thirdnerve

Thirdnerve, more properly known as the oculomotor nerve or cranial nerve III, is a mixed nerve with motor and parasympathetic components. It carries somatic motor fibers from the oculomotor nucleus to four extraocular muscles—superior rectus, inferior rectus, medial rectus, and inferior oblique—and to the levator palpebrae superioris, which elevates the upper eyelid. It also carries parasympathetic preganglionic fibers from the Edinger-Westphal nucleus that travel to the ciliary ganglion; postganglionic fibers then innervate the sphincter pupillae muscle (pupil constriction) and the ciliary muscle (lens accommodation).

Anatomically, the nerve originates in the midsagittal region of the brainstem and exits the midbrain at the interpeduncular fossa between the posterior cerebral and superior cerebellar arteries. After a short intracranial course, it travels in the lateral wall of the cavernous sinus and enters the orbit via the superior orbital fissure, where it divides into superior and inferior divisions. The superior division primarily innervates the levator palpebrae superioris and the superior rectus, while the inferior division innervates the medial rectus, inferior rectus, inferior oblique, and carries the parasympathetic fibers to the ciliary ganglion.

Clinically, disruption of third nerve function produces ptosis, a "down and out" eye position due to unopposed lateral rectus and superior oblique activity, and diplopia. If the pupil is involved, it suggests a compressive lesion such as an aneurysm of the posterior communicating artery; if the pupil is spared, ischemic microvascular causes (e.g., diabetes) are more likely. Diagnosis typically uses neuroimaging and targeted neuro-ophthalmologic testing.