The nervous system has several parts, but the autonomic nervous system is what controls a number of our behaviors through the flight or fight branch or the slow down branch. The fight or flight response, also called the sympathetic branch, is what causes someones heart to pound in fear or to sweat in a moment of stressful situation. The fight or flight response causes the body to produce more adrenaline and moves the blood faster so that the person can escape any danger that may be present.
While the sympathetic branch deals with fear, the parasympathetic branch tells the body to slow down and relax. This system plays a large role in preparing the body to rest and for digestion. It also plays a part in reproduction, crying, salvation and the release of bodily waste. Both of these behavioral responses to the environment and stimuli are part of the central nervous system. The central nervous system is made up of the brain and the spinal cord. The placement of these nerve centers is one of the reasons why people mention a shiver running down their spines when they are afraid or say that their mind is racing when faced with a problem. The systems are not normally controllable and happen on instinct, though some people are able to train their bodies to react specific ways during stressful situation.