The Autonomic Nervous System

All the nerves in your body that extend beyond your brain and spine belong to the peripheral nervous system. The autonomic nervous system (ANS), which controls the organs and muscles inside the body, is part of that network. Most of the time, we aren’t aware of the workings of the ANS, since it usually works in an involuntary, reflexive manner, widening or constricting your blood vessels, raising or lowering your heart rate, or prompting your intestines to move and digest your food.

The ANS is most important in two situations: It responds immediately in emergencies that cause stress, requiring us to "fight" or "take flight" (that is, run away). The ANS also works during nonemergencies, relaying messages that allow us to "rest and digest." Of course, this system is constantly acting to maintain the body’s normal internal working, which is why it’s an integral part of the set point.

Source: George L. Blackburn, M.D., Ph.D., "Break Through Your Set Point: How to Finally Lose the Weight You Want and Keep It Off," 2008