Oculomotor (III), Trochlear (IV), and Abducens (VI) Nerves

Oculomotor Nuclei. The somatic and parasympathetic efferent fibers in the oculomotor nerve are the axons of cells located in the complex oculomotor nuclei situated anterolateral to the upper end of the cerebral aqueduct. The nuclei are composed of groups of large and small multipolar cells. The main groups of large cells are arranged in two columns of posterolateral, intermediate, and anteromedial nuclei, one on each side of the midline, which control the rectus and oblique extraocular muscles. A single median nucleus, composed of similar cells and partly overlying the caudal and posterior aspects of the bilateral columns, controls the levator muscles of the upper eyelids. Cranial to the median nucleus, and also partially overlying the posterior aspects of the main bilateral columns, are two narrow, wing-shaped nuclei, which are interconnected across the midline at their cranial ends—the accessory (autonomic) nuclei (Edinger-Westphal). They are the source of parasympathetic preganglionic fibers for the ciliary ganglion. The multiple subnuclei of the oculomotor nucleus each project ipsilaterally via the oculomotor nerve to the individual muscles that they innervate, with the exception of the superior rectus subnucleus, which projects contralaterally via the contralateral oculomotor nerve to the contralateral superior rectus muscle.

Oculomotor Nerve. The axons from the bilateral oculomotor nuclear cells form minute bundles, which run through the mesencephalic tegmentum, traversing the red nuclei to emerge from the mesencephalic oculomotor sulcus as the oculomotor nerve rootlets.

Each oculomotor nerve runs forward between the posterior cerebral and superior cerebellar arteries and lateral to the posterior communicating artery in the interpeduncular subarachnoid cistern. It pierces the arachnoid and dura mater in the angle between the free and attached margins of the tentorium cerebelli to enter first the roof of the cavernous sinus and then its lateral wall. Continuing forward above the trochlear nerve, the oculomotor nerve divides into superior and inferior rami as it enters the orbit through the superior orbital fissure.

The smaller superior division supplies the superior rectus muscle and the main superficial (voluntary, or striated, muscular) lamina of the levator palpebrae superioris. The deep lamina is a tenuous layer of involuntary, or unstriated, fibers—the superior tarsal muscle; a similar but even more tenuous inferior tarsal muscle is present in the lower eyelid, and both these tarsal muscles are innervated by sympathetic fibers. The larger inferior division supplies the medial and inferior recti and the inferior oblique muscles.

CILIARY GANGLION

The ciliary ganglion is tiny and lies in the posterior part of the orbit between the optic nerve and the lateral rectus muscle. Only the first of its three roots is constant because the sensory and/or sympathetic roots may bypass the ganglion.

Motor Root. The ciliary ganglion is the relay station for preganglionic parasympathetic fibers, which originate in the accessory (autonomic) oculomotor nucleus and reach the ganglion through a short offshoot from the oculomotor branch to the inferior oblique muscle. The postganglionic fibers form the 12 to 20 delicate short ciliary nerves that penetrate the sclera around the optic nerve and continue forward in the perichoroidal space to supply the ciliaris and sphincter pupillae muscles and the intraocular vessels.

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