The dilator pupillae is a thin sheet of radially oriented smooth muscle fibres in the posterior iris stroma. It is innervated by postganglionic sympathetic fibres from the superior cervical ganglion, travelling in the nasociliary nerve and long ciliary nerves. Sympathetic activation dilates the pupil in dim light, fear, and pain responses. The long three-neuron sympathetic pathway (hypothalamus to superior cervical ganglion via the stellate ganglion region) makes this muscle a sensitive indicator of sympathetic interruption anywhere along the pathway.
| Origin | Radial smooth muscle fibres from the iris root |
|---|---|
| Insertion | Sphincter pupillae zone at the pupillary margin (radial fibres meeting the circular sphincter) |
| Nerve Supply | Sympathetic fibres via the long ciliary nerves (superior cervical ganglion pathway) |
| Blood Supply | Long posterior ciliary arteries |
| Actions |
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|---|
It widens the pupil in dim light and under sympathetic stimulation, such as during fear or excitement.
Dilator pupillae denervation anywhere along the sympathetic pathway produces Horner syndrome: miosis (small pupil from unopposed sphincter action), ptosis (from superior tarsal muscle paralysis), and anhidrosis (if preganglionic). Pharmacological testing with cocaine eye drops (dilates normally innervated pupil, fails in Horner) and hydroxyamphetamine (fails in postganglionic lesion, succeeds in preganglionic) localises the lesion level. Dilating drops for eye examination (tropicamide, phenylephrine) stimulate or bypass the dilator pathway.
Not palpable; assessed by pupil size measurement and pharmacological testing.
Interruption of the sympathetic pathway to the dilator pupillae at any level (central, preganglionic, or postganglionic) produces miosis, ptosis, and anhidrosis; the location is determined pharmacologically, and central and preganglionic causes (Pancoast tumour, carotid dissection, brainstem stroke) require urgent investigation.
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