Stress vs. Rest
The autonomic nervous system controls conditions throughout your body, automatically, in response to your environment. It has two parts: the parasympathetic nervous system, which guides activities that occur when the body is at rest, and the sympathetic nervous system, which kicks in to respond to stress.
At Rest: The Parasympathetic Nervous System
The autonomic nervous system (ANS) is divided into two networks of nerves. The network that sends messages to the brain resulting in the regulation of our bodily activity is vast and complex. The involuntary processes that go on throughout our days fall under the jurisdiction of the parasympathetic nervous system. This network keeps our digestive system chugging along without our thinking about it much. It signals our eyes to produce adequate tears and our sex organs to respond to arousal. The parasympathetic system's mission is to keep our body's functions in balance under normal, restful conditions.
On Alert: The Sympathetic Nervous System
The other part of the ANS, the sympathetic nervous system, regulates the high-alert response when we face a stressor. The nerves send messages that it's not time to waste energy on digestion or going to the bathroom, it's time for a burst of energy that you will need for the fight-or-flight response. The nerves of the SNS, which originate in the spinal cord, send messages at lightning speed to effect the changes in hormone levels and cardiovascular conditions needed to respond to a stressor.