Anatomy

Origin

The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

Nerve

It is controlled by parasympathetic fibers of the muscarinic acetylcholine receptor (M3) that originate from the Edinger-Westphal nucleus, travel along the oculomotor nerve (CN III), synapse in the ciliary ganglion, and then enter the eye via the short ciliary nerves. The short ciliary nerves then run forward and pierce the sclera at the back of the eye, traveling between the sclera and the choroid to innervate the iris sphincter muscle.

Actions

In humans, it functions to constrict the pupil in bright light (pupillary light reflex) or during accommodation. In lower animals, the muscle cells themselves are photosensitive causing iris action without brain input.