Physiology and Pathophysiology of the Human Sympathoadrenal Neuroendocrine System

THE sympathoadrenal system is the prototype neuroendocrine system. Epinephrine is a hormone in the traditional sense. It is secreted from the adrenal medullae into the circulation and transported to its various target cells. In contrast, norepinephrine is primarily a neurotransmitter. It is released from axon terminals of sympathetic postganglionic neurons and deposited directly at innervated target cells. The adrenal medullae also release some norepinephrine1 and can become major sources of norepinephrine in plasma under certain conditions such as hypoglycemia. Since plasma norepinephrine concentrations occasionally rise to levels that produce biologic effects, 2 norepinephrine may also act as a hormone.The catecholamines . . .