pupilconstricting

Pupil constriction, commonly called miosis, is the narrowing of the pupil due to contraction of the iris sphincter muscle (sphincter pupillae). This process is driven by the parasympathetic division of the autonomic nervous system. Signals travel from the oculomotor nerve (cranial nerve III) to the ciliary ganglion and then along short ciliary nerves to the iris, where acetylcholine activates muscarinic receptors to tighten the sphincter.

Two related reflexes regulate pupil size: the pupillary light reflex and the near (accommodation) reflex. In the light reflex, retinal input proceeds to the pretectal area and Edinger–Westphal nucleus; parasympathetic fibers travel with CN III to the iris, producing constriction. The near response involves accommodation for near vision, where convergence and lens adjustment are accompanied by pupil constriction.

Clinical and practical aspects include assessment of brain function and ocular health. Pupillary constriction can be tested by exposing the eye to light to observe a brisk, consensual response. Drugs influence pupil size: cholinergic agonists (for example, pilocarpine) cause miosis, whereas anticholinergic agents or sympathetic stimulation cause mydriasis. Abnormal patterns of constriction or unreactive pupils can indicate neurological injury or disease and require clinical evaluation.

Variants and related conditions include anisocoria with differential diagnoses such as Horner syndrome (constricted pupil on one side with ptosis), Adie’s tonic pupil (slow, irregular constriction), and Argyll Robertson pupil (poor light response but preserved near response). Understanding pupil constriction helps in neurological assessment and in diagnosing ocular and systemic conditions.