The Autonomic Nervous System:

The brain functions we have discussed so far are primarily voluntary functions that enable us to think and to function in relation to the world outside of our bodies. There is, however, a part of the nervous system that is concered with regulating internal bodily functions that are generally considered to be involuntary or automatic, such as digestion, heart rate, blood pressure, glandular secretions, and tear production.  This part of the nervous system is called the autonomic nervous system.  The autonomic nervous system is made up of two parts with opposite functions:  the sympathetic nervous system and the parasympathetic nervous system.

autonomic nervous system =

sympathetic nervous system + parasympathetic nervous system

The parasympathetic nervous system is sometimes referred to as the "vegetative" system.  It  nurtures and rebuilds the body.  The parasympathetic nervous system is active during relaxation and rest.  Activity in the parasympathetic nervous system decreases heart rate and blood pressure and promotes digestion.

The sympathetic nervous system tends to be overly active in people who are anxious, including people with "anxiety disorders."  Overactivity in the sympathetic nervous system may predispose people to high blood pressure, stroke, and heart problems.  Meditation and relaxation tend to shut off the activity in the sympathetic nervous system, and turn on activity of the parasympathetic nervous system.  When the parasympathetic nervous system is active, digestive activity increases and  heart rate and blood pressure decrease.

Check out the following link to increase your comprehension of the autonomic nervous system:

The Autonomic Nervous System - Succinct summary of information about the structure and function of the autonomic nervous system, prepared by Eric Chudler http://faculty.washington.edu/chudler/auto.html