Chapter 16 The Endocrine System Endocrine System interacts with the nervous system to coordinate and integrate the activity of body cells NS regulates the activity of muscles and glands via electrochemical impulses delivered by neurons those organs respond within milliseconds The endocrine system influences metabolic activity by means of hormones which are chemical messengers released into the blood to be transported throughout the body o Hormone long distance chemical signals that travel in blood or lymph throughout the body o Binding of a hormone to cell receptors initiates responses that typically occur after a lag period of seconds or even days But once initiated those responses tend to be much more prolonged than those induced by the NS Major processes of hormones reproduction maintenance of electrolyte water and nutrient balance of the blood and regulation of cell metab and energy balance Scientific study of hormones and the endocrine organs is called endocrinology The Endocrine System Overview Organs of this system are small and unimpressive Anatomical continuity typical of most organ systems doesn t exist in the endocrine system o Instead endocrine organs are widely scattered about the body Endocrine glands produce hormones and lack ducts o Release their hormones into the surrounding tissue fluid they typically have a rich vascular lymphatic drainage that receives their hormones o Most of the hormone producing cells in endocrine glands are arranged in cords and branching networks a situation that MAXIMIZES contact btw them and the capillaries surrounding them o Include pituitary thyroid parathyroid adrenal pineal thymus glands Also includes pancreas and gonads o Hypothalamus along with its neural functions produces and releases hormones so we can consider the hypothalamus a neuroendocrine organ o Other organs secrete hormones as well adipose cells release leptin and kidneys stomach small intestine and heart have hormone producing cells Autocrines chemicals that exert their effects on the same cells that secrete them o Ex certain prostaglandins released by smooth muscle cells cause those smooth muscle cells to contract Paracrines act locally but affect cell types other than those release the paracrine chemicals o Ex somatostatin released by one population of pancreatic cells inhibits the release of insulin by a diff population of pancreatic cells 1 Hormones Chemistry of Hormones o Hormones are chemical substances secreted by cells into the extracellular fluids that regulate the metabolic function of other cells in the body Can be classified as AA based or steroids Have lag times ranging from seconds to hours o Most hormones are AA based Vary in size Simple AA derivatives amines thyroxines constructed from the AA tyrosine Peptides PRO o Steroid based hormones Synthesized from CHOLESTEROL Of the hormones produced from the major endocrine organs gonadal and adrenocortical hormones are steroids o Eicosanoids biologically active lipids Include leukotrienes and prostaglandins Made from arachidonic acid are released by nearly all cell membranes Leukotrienes are signaling chemicals that mediate inflammation and some allergy reactions Prostaglandins have multiple targets and effects ranging from BP and the expulsive uterine contractions of birth to enhancing blood clotting pain and inflammation Effects are highly localized affecting only nearby cells generally act as paracines and autocrines and NOT AS TRUE HORMONES influence distant targets o Target cells influences activities of only certain tissue cells o Hormones bring about their characteristic effects on target cells by altering cell activity they or the rates of normal cell processes Precise response depends on target cell type o Hormonal stimulus typically produces one or more of the following changes Alters plasma membrane permeability or membrane potential or both by opening or closing ion channels Stimulates synthesis of PRO or regulatory molecules such as enzymes w in a cell Activates or deactivates enzymes Induces secretory activity Stimulates mitosis o 2 main mechanisms account for how a hormone communicates with its target cells that is how hormone receptor binding is harnessed to the intracell machinery needed for hormone action 2 Mechanisms of Hormone Action Mechanism usually depends on the chemical nature of the hormone and the cell location of the receptor o Water soluble hormones all AA based except thyroid act on receptors in the plasma membrane coupled via regulatory molecules called G PRO to 1 intracell 2nd messengers which mediate the target cell response o Lipid soluble hormones steroid and thyroid act on intracell receptors directly activating genes o Plasma membrane receptors and 2 nd messenger systems All AA based hormones except thyroid exert their signaling effects thorough intracell 2nd messengers generated when a hormone binds to a receptor on the plasma membrane Cyclic AMP used by neurotransmitters and olfactory and gustatory receptors Cyclic AMP signaling mechanism Hormone acting as first messenger binds to its receptor Hormone binding causes the receptor to change shape allowing it to bind a nearby inactive G PRO o G PRO is activated as the guanosine diphosphate GDP bound to it is displaced by the energy compound GTP o G PRO behaves like light switch off when GDP is bound and on when GTP is bound to it Activated G PRO binds the effector enzyme adenylate cyclase o Some G PRO Gs stimulate or inhibit Gi adenylate cyclase 3 o Eventually GTP bound to the G PRO is hydrolyzed to GDP and the G PRO become inactive once again If inactivated adenylate cyclase generates the 2nd messenger cAMP from ATP cAMP which is free to diffuse throughout the cell triggers a cascade of chemical reactions beginning with the activation of PRO kinase A o PRO kinase are enzymes that phosphorylate various PRO many of which are other enzymes o b c phosphorylation activates some of the PRO and inhibits others a variety of reactions may occur in the same target cell at the same time This type of intracell enzymatic cascade has a huge amplification effect Each activated adenylate cyclase generates large of cAMP molecules and a single kinase enzyme can catalyze hundreds of reactions as the reaction cascades through one enzyme intermediate after another the number product molecules increase dramatically at each step Sequence of reactions set into motion by cyclic AMP depends on the type of target cell the specific kinases it
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