The autonomic nervous system resides outside the central nervous system and controls not voluntary muscles as the peripheral nervous system does, but the heart, glands, and organs with smooth muscles (not under voluntary control) such as the intestines.
The autonomic nervous system consists of two divisions, the sympathetic and parasympathetic that often act in opposition to each other. The sympathetic system prepares the body for immediate action (fight or flight), at the expense of body regulating functions such as digestion.
These two branches use two different neurotransmitters — norepinephrine (noradrenaline) for the sympathetic branch and acetylcholine for the parasympathetic branch — that tend to have opposite effects on the target organs.For example, norepinephrine speeds up the heart while acetylcholine slows it. Norepinephrine also dilates the pupil and lung bronchi, decreases digestive functions, and inhibits bladder contraction and blood flow to the genitals.
Many of these actions are mediated through the adrenal medulla. However, overstimulation of the sympathetic system from excessive stress is hard on the body and tends to be associated with heart disease and other stress-related chronic illnesses.
Because social conflicts can also trigger the sympathetic system, the chronic stress of modern life may result in sympathetic overstimulation and its associated long-term effects.