 Broadly speaking, the function of the sympathetic nervous system is to mobilize the body for action; the phrase often invoked to describe it is fight-or-flight.The function of the parasympathetic nervous system is to put the body in a state conducive to rest, regeneration, digestion, and reproduction; the phrase often invoked to describe it is "rest and digest" or "feed and breed".Both of these aforementioned systems use acetylcholine, but in different ways.