This is a schematic diagram of the pupillary light reflex. The afferent limb originates in the retinal photoreceptors, which convert light energy to a neural signal.
Pupillary information is conveyed from the eye to the brain by the melanopsin-expressing retinal ganglion cells, and their axons project to the dorsal mid brain, synapsing in the pretectal olivary nucleus.
Each pretectal olivary nucleus distributes the afferent pupillary impulses to the ipsilateral and contralateral Edinger-Westphal subnucleus of the oculomotor nuclear complex.
The neurons of the Edinger-Westphal subnucleus initiate the efferent limb of the pupillary light reflex, that is, pupilloconstriction.
Efferent pupillomotor impulses travel in the parasympathetic fibers of the oculomotor nerve, synapse in the ciliary ganglion of the orbit, and then pass via the short ciliary nerves to innervate the iris sphincter muscle.