The iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated by the sympathetic nervous system, the cells contract, widening the pupil and allowing for more light to pass through the eye. Pupillary dilation occurs in two situations: when light intensity changes and when we shift our gaze between distant and nearby objects. The pupillary dilator acts as an antagonist to the pupillary constrictor which narrows the pupil and admits less light to the eye. It has its origin from the anterior epithelium. It is innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on α1-receptors. Thus, when presented with a threatening stimuli that activates the fight-or-flight response, this innervation contracts the muscle and dilates the iris, thus temporarily letting more light reach the retina. The pupillary dilator acts as an antagonist to the pupillary constrictor which narrows the pupil and admits less light to the eye.
The dilator muscle is innervated more specifically by postganglionic sympathetic nerves arising from the superior cervical ganglion as the Sympathetic root of ciliary ganglion. They will follow both short ciliary and long ciliary nerves to reach the dilator muscle.
Additional images 
Scheme showing sympathetic and parasympathetic innervation of the pupil and sites of lesion in a Horner's syndrome.
Sympathetic connections of the ciliary and superior cervical ganglia.
The iris dilator muscle fibers course radially through the iris.
See also 
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