Iris sphincter muscle

The iris sphincter muscle 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.
The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle sometimes are called intrinsic ocular muscles or intraocular muscles.

Comparative anatomy

This structure is found in vertebrates and in some cephalopods.

All the myocytes are of the smooth muscle type.
Its dimensions are about 0.75 mm wide by 0.15 mm thick.

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

It is controlled by parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor of iris sphincter muscle. Preganglionic fibers originate from the Edinger–Westphal nucleus, travel along the oculomotor nerve, and make nicotinic cholinergic synapses on neurons in the ciliary ganglion. Those neurons' postganglionic parasympathetic fibers 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.