Final answer:
The parasympathetic nervous system causes pupillary constriction by releasing acetylcholine, which acts on circular muscle fibers in the iris.
Step-by-step explanation:
The effect that the parasympathetic nervous system exerts at the iris is pupillary constriction. This is a homeostatic response to bright light. When light hits the retina, specialized cells send signals via the optic nerve to the brain, which triggers the parasympathetic nervous system. The parasympathetic fibers, traveling through the oculomotor nerve to the ciliary ganglion and then to the iris, release the neurotransmitter acetylcholine. Acetylcholine acts on circular muscle fibers of the iris, causing them to contract and thus constrict the pupil, reducing the amount of light entering the eye.