Corpus: Oculomotor nerve

The oculomotor nerve is the 3rd cranial nerve and carries somatic motor and general visceral motor fibers. Along with the trochlear and abducens nerves, it is responsible for eye movement.

The oculomotor nerve originates in the interpeduncular fossa, located between the two cerebral crura. It then travels forward (ventrally) to the edge of the sella turcica and the posterior clinoid process, where it pierces the dura mater. From there, it continues ventrally along the lateral wall of the cavernous sinus and enters the orbit medially through the superior orbital fissure. After passing through the origin of the eye muscles, the common tendinous ring, it divides into three branches: the superior branch (somatic motor), the inferior branch, and a general visceral motor branch to the ciliary ganglion.

3.1. Superior branch

The superior branch supplies the levator palpebrae superioris muscle and the superior rectus muscle.

3.2. Inferior branch

The inferior branch innervates the medial rectus muscle, the inferior rectus muscle, and the inferior oblique muscle.

3.3. Branch to the ciliary ganglion

This branch connects to the postganglionic neuron in the ciliary ganglion, which then innervates the ciliary muscle and the sphincter pupillae muscle via the ciliary nerves.

4.1. Somatc motor fibers

The nucleus of the somatic motor fibers is located medially at the level of the superior colliculi in the tegmentum of the midbrain (mesencephalon) and is known as the oculomotor nerve nucleus. Each muscle innervated by the oculomotor nerve has its own subnucleus, except for the levator palpebrae superioris muscle, which has an unpaired subnucleus. This anatomical feature makes it difficult to keep one eye open while the other is closed.

4.2. General visceral motor fibers

The nucleus of the general visceral motor fibers, known as the Edinger-Westphal nucleus, is situated medio-dorsally to the oculomotor nerve nucleus. These fibers control the sphincter pupillae muscle, causing pupil constriction (miosis).

If the oculomotor nerve is damaged, the eyeball turns outward and downward due to the unopposed action of the still functional superior oblique and lateral rectus muscles. Loss of function in the levator palpebrae superioris muscle leads to drooping of the eyelid (ptosis). Additionally, damage to the general visceral motor fibers results in a dilated pupil (mydriasis) due to the unopposed action of the sympathetically innervated dilator pupillae muscle. Furthermore, the affected eye cannot accommodate due to the loss of function in the ciliary muscle.