Like the dilator muscle, the sphincter muscle is derived from neuroectoderm. It is composed of a circular band of smooth muscle fibers and is located near the pupillary margin in the deep stroma, anterior to the posterior pigmented epithelium of the iris (see Fig 2-20). The sphincter muscle receives its primary innervation from parasympathetic nerve fibers that originate in the Edinger-Westphal nucleus and travel with the oculomotor nerve, and it responds pharmacologically to muscarinic stimulation. The reciprocal sympathetic innervation to the sphincter appears to serve an inhibitory role, helping relax the sphincter in darkness. See BCSC Section 5, Neuro-Ophthalmology, for additional discussion of physiology and pathology of the sphincter muscle.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.