The sympathetic nervous system is a part of the Peripheral nervous system which acts when there is a threatening stimulus. In this situation, the sympathetic nervous system acts by increasing heart rate, the activity of sweat glands, dilating pupils etc. And decreases the activity of actions that are less important in that emergency situation such as digestion. The parasympathetic nervous system is another part of the Peripheral nervous system which maintains the normal activity of the body and acts when an emergency situation is passed. It reduces heart rate to normal, constrict pupils, reduce sweating etc.