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University of Jordan The Endocrine System Ebaa M Alzayadneh, DDS, PhD Integrative Physiology and Pharmacology 1 University of Jordan Overview of the endocrine system 2 Overview of the endocrine system Hormones are chemical messengers that are carried by the bloodstream to different cells in the body. Hormones act at a distance from their place of production directly on the target cell, inciting it to increase or decrease the expression of specific genes. Hormones can be either lipid-based, also known as steroidal, or protein-based, known as non-steroidal. Lipid hormones enter the target cell directly through the cell membrane, and then travel to the nucleus and directly affect expression of target genes. Protein hormones cannot enter the cell, so they bind to receptors on the cell membrane, triggering a secondary messenger system within the cell. University of Jordan 3 Overview of the endocrine system Hormone release by the endocrine system is regulated largely through negative feedback loops, and rarely through positive feedback loops. In negative feedback, increases in hormone activity lead to a decrease in the production of that hormone. Endocrine glands in turn have no duct and release their secretions directly into the intercellular fluid or the blood. The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads. 4 University of Jordan Nervous and Endocrine Systems: Act together to coordinate functions of all body systems Nervous system • Responds to stimuli by sending electrical action potentials along neurons using neurotransmitters, the chemical messenger of the nervous system. • This response to stimuli is near instantaneous. • The nervous system responds rapidly to short-term changes by sending electrical impulses. • They may not last for long time (briefer) • Specific target Endocrine system • Hormones are synthesized at a distance from their target cells, and travel through the bloodstream or intercellular fluid • This process takes significantly longer • effects are long lasting. Additionally, target cells can respond to minute quantities of hormones and are sensitive to subtle changes in hormone concentration. • Broad spectrum of targets 5 Endocrine Glands • Glands are either: • Secrete hormones into interstitial fluid, diffuse into blood • Endocrine glands: • Pituitary, thyroid, parathyroid, adrenal and pineal glands • Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta not exclusively endocrine glands University of Jordan • Exocrine – have ducts • Endocrine – ductless 6 7 University of Jordan Hormone Activity University of Jordan • Hormones affect only specific target tissues with specific receptors • Receptors are constantly synthesized and broken down 8 Hormones types • Circulating – circulate in blood throughout body • Local hormones – act locally • Paracrine – act on neighboring cells • Autocrine – act on the same cell that secreted them 9 • Lipid-soluble – use transport proteins • Steroid • Thyroid • Nitric oxide (NO) • Water-soluble – circulate in “free” form • Amine • Peptide/ protein • Eicosanoid University of Jordan Chemical classes of hormones 10 University of Jordan • Lipid-soluble – use transport proteins • Steroid • Thyroid • Nitric oxide (NO) • Water-soluble – circulate in “free” form • Amine • Peptide/ protein • Eicosanoid 11 Chemical classes of hormones Chemical classes of hormones University of Jordan 12 • University of Jordan 13 Chemical classes of hormones • Response depends on both hormone and target cell • Lipid-soluble hormones bind to receptors inside target cells • Water-soluble hormones bind to receptors on the plasma membrane • Activates second messenger system • Amplification of original small signal • Responsiveness of target cell depends on • Hormone’s concentration • Abundance of target cell receptors • Influence exerted by other hormones: • Permissiveness is the situation in which a hormone cannot exert its full effects without the presence of another hormone. • Synergism occurs when two or more hormones produce the same effects in a target cell and their results are amplified. • Antagonism occurs when a hormone opposes or reverses the effect of another hormone. University of Jordan Mechanisms of Hormone Action 14 Blood capillary 1 Binding of hormone (first messenger) to its receptor activates G protein, which activates adenylate cyclase Water-soluble hormone Adenylate cyclase Receptor Second messenger G protein ATP cAMP 2 Activated adenylate cyclase converts ATP to cAMP inactivates cAMP 3 cAMP serves as a Activated protein kinases second messenger to activate protein kinases 4 Activated protein Protein kinases phosphorylate cellular proteins ATP University of Jordan 6 Phosphodiesterase Protein kinases ADP Protein— P 5 Millions of phosphorylated proteins cause reactions that produce physiological responses Water-soluble Hormones Target cell 15 Free hormone Blood capillary 1 Lipid-soluble hormone diffuses into cell 2 Activated Nucleus Receptor receptor-hormone complex alters gene expression DNA Cytosol mRNA 3 Newly formed mRNA directs synthesis of specific proteins on ribosomes Ribosome University of Jordan Transport protein New protein 4 New proteins alter cell's activity 16 Lipid-soluble Hormones Target cell • Regulated by • Signals from nervous system • Chemical changes in the blood • Other hormones • Most hormonal regulation by negative feedback • Few examples of positive feedback University of Jordan Control of Hormone Secretion 17