Pupillary constriction or constriction of the pupil is a physiological response that decreases the size of the pupil of the eye.
When bright light is shone on the eye, it will automatically constrict. The pupil opens wide when dark and small when light. This is the pupillary reflex, which is an important test of brainstem function.
The oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.
The iris is a contractile structure, consisting mainly of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When they contract, the iris decreases or constricts the size of the pupil. These muscles are sometimes referred to as intrinsic eye muscles.