ABSTRACT
Nowhere in the human body is the intimate relationship that exists between the nervous system and the endocrine system more apparent than in the connection between the sympathetic nervous system itself and the adrenal medulla (the sympathoadrenal system). This concept was postulated as early as Aristotle’s time, with the term ‘‘neurohumor’’ (a word demonstrating the involvement of both systems) used to describe substances released by a system for use at a distant site. Even today this definition can be superficially used to describe the regulation of homeostasis by the catecholamine neurotransmitters (1). The sympathetic nervous system allows for rapid adjustment to change by elaboration of the transmitter, which acts quickly at the site of release. On the other hand, the substance secreted by the endocrine system acts more slowly and at distant sites. Classically, the three naturally occurring catecholamines in the human subject that function as neurotransmitters for major portions of the central and autonomic nervous systems are norepinephrine (NE, noradrenaline, levarterenol), epinephrine (E, adrenaline), and dopamine (DA). One would expect that these compounds would participate in the metabolic changes associated with innumerable pathophysiological situations. NE is the transmitter released from postganglionic axons of the sympathetic nervous system as well as from central nervous system noradrenergic neurons. It exerts a direct effect where it is released at the junctional cleft, while E
elaborated from the adrenal medulla affects sites far distant from where it is secreted. Although it is postulated that a peripheral dopaminergic system exists, it has not been fully elucidated (2,3).
