Figure 64: The Course of the 3rd (III) Nerve

The 3rd nucleus lies at the ventral border of the periaqueductal gray matter, at the level of the superior colliculus. In between the two nuclei is the midline central caudal nucleus (CCN), which innervates bilateral levator palpebrae muscles (explaining how a unilateral nuclear 3rd can cause bilateral ptosis). The 3rd nerve fascicle (fibers responsible the pupillary sphincter [P], inferior rectus [IR], levator palpebrae [LP], medial rectus [MR], superior rectus [SR], and inferior oblique [IO] are located medial to lateral) travels anteriorly through the tegmentum, the red nucleus, the substantia nigra, and finally exits the midbrain medially from the cerebral peduncles. The peripheral portion of the 3rd nerve courses between the superior cerebellar and posterior cerebral arteries and then passes the posterior communicating artery (PCOM aneurysm causes compression here) and the temporal lobe uncus (uncal herniation can cause compression here), then above the petroclinoid (Gruber's) ligament (while the 6th nerve travels under) and into the cavernous sinus where it is located laterally. The separation into superior (SR, LP) and inferior (P and ciliary body, MR, IR, IO) divisions occurs in the anterior sinus, and these branches then travel through the superior orbital fissure to enter the orbit, passing through the annulus of Zinn before innervating their respective muscles. Castro O, Johnson LN, Mamourian AC. Isolated inferior oblique paresis from brain-stem infarction. Perspective on oculomotor fascicular organization in the ventral midbrain tegmentum. Arch Neurol. 1990;47(2):235-7.