What is Hormone

Hormones

A hormone is a chemical signal produced in one part of the body that is stable enough to be transported in active form far from where it is produced and that typically acts at a distant site. There are three big advantages to using chemical hormones as messengers rather than speedy electrical signals (like those used in nerves) to control body organs. First, chemical molecules can spread to all tissues via the blood (imagine trying to wire every cell with its own nerve!) and are usually required in only small amounts. Second, chemical signals can persist much longer than electrical ones, a great advantage for hormones controlling slow processes like growth and development. Third, many different kinds of chemicals can act as hormones, so different hormone molecules can be targeted at different tissues. For all these reasons, hormones are excellent messengers for signaling widespread, slow-onset, long duration responses.

Hormones, in general, are produced by glands, most of which are controlled by the central nervous system. Because these glands are completely enclosed in tissue rather than having ducts that empty to the outside, they are called endocrine glands (from the Greek, en don, within). Hormones are secret-ed from them directly into the bloodstream (this is in contrast to exocrine glands, which, like sweat glands, have ducts). Your body has a dozen principal endocrine glands, that together make up the endocrine system. 

The endocrine system and the motor nervous system are the two main routes the central nervous system (CNS) uses to issue commands to the organs of the body. The two are so closely linked that they are often considered a single system—the neuroendocrine system. The hypothalamus can be considered the main switchboard of the neuroendocrine system. The hypothalamus is continually checking conditions inside the body to maintain a constant internal environment, a condition known as homeostasis. Is the body too hot or too cold? Is it running out of fuel? Is the blood pressure too high? If homeostasis is no longer maintained, the hypothalamus has several ways to set things right again. For example, if the hypothalamus needs to speed up the heart rate, it can send a nerve signal to the medulla oblongata, or it can use a chemical command, causing the adrenal gland to produce the hormone adrenaline, which also speeds up the heart rate. Which com-mand the hypothalamus uses depends on the desired duration of the effect. A chemical message is typically far longer last-ing than a nerve signal.

The Chain of Command 

The hypothalamus issues commands to a nearby gland, the pituitary, which in turn sends chemical signals to the various hormone-producing glands of the body. The pituitary is suspended from the hypothalamus by a short stalk, across which chemical messages pass from the hypothalamus to the pituitary. The first of these chemical messages to be discovered 

was a short peptide called thyrotropin-releasing hormone (TRH), which was isolated in 1969. The release of TRH from the hypothalamus triggers the pituitary to release a hormone called thyrotropin, or thyroid: stimulating hormone and which travels to the thyroid and causes the thyroid gland to release thyroid hormones.

 Several other hypothalamic hormones have since been isolated, which together govern the pituitary. Thus, the CI\1 regulates the body's hormones through a chain of command. The "releasing" hormones made by the hypothalamus cat's! the pituitary to synthesize a corresponding pituitary horror': which travels to a distant endocrine gland and causes thiaet gland to begin producing its particular endocrine hornThe hypothalamus also secretes inhibiting hormones that keep the pituitary from secreting specific pituitary hormones.