Answer: When norepinephrine binds to alpha receptors of the iris of the eye, it causes contraction of the ciliary muscles of the eye which brings about pupil dilation.
Stimulation of alpha receptors of the smooth muscle vasculature results in vasoconstriction of blood vessels.
Explanation: Alpha receptors are adrenergic receptors that respond to norepinephrine. They are subdivided into two (2) types:
Alpha 1, found in smooth muscle heart, liver, pregnant uterus, male sexual organs and blood vessels, with effects including vasoconstriction, intestinal relaxation, uterine contraction and papillary dilation.
Alpha 2, found in platelets, vascular smooth muscle, nerve termini and pancreatic islets, with effects including platelet aggregation, vasoconstriction and inhibition of norepinephrine release and of insulin secretion.
