The autonomic nervous system is divided into three parts: 1) the sympathetic nervous system; 2) the parasympathetic nervous system; and 3) the enteric nervous system. The parasympathetic and sympathetic nervous systems usually have opposing actions. For example, while the sympathetic nervous system controls the “fight or flight” responses, which increase the heart rate under stress, the parasympathetic nervous system will slow the heart rate. The enteric nervous system consists of nerve cells in the gastrointestinal tract.