Neuroclinical Anatomy of the third Cranial Nerve

The oculomotor (third) cranial nerve arises from the upper part of the mesencephalon (midbrain). It carries both a somatic efferent (motor) supply to some of the extra-ocular muscles and a general visceral efferent (parasympathetic) supply to the sphincter pupillae and ciliaris muscles. Its contribution to the pupillary light reflex represents an important aspect of its function and is an essential component of the evaluation of neurological patients. This paper reviews the neuroclinical aspects of the nerve�s anatomy and and provides easily understandable guidance to an effective assessment of its dysfunction. The contributions of the third cranial nerve to pupillary reaction to light, the changes in pupillary size reflecting the various stages of its dysfunction in intracranial pathologies, and its association with neurotrauma as well as some of the eponymous syndromes associated with it are discussed.