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Chapter 18 - Introduction to the Endocrine System

Chapter 18 - Introduction to the Endocrine System - Chapter...

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Chapter 18 - Introduction to the Endocrine System The Endocrine System regulates long-term processes; e.g., Growth, Development & Reproduction Uses chemical messengers to relay information and instructions between cells Homeostasis and Intercellular Communication a. Direct Communication : Exchange of ions and molecules between adjacent cells across gap junctions: occurs between two cells of same type, highly specialized and relatively rare b. Paracrine Communication : Uses chemical signals to transfer information from cell to cell within single tissue; most common form of intercellular communication c. Endocrine Communication: Endocrine cells release chemicals ( hormones ) into bloodstream: alters metabolic activities of many tissues and organs simultaneously 4. Synaptic communication : Across synaptic clefts - neurotransmitters Target Cells - specific cells that possess receptors needed to bind and “read” hormonal messages Hormones i. Stimulate synthesis of enzymes or structural proteins ii. Increase or decrease rate of synthesis iii. Turn existing enzyme or membrane channel “on” or “off” Hormones can be divided into three groups i. Amino acid derivatives – T4, epinephrine, melatonin ii. Peptide hormones – FSH, ADH, PRL iii. Lipid derivatives – Testosterone, estrogen, cortisol Circulate freely or bound to transport proteins Secretion and Distribution of Hormones: free Hormones: remain functional for less than 1 hour 1. Diffuse out of bloodstream - bind to receptors on target cells 1. Are broken down and absorbed - by cells of liver or kidney 1. Are broken down by enzymes - in plasma or interstitial fluids Thyroid and Steroid Hormones: remain in circulation much longer; enter bloodstream & more than 99% become attached to special transport proteins; bloodstream contains substantial reserve of bound hormones Mechanisms of Hormone Action Hormone Receptor is a protein molecule to which a particular molecule binds strongly ii. Responds to several different hormones iii. Different tissues have different combinations of receptors iv. Presence or absence of specific receptor determines hormonal sensitivity Hormones and Plasma Membrane Receptors Catecholamines and peptide hormones 1. Are not lipid soluble & unable to penetrate plasma membrane 2. Bind to receptor proteins at outer surface of plasma membrane (extracellular receptors) Cannot have direct effect on activities inside target cell Use intracellular intermediary to exert effects 1 st messenger: leads to 2 nd messenger; may act as enzyme activator, inhibitor or cofactor; results in change in rates of metabolic reactions Important Second Messengers i. Cyclic-AMP (cAMP) - Derivative of ATP ii. Cyclic-GMP (cGMP) - Derivative of GTP iii. Calcium ions Process of Amplification: binding of a small number of hormone molecules to membrane receptors - leads to thousands of 2 nd messengers in cell; magnifies effect of hormone on target cell Down-regulation: presence of a hormone triggers in number of hormone receptors; when levels
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