[citation needed] All the myocytes are of the smooth muscle type.

[citation needed] In lower animals, the muscle cells themselves are photosensitive causing iris action without brain input.

[6] Preganglionic fibers originate from the Edinger–Westphal nucleus, travel along the oculomotor nerve (CN III), and make nicotinic cholinergic synapses on neurons in the ciliary ganglion.

[7] 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.