Sympathetic is to release and as Parasympathetic to replenish in the autonomic nervous system. Sympathetic fibers release acetylcholine, substance P, neuropeptide Y, and norepinephrine. Parasympathetic tone holds the resting heart rate below its intrinsic rate. Sympathetic effects tend to last longer than parasympathetic effects. One reason is that norepinephrine can diffuse into the bloodstream without being broken down.
