The pupil reflex is a response of the eye that controls the size of the pupil, which is the opening in the center of the iris. This reflex helps regulate the amount of light entering the eye, protecting the retina from excessive brightness and improving vision in low-light conditions. When exposed to bright light, the pupils constrict (become smaller), while in dim light, they dilate (become larger). The reflex is primarily controlled by the autonomic nervous system, specifically the parasympathetic and sympathetic branches. The constriction is mediated by the oculomotor nerve, while dilation is influenced by the sympathetic nervous system. This automatic response is crucial for maintaining optimal vision and eye health.