Cranial Nerve III (Oculomotor Nerve)

Synonyms

CNIII; Cranial nerve III; Oculomotor nerve

Definition

The oculomotor nerve (cranial nerve III) begins in the midbrain, provides motor innervation to many of the extraocular muscles, and provides parasympathetic innervation to the eye. The third nerve begins in the third cranial nerve nucleus located at the level of the superior colliculus in the dorsal midbrain. The fascicular part of cranial nerve III then projects from the nucleus and travels ventrally, passing by the red nucleus, the corticospinal tracts, and other important brainstem structures. The fascicle exits the midbrain into the interpeduncular fossa and travels in the subarachnoid space medial to the cerebral peduncle.

In the subarachnoid space, the oculomotor nerve travels in close association to arteries of the circle of Willis. Specifically, CNIII travels in-between the posterior cerebral artery (PCA) and superior cerebellar artery (SCA). Aneurysms in the PCA or SCA can rarely compress CNII at this point and...