The Diameter of the Pupil Is Controlled by the Autonomic Nervous System
The iris of the eye contains two sets of smooth muscle fibers. One set, arranged in a circular pattern around the pupil, causes the pupil to become smaller (constrict) when the fibers contract.
These constrictor fibers are innervated by postganglionic parasympathetic neurons whose cell bodies lie in the ciliary ganglion, just behind the eye, and which secrete acetylcholine as the neurotransmitter to the muscle. These parasympathetic postganglionic neurons are activated by parasympathetic preganglionic neurons whose axons travel in the oculomotor cranial nerve (cranial nerve Ill) and whose cell bodies reside in the Edinger-Westphal nucleus of the midbrain.The other smooth muscle fibers of the iris are arranged radially from the pupil, resembling spokes of a wheel. When these radial smooth muscle fibers contract, they cause the pupil to become larger (dilate). These dilator fibers are innervated by the sympathetic nervous system. Sympathetic preganglionic neurons begin in the first two thoracic segments and course Cranially in the vagosympathetic trunk to synapse in the superior cervical ganglion of the neck. The sympathetic postganglionic axons originate in cells of this ganglion and course to the region of the eye, where they innervate the dilator fibers of the iris, in addition to the muscle that helps lift the upper eyelid and the muscle that
helps keep the “third eyelid” in place at the medial canthus of the eye. The sympathetic postganglionic axons also innervate sweat glands and vascular smooth muscle to the face.