18.2 Endocrine System
18.2 Endocrine System BMS 508
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This 6 page Class Notes was uploaded by Katherine Loiselle on Monday February 22, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 21 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 02/22/16
18.2 The endocrine system regulates physiological processes through the binding of hormones to receptors Endocrine System All the endocrine cells and tissues of the body that produce hormones or paracrine factors with effects beyond their tissues of origins Endocrine Cells Glandular secretory cells that release their secretions into the extracellular fluid o May affect only nearby cells (paracrine factors) or may affect cells throughout the body Organs and Tissues of the Endocrine System Organs with Primary Endocrine Functions o Hypothalamus Production of ADH, OXT, and regulatory hormones o Pituitary Gland Anterior Lobe Adrenocorticotropic hormone (ACTH) Thyroid-stimulating hormone (TSH) Growth hormone (GH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Melanocyte-stimulating hormone (MSH) Posterior Lobe Release of oxytocin (OXT) and antidiuretic hormone (ADH) o Thyroid Gland Thyroxin (T 4 Triiodothyronine (T 3 Calcitonin (CT) o Adrenal Glands Medulla Epinephrine (E) Norepinephrine (NE) Cortex Cortisol Corticosterone Aldosterone Androgens o Pancreatic Islets Insulin Glucagon o Pineal Gland Melatonin o Parathyroid Glands Located on posterior surface of the thyroid gland Parathyroid hormone (PTH) Organs with Secondary Endocrine Functions o Heart Atrial natriuretic peptide (ANP) Brain natriuretic peptide (BNP) o Thymus Undergoes atrophy during adulthood Thymosins o Adipose Tissue Leptin o Digestive Tract Secrets numerous hormones involved in the coordination of system functions, glucose metabolism, and appetite o Kidneys Erythropoietin (EPO) Calcitrol o Gonads Testes (Male) Androgens (especially testosterone) Inhibin Ovaries (Female) Estrogens Progesterone Inhibin Classes of Hormones Three groups based on chemical structure 1. Amino acid derivatives 2. Peptide Hormones 3. Lipid Derivatives Amino acid derivatives o biogenic amines o small o structurally related to amino acids o synthesized from amino acids tyrosine and tryptophan tyrosine thyroid hormones epinephrine norepinephrine dopamine tryptophan melatonin Peptide Hormones o Glycoproteins o Short polypeptides and small proteins Lipid Derivatives o Eicosanoids signaling molecules leukotrienes prostaglandins thromboxanes prostacyclins o Steroid hormones lipids structurally similar to cholesterol Differ in side chains attached to basic structure Liver absorbs and converts to soluble form to be excreted Secretion and Distribution of Hormones Hormones are typically released where capillaries are abundant o Quickly enter bloodstream for distribution o Within blood, may circulate freely Travel bound to special carrier proteins o Freely circulating hormone is inactivated when it diffuses out of the bloodstream and binds to receptors on target cells it is absorbed and broken down by cells of the liver or kidneys it is broken down by enzymes in the blood or interstitial fluids o Thyroid hormones and steroid hormones are bound o For each hormone, an equilibrium state exists between free and bound forms As free hormones are released and inactivated, bound hormones are released to replace them Mechanisms of Hormone Action Hormones o Coordinate cell, tissue, and organ activities on a sustained basis o Circulate in the extracellular fluid o Bind to specific receptors on plasma membranes or within target cells Modifies cellular activities by: altering membrane permeability activating or inactivating key enzymes changing genetic activity o To affect a target cell, a hormone must first interact with an appropriate receptor Hormone Receptor Protein molecule to which a particular molecule binds strongly Located either on the plasma membrane or inside the cell Hormones and Plasma Receptors Receptors for catecholamines, peptide hormones, and eicosanoids are in the plasma membrane of the target cells o Catecholamines and peptide hormones are lipid insoluble o Cannot penetrate plasma membrane o Bind to receptor proteins at outer surface of membrane (extracellular receptors) Eicosanoids are lipid soluble o Diffuse across plasma membrane to reach receptors on inner surface of membrane (intracellular receptors) First and Second Messengers Communication between hormone and cell First Messenger o A hormone that binds to a receptor on the plasma membrane surface Second Messenger o An intermediary molecule that appears due to a hormone-receptor interaction cyclic AMP (cAMP) cyclic GMP (cGMP) calcium ions When a small number of hormone molecules bind to membrane receptors, thousands of second messengers may appear in the cell o amplification o magnifies effect of a hormone on the target cell Arrival of single hormone may promote release of more than one type of second messenger or the production of a linked sequence of enzymatic reactions o receptor cascade o alters many aspects of the cell function at the same time Presence of absence of a hormone can also affect the nature and number of hormone receptor proteins in the plasma membrane o Down regulation a process in which the presence of a hormone triggers a decrease in the number of hormone receptors Cells become less sensitive to high levels of a particular hormone o Up regulation a process in which the absence of a hormone triggers increase in the number of hormone receptors cells become more sensitive to low levels of a particular hormone Link between first and second messengers o G protein an enzyme complex coupled to a membrane receptor holds GTP Activated when a hormone binds to its receptor at the membrane surface Three major patterns of response to activation Three major patterns of response to G protein activation o Increasing cAMP levels activated G protein activates adenylate cyclase adenylate cyclase converts ATP to cAMP cAMP functions as second messenger activates kinase o phosphorylates proteins opens ions channels activates enzymes Short lived cytoplasm contains phosphodiesterase (PDE) inactivates cAMP by converting it to AMP o Decreasing cAMP levels activated G protein stimulates PDE activity and inhibits adenylate cyclase activity levels of cAMP declines because cAMP breakdown accelerates while cAMP synthesis is prevented Without phosphorylation, key enzymes remain inactive o Calcium Ions Activated G protein can trigger the opening of calcium ion channels in the plasma membrane or the release of calcium ions from intracellular compartments G protein activates phospholipase C (PLC) triggers a receptor cascade that begins with production of diacylglycerol (DAG) and inositol triphosphate (IP ) from 3 membrane phospholipids IP3diffuses into cytoplasm o triggers release of calcium ions from intracellular reserves DAG and calcium ions activates protein kinase C (PKC) o leads to phosphorylation of calcium channel proteins o Positive feedback loop elevates intracellular calcium concentration o Calcium ions serve as messengers with calmodulin activates specific cytoplasmic enzymes Hormone and Intracellular Receptors Steroid hormones diffuse across the lipid part of the plasma membrane and bind to receptors in the cytoplasm or nucleus o activate or deactivate specific genes o Can alter rate of DNA transcription in the nucleus Changes pattern of protein synthesis Affects metabolic activity and structure of target cell Thyroid hormones cross plasma membrane by a transport mechanism o bind to receptors within the nucleus and on mitochondrion o activates specific genes or changes the rate of transcription affects metabolic activities of the cell by increasing or decreasing the concentration of specific enzymes o thyroid hormones bound to mitochondria increase mitochondrial rates of ATP production Control of Endocrine Activity by Endocrine Reflexes Endocrine Reflexes o functional counterparts of neural reflexes o can be triggered by humeral stimuli changes in the composition of the extracellular fluid hormonal stimuli arrival or removal of specific hormone neural stimuli arrival of neurotransmitter at neuroglandular junctions o Controlled by negative feedback a stimulus triggers the production of a hormone and the direct or indirect effects of the hormone reduce the intensity of the stimulus o Simple endocrine reflexes involve one hormone cells involved respond directly to changes in the composition of the extracellular fluid secreted hormone adjusts activity of target cells and restores homeostasis controls hormone secretion by the heart, pancreas, parathyroid glands, and digestive tract o Complex endocrine reflexes involve one or more intermediary steps and two or more hormones o Hypothalamus provides highest level of endocrine control integrates activities of nervous and endocrine systems in three ways acts as an endocrine organ o hypothalamus neurons synthesize hormones and transports them along axons to the posterior lobe at the pituitary gland, when they are released into circulation secretes regulatory hormones o control endocrine cells in the pituitary gland (anterior lobe) control activities of endocrine cells in the thyroid, adrenal cortex, and reproductive organs contains autonomic centers that exert direct neural control over the endocrine cells of the adrenal medullae o when sympathetic division is activated, releases E and NE into bloodstream o Hypothalamus secretes regulatory hormones and ADH in response to changes in the composition of the circulating blood secretion of OXT, E, and NE involves both neural and hormonal mechanisms neuroendocrine reflexes o Endocrine system sends complex commands by changing the amount of hormone secreted and the pattern of hormone release simple endocrine reflex releases hormones continuously hypothalamic and pituitary hormones are released in pulses target cells vary response with frequency of pulses
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