Final answer:
Alpha-1 adrenergic receptor stimulation by norepinephrine leads to contraction of the radial muscles in the iris, resulting in pupillary dilation or mydriasis. This is part of the sympathetic nervous system response. In contrast, the parasympathetic nervous system, via acetylcholine release, causes pupillary constriction.
Step-by-step explanation:
Alpha-1 stimulation by norepinephrine results in the contraction of the radial muscles of the iris. This action, governed by the sympathetic nervous system, causes pupillary dilation, also known as mydriasis. When the sympathetic postsynaptic neuron releases norepinephrine, it binds to the alpha-1 adrenergic receptors in the radial muscle fibers, leading to their contraction. The use of phenylephrine in a doctor's office can mimic this action, as it binds to the same receptors, causing a similar dilation effect on the pupils.
In contrast, the parasympathetic nervous system, through the action of acetylcholine (ACh), results in the contraction of the circular iris muscles, leading to pupillary constriction. The interaction between these two systems allows the body to regulate the size of the pupil in response to various stimuli, such as light levels, to protect the retina and optimize vision.