General Information
The original definition of a hormone, given by Starling in 1905, was: "a hormone is a substance produced by endocrine glands that serves to transmit signals through the blood to target organs."
Today, this definition is considered somewhat inaccurate, but more recent definitions also do not encompass the entire significance of hormones as specific information carriers.
One of the main nuances is that there is no distinction between hormones and other signaling molecules, such as cytokines, growth factors, or autacoid compounds.
It seems that an exact definition is even impossible, since some cytokines and growth factors, such as the cytokines erythropoietin, lipocalin-2, and asprosin, or fibroblast growth factor 23, act like hormones under certain conditions.
History
At the turn of the 20th century, an important medical discipline was formulated that significantly expanded our understanding of various regulatory processes in the body — endocrinology. Research was conducted on the effects of extracts from the adrenal cortex, thyroid gland, spleen, and pituitary gland.
Compounds with regulatory functions have been called hormones since 1905, after the term was first used by physiologist Ernest Henry Starling in June of that year during a lecture for the Royal College of Physicians in London.
In his study of the effect of duodenal secretions on the pancreas, he discovered the compound secretin, which caused an effect even after all the nerves were removed from the duodenum, thus showing that the signal travels not through nerves but through the blood.
Thus, a hormone was defined as a compound produced in secretory tissue and transported by the circulatory system to target tissues, where it causes functional changes.
This concept of a hormone later appeared in textbooks with the formulation: "a hormone is a substance produced by endocrine glands that serves to transmit signals through the blood to target organs."
However, hormones can be produced in tissues other than specialized endocrine glands, and their transport to the site of action is not related to the circulatory system.
In plants, every cell is capable of producing hormones. Hormones are indeed information carriers, but there is a vast number of signaling compounds that we do not consider hormones.
Alternative Formulations
Hormone — an organic substance secreted by plants and animals that is involved in the regulation of physiological activity and the maintenance of homeostasis.
Hormone — a biological compound used by multicellular organisms to organize, coordinate, and control the functions of their cells and tissues.
These chemicals can control many processes, from metabolism to behavior, and are essential for the organism's survival and reproduction.
Sport
There is great interest in studying the mechanisms of the endocrine system not only among healthcare professionals but also among representatives of various sports disciplines. Today it is known that the impact of hormones on sports performance is quite significant.
Some substances from the group of hormones can have a powerful effect on increasing the performance of the body. Therefore, some hormonal drugs are of great value to sports pharmacology
Neurohormones
Hormonal regulation is closely related to regulation by the nervous system, and these two processes usually differ in speed, duration, and degree of action.
That is, the effects of endocrine regulation can develop slowly but have a prolonged and widespread influence throughout the body, while nervous regulation is usually associated with rapid responses that are short-lived and localized in their spectrum of action.
Modern scientific achievements have somewhat changed these perceptions.
Nerve cells are secretory, as responses to nerve impulses depend on the production of chemical transmitters or neurotransmitters, such as acetylcholine and norepinephrine. Neurotransmitters are released at nerve endings in small amounts and have only a short-term effect.
However, it has been established that some specialized nerve cells, called neurosecretory cells, can convert nerve signals into chemical stimuli in the form of substances called neurohormones.
Neurohormones are often polypeptides — compounds similar to proteins but consisting of fewer amino acids. Neurohormones travel along the nerve cell processes, or axons, and are usually released into the bloodstream in special areas called neurohemal organs, where axons are in close contact with blood capillaries.
Released in this way, neurohormones act similarly to hormones transmitted in the bloodstream and synthesized in endocrine glands.
The distinctions between nervous and endocrine regulation are no longer as clear-cut as they once seemed, further blurred by the fact that neurosecretory nerve endings sometimes are so close to their target cells that vascular transmission becomes unnecessary.
There is convincing evidence that hormonal regulation occurs by diffusion in plants and lower animals (e.g., cnidarians) that lack a vascular system
Peptide Hormones
Peptide hormones are water-soluble molecules composed of amino acids linked by amide bonds. They exist as single polypeptide chains or as multimeric proteins ranging from 3 to 200 amino acids.
Peptide hormones act on cell surface receptors and therefore must be capable of exocytosis after translation. These hormones vary widely in size and structure, but several families of peptides should be noted.
Glycoproteins are a group of glycosylated heterodimers of alpha and beta subunits. The alpha subunit consists of 92 amino acids and is common to all peptides of this class.
This family includes the thyroid-stimulating hormone (TSH) and the gonadotropins: follicle-stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG).
Proopiomelanocortin is a family consisting of hormones derived from a single protein product that is cleaved at various lysine residues into several active peptides, including adrenocorticotropic hormone (ACTH) and beta-lipotropin.
The posterior pituitary produces two peptide hormones that differ by only two amino acids: oxytocin and antidiuretic hormone (ADH). Both are nonapeptides with a disulfide bridge, packaged with carrier proteins called neurophysins.
Insulin and insulin-like growth factors (IGFs) are also part of the peptide hormone family.
Insulin consists of 51 amino acids and is a hormone composed of two polypeptide chains linked by a disulfide bond, whereas IGFs are single polypeptide chains.
The parathyroid hormone family includes parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), as well as the calcitonin family, which includes calcitonin, adrenomedullin, and amylin.
Neuroendocrinology
Modern research has shown that steroid hormones are capable of directly influencing gene expression in the central nervous system. Several hormone receptors, such as estrogen and progesterone receptors, have been found in the hypothalamus and pituitary gland.
There are also mechanisms for influencing the release of neurotransmitters and neuropeptides by steroid hormones. The hypothalamus synthesizes and secretes neurohormones into the bloodstream that stimulate or inhibit the secretion of pituitary hormones. The hypothalamus receives regulatory inputs from both the central nervous system and from steroid feedback from the blood.
Most endocrine functions are regulated by the hypothalamic-pituitary axis. Hormones secreted by the hypothalamus include corticotropin-releasing hormone (CRH), gonadotropin-releasing hormone (GnRH), and growth hormone-releasing hormone (GHRH).
Each of these hormones acts on specific cells in the pituitary gland to control the secretion of various pituitary hormones.
