The sphincter pupillae is a circular smooth muscle ring 0.75 mm wide situated in the posterior iris stroma at the pupillary margin. It is innervated by postganglionic parasympathetic fibres from the ciliary ganglion, reaching the muscle via the short ciliary nerves. Contraction narrows the pupil (miosis) in the light reflex and near reflex. It works in opposition to the dilator pupillae.
| Origin | Circular muscle fibres arising from within the iris stroma |
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| Insertion | Forms a complete circular ring around the pupillary margin |
| Nerve Supply | Parasympathetic fibres from the ciliary ganglion via the short ciliary nerves (CN III pathway) |
| Blood Supply | Long posterior ciliary arteries |
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It constricts the pupil in bright light and during near vision, reducing the amount of light entering the eye and increasing depth of focus.
Sphincter pupillae function is assessed clinically by the direct and consensual pupillary light reflex. Parasympathetic denervation (from oculomotor nerve palsy or ciliary ganglion damage) abolishes the light reflex and produces a fixed dilated pupil. Pharmacological miosis with pilocarpine stimulates the sphincter to treat elevated intraocular pressure in angle-closure glaucoma. The Argyll Robertson pupil (loss of light reflex with preserved near reflex) reflects selective loss of the reflex arc to the sphincter while the near reflex pathway remains intact, historically associated with neurosyphilis.
Not palpable; assessed by ophthalmoscopy and pupillary reflex testing.
Compression of CN III (by herniation or aneurysm) interrupts parasympathetic outflow to the sphincter pupillae, causing a fixed dilated non-reactive pupil that is a neurosurgical emergency requiring immediate CT angiography to exclude posterior communicating artery aneurysm.
Degeneration of the ciliary ganglion or short ciliary nerves causes denervation hypersensitivity of the sphincter pupillae, producing a large tonic pupil with absent direct light reflex but intact, slow, and tonic near reflex that constricts to dilute pilocarpine 0.1%.
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