The pupillary light reflex is a natural response of the eye that occurs when light is shone into it. When bright light enters the eye, the pupils constrict, or become smaller, to limit the amount of light that reaches the retina. This reflex helps protect the eye from excessive brightness and aids in maintaining optimal vision. This reflex involves a neural pathway that includes the retina, optic nerve, and brainstem. When light stimulates the retina, signals are sent to the brain, which then sends signals back to the muscles controlling the pupil size. This process occurs quickly and automatically.