What is Hormones?
Daily physiological and behavioral patterns of activity observed in most vertebrates. Both of these systems interact and are great communication networks. Mammals, including man, are regulated by two integrated systems, nervous system, and endocrine system. The nervous system transmits its messages by means of electro-chemical impulses, which and where it is produced to the cells of the target organ, travel quickly to the muscles and glands, whereas the endocrine system employs chemical messengers, called hormones. Hormones help to maintain a constant environment inside the body, adjusting the amount of salt and water in our tissues, sugar in our blood, and salt in our sweat to suit particular conditions around us. Hormones produce both long-term changes and rhythmic changes. Most hormones command their target cells to do more or less of what they normally do. They do this by altering enzyme activity, either directly by taking control of enzyme synthesis or indirectly by initiating a series of reactions catalyzed by enzymes already present in the cytoplasm. Not only that, additionally almost all vertebrate hormones have some effect on the permeability of plasma membrane to ions and other dissolved substances.
Exocrine and Endocrine Glands
There have mainly two types of glands in our body, exocrine and endocrine. Exocrine glands secrete their products into ducts or channels that carry them either to the outside of the body or into body cavities. Salivary glands and sweat glands are common examples of exocrine glands. Endocrine glands empty their products —hormones— inside the body. They have no ducts; instead, their secretions flow directly into blood-stream. These glands are also said to be ductless glands or glands of internal secretion. Pituitary, thyroid and adrenals are common endocrine glands. However some glands like pancreas perform both exocrine and endocrine functions.
Nature of Hormones
Hormones do not belong to a single chemical group but are varied in their composition. They are medium-sized molecules and may be polypeptides, proteins, amines or steroids. The chemistry of a particular hormone is very comparable in all species although its effects may vary from animal to animal.
Steroid hormones are small lipid molecules that move freely through the lipid bilayer of a plasma membrane. Only their target cells contain the specific receptor molecules (large proteins) that allow hormones to regulate cellular activities. Each hormone interacts with a different type of receptor molecule. The hormone and the receptor form a complex that moves from the cytoplasm into the nucleus, where it attaches to a specific site on a chromosome. The hormone-receptor complex exerts an influence on gene activity at the site by triggering or suppressing transcription of the genetic code into m-RNA.
Non-steroid hormones, usually derived from proteins, exert control over their target cells by attaching themselves to receptor sites on the plasma membrane. The binding of a hormone to its receptor activates the enzyme adenylatecyclase. Once activated, this enzyme promotes the synthesis of cyclic AMP from ATP. The hormone is released rapidly after it is bound to the receptor and is quickly destroyed by enzymes.
How the Hormones Release?
Most of the hormones are released in short bursts, with little or no secretion between the bursts. When increasingly stimulated, an endocrine gland will release its hormone in more frequent bursts, and thus blood level of the hormone increases. In the absence of stimulation, bursts are minimal or subdued, and blood level of the hormone decreases. Regulation of secretion normally maintains homeostasis and prevents overproduction or underproduction of a particular hormone. Unfortunately, there are times when the regulating mechanisms do not operate properly, and hormonal levels are excessive or deficient. Hormone secretion is stimulated and inhibited by (1) signals from the nervous system, (2) chemical changes in the blood, and (3) other hormones.
When hormones are released in response to nervous stimulation, the control of the release is simple. If the gland is stimulated, hormone is released. If it is not stimulated, no hormone is released. The level of stimulation determines the level of response. The situation is slightly more complex when hormones are released in response to a chemical stimulus such as another hormone or glucose. The presence of the suitable chemical in the blood stimulates the release of the hormone. As the hormone levels rise, the amount of stimulating chemical in the blood drops. As a result, the endocrine gland receives less stimulation and so the hormone levels this kind of reaction mechanism is very common, and it gives a sensitive level of control which can be constantly adjusted to the needs of the body.
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