Inervation of the heart

Answer 1

Although the heart is often sast to work Autonomously (which it does) it also receives signals from the Autonomic Nervous System. This system has to parts the Sympathetic and the Parasympathetic systems. The sympathetic system is activated when the body is in danger and stimulates the body to prepare for action (the "fight of Flight" response). This involves an increase in the heart rate. There is not name for the sypmathetic branches that innervate the heart by they increase the heart rate by releasing Nor-epinephrine onto the SA node (the region that controls the heart rate). It also stimulates the release of Epinephrin from the Adrenal Medulla in the Kidneys. Epinephrin and nor epinephrine both stimulate the increase in heart rate (the mechanisms stray too far into physiology to answer this question here). The parasympathetic innervation of the heart comes from a nerve called the Vagus Nerve (CN x). It acts like a limitter on the heart rate holding it at a steady pace, its action decreases when the sympathetic innervation starts.