Illustration shows the effects of the sympathetic and parasympathetic systems on target organs, and the placement of the preganglionic neurons that mediate these effects. The parasympathetic system causes pupils and bronchi to constrict, slows the heart rate, and stimulates salivation, digestion, and bile secretion. Preganglionic neurons that mediate these effects are all located in the brain stem. Preganglionic neurons of the parasympathetic system that are located in the sacral cause the bladder to contract. The sympathetic system causes pupils and bronchi to dilate, increases heart rate, inhibits digestion, stimulates the breakdown of glycogen and the secretion of adrenaline and noradrenaline, and inhibits contraction of the bladder. The preganglionic neurons that mediate these effects are all located in the spine.
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