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Anatomy · Anatomical structures

Sphincter pupillae

m. sphincter pupillae

Sphincter pupillae muscle (m. sphincter pupillae) is an intrinsic smooth muscle of the eye of neuroglial origin, located within the iris. It runs circularly around the pupillary margin and is responsible for constriction of the pupil (miosis). The muscle's function is regulated by the parasympathetic nervous system, providing protection of the retina from bright light as well as focusing vision on near objects.

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LocationIris (circularly at the pupillary margin)
InnervationParasympathetic (CN III)
FunctionPupil constriction (miosis)
OriginNeuroglial
3 min readFor medical studentsUpdated 09/24/2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesReviewed by medical editor: Aulov A. A., general practitioner · 09/24/2026

Where is the muscle located and how is it structured?

The sphincter pupillae is a part of the iris (iris), the anterior portion of the vascular tunic of the eye. The muscle lies in the anterior connective tissue layer (stroma) of the iris, directly adjacent to the pupillary margin.

The structure is of neuroglial origin. At the cellular level, myocyte cell bodies are located within the epithelium and are incapable of contraction. Contractile processes extend from them into the thickness of the iris. The sphincter fibers are arranged circularly (in a circle, parallel to the pupillary margin), which distinguishes them from the radial fibers of the dilator pupillae.

How is the sphincter pupillae innervated?

Innervation is provided by the parasympathetic division of the autonomic nervous system. The efferent pathway includes the following stages:

  • Center — Edinger-Westphal nucleus (accessory parasympathetic nucleus) located in the midbrain.
  • Preganglionic fibers — run as part of the oculomotor nerve (n. oculomotorius, CN III), which enters the orbit through the superior orbital fissure.
  • Ganglion — impulse relay takes place in the ciliary ganglion (ganglion ciliare).
  • Postganglionic fibers — short fibers leave the ganglion and run inside the eyeball directly to the sphincter pupillae.
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What is its function?

The primary task of the muscle is pupil constriction (miosis). This process can occur reflexively or voluntarily in the following situations:

  • Pupillary light reflex: upon illumination, the sphincter contracts, protecting the retina from excessive light flux. Due to bilateral connections in the brain, illumination of one eye causes pupil constriction in both eyes (direct and consensual responses).
  • Accommodation and convergence: when fixing gaze on a near object, the pupil constricts. This works on the principle of a camera aperture: narrowing the opening increases depth of field and ensures a sharp image on the retina.

Clinical significance and symptoms of lesion

Disorders of sphincter pupillae function may be associated with pathology of the neural pathways:

  • Mydriasis (pupillary dilation): occurs when the efferent parasympathetic link (oculomotor nerve or ciliary ganglion) is damaged. Impulse conduction to the sphincter is interrupted, and the pupil on the affected side ceases to constrict to light. Mydriasis can also occur during severe pain or emotional arousal due to decreased parasympathetic activity.
  • Anisocoria: pathologic difference in pupil diameter. May indicate a unilateral intracranial space-occupying lesion compressing the oculomotor nerve.
  • Medication effects: eye drops can cause drug-induced anisocoria. The use of such drugs in comatose patients should be avoided, as it distorts neurological symptoms.

Class and exam questions

Why do both pupils constrict when one eye is illuminated?

This happens due to the consensual light reflex. Fibers travel from the pretectal nucleus to the parasympathetic nuclei of the oculomotor nerves on both the ipsilateral and contralateral sides.

How does the sphincter pupillae participate in near vision?

When viewing near objects, the sphincter contracts together with the ciliary muscle and medial rectus muscles. Pupil constriction acts as an aperture, increasing depth of field and helping focus the image on the retina.

What happens to the sphincter when the oculomotor nerve is compressed?

Parasympathetic impulse delivery to the sphincter will be interrupted. This leads to pupil dilation (mydriasis) on the affected side and absence of its reaction to light.

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What else to study on this topic

  • Oculomotor nerve (n. oculomotorius)
  • Dilator pupillae muscle (m. dilatator pupillae)
  • Ciliary ganglion (ganglion ciliare)
  • Iris (iris)

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