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 norepinephrine 1 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 . . .