The dilator pupillae muscle is a ring of contractile cells within the iris. These cells are arranged radially, such that their contraction facilitates pupillary dilation (mydriasis). The dilator pupillae muscle receives innervation from the sympathetic nervous system.
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Gross anatomy
The dilator pupillae muscle is a circumferential, spoke-like arrangement of contractile myoepithelial cells located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. Like the sphincter pupillae muscle, it is located posterior to the rich connective tissue stroma and neurovasculature of the anterior iris leaf 1.
Innervation
The dilator pupillae muscle is innervated by the sympathetic nervous system. Postganglionic sympathetic fibers project from the superior cervical ganglion to join the carotid plexus. These fibers then course with the ophthalmic artery, forming a number of long ciliary nerves that supply the dilator pupillae muscle 2.
Action
Contraction of the dilator pupillae muscle facilitates dilation of the pupil (mydriasis). This increases the amount of light impinging on the retina 2.
Clinical importance
- alongside endogenous sympathetic stimulation, sympathomimetic drugs (e.g. stimulants) may act on the dilator pupillae muscle, resulting in mydriasis 2
- prolonged significant mydriasis narrows the iridocorneal angle for aqueous humor drainage, increasing the risk of closed angle glaucoma
- the miosis seen in patients with Horner syndrome is mediated by inactivation of the dilator pupillae muscle due to a lesion interrupting sympathetic outflow 3
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