How the Nervous System Works
During intense emotion, the whole organism appears to be involved, but many, of the most significant reactions may be traced to the autonomic nervous system. The autonomic system contains nerve groups that are located outside the brain and function with some independence of the brain and central nervous system. This somewhat self-regulated (autonomous) system is divided into two parts. The sympathetic system is most active during emotional states. The craniosacral division, known as the parasympathetic system, controls functions that go on during quiet states-digestion, for example. Typically, the sympathetic system works in opposition to the parasympathetic.
Thus if one system accelerates the activity of some body organ, the other system inhibits the functioning of the organ.
In emotions the sympathetic system accelerates the heartbeat, whereas the parasympathetic checks the rate of heartbeat. Secretions of the adrenal gland are accelerated by the sympathetic system. Many physiological changes observed during certain emotions are responses to the sympathetic division of the autonomic nervous system: speeding of the heartbeat, increase in rate of respiration, increased perspiration, increased supply of epinephrine (adrenalin) in the bloodstream, concentration of the blood supply in the skeletal muscles, and increase in the amount of blood sugar released from the liver.