The autonomic nervous system can further be subdivided into the sympathetic and parasympathetic nervous systems.The sympathetic nervous system is responsible for the fight-or-flight response and generally results in activity being increased. The neurotransmitter used is noradrenaline. The parasympathetic nervous system is responsible for relaxing responses (rest and digest) e.g. when the heart and breathing rate is reduced after a period of vigorous exercise and the neurotransmitter is acetylcholine.