Location of motoneurons in the oculomotor nucleus and the course of their axons in the oculomotor nerve.

Subdivisions of the oculomotor nucleus, and the course of axons in the brainstem and more peripherally in the oculomotor nerve of the cat, were studied by directly applying horseradish peroxidase solution to the transected nerve-branch stump in the orbit. The medial rectus subdivision consisted of two subgroups, and intermingling between subdivisions was found. About 20% of the motoneurons controlling the medial rectus muscle were scattered in the medial longitudinal fasciculus or a more ventrolateral area. A few motoneurons controlling the inferior rectus or inferior oblique muscle were also located in the medial longitudinal fasciculus. Axons to the superior branch that supplied the superior rectus and levator muscle coursed in the dorsolateral half of the oculomotor nerve. In contrast, those to the medial rectus, inferior rectus, and inferior oblique muscles were scattered diffusely in the oculomotor nerve.