Final answer:
Acetylcholine is released by parasympathetic neurons and it causes constriction of the pupil. Norepinephrine is released by the sympathetic nervous system. Nitric oxide is involved in vasodilation.
Step-by-step explanation:
The neurotransmitter that is released by parasympathetic neurons is acetylcholine. This contrasts with the sympathetic nervous system, where norepinephrine is the primary neurotransmitter released by postganglionic neurons. As for the pupil, it is the constriction of the pupil, or miosis, that is primarily mediated by acetylcholine.
Additionally, acetylcholine is the neurotransmitter released at the neuromuscular junction, which is essential for muscle contraction. When looking at vasodilation, nitric oxide is one of the chemicals that can induce this effect.