The fibers of the third-nerve nucleus course ventrally in the midbrain, crossing the medial longitudinal fasciculus, red nucleus, substantia nigra, and medial part of the cerebral peduncle successively. Lesions involving these structures therefore interrupt oculomotor fibers in their intramedullary (fascicular) course and give rise to several crossed syndromes with ipsilateral ocular palsy. With regard to the oculomotor subnuclei, schematic arrangements of their projections have been derived from various sources, mainly experimental but some clinical, and are shown in the figure from Ksiazek and colleagues (Fig. 13-3). The emerging fibers can be considered as situated in medial, lateral and rostrocaudal groups, with the pupillary fibers occupying the rostromedial aspect. This location of axons destined for the pupil continues through the third nerve. This information becomes useful in recognizing that combined pupillary and inferior and medial rectus palsies on one side may be the result of a fascicular lesion of the oculomotor nerve.
The oculomotor nerve, soon after it emerges from the brainstem, passes between the superior cerebellar and posterior cerebral arteries. The nerve (and sometimes the posterior cerebral artery) may be compressed at this point by herniation of the uncal gyrus of the temporal lobe through the tentorial opening (see Chap. 16). Just posterior and superior to the cavernous sinus, the oculomotor nerve crosses the terminal portion of the internal carotid artery at its junction with the posterior communicating artery. An aneurysm at this site frequently damages the third nerve; this serves to localize the site of compression or bleeding. When infraclinoid retrocavernous compressive lesions, such as aneurysms and tumors, affect the oculomotor nerve, they tend to also involve all three divisions of the trigeminal nerve. In the posterior portion of the cavernous sinus, the first and second trigeminal divisions are involved along with the ocular motor nerves; in the anterior portion, only the ophthalmic division of the trigeminal nerve is affected since the third trigeminal division does not pass through the cavernous sinus.
As the oculomotor nerve enters the orbit, it divides into superior and inferior branches, although a functional separation of nerve bundles occurs well before this anatomic bifurcation. The superior branch supplies the superior rectus and the voluntary (striated) part of the levator palpebrae (the involuntary part is under the control of sympathetic fibers of Müller); the inferior branch supplies the pupillary and ciliary muscles and all the other extrinsic ocular muscles except, of course, two—the superior oblique and the lateral rectus which are innervated by the trochlear and abducens nerves, respectively. Superior branch lesions of the oculomotor nerve caused by an aneurysm or more commonly by diabetes, result in ptosis and uniocular upgaze paresis.