Final answer:
The dilation of the pupil is caused by the paralysis of the iris sphincter muscle. This is primarily caused by the activation of the sympathetic nervous system, which releases norepinephrine onto the radial fibers of the iris smooth muscle. Phenylephrine, a medication used in eye drops, can mimic this action.
Step-by-step explanation:
The dilation of the pupil is caused by the paralysis of the sphincter muscle of the iris. This paralysis is primarily caused by the activation of the sympathetic nervous system, which releases norepinephrine onto the radial fibers of the iris smooth muscle. Norepinephrine causes the radial fibers to contract, resulting in the dilation of the pupil. Phenylephrine, a medication used in eye drops, can mimic this action by binding to the same adrenergic receptor as norepinephrine. By paralyzing the sphincter muscle of the iris, phenylephrine causes pupillary dilation.