Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intracellular and Intercellular Communications
Characteristics of the Endocrine System
General Properties

A characteristic feature of Multicellular Organisms is the presence of differentiated Tissues, which possess specialized Functions essential for the Organism's survival.

To coordinate tissue responses to changing environmental and internal conditions, Intercellular Communication mechanisms are required. Throughout evolution, two primary systems have developed to perform this function: The Nervous system, traditionally viewed as a hardwired signal-conduction network, and the Endocrine System, which utilizes a variety of mobile messengers known as Hormones. These hormones are secreted by specific glands and subsequently transported to act on adjacent or distant tissues.

It is now evident that these two regulatory systems remarkably overlap and intertwine. Nervous regulation plays a vital role in endocrine function; for instance, postganglionic Cells of The adrenal medulla synthesize and secrete adrenaline, while vasopressin is synthesized in the Hypothalamus, transported via axons to the posterior pituitary, and secreted from there. Similarly, many Neurotransmitters (such as catecholamines, dopamine, and acetylcholine) share similarities with hormones in their modes of synthesis, release, transport, and MECHANISM OF ACTION. Thus, catecholamines act as neurotransmitters in one tissue and as hormones in another. Another example of the interplay between hormones and neurotransmitters is that certain metabolites of adrenal Steroid Hormones have recently been shown to modulate y-aminobutyric acid (GABA) receptor function in the Brain, much like barbiturates do. Finally, many hormones—including Insulin, ACTH, vasoactive intestinal peptide (VIP), Somatostatin, thyrotropin-releasing hormone (TRH), and cholecystokinin—have been detected in the brain. It remains unclear whether these compounds are synthesized within the brain itself or whether they function there as neuromodulators or neurotransmitters. Given that specific receptors for many of these hormones have been identified in the brain, it is highly likely that they exert physiological effects on this tissue.

The word "hormone" is derived from the Greek word meaning "to set in motion" or "to urge on." According to the classical definition, a hormone is a substance synthesized in one tissue, transported via the bloodstream, and acting upon another organ. However, this original definition is too narrow; it is now established that hormones can also act on neighboring cells within the same tissue (paracrine effect) as well as on the very cells in which they are synthesized (autocrine effect).