Endocrine Regulation

WebLecture Topics

• The Endocrine System Components: Glands
• Hormones
• Hormone Secretion Regulation
• Hormones and Proper Metabolic Functions

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We've talked before about homeostasis, the tendency of an organism to try and keep its internal environment within certain boundaries. To that end, the organism's nervous system constantly monitors the temperature, concentration levels, and other indicators for changes that would take the organs out of "safe operating conditions". We've seen how this works in monitoring temperature and water absorption and elimination. In the next chapter, we'll look at how the body adapts when infections occur, but for now we turn to the finer controls required for normal cell function regulations like growth and response to exercise.

The Endocrine System Components: Glands

Hormones can be produced by a number of different glands in the human body; they can also be produced in some cases by nerve cells.

The hypothalamus at the base of the brain is sometimes considered the "master" endocrine gland, because it produces hormones that stimulate further hormone production in other endocrine glands, particularly the pituitary gland next to it, which is itself responsible for embryo growth and development and for transformation of reproductive organs to adulthood functions. The hypothalamus controls body temperature, sensations of hunger and thirst, sleep and other autonomic nervous system responses. Some very controversial studies have also claimed to indicate that the size of the hypothalamus varies by gender and stated sexual orientation.

Obviously, genetic defects that affect the hypothalamus can lead to developmental problems. A defect on chromosome 15 that truncates the strand can cause Prader-Willi syndrome (PWS) once in every 20000 live births or so. The lost genes control development of the hypothalamus, so the defect disrupts normal development and functioning of the hypothalamus that trigger a cascade of malfunctions in the pituitary gland as well. Children born with PWS have low muscle tone, are usually short, always hungry, and often have learning disabilities.

Other hormone-producing glands work in isolation, in pairs, or in systems to regulate specific functions.

• The thyroid controls metabolic rates; lack of thyroid secretions causes a slow-down of all metabolic functions (myxedema).
• The parathyroid glands regulate the amount of calcium in the blood and interstitial fluids, which in turn controls muscle relaxation and cramping.
• The pancreas secretes insulin, which controls the amount of glucose in the blood. An insufficiency of insulin results in diabetes, a life-threatening disease in its extreme form. Hypoglycemia (too much sugar) results from too much insulin secreted in response to sugar in the diet; the patient may lapse into coma.
• The adrenal glands help the body adjust to stress by releasing epinephrine, norepinephrine, and adrenalin (the "fight or flight" response).

Hormones

Hormones themselves come in four flavors.

Chemical Group	Source	Examples	Purpose
Steroids	adrenal cortex, testis, ovary, placenta	Progesterone, testosterone, estradiol, molting hormone	Regulate menstrual periods Initiate exoskeleton molting
Amino Acid derivatives	Thyroid, adrenal gland	epinephrine,	Neural response
Proteins and peptides	hypothalamus   Pancreas	oxytocin, antidiuretic hormones insulin	Thirst, sugar content in blood
Fatty acid derivatives		Prostaglandins

Hormones Secretion Regulation

Most hormone secretion in vertebrates is continuous; the amount is controlled by negative feedback mechanisms, where the effects are opposite to the stimulus (more of the stimulant chemical leads to less of the hormone). Hormones usually target specific cells, which may either absorb the hormone directly by diffusion through the cell membrane, or which have surface molecules that detect the presence of the hormone and fire off a messenger internally to start the metabolic function governed by the hormone. One of the primary examples of these "secondary messengers" is cyclic AMP.

1. The endocrine gland releases a hormone, which travels through the body to
2. a target tissue, where the hormone binds to then surface receptor of a cell.
3. The surface receptor breaks with the protein molecule, which is then free to
4. attach to enzyme (adenyl cyclase), which is activated to
5. convert AMP to cyclic AMP, which then activates any one of several
6. protein enzymes (kinases), which may activate/inhibit other enzymes.

Since each protein kinase reacts with a specific enzyme, different metabolic functions can be affected. Sometimes the cell uses ions instead of cyclic AMP as secondary messengers.

Prostaglandins are a special case of hormone, since they usually affect nearby tissues, and often interact with other hormones. They control or affect many different metabolic processes.

Hormones in invertebrates can regulate growth, development, metabolic process rates, reproduction, pigmentation, and in insects, molting. In vertebrates, they affect growth, development, fluid balance, metabolic process rates, and reproduction.

Hormones and Proper Metabolic Functions

Excess (hyper-secretion) or lack (hypo-secretion) of hormones can result in serious problems for the organism. For example, the hypothalamus can release both inhibitory and stimulating hormones which control the function of the pituitary gland. Among the hormones released by the pituitary is the growth hormone, oxytocin (which controls the onset of labor and lactation), and hormones which control other hormone-releasing glands. This cascade affect means a disfunction of the hypothalamus can affect many different systems.

Note that proper hormonal function is not enough for certain kinds of development; the hormones released by the endocrine system are only a small group of chemicals released by different organs in the body. Children who have functional pituitaries but lack emotional support will not thrive and grow as well as children who are cuddled and praised, because they do not experience the triggers that cause other important hormone or neurotransmitter releases.

We are only beginning to realize the complexity of hormone interactions for proper development, or even for response to pain and depression. In the 1970s, Norman Cousins, who was a well-respected journalist and editor, developed what doctors concluded was a fatal heart disease. In order to handle his pain, Cousins discovered that sustained genuine laughter (ten minutes or more) was as effective as a heavy morphine dose to relieve pain. His description of his experience, Anatomy of an Illness led to the founding of the Cousins Center for Psychoneuroimmunology at UCLA in 1980, an institute with the specific mission of discovering and identifying the relationship between brain functions and response to disease. The nearly simultaneous discovery of endorphins by researchers in England and the US forced biologists to recognize that their description of hormone functions was far too simple, and that behavior and emotional states have a huge impact on disease response and immune system functions.

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