ABSTRACT
Acetylcholine (ACh) is the neurotransmitter released from postganglionic parasympathetic nerves ending on sinoatrial (SA) and atrioventricular node myocytes. There is some parasympathetic innervation of atrial myocytes, but sparse parasympathetic innervation of ventricular myocytes. The chronotropic effect of parasympathetic activity is due to actions of ACh on SA node myocytes. The net result is that diastolic depolarization starts from a more negative level, rises more slowly and must rise to a higher threshold to induce an action potential. This combination of effects results in a longer interval between beats and a reduced heart rate. Norepinephrine is the sympathetic neurotransmitter and is released at sympathetic nerve endings on myocytes. Sympathetic nerves innervate all heart muscle cells. Also, sympathetic stimulation of the adrenal medulla results in the release of mostly epinephrine and some norepinephrine, which circulate as hormones and interact with cardiac myocytes.
