Dilator pupillae

The dilator pupillae muscle is a thin layer of radially arranged smooth muscle fibres located in the anterior part of the posterior epithelium of the iris.

Structurally, its fibres extend radially from the pupillary margin toward the periphery of the iris.

Function

When these fibers contract, they pull the iris outward, resulting in dilation of the pupil (mydriasis). Functionally, the dilator pupillae increases the amount of light entering the eye, particularly in low light conditions. It also plays a role in the fight or flight response, where pupil dilation enhances visual awareness.

Innervation

The muscle is supplied by sympathetic fibres originating from the superior cervical ganglion. These fibers reach the eye via the long ciliary nerves.

Clinical correlate

Clinically, disruption of sympathetic supply (e.g., in Horner's syndrome) leads to miosis (constricted pupil) due to unopposed action of the sphincter pupillae. Conversely, excessive sympathetic stimulation results in mydriasis.