A hormone refers to any member of a class of signaling molecules, produced by glands transported by the circulatory system to target distant organs to regulate physiology and behavior.



Hormones have diverse chemical structures.



Hormone structures are mainly of three classes:









amino acid/protein derivatives-amines, peptides, and proteins



Different types of hormones are secreted with different biological roles and functions.



Hormones secreted by glands comprise the endocrine signaling system. 



Hormone sometimes refers to include chemicals produced by cells that affect the same cell, autocrine, or nearby cells, paracrine signaling.



Hormones communicate between organs and tissues.



Hormones provide physiological regulation and behavioral activities of: digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress induction, growth and development, movement, reproduction, and mood change.



Hormones bind to specific receptor proteins in the target cell, resulting in a change in cell function. 



Hormonal binding  to the receptor results in the activation of a signal transduction pathway that typically activates gene transcription, resulting in increased expression of target proteins.



Amino acid–based hormones are amines, peptide or protein hormones.



Amino acid–based hormones are water-soluble and act on the surface of target cells via second messengers.



Steroid  hormones are lipid-soluble, move through the plasma membranes of target cells to act within their nuclei.



Endocrine glands are the major source of hormones, but specialized cells in various other organs also secrete hormones. 



Biological signals from a wide range of systems cause hormone secretion: 


serum calcium concentration affects parathyroid hormone synthesis, blood sugar affects insulin synthesis, the amounts of gastric juice and pancreatic juice become the input of the small intestine, the small intestine secretes hormones to stimulate or inhibit the stomach and pancreas.



Regulation of hormone synthesis of gonadal hormones, adrenocortical hormones, and thyroid hormones often depends direct-influence and feedback interactions involving the hypothalamic-pituitary-adrenal (HPA), -gonadal (HPG), and -thyroid (HPT) axes.



Some secreted hormones (protein hormones and catecholamines) are water-soluble and are easily transported through the circulatory system. 



Steroid and thyroid hormones, are lipid-soluble and can achieve widespread distribution.



Steroid and thyroid hormones must bond to carrier plasma glycoproteins, such as thyroxine-binding globulin (TBG)) to form ligand-protein complexes. 



Some hormones are immediately fully  active when released into the bloodstream-insulin and growth hormones.



Other hormones are prohormones.



Prohormones must be activated in specific activation steps that are commonly highly regulated. 



The endocrine system secretes hormones directly into the bloodstream, usually  by fenestrated capillaries.



The exocrine system secretes its hormones indirectly using ducts. 



Paracrine function hormones 


diffuse through the interstitial spaces to nearby target tissue.



Recognition of a  hormone occurs by an associated cell membrane or intracellular receptor protein.



Relay and amplification of the received hormonal signal via 



A signal transduction process relays and amplifies hormonal signals leading to a cellular response. 



The homeostatic negative feedback occurs when reacting target cells are recognized by the original hormone-producing cells, leading to a downregulation in hormone production. 



Hormone gland cells are specialized cell types, residing within a particular endocrine gland.



The hormones from individual glands 


exit their cell of origin by exocytosis or other membrane transports. 



Hormones trigger a diverse range of systemic physiological effects, and different tissue types may also respond differently to the same hormonal signal.



Hormonal effects depend  on where and how they are released.



Hormone signaling types are:















Endocrine hormones act on the target cell after being released into the bloodstream.



Paracrine  hormones act on a nearby cell and does not have to enter general circulation.



Autocrine hormones  affect the cell type that secreted it and causes a biological effect.



Intracrine  hormones acts intracellularly on the cell that synthesized it.



Hormones have diverse chemical structures. 



Peptide hormones consist of a chain of amino acids that can range from just 3 to hundreds of amino acids. 



Peptide hormones include oxytocin and insulin.



Peptide hormones are encoded in DNA and can be modified by splicing and/or post-translational modification.



Peptide hormones are packed in vesicles and are hydrophilic.



Peptide hormones are soluble in water, and can only bind to receptors on the cell membrane.



Some hormones can bind to intracellular receptors through an intracrine mechanism.



Amino acid hormones are derived from amino acids.



Amino acid hormones are most commonly derived from the amino acid tyrosine.



Amino acid hormones are stored in vesicles, and include melatonin and thyroxine.



Eicosanoids hormones are derived from lipids: arachidonic acid, lipoxins and prostaglandins. 



Eicosanoid hormones include prostaglandin and thromboxane. 



Eicosanoid hormones are produced by cyclooxygenases and lipoxygenases. 



Eicosanoid hormones are hydrophobic and act on membrane receptors.



Eicosanoids are considered to act as local hormones, because they possess specific effects on target cells close to their site of formation. 



Eicosanoid hormones have a rapid degradation cycle, making sure they stay locally.



Steroid hormones are derived from cholesterol: sex hormones estradiol and testosterone and cortisol.



Steroid hormones contain four fused rings, and are lipophilic.



Steroid hormones can cross membranes to bind to intracellular nuclear receptors.



Hormones usually initiate a cellular response by binding to either cell membrane associated or intracellular receptors. 



Cells may have several different receptor types that recognize the same hormone, but activate different signal transduction pathways.



In addition, cells may have several different receptors able to recognize different hormones and activate via the same biochemical pathway.



Most peptide and many eicosanoid hormones are embedded in the plasma membrane surface of the cell.



 The majority of plasma membrane receptors belong to the G protein-coupled receptor (GPCR) class.



The G protein-coupled receptor (GPCR) class has seven alpha helix transmembrane proteins. 



The hormone and its receptor 


typically trigger  secondary effects within the cytoplasm of the cell.



This process is described as signal transduction.



Signal transduction often involves  phosphorylation or dephosphorylation of cytoplasmic proteins, changes in ion channel permeability, or increased concentrations of intracellular molecules that may act as secondary messengers.



Some hormones interact by an intracrine mechanism affecting intracellular receptors located in the cytoplasm or nucleus of cells:steroid or thyroid hormone receptors are located inside the cytoplasm of the target cell. 



Intracellular receptors belong to the nuclear receptor family of ligand-activated transcription factors, and hormones that bind to them 


must  cross the cell membrane. 



As noted above, such hormones can cross the cell  membrane because they are lipid-soluble. 



A hormone-receptor complex then moves across the nuclear membrane into the nucleus of the cell.



In the nucleus of the cell the hormone-receptor complex binds to specific DNA sequences, and regulates  the expression of certain genes, and thereby increasing the levels of the proteins encoded by these genes.



Some steroid receptors are located inside the cell, while others are associated with the plasma membrane.



Hormonal effects include: 



stimulation or inhibition of growth



wake-sleep cycle and other circadian rhythms



mood swings



induction or suppression of apoptosis 



activation or inhibition of the immune system



regulation of metabolism



preparation of the body for mating, fighting, fleeing, and other activities



preparation of the body for a new phases of life: puberty, parenting, and menopause



control of the reproductive cycle



hunger cravings



regulation in  the production and release of other hormones. 



Control the internal environment of the body through homeostasis.



Homeostatic negative feedback control mechanisms determine the rate of hormone  synthesis and secretion.



Higher hormone concentration alone cannot trigger the negative feedback mechanism. 



The negative feedback is triggered by overproduction of the effects of the hormone.



Hormone secretion can altered-stimulated and inhibited by:



Other hormones 



Plasma concentrations of ions or nutrients, as well as binding globulins



Neurons and mental activity



Environmental changes



Tropic hormones that stimulate the hormone production of other endocrine glands (TSH).



To release active hormones quickly into the circulation, hormones are produced and stored biologically as inactive forms of pre- or prohormones. 



These inactive forms can quickly be 


converted into their active hormone form in response to a particular stimulus.



Hormones are also regulated by receptor agonists, and act as ligands.



Hormone effects can be regulated, by competing ligands that bind to the same target receptor on the hormone, preventing hormone binding and preventing their effects.



Competing ligands are referred to as antagonists of the hormone.



Hormones and their structural and functional analogs are used as medication. 



The most commonly prescribed hormones: estrogens and progestogens as methods of hormonal contraception and as hormonal replacement therapy, thyroxine, steroids, and Insulin.



Steroid and vitamin D creams are used extensively in dermatological practice.



Hormones influence behavior, but behavior and the environment influence hormones in a feedback loop.



Hormones have functions over a larger range and temporal scale than can a neurotransmitter, as they can travel virtually anywhere in the circulatory system, whereas neural signals are restricted to pre-existing nerve tracts.



Neural signals can be transmitted more quickly, in the range of milliseconds, than can hormonal signals, which are in the range of seconds, minutes, or hours.



Hormonal signalling is an action that can be  variable as dependent upon hormone concentration.



Neurohormones are a type of hormone that are produced by endocrine cells that receive input from neurons, or neuroendocrine cells.



Both classic hormones and neurohormones are secreted by endocrine tissue.



Neurohormones are the result of a combination between endocrine reflexes and neural reflexes, creating a neuroendocrine pathway.



Endocrine pathways produce chemical signals in the form of hormones.



The neuroendocrine pathway involves the electrical signals of neurons.



The neuroendocrine pathway


 results  fro the electrical signal produced by a neuron with the release of a chemical neurohormone to the bloodstream.



Hormone transport and binding proteins are essential to the function of hormones. 



When hormones complex with a binding protein half-life of the bound hormone is increased, a reservoir of bound hormones is created, evening  the variations in concentration of unbound hormones.





Leave a Reply

Your email address will not be published. Required fields are marked *