digplanet beta 1: Athena
Share digplanet:

Agriculture

Applied sciences

Arts

Belief

Business

Chronology

Culture

Education

Environment

Geography

Health

History

Humanities

Language

Law

Life

Mathematics

Nature

People

Politics

Science

Society

Technology

Different types of hormones are secreted in the body, with different biological roles and functions.

A hormone (from Greek ὁρμή, "impetus") is any member of a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behaviour. Hormones have diverse chemical structures that include eicosanoids, steroids, amino acid derivatives, peptides, and proteins. The glands that secrete hormones comprise the endocrine signaling system. The term hormone is sometimes extended to include chemicals produced by cells that affect the same cell (autocrine or intracrine signalling) or nearby cells (paracrine signalling).

Hormones are used to communicate between organs and tissues to regulate physiological and behavioral activities, such as digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress, growth and development, movement, reproduction, and mood.[1][2] Hormones affect distant cells by binding to specific receptor proteins in the target cell resulting in a change in cell function. When a hormone binds to the receptor, it results in the activation of a signal transduction pathway. This may lead to cell type-specific responses that include rapid non-genomic effects or slower genomic responses where the hormones acting through their receptors activate gene transcription resulting in increased expression of target proteins.

Hormone synthesis may occur in specific tissues of endocrine glands or in other specialized cells. Hormone synthesis occurs in response to specific biochemical signals induced by a wide range of regulatory systems. For instance, ionized calcium concentration affects PTH synthesis, whereas glucose concentration affects insulin synthesis. Regulation of hormone synthesis of gonadal, adrenal, and thyroid hormones is often dependent on complex sets of direct influence and feedback interactions involving the hypothalamic-pituitary-adrenal (HPA), -gonadal (HPG), and -thyroid (HPT) axes.

Upon secretion, certain hormones, including protein hormones and catecholamines, are water soluble and are thus readily transported through the circulatory system. Other hormones, including steroid and thyroid hormones, are lipid soluble; to allow for their widespread distribution, these hormones must bond to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes. Some hormones are completely active when released into the bloodstream (as is the case for insulin and growth hormones), while others must be activated in specific cells through a series of activation steps that are commonly highly regulated. The endocrine system secretes hormones directly into the bloodstream typically into fenestrated capillaries, whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissue.

Overview[edit]

Further information: Signal transduction

Hormonal signaling involves the following steps:[3]

  1. Biosynthesis of a particular hormone in a particular tissue
  2. Storage and secretion of the hormone
  3. Transport of the hormone to the target cell(s)
  4. Recognition of the hormone by an associated cell membrane or intracellular receptor protein
  5. Relay and amplification of the received hormonal signal via a signal transduction process: This then leads to a cellular response. The reaction of the target cells may then be recognized by the original hormone-producing cells, leading to a down-regulation in hormone production. This is an example of a homeostatic negative feedback loop.
  6. Breakdown of the hormone.

Hormone cells are typically of a specialized cell type, residing within a particular endocrine gland, such as the thyroid gland, ovaries, and testes. Hormones exit their cell of origin via exocytosis or another means of membrane transport. The hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal.

Regulation[edit]

The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors that influence the metabolism and excretion of hormones. Thus, higher hormone concentration alone cannot trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.

Hormone secretion can be stimulated and inhibited by:

  • Other hormones (stimulating- or releasing -hormones)
  • Plasma concentrations of ions or nutrients, as well as binding globulins
  • Neurons and mental activity
  • Environmental changes, e.g., of light or temperature

One special group of hormones is the tropic hormones that stimulate the hormone production of other endocrine glands. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland, the thyroid, which increases output of thyroid hormones.

To release active hormones quickly into the circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus.

Eicosanoids are considered to act as local hormones.

Receptors[edit]

The left diagram shows a steroid (lipid) hormone (1) entering a cell and (2) binding to a receptor protein in the nucleus, causing (3) mRNA synthesis which is the first step of protein synthesis. The right side shows protein hormones (1) binding with receptors which (2) begins a transduction pathway. The transduction pathway ends (3) with transcription factors being activated in the nucleus, and protein synthesis beginning. In both diagrams, a is the hormone, b is the cell membrane, c is the cytoplasm, and d is the nucleus.

Most hormones initiate a cellular response by initially binding to either cell membrane associated or intracellular receptors. A cell may have several different receptor types that recognize the same hormone but activate different signal transduction pathways, or a cell may have several different receptors that recognize different hormones and activate the same biochemical pathway.

Receptors for most peptide as well as many eicosanoid hormones are embedded in the plasma membrane at the surface of the cell and the majority of these receptors belong to the G protein-coupled receptor (GPCR) class of seven alpha helix transmembrane proteins. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, often involving phosphorylation or dephosphorylation of various other cytoplasmic proteins, changes in ion channel permeability, or increased concentrations of intracellular molecules that may act as secondary messengers (e.g., cyclic AMP). Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.

For steroid or thyroid hormones, their receptors are located inside the cell within the cytoplasm of the target cell. These receptors belong to the nuclear receptor family of ligand-activated transcription factors. To bind their receptors, these hormones must first cross the cell membrane. They can do so because they are lipid-soluble. The combined hormone-receptor complex then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific DNA sequences, regulating the expression of certain genes, and thereby increasing the levels of the proteins encoded by these genes.[4] However, it has been shown that not all steroid receptors are located inside the cell. Some are associated with the plasma membrane.[5]

Effects[edit]

A variety of exogenous chemical compounds, both natural and synthetic, have hormone-like effects on both humans and wildlife. Their interference with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body can change the homeostasis, reproduction, development, and/or behavior, similar to endogenously produced hormones.[6]

Hormones have the following effects on the body:

A hormone may also regulate the production and release of other hormones. Hormone signals control the internal environment of the body through homeostasis.

Chemical classes[edit]

As hormones are defined functionally, not structurally, they may have diverse chemical structures. Hormones occur in multicellular organisms (plants, animals, fungi, brown algae and red algae). These compounds occur also in unicellular organisms, and may act as signaling molecules,[7][8] but there is no consensus if, in this case, they can be called hormones.

Animal[edit]

Further information: List of human hormones

Vertebrate hormones fall into three main chemical classes:

Compared with vertebrate, insects and crustaceans possess a number of structurally unusual hormones such as the juvenile hormone, a sesquiterpenoid.[9]

Plant[edit]

Plant hormones include abscisic acid, auxin, cytokinin, ethylene, and gibberellin.

Therapeutic use[edit]

Many hormones and their analogues are used as medication. The most commonly prescribed hormones are estrogens and progestogens (as methods of hormonal contraception and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.

A "pharmacologic dose" or "supraphysiological dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful, though not without potentially adverse side effects. An example is the ability of pharmacologic doses of glucocorticoids to suppress inflammation.

Hormone-behavior interactions[edit]

At the neurological level, behavior can be inferred based on: hormone concentrations; hormone-release patterns; the numbers and locations of hormone receptors; and the efficiency of hormone receptors for those involved in gene transcription. Not only do hormones influence behavior, but also behavior and the environment influence hormones. Thus, a feedback loop is formed. For example, behavior can affect hormones, which in turn can affect behavior, which in turn can affect hormones, and so on.

Three broad stages of reasoning may be used when determining hormone-behavior interactions:

  • The frequency of occurrence of a hormonally dependent behavior should correspond to that of its hormonal source
  • A hormonally dependent behavior is not expected if the hormonal source (or its types of action) is non-existent
  • The reintroduction of a missing behaviorally dependent hormonal source (or its types of action) is expected to bring back the absent behavior

Comparison with neurotransmitters[edit]

There are various clear distinctions between hormones and neurotransmitters:

  • A hormone can perform functions over a larger spatial and temporal scale than can a neurotransmitter.
  • Hormonal signals can travel virtually anywhere in the circulatory system, whereas neural signals are restricted to pre-existing nerve tracts
  • Assuming the travel distance is equivalent, neural signals can be transmitted much more quickly (in the range of milliseconds) than can hormonal signals (in the range of seconds, minutes, or hours). Neural signals can be sent at speeds up to 100 meters per second.
  • Neural signaling is an all-or-nothing (digital) action, whereas hormonal signaling is an action that can be continuously variable as dependent upon hormone concentration

See also[edit]

References[edit]

  1. ^ Neave N (2008). Hormones and behaviour: a psychological approach. Cambridge: Cambridge Univ. Press. ISBN 978-0521692014. Lay summaryProject Muse. 
  2. ^ "Hormones". MedlinePlus. U.S. National Library of Medicine. 
  3. ^ Nussey S, Whitehead S (2001). Endocrinology: an integrated approach. Oxford: Bios Scientific Publ. ISBN 978-1-85996-252-7. 
  4. ^ Beato M, Chavez S and Truss M (1996). "Transcriptional regulation by steroid hormones". Steroids 61 (4): 240–251. doi:10.1016/0039-128X(96)00030-X. PMID 8733009. 
  5. ^ Hammes SR (2003). "The further redefining of steroid-mediated signaling". Proc Natl Acad Sci USA 100 (5): 21680–2170. doi:10.1073/pnas.0530224100. PMC 151311. PMID 12606724. 
  6. ^ Crisp TM, Clegg ED, Cooper RL, Wood WP, Anderson DG, Baetcke KP, Hoffmann JL, Morrow MS, Rodier DJ, Schaeffer JE, Touart LW, Zeeman MG, Patel YM (1998). "Environmental endocrine disruption: An effects assessment and analysis". Environ. Health Perspect. 106 (Suppl 1): 11–56. doi:10.2307/3433911. PMC 1533291. PMID 9539004. 
  7. ^ Lenard J (1992). "Mammalian hormones in microbial cells". Trends Biochem. Sci. 17 (4): 147–50. doi:10.1016/0968-0004(92)90323-2. PMID 1585458. 
  8. ^ Janssens PM. "Did vertebrate signal transduction mechanisms originate in eukaryotic microbes?". Trends in Biochemical Sciences 12: 456–459. doi:10.1016/0968-0004(87)90223-4. 
  9. ^ Heyland A, Hodin J, Reitzel AM (2005). "Hormone signaling in evolution and development: a non-model system approach". Bioessays 27 (1): 64–75. doi:10.1002/bies.20136. PMID 15612033. 

External links[edit]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Hormone — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.

4203045 news items

 
Newsmax
Mon, 30 Mar 2015 07:03:45 -0700

Hormone replacement therapy was a standard treatment for women to reduce menopause symptoms until a 2002 study warned about possible side effects, decreasing its use. However, more research has shown the positive effects and standard hormone ...

News-Medical.net

News-Medical.net
Tue, 31 Mar 2015 01:48:45 -0700

Research has discovered a role for prolactin, the hormone that stimulates milk production in nursing mothers, in the bond between parents. The study relied on hormone analyses of urine from cotton-top tamarins, a small, endangered monkey native to ...

The News Journal

The News Journal
Sun, 29 Mar 2015 18:30:00 -0700

A new editorial, released last week in the Journal of the American Geriatrics Society, makes the case that “disease mongering” created a surge in the prescribing and the use of testosterone and human growth hormone in the United States. So what exactly ...
 
Newsmax
Fri, 27 Mar 2015 09:07:30 -0700

Hormone replacement therapy (HRT) became a popular treatment for women who were going through menopause beginning in the 1950s. The therapy uses synthetic hormones to replace natural hormones lost in menopause. HRT was believed to reduce ...

Today.com

Today.com
Thu, 26 Mar 2015 10:24:00 -0700

It's unclear what specific types of hormones Jolie might be taking, but without hormone replacement therapy, she'd cope with the typical symptoms of menopause: hot flashes, vaginal dryness, insomnia and mood swings. Because she's experiencing ...

Newsmax

Newsmax
Fri, 27 Mar 2015 10:26:15 -0700

Stunning TV star and health promoter Suzanne Somers still turns heads wherever she goes. She holds onto her youthful beauty and enthusiasm at age 68, thanks to hormone therapy. "I think it's the new way to age," she announced on her website, praising ...

Medical Xpress

Medical Xpress
Mon, 30 Mar 2015 05:52:29 -0700

The hormone oxytocin is made at different levels in different people and it plays a role in regulating social behavior. A new University of Virginia study involving brain imaging finds that people with naturally higher levels of oxytocin in their blood ...

ABC News

ABC News
Wed, 18 Mar 2015 11:45:00 -0700

Turns out that dogs could have evolved into man's best friend thanks to the "love hormone" oxytocin, according to a new study. Dogs given extra doses of oxytocin -- the same hormone linked to pair-bonding in humans -- were better at performing simple ...
Loading

Oops, we seem to be having trouble contacting Twitter

Support Wikipedia

A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia. Please add your support for Wikipedia!

Searchlight Group

Digplanet also receives support from Searchlight Group. Visit Searchlight