Pupil

The pupil is the opening in the center of the iris that regulates the amount of light reaching the retina. It appears as a circular, black aperture in the eye. Although the pupil itself is a hole, its apparent size is controlled by the iris muscles: the dilator pupillae, a radial muscle that enlarges the pupil under sympathetic stimulation, and the sphincter pupillae, a circular muscle that constricts the pupil under parasympathetic stimulation. In normal conditions the pupil size ranges roughly from 2 to 4 millimeters in bright light to 4 to 8 millimeters in dim light, with some variation by age and individual.

Light exposure triggers the pupillary light reflex. Bright light causes constriction of the illuminated pupil and, via a consensual reflex, constriction of the opposite pupil as well. The reflex involves the optic nerve as the afferent limb and the oculomotor nerve to the iris as the efferent limb. The degree of constriction reflects both the intensity of light and the integrity of the neural pathways.

During accommodation for near vision, the pupils also constrict in a coordinated response called the near triad, along with lens thickening and eye convergence. Pupil size can be influenced by non-light factors such as arousal, cognitive load, and certain drugs or medications.

Abnormal pupil size or reactivity can indicate neurological or ocular disorders. Clinicians assess pupil size, symmetry, and reactivity as part of a neurological examination. Pharmacologic agents, including pilocarpine, atropine, and phenylephrine, can induce constriction or dilation for diagnostic or therapeutic purposes. The term pupil also appears in other contexts, notably as a synonym for a student or learner in some varieties of English.