Parasympathetic nerves control the muscle cells of the pupillary sphincter of the iris - the constrictor pupillae. These are circular muscle cells which cause pupil constriction when they contract. Pre-ganglionic nerves in the inferior root of the oculomotor nerve synapse in the ciliary ganglion. Post-ganglionic nerves travel with the short ciliary nerves and innervate the circular muscle cells of the pupillary constrictor which causes a narrowing of the aperture of the iris and a reduction in the amount of light reaching the retina. Acetylcholine is the neurotransmitter at both the ganglion (acts on nicotinic receptors) and post-ganglionic synapse (acts on muscarinic M3 receptors).