part_of part of part of develops from has muscle origin has muscle insertion has muscle antagonist system sphincter pupillae dilatator pupillae http://upload.wikimedia.org/wikipedia/commons/2/2f/Gray878.png It is innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on alpha1-receptors. Thus, when presented with a threatening stimuli that activates the fight-or-flight response, this innervation dilates the iris, thus temporarily letting more light reach the retina iris neurectoderm iris smooth muscle