part_of part of is part of part of develops from has muscle origin has muscle insertion has muscle antagonist anatomical structure connected anatomical structure system sphincter pupillae dilatator pupillae http://upload.wikimedia.org/wikipedia/commons/2/2f/Gray878.png radial muscle of iris radiating fibers uberon iris dilator dilator muscle of pupil NCIT:C32463 dilator pupillae muscle FMA:49158 pupillary dilator musculus dilatator pupillae 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 BTO:0001371 dilator of pupil pupillary dilator muscle dilator pupillae radial muscle pupillary muscle pupil dilator MA:0001288 UMLS:C0229190 musculus dilator pupillae dilator pupillae muscle UBERON:0001608 pupil dilator muscle A smooth muscle of the eye, running radially in the iris that functions as a dilator. [WP,unvetted]. musculus dilatator pupillae EMAPA:35286 iris dilator muscle iris neurectoderm iris smooth muscle