Hormone

Among the substances that can be considered hormones, are eicosanoids (e.g. prostaglandins and thromboxanes), steroids (e.g. oestrogen and brassinosteroid), amino acid derivatives (e.g. epinephrine and auxin), protein or peptides (e.g. insulin and CLE peptides), and gases (e.g. ethylene and nitric oxide).

In vertebrates, hormones are responsible for regulating a wide range of processes including both physiological processes and behavioral activities such as digestion, metabolism, respiration, sensory perception, sleep, excretion, lactation, stress induction, growth and development, movement, reproduction, and mood manipulation.

[6] Water-soluble hormones (such as peptides and amines) generally act on the surface of target cells via second messengers.

Lipid soluble hormones, (such as steroids) generally pass through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei.

Hormone secretion occurs in response to specific biochemical signals and is often subject to negative feedback regulation.

For instance, high blood sugar (serum glucose concentration) promotes insulin synthesis.

Lipid-soluble hormones must bond to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes.

Other hormones, called prohormones, must be activated in certain cells through a series of steps that are usually tightly controlled.

For example, the hormone auxin is produced mainly at the tips of young leaves and in the shoot apical meristem.

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.

[citation needed] Arnold Adolph Berthold was a German physiologist and zoologist, who, in 1849, had a question about the function of the testes.

[12][13] Although known primarily for his work on the Theory of Evolution, Charles Darwin was also keenly interested in plants.

[14][15][16] British physician George Oliver and physiologist Edward Albert Schäfer, professor at University College London, collaborated on the physiological effects of adrenal extracts.

[17][18] Though frequently falsely attributed to secretin, found in 1902 by Bayliss and Starling, Oliver and Schäfer's adrenal extract containing adrenaline, the substance causing the physiological changes, was the first hormone to be discovered.

They knew that the pancreas was involved in the secretion of digestive fluids after the passage of food from the stomach to the intestines, which they believed to be due to the nervous system.

The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, described as signal transduction, 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.

The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism.

They are considered to be "local" because they possess specific effects on target cells close to their site of formation.

Hormones are ligands, which are any kinds of molecules that produce a signal by binding to a receptor site on a protein.

Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.

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.

[42] Three broad stages of reasoning may be used to determine if a specific hormone-behavior interaction is present within a system:[citation needed] Though colloquially oftentimes used interchangeably, there are various clear distinctions between hormones and neurotransmitters:[43][44][36] Neurohormones are a type of hormone that share a commonality with neurotransmitters.

[36] In this pathway, the result of the electrical signal produced by a neuron is the release of a chemical, which is the neurohormone.

Left: A hormone feedback loop in a female adult. (1) follicle-stimulating hormone , (2) luteinizing hormone , (3) progesterone , (4) estradiol . Right: auxin transport from leaves to roots in Arabidopsis thaliana
Different types of hormones are secreted in the human body, with different biological roles and functions.
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.
Blood glucose levels are maintained at a constant level in the body by a negative feedback mechanism. When the blood glucose level is too high, the pancreas secretes insulin and when the level is too low, the pancreas then secretes glucagon. The flat line shown represents the homeostatic set point. The sinusoidal line represents the blood glucose level.
This is a diagram that represents and describer what hormones are and their activity in the bloodstream. Hormones flow in and out of the bloodstream and are able to bind to Target cells to activate the role of the hormone. This is with the help of the bloodstream flow and the secreting cell. Hormones regulate: metabolism, growth and development, tissue function, sleep, reproduction, etc. This diagram also lists the important hormones in a human body.