How does the autonomic nervous system work?

There are two branches of the autonomic nervous system (ANS) -- the sympathetic branch and the parasympathetic branch -- which act in opposition to each other. For example, when the sympathetic branch is activated in response to stressors, the parasympathetic branch is inactive, or suppressed. When the parasympathetic branch is activated, the sympathetic branch is suppressed.

Both branches of the ANS originate from the hypothalamus of the emotional motor system of the central nervous system (CNS). They branch out to interface with virtually every organ, blood vessel and sweat gland in your body. The ANS, for example, is at work when a scary event causes your hair to stand on end. When a fear signal is sent from your mind/brain down the "wires" of your ANS, little muscles at the base of each hair follicle are stimulated to raise your hair! Goosebumps occur by the same mechanism.