sympathetic nervous system
one of the two divisions of the autonomic nervous system (ANS), the other being the parasympathetic nervous system. Anatomically it consists of preganglionic autonomic neurons whose fibers run from the thoracic and lumbar regions of the spinal cord to the chains of sympathetic ganglia. From these arise the fibers of postganglionic autonomic neurons, which innervate organs ranging from the eyes to the reproductive organs. It is defined functionally in terms of its ability to act as an integrated whole in affecting a large number of smooth muscle systems simultaneously, usually in the service of enhancing the fight-or-flight response. Typical sympathetic changes include dilation of the pupils to facilitate vision, constriction of the peripheral arteries to supply more blood to the muscles and the brain, secretion of epinephrine to raise the blood-sugar level and increase metabolism, and reduction of stomach and intestinal
activities so that energy can be directed elsewhere. Thus, the sympathetic nervous system tends to antagonize the effects of the parasympathetic nervous system. Also called sympathetic division.