Miosis

Constriction or narrowing of the pupil of the eye, reducing the size of the pupillary opening. This is the opposite of mydriasis.

The parasympathetic nervous system activates 'rest and digest' responses by decreasing heart rate, increasing digestion, and promoting energy conservation. It opposes sympathetic activation through antagonistic control of organs like the heart, digestive tract, and eyes—for example, parasympathetic fibers via the oculomotor nerve (CN III) constrict the pupil (miosis) to limit light exposure. This system is essential for maintaining homeostasis during calm, normal conditions.

The circular muscle of the iris responsible for pupil constriction when stimulated by parasympathetic innervation.

Miosis occurs when the eye focuses on near objects as part of the accommodation process, along with lens thickening and convergence.

The pupil normally constricts in response to increased light exposure to protect the retina and improve visual acuity.

Strong pupillary constriction caused by opioid drugs, a characteristic sign often observed in opioid overdose or toxicity.

A neurological condition causing miosis on one side of the face, along with ptosis and anhidrosis, due to sympathetic pathway disruption.

Drugs like pilocarpine and carbachol stimulate the parasympathetic system and cause miosis by directly activating the sphincter pupillae.

Miosis is a valuable diagnostic sign in assessing neurological conditions, drug toxicity, and autonomic nervous system function.

Light stimulates the retina, signals travel via CN II to the midbrain, then CN III carries parasympathetic signals causing miosis.