<P> Its dimensions are about 0.75 mm wide by 0.15 mm thick . </P> <P> 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 . </P> <P> 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 . </P>

The circular muscle of the iris uses which parasympathetic receptor