Final answer:
The sympathetic nervous system causes contraction of the smooth muscles of the upper eyelid, leading to the opening of the eyelids, which is similar to its action of causing pupillary dilation in low light conditions.
Step-by-step explanation:
The effect of the sympathetic nervous system on the upper eyelid is that it causes a contraction of the smooth muscles, leading to the opening of the eyelids. This action is part of the body’s "fight or flight" response, which prepares the body for alertness and acute stress. The neurotransmitter norepinephrine, when released by the sympathetic nervous system, binds to adrenergic receptors in the smooth muscle fibers, resulting in muscle contraction and eyelid elevation. This is similar to its effect on the pupillary dilation, or mydriasis, where norepinephrine causes the radial fibers of the iris to contract, leading to pupil dilation and allowing more light to enter the eye in low-light conditions.