Sphincter pupillae muscle

Last revised by Daniel J Bell on 24 May 2022

The sphincter pupillae muscle is a circular ring of smooth muscle within the iris responsible for constriction of the pupil (miosis). The structure is stimulated by the parasympathetic nervous system causing the muscle to decrease in diameter as it contracts.

The sphincter pupillae is a round structure comprised of smooth muscle located within the posterior iris anterior to the pigmented epithelium 1. The structure interlocks with the surrounding stroma of the iris.

Parasympathetic nerve fibers originating in the Edinger-Westphal nucleus travel along the oculomotor nerve before synapsing at the ciliary ganglion 2. Postganglionic fibers arrive at the sphincter pupillae via the short ciliary nerves

The muscle facilitates constriction of the pupil (miosis) in turn limiting the amount of light reaching the retina

  • sphincter pupillae has a role in the accommodation and pupillary light reflexes 2
  • compression of the oculomotor nerve (III) may also result in pinching of peripherally located parasympathetic nerve fibers that supply the sphincter pupillae causing relaxation of the muscle and dilation of the pupil (see oculomotor nerve palsy3
  • drugs that have cholinergic effects may act to stimulate the smooth muscle of the sphincter pupillae resulting in miosis (e.g. pilocarpine, bethanechol, and metoclopramide) 1

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