During which phase in the control of the digestive system would bicarbonate and bile be stimulated? cephalic phase gastric phase intestinal phase Norepinephrine is the neurotransmitter released by which fibers? sympathetic postganglionic fibers interneurons of the enteric nervous system both sympathetic and parasympathetic preganglionic fibers parasympathetic postganglionic fibers Enteric interneurons that are inhibitory to smooth muscle use which of the following neurotransmitters? Ach (acetylcholine)