User:Powell Cat/Hormone

Article body
Completely active hormones can be released into the bloodstream (as seen in insulin and growth hormones, but some travel as prohormones that must be activated in specific cells through a series of activation steps that are commonly highly regulated.

Introduction and overview
Hormone producing cells are found in the endocrine glands, such as the thyroid gland, ovaries, and testes. Exocytosis and other methods of membrane transport are used to secrete hormones when the endocrine glands are signaled. 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.

Hormone-Behavior Interactions (part of section)
At the neurological level, behavior can be inferred based on hormone concentration, which in turn are influenced by hormone-release patterns; the numbers and locations of hormone receptors; and the efficiency of hormone receptors for those involved in gene transcription. Hormone concentration does not incite behavior, as that would undermine other external stimuli; however, it influences the system by increasing the probability of a certain event to occur.

Not only can hormones influence behavior, but also behavior and the environment can influence hormone concentration. Thus, a feedback loop is formed, meaning behavior can affect hormone concentration, which in turn can affect behavior, which in turn can affect hormone concentration, and so on. For example, hormone-behavior feedback loops are essential in providing constancy to episodic hormone secretion, as the behaviors affected by episodically secreted hormones directly prevent the continuous release of said hormones.

Comparison with Neurotransmitters
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, which often acts in micrometer-scale distances.
 * 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 signalling is an all-or-nothing (digital) action, whereas hormonal signalling is an action that can be continuously variable as it is dependent upon hormone concentration.

Neurohormones are a type of hormone that share a commonality with neurotransmitters. They are produced by endocrine cells that receive input from neurons, or neuroendocrine cells.

Binding proteins
Hormone transport and the involvement of binding proteins is an essential aspect when considering the function of hormones.

The formation of a complex with a binding protein has several benefits: the effective half-life of the bound hormone is increased, and a reservoir of bound hormones is created, which evens the variations in concentration of unbound hormones (bound hormones will replace the unbound hormones when these are eliminated). An example of the usage of hormone-binding proteins is in the thyroxine-binding protein which carries up to 80% of all thyroxine in the body, a crucial element in regulating the metabolic rate.

Topic:
A prohormone is a committed precursor of a hormone, usually having minimal hormonal effect by itself but rather circulating in the blood stream as a hormone in an inactivated form, ready to be activated later by post-translational modification. Examples of natural, human prohormones include proinsulin and pro-opiomelanocortin.

For peptide hormones, the conversion process from prohormone to hormone (pro-protein to protein) typically occurs after being exported to the endoplasmic reticulum and often requires multiple processing enzymes. Proamylin, which is cosecreted with proinsulin, requires the above three factors and an amidating monooxygenase.

References:

 * 1) Miller, Benjamin Frank (1997). Miller-Keane Encyclopedia & dictionary of medicine, nursing & allied health. Claire Brackman Keane (6th ed.). Philadelphia: Saunders. ISBN  0-7216-6278-1 . OCLC 36465055.
 * 2) "Amylin Proprotein Processing Generates Progressively More Amyloidogenic Peptides that Initially Sample the Helical State". Biochemistry. 47: 9900–9910. August 19, 2008 – via American Chemical Society.

____

____

____