The iris sphincter muscle
, circular muscle of iris
, circular fibers
) is a muscle
in the part of the eye
called the iris. It encircles the eye, appropriate to its function as a constrictor.
It is found in vertebrates and some cephalopods.
In humans, it functions to constrict the pupil in bright light (pupillary reflex) or during accommodation. Its dimensions are about 0.75 mm wide by 0.15 mm thick.
It is controlled by parasympathetic fibers 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.
Initially, all the myocytes are of the smooth muscle type, but hatch later in life, so that ultimately most cells are of the striated muscle type.