Final answer:
The neurotransmitter associated with the parasympathetic nervous system is Acetylcholine. It causes a decrease in heart rate and promotes digestion by binding to muscarinic receptors, whereas norepinephrine, linked to the sympathetic nervous system, has the opposite effect.
Step-by-step explanation:
The neurotransmitter associated with the parasympathetic nervous system response, which includes an increase in digestion and a decrease in heart rate, is Acetylcholine (ACh). This neurotransmitter is released by the parasympathetic postganglionic fibers and binds to muscarinic receptors on target organs. When ACh binds to these receptors on the heart, it causes the cardiac cells to hyperpolarize, resulting in a slower heart rate. Meanwhile, in digestive organs, ACh release promotes increased digestion. In contrast, the release of norepinephrine in the sympathetic nervous system results in increased heart rate and blood pressure, which are opposite effects of the parasympathetic response. Therefore, the correct option is 1) Acetylcholine.
It is also important to note that incorrect pairings such as acetylcholine decreasing digestion or norepinephrine being responsible for parasympathetic effects are commonly misunderstood. Acetylcholine actually promotes digestion and norepinephrine is associated with the sympathetic nervous system, which increases heart rate and blood pressure.