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Autonomic Nervous System

The autonomic nervous system controls the insides of the body: the viscera or gut. It carries information about the inside of the body to the CNS and controls the action of internal organs, including the gut, the heart, the secretion of epinephrine (adrenalin) and norepinephrine (noradrenalin) from the medulla (middle part) of the adrenal gland, etc.

The autonomic nervous system plays an essential role in keeping the body's internal environment (temperature, salt concentration, blood sugar, oxygen and carbon dioxide level in blood, etc) in proper balance, a condition called homeostasis. The autonomic nervous system also plays a major part in emotional experience and expression. When you are emotionally excited, the body shows many changes: blood pressure and heart beat increase, mouth is often dry, stomach has "butterflies" in it. These and other body actions are controlled by the autonomic nervous system.

The autonomic nervous system also has two divisions: the sympathetic division and the parasympathetic division. These two divisions have antagonistic (opposing) effects on the internal organs they innervate (send nerves to = act on).

The sympathetic division, shown at the left, is the emergency system. It prepares the body to put out energy and to protect it from effects of injury. It shuts the gut down, speeds up the heart, increases blood pressure, dilates (makes bigger) the pupils of the eyes, makes more glucose (blood sugar) available in the blood for energy, etc. Cannon described these reactions as preparation for fight or flight (running away).

The parasympathetic division, shown at the right, is the "housekeeping" division. It acts to replace and recover from the activities of living. Its action is (almost always) the opposite of the sympathetic division. It activates the gut for digestion, slows the heart rate, decreases the blood pressure, etc.