The eye is innervated by both parasympathetic and sympathetic nerve fibers, as shown in Figure 1. The parasympathetic preganglionic fibers arise in the Edinger Westphal nucleus (the visceral nucleus portion of the third cranial nerve) and then pass in the third nerve to the ciliary ganglion, which lies immediately behind the eye. There, the preganglionic fibers synapse with postganglionic parasympathetic neurons, which in turn send fibers through ciliary nerves into the eyeball. These nerves excite (1) the ciliary muscle that controls focusing of the eye lens and (2) the sphincter of the iris that constricts the pupil.
Fig1. Autonomic innervation of the eye, showing also the reflex arc of the light reflex. N., nerve. (Modified from Ranson SW, Clark SL: Anatomy of the Nervous System: Its Development and Function, 10th ed. Philadelphia: WB Saunders, 1959.)
The sympathetic innervation of the eye originates in the intermediolateral horn cells of the first thoracic segment of the spinal cord. From there, sympathetic fibers enter the sympathetic chain and pass upward to the superior cervical ganglion, where they synapse with postganglionic neurons. Postganglionic sympathetic fibers from these then spread along the surfaces of the carotid artery and successively smaller arteries until they reach the eye.
There, the sympathetic fibers innervate the radial fibers of the iris (which open the pupil), as well as several extra ocular muscles of the eye, which are discussed subsequently in relation to Horner’s syndrome.
