organic substance secreted by plants and animals that functions in the regulation of physiological activities and in maintaining homeostasis. Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them. The classical view of hormones is that they are transmitted to their targets in the bloodstream after discharge from the glands that secrete them. This mode of discharge (directly into the bloodstream) is called endocrine secretion. The meaning of the term hormone has been extended beyond the original definition of a blood-borne secretion, however, to include similar regulatory substances that are distributed by diffusion across cell membranes instead of by a blood system.
Hormonal regulation is closely related to that exerted by the nervous system, and the two processes have generally been distinguished by the rate at which each causes effects, the duration of these effects, and their extent; i.e., the effects of endocrine regulation may be slow to develop but prolonged in influence and widely distributed through the body, whereas nervous regulation is typically concerned with quick responses that are of brief duration and localized in their effects. Advances in knowledge, however, have modified these distinctions.
Nerve cells are secretory, for responses to the nerve impulses that they propagate depend upon the production of chemical transmitter substances, or neurohumors, such as acetylcholine and noradrenaline (norepinephrine), which are liberated at nerve endings in minute amounts and have only a momentary action. It now has been established, however, that certain specialized nerve cells, called neurosecretory cells, can translate neural signals into chemical stimuli by producing secretions called neurohormones. These secretions, which are often polypeptides (compounds similar to proteins but composed of fewer amino acids), pass along nerve-cell extensions, or axons, and are typically released into the bloodstream at special regions called neurohemal organs, where the axon endings are in close contact with blood capillaries (Figure 1A
). Once released in this way, neurohormones function in principle similar to hormones that are transmitted in the bloodstream and are synthesized in the endocrine glands.
The distinctions between neural and endocrine regulation, no longer as clear-cut as they once seemed to be, are further weakened by the fact that neurosecretory nerve endings are sometimes so close to their target cells that vascular transmission is not necessary (). There is good evidence that hormonal regulation occurs by diffusion in plants and (although here the evidence is largely indirect) in lower animals (e.g., coelenterates), which lack a vascular system.
The-release-of-neurohormones-from-neurosecretory-nerve-cellsFigure 1: The release of neurohormones from neurosecretory nerve cells.
Elements-in-a-generalized-mammalian-pituitary-glandFigure 2: Elements in a generalized mammalian pituitary gland.[Credits : Encyclopædia Britannica, Inc.]
Major-pathways-involved-in-the-biosynthesis-of-steroid-hormonesFigure 5: Major pathways involved in the biosynthesis of steroid hormones.[Credits : Encyclopædia Britannica, Inc.]
The-structures-of-plant-hormonesFigure 6: The structures of plant hormones.
Hormones-and-active-metabolites-bind-to-different-types-of-receptorsHormones and active metabolites bind to different types of receptors. Water-soluble molecules …
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