<P> The autonomic nervous system controls involuntary responses to regulate physiological functions . The brain and spinal cord from the central nervous system are connected with organs that have smooth muscle, such as the heart, bladder, and other cardiac, exocrine, and endocrine related organs, by ganglionic neurons . The most notable physiological effects from autonomic activity are pupil constriction and dilation, and salivation of saliva . The autonomic nervous system is always activated, but is either in the sympathetic or parasympathetic state . Depending on the situation, one state can overshadow the other, resulting in a release of different kinds of neurotransmitters . There is a lesser known division of the autonomic nervous system known as the enteric nervous system . Located only around the digestive tract, this system allows for local control without input from the sympathetic or the parasympathetic branches, though it can still receive and respond to signals from the rest of the body . The enteric system is responsible for various functions related to gastrointestinal system . </P> <P> The sympathetic system is activated during a "fight or flight" situation in which great mental stress or physical danger is encountered . Neurotransmitters such as norepinephrine, and epinephrine are released, which increases heart rate and blood flow in certain areas like muscle, while simultaneously decreasing activities of non-critical functions for survival, like digestion . The systems are independent to each other, which allows activation of certain parts of the body, while others remain rested . </P> <P> Primarily using the neurotransmitter acetylcholine (ACh) as a mediator, the parasympathetic system allows the body to function in a "rest and digest" state . Consequently, when the parasympathetic system dominates the body, there are increases in salivation and activities in digestion, while heart rate and other sympathetic response decrease . Unlike the sympathetic system, humans have some voluntary controls in the parasympathetic system . The most prominent examples of this control are urination and defecation . </P> <P> Diseases of the peripheral nervous system can be specific to one or more nerves, or affect the system as a whole . </P>

Parts of peripheral nervous system and their functions