Anatomy and Physiology: Feedback: Not Just for Guitars

Feedback: Not Just for Guitars

One of the most important things to understand is that hormones do not work alone. To regulate the release of hormones, the endocrine organ that's reweleasing the hormone needs to be able to adjust its output by responding to chemical changes, be it altered calcium ion levels, in the case of calcitonin, or by altering levels of another hormone, as in estrogen inhibiting a hormone that regulates gamete production.

Regardless of the chemicals involved, the mechanism is pretty much the same: Hormones are regulated by feedback loops—receptor, control center, and effector. With the few exceptions, such as oxytocin and labor contractions, hormones are regulated by negative feedback loops, which is ideal in terms of maintaining equilibrium.

In terms of hormones, the effector gets another name: target organ or tissue. Every hormone is produced by an organ, some of which also belong to other systems (such as the pancreas, which you might remember is also digestive). Receptors on the surface of cells in the endocrine organ act to receive information from the environment and then signal the control center. The organ that produces the hormone is the control center; more specifically, operons in the nuclei of cells in that organ act as the control center, turning a gene on or off and thus instructing the cell to either make or stop making a hormone.

The hormone, in turn, after being picked up by veins leaving the endocrine organ, travels all over the body thanks to the cardiovascular system. Eventually some of the hormone makes it to an artery entering the target organ (or tissue), which in turn responds to the hormone. That response could be as simple as increasing the growth rate, as in the case of growth hormone, or it could mean the production of another hormone!

For example, a hormone from the hypothalamus (I'll call it hormone A) targets the hypophysis (pituitary gland), and the pituitary gland then produces its own hormone (hormone B). That pituitary hormone (hormone B), of course, targets another organ, which in turn produces its own hormone (hormone C)! As if that isn't bad enough, that last hormone (hormone C) is part of a feedback loop and targets the hypothalamus. It turns out that hormone C inhibits the production of hormone A by the hypothalamus, the one that started this whole mess off!

When the production of hormone A is inhibited, the level of hormone B drops, which in turn stops (or at least slows down) the production of hormone C. The low level of hormone C is not able to inhibit production of hormone A, which is then produced, and the whole merry-go-round starts all over again! To be fair, hormones, especially hormone C in this scenario, will affect other organs and tissues: regulating ion levels, blood pressure, you name it! It is, nonetheless, important to understand the role of hormones (such as hormone A and B) in the regulation of hormones themselves!

Types of Hormones

Hormones are classified in different ways. One way depends on the location of the target cells, which divides them into autocrines, paracrines, and endocrines (see Figure 18.1). Autocrines, simply put, are hormones that act only on the cell that produces them (auto = self); the T-cell costimulator interleuken-2 is one example, which is released in conjunction with antigen attachment to trigger T cell function. Paracrines are local hormones that act on neighboring tissues (para = next to); histamine—which is released in response to an allergen, such as the toxin in a mosquito bite—causes swelling only in the area next to the allergen. Endocrines, the hormones of this section, are hormones that act on tissues and organs that may be quite distant from the organ producing the hormone, and they travel via blood vessels.

Figure 18.1Hormones differ by the location of the target cells.

Exocrine glands, by contrast, release their products directly into ducts. Lacrimal glands (see The Senses), for example, release tears into tear ducts. You have also seen such glands in the mouth (salivary glands, see The Digestive System) and the skin (sudoriferous, sebaceous, and ceruminous glands. The pancreas, my favorite, is both endocrine (insulin and glucagon) and exocrine (digestive enzymes), one of many organs that fit into more than one system.

Hormones also differ in terms of their basic chemical structure. Biogenic amines are modified amino acids and are the smallest of the hormones; some examples of biogenic amines are two thyroid hormones, T3 and T4, and epinephrine (adrenaline). Eicosanoids come from arachidonic acid (one of the fatty acids) and are primarily local hormones, such as prostaglandin. Peptides (short amino acid chains) and proteins (longer amino acid chains, with more complex structure) are the largest of the hormone types; most endocrine hormones are from this group, including insulin, glucagon, and all hormones from the hypothalamus and hypophysis. Steroids are lipid-based hormones that are all related to the four-ring structure of cholesterol; these hormones include the sex hormones, estrogen and testosterone.

Before going too far, I should let you know a little bit about naming hormones. Some of the names get very long. Because of this, physiologists found a simple way to speak in a sort of shorthand. Most hormones have the word hormone, so a capital H always means “hormone.” Since many hormones were first discovered in other animals, a handful of hormones have a lowercase h, which means “human.” In this way you can easily distinguish human growth hormone (hGH) from its analog in cows, bovine growth hormone (bGH).

Some hormones stimulate an organ to act or to release a hormone (S = stimulating, as in FSH = follicle-stimulating hormone) or both, whereas others inhibit the release of a hormone (I = inhibiting, as in PIH = prolactin-inhibiting hormone). Stimulating hormones are a product of the pituitary, whereas hormones from the hypothalamus that stimulate the release of pituitary hormones are called releasing hormones (R = releasing, as in GnRH = gonadotropic releasing hormone). The following table provides a quick summary of shortcuts used to name hormones.

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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