iris sphincter muscle iris sphincter muscle 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. iris sphincter muscleiris, front view. (muscle visible but not labeled.)the upper half of a sagittal section through the front of the eyeball. ("sphincter of pupil" labeled near bottom-center.)detailsoriginencircles irisinsertionencircles irisarterylong posterior ciliary arteriesnerveshort ciliary nervesactionsconstricts pupilantagonistiris dilator muscleidentifierslatinmusculus sphincter pupillaeta98a15.2.03.029ta26762fma49157anatomical terms of muscle comparative anatomy this structure is found in vertebrates and in some cephalopods. general structure all the myocytes are of the smooth muscle type. its dimensions are about 0.75 mm wide by 0.15 mm thick. mode of action 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. innervation 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 through 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.