Sphincter pupillae

Nerve: Short ciliary nerves

Action: Constricts pupil

Antagonist: Iris dilator muscle

The sphincter pupillae is a circular band of smooth muscle located within the stroma of the iris, surrounding the margin of the pupil. It forms a ringlike structure close to the pupillary border.

Functionally, the sphincter pupillae is responsible for constriction of the pupil (miosis). When it contracts, the diameter of the pupil decreases, thereby limiting the amount of light entering the eye. This action is particularly important in bright light conditions and during the near response (accommodation reflex), where pupil constriction improves depth of focus.

Innervation

The sphincter pupillae muscle is supplied by parasympathetic fibers originating from the oculomotor nerve (cranial nerve III). These fibers synapse in the ciliary ganglion, and postganglionic fibers reach the muscle via the short ciliary nerves.

Clinical correlate

Clinically, impairment of the sphincter pupillae leads to pupil dilation (mydriasis) and loss of the normal light reflex, which may occur in lesions affecting the oculomotor nerve or parasympathetic pathways.