Figure 2: Parasympathetic Pathway for Pupillary Constriction

A bright light is shone in one eye, light enters the pupil and hyperpolarizes retinal photoreceptors which activates retinal ganglion cells. These signals propagate along the optic nerves, chiasm, optic tracts, and fibers responsible for the light reflex then synapse in the dorsal midbrain (prior to reaching the lateral geniculate nucleus) at the pretectal nuclei, then to the Edinger-Westphal nucleus (EWN) of the oculomotor nucleus. From here, efferent fibers travel with the oculomotor nerve to the ciliary ganglion, and finally innervate the constrictor (sphincter) muscles for bilateral pupillary constriction.