Oculomotor nerve is the third of the 12 paired cranial nerves (i.e. cranial nerve III). It innervates four extraocular muscles, a muscle to upper eyelid. It also controls the constriction of the pupils (light reflex | pupillary reflex) and the thickening of the lens of the eye (accommodation reflex).
The oculomotor nerve emerges from the anterior aspect of midbrain. It is then invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid mater.
The nerve then passes between the superior- and posterior- cerebral arteries. It then pierces the dura mater passing between the free and attached borders of the tentorium cerebelli.
The nerve then runs along the lateral wall of the cavernous sinus superior to the zygomatic nerve. It is here that the oculomotor nerve receives sympathetic fibers from the cavernous plexus. It also receives a communicating branch from the ophthalmic division of the trigeminal nerve.
Afterwards, the nerve divides into two rami; superior ramus and inferior ramus. Both rami enter the orbit through the superior orbital fissure, between the two heads of the lateral rectus muscle.
Superior ramus: It supplies the superior rectus muscle, then pierces it, and supplies the levator palpebrae superioris muscle.
Inferior ramus: It supplies the inferior rectus muscle, the medial rectus muscle and the inferior oblique muscle. The part to the inferior oblique gives off a branch that passes to the ciliary ganglion and carries parasympathetic fibers to the pupillary sphincter and the ciliary muscle.
As is clear from the above discussion, the oculomotor nerve innervates the following structures;
- Superior rectus muscle (via superior ramus)
- Levator palpebrae superioris muscle (via superior ramus)
- Inferior rectus muscle (via inferior ramus)
- Medial rectus muscle (via inferior ramus)
- Inferior oblique muscle (via inferior ramus)
- Pupillary reflex, also called the light reflex (via inferior ramus)
- Accommodation reflex (via inferior ramus)