Final answer:
Beta-2 stimulation leads to the contraction of radial fibers in the iris smooth muscle and results in pupillary dilation, whereas the parasympathetic system causes pupillary constriction.
Step-by-step explanation:
The student's question relates to the effects of beta-2 stimulation on the ciliary muscle and pupillary size. When the sympathetic nervous system is activated, norepinephrine is released and binds to adrenergic receptors, specifically the beta-2 receptors which are present in the radial fibers of the iris. This does not directly affect the ciliary muscle, which is involved in lens shape for focusing, but rather causes the radial fibers of the iris to contract, leading to pupillary dilation, also known as mydriasis. This sympathetic response allows more light to enter the eye in low-light conditions. Conversely, the parasympathetic nervous system releases acetylcholine to the circular muscles of the iris, causing pupillary constriction, or miosis, which is part of the pupillary light reflex to decrease light entry under bright conditions.