Non-PRO inglés al español Medicina Medicina (general) .

The autonomic nervous system is composed of two complementary branches acting, in the main, in opposition to each other. These two branches are called the sympathetic and parasympathetic nervous system. The heartbeat, for example, normally occurs at a rate kept within certain bounds by the balance struck between these two branches of the autonomic nervous system. However, under the influence of physical stress (exertion, effort) or emotional stress (fear, anger, excitement), the sympathetic nervous system will override the parasympathetic system and the heart rate will speed up. Not unlike the fight or flight syndrome, the body will react to compensate for exigent circumstance.