Depends on what part of the body your actually talking about. The sympathetic and parasympathetic are two different nerve pathways to organs in your body. Depending on situation of the body, nerves will take one of the two paths to organs. In times of stress the sympathetic nerve pathway is used. This has the effect of increasing blood flow to muscles, the brain etc, all the things you need to either fight or run from a lion.
Both the sympathetic and parasympathetic are part of of the autonomic nervous system. In the sympathetic system, the nerves originate in the lumbar and thoracic regions and mediate involuntary responses. In the parasympathetic system, the nerves originate in the midbraine, hindbrain and sacral region. Functions include constriction of pupils, slowing heartbeat, and stimulation of digestive glands.
The difference between the sympathetic and parasympathetic nerves, lies in the fact that they are often in opposition to one another. Stimulation by the parasympathetic system decreases the contractions of the heart, while the sympathetic does the opposite. If both systems are required for an organs function, the two systems, parasympathetic and sympathetic, act simultaneously, although one may operate faster than the other. This action of the two nervous systems has been compared to both the accelerator and brake pedals of a car, being pushed down at the same time. These two systems are highly intricate, and require a finely tuned nervous system that medical science is still learning about.
