Iris dilator muscle

The iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a smooth muscle of the eye. It is innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on α1-receptors. . Thus, when presented with a threating stimuli that activates the fight-or-flight response, this innervation dilates the iris, thus temporarily letting more light reach the retina.

It has the opposite effect to the iris sphincter muscle