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# There are two main systems that transmit physiological information over longer distances within the bodies of animals: the nervous system and the endocrine system.

After looking at the endocrine system in general, we discuss the types of hormones (chemical messengers), their features, and their functions in an organism. We conclude by noting that some hormonal mechanisms have remained unchanged over a long period of evolutionary time and now occur in the same form in a wide range of different organisms, even though they have acquired different physiological functions. 
 
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<center>http://www.ai.rug.nl/~lambert/papers/thesis-schomaker-1991/gifs/p3-fig1.gif</center>

## Homeostasis depends on the ability of different biological components in an organism—cells, tissues, or organs—to communicate with each other.  


For homeostasis to work, receptors need to send information to a comparator and the comparator needs to send information to effectors. Indeed, transmitting physiological information from one tissue or organ to another, sometimes over very long distances, is essential for the existence of complex multicellular organisms.  Although mechanisms for long-distance internal communication have evolved in all multicellular organisms, they are most sophisticated in animals. Because animals tend to be able to move, their external environment can change rapidly, which means they especially need to be able to obtain, process, and transmit information about the external environment in order to respond in an adaptive fashion. 


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## Two structurally and functionally overlapping systems of internal communication have evolved in animals. 


The nervous system is a high-speed, high-performance information transmission and processing system. Its cells are specialized to transmit electrical impulses over long distances and to send chemical signals between adjacent cells. The nervous system is most obviously involved in the immediate control of animal behavior.  

The endocrine system is a slower information transmission system that involves cells and tissues that produce chemical signals called hormones, which are compounds specialized for long-range communication between tissues. The endocrine system also affects animal behavior, but it tends to have more global effects, and effects that are generally slower than those of the nervous system. Hormones also play a key role in the homeostatic regulation of many physiological factors and the control of developmental processes. Whereas nervous systems are solely found in animals, both plants and animals use hormones for internal communication.  


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<center>http://www.untamedscience.com/science/wp-content/uploads/2013/10/Endocrine.jpg</center>

## The endocrine system transmits chemical signals throughout the entire organism. 


 Endocrine cells are usually assembled into organs called endocrine glands, which typically include some mechanism for releasing the hormones they produce directly into the circulatory system (bloodstream) for efficient delivery to the rest of the organism. Because they are released into the bloodstream, hormones exert their action over long distances, reaching targets throughout the body.  



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<center>https://moodle.kent.ac.uk/external/pluginfile.php/4201/mod_book/chapter/78/Paracrine.png</center>


## Not all chemical signals exert influence over long distances; paracrine signals are chemical signals that behave much like hormones but only in the immediate neighborhood of the cells that produce them. 

An example of a paracrine, or local regulator, is histamine, which causes inflammation (a tissue response that helps protect organisms from invasion by foreign materials or microorganisms).   When a damaged tissue is inflamed, it becomes red, hot, and swollen. 

Histamine does this by causing small blood vessels (capillaries) in the tissue to dilate (expand) and become leaky. This allows protective blood proteins and white blood cells to leave the capillaries more easily and begin to repair the injured area. Although local responses to histamine are protective, histamine responses sometimes can spread across the body, causing such problems as hay fever. 

A different kind of close-range chemical signaling is synaptic signaling, which involves the release of chemical signals called neurotransmitters from nerve cells. Although neural communication is long distance, synaptic signaling is specialized for close-range communication between pairs of cells that essentially touch each other. 

Although hormones, local regulators, and neurotransmitters are considered functionally separate classes of chemical signals, the lines separating them are not always distinct. For example, some chemicals function as hormones in one context and as neurotransmitters in a different context. In addition, these categories of chemical signals have many molecular mechanisms in common. 


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## Although the number and diversity of hormones is enormous, they can be classified broadly into two major groups. 

Lipid-soluble hormones are primarily types of steroids (a kind of lipid). The most notable steroids are the sex steroids, including androgens and estrogens, which are involved in the expression of secondary sex characteristics, the regulation of reproductive physiology, and the control of sexual behavior.

Water-soluble hormones include peptides and other amino-acid derivatives; these are sometimes called peptide hormones.  Because water-soluble hormones are large electrically charged molecules, they cannot pass through cell membranes and must act by binding to receptors on the target cell’s surface.  

As discussed previously, one advantage of intracellular signal transduction is that a small signal can be greatly amplified. Hormones are generally released in small quantities that spread through the body, resulting in a very small concentration in target tissues. Signal transduction cascades amplify these weak signals significantly.  In contrast to water-soluble hormones, steroids can pass through the cell membrane; therefore, steroid receptors are located in the cell’s cytoplasm. Specific receptor molecules bind to the steroid once it enters the cell, forming a hormone-receptor complex that can then enter the nucleus and affect gene transcription. 


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## The fact that hormones circulate to every cell in an organism brings into focus several features of their physiological action.  

First, although every cell in an organism may be exposed to the same hormones, only specific target cells will respond to a given hormone. This can be explained by the fact that only target cells have receptor proteins for a particular hormone, not because the hormones themselves specifically target particular cells. 

Second, different kinds of cells can respond to the same hormone in different ways.  Some hormones have highly specific functions and influence only a single kind of target cell, but other hormones have a variety of different effects on different kinds of target cells.  When released into the bloodstream, epinephrine initiates a series of physiological responses—increasing heart rate, constricting blood vessels in the gut, and so forth—because different kinds of cells in different tissues have receptors for it, and the specific response of each cell depends on its type.  

The third general feature is that many hormones function in antagonistic pairs; one hormone will have one effect on a system, while a different hormone will have the opposite effect. For example, calcitonin produces several effects that reduce the level of calcium in the blood, while parathyroid hormone increases calcium in the blood by producing exactly the opposite responses in the same tissues.  Antagonistic control offers two advantages over a single hormone.  The first advantage is speed—releasing an antagonistic hormone to stop or down-regulate the system is faster than waiting for the effects of a single hormone to disappear.  The second advantage is precision—a system that depends on the ratio of one hormone to another can be precisely regulated even when the absolute concentrations of the hormones are very low, which is usually the case. 

The fourth general feature is that different hormones may have the same effect on a particular kind of tissue. For example, epinephrine causes blood sugar levels to increase by increasing production of glucose; glucagon has exactly the same effect. The difference is that glucagon functions in long-term control of blood sugar levels, whereas epinephrine is involved in short-term emergency responses. 


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## One quite remarkable feature of hormones is that virtually identical chemical signals are often found in widely divergent species. 

For example, a hormone called thyroxin is found in animals as different from one another as clams, frogs, and humans. Apparently, many fundamental chemical signals evolved very early in evolutionary history, then remained unchanged, in spite of the fact that the organisms using these compounds diverged and diversified. Even more interesting is the fact that, although the structures of these chemical signals have remained the same, their physiological functions have changed dramatically. Thyroxin seems to be associated with feeding in invertebrates; it helps control the transition from tadpole to adult in amphibians; and in mammals, it helps regulate cellular metabolism.  It appears that because certain kinds of molecules are good at conveying information, their chemical structures do not change over evolutionary time; they simply acquire different physiological functions. 


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<center>http://sciencelearn.org.nz/var/sciencelearn/storage/images/contexts/digestion-chemistry/sci-media/images/hormone-action/489628-6-eng-NZ/Hormone-action.jpg</center>


# One approach to treating a hormonal disorder could be to increase or decrease the amount of hormone in the bloodstream. Based on your acquired knowledge  of cell signaling mechanisms, can you think of a fundamentally different kind of approach that could be taken? 


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Sources: [1](https://en.wikipedia.org/wiki/Homeostasis) 


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Image Sources:

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https://cdn.brainpop.com/health/bodysystems/endocrinesystem/screenshot1.png
http://www.ai.rug.nl/~lambert/papers/thesis-schomaker-1991/gifs/p3-fig1.gif
https://s-media-cache-ak0.pinimg.com/originals/75/73/a5/7573a592f1442d3cb781688f1fe9e339.jpg
http://www.untamedscience.com/science/wp-content/uploads/2013/10/Endocrine.jpg
https://moodle.kent.ac.uk/external/pluginfile.php/4201/mod_book/chapter/78/Paracrine.png
http://images.slideplayer.com/24/7517841/slides/slide_18.jpg
https://classconnection.s3.amazonaws.com/42/flashcards/4466042/png/preview009-1488FA3330B4EB5932B.png
http://editions.sciencetechnologyaction.com/lessons/6/95/fig2.jpg
http://sciencelearn.org.nz/var/sciencelearn/storage/images/contexts/digestion-chemistry/sci-media/images/hormone-action/489628-6-eng-NZ/Hormone-action.jpg
Bird [Giphy](https://giphy.com/gifs/blue-5lDik5jPpRw2Y)
```


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