic nuclei. In this context, it should be noted that dilation of the pupil is accomplished by the action of sympathetic innervation that arises from preganglio-nic neurons in the intermediolateral column of the spinal cord and affects the dilator pupillae muscle fibers via postganglionic neurons of the superior cervical ganglion.

Damage to the ventral region of the midbrain may result in the syndrome called superior alternating hemiplegia, which is motor impairment on the contralateral side of the body (due to damage to the corticospinal fibers in the crus cerebri) combined with lateral and downward deviation of the eye on the ipsilateral (i.e., alternate) side due to a lower motor neuron lesion of the oculomotor nerve. In this situation, the eye deviates laterally due to the unopposed contraction of the lateral rectus muscle, which is innervated by cranial nerve VI. Supranuclear innervation of the oculomotor nuclei is mediated by cortical eye fields and the superior colliculus via gaze centers in the brain stem and is coordinated among the oculomotor, trochlear, and abducens nuclei and the vestibular system via the medial longitudinal fasciculus. Damage to the oculomotor nuclear complex or nerve also results in drooping of the eyelid (ptosis) and pupillary dilation.

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