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Transcript
Endocrine System General Functions Communication Integration Control Chemical Messengers Hormone – substance secreted by an endocrine gland (sometimes from neurosecretory tissues) into the bloodstream that acts on a specific target tissue to produce a given response. Neuroendocrine System Both the nervous and endocrine system function to maintain stability of the internal environment Names and Locations of Major Endocrine Glands Name Location Hypothalamus Cranial cavity (brain) Pituitary gland Cranial cavity Pineal gland Crainal cavity (brain) Thyroid gland Neck Parathyroid glands Neck Thymus Mediastinum Adrenal glands Abdominal cavity Pancreatic Islets Abdominal cavity (pancrease) Ovaries Pelvic cavity Testes Scrotum Placenta Pregnant uterus Classification of Hormones Classification by function: Tropic hormones – hormones that target other endocrine glands and stimulate their growth and secretion Sex hormones – target reproductive tissues Anabolic hormones Classification by Chemical Structure Steroid hormones Nonsteroid hormones Steroid Hormones Steroid hormone molecules are made by endocrine cells from cholesterol, an important lipid All have a characteristic chemical group at the core of each molecule Lipid-soluble, thus they can easily pass through the phospholipid plasma membrane of target cells. Examples: cortisol, aldosterone, estrogen, progesterone, and testosterone Steroid Hormone Structure As these examples show, steroid hormone molecules are very similar in structure to cholesterol (top), from which they are all derived. Nonsteroid Hormones Synthesized mainly from amino acids Some are proteins (long folded chains of amino acids Glycoprotein hormones have carbohydrate groups attached to their amino acid chains Peptide hormones are smaller than protein hormones Amino acid derivative hormones are derived from a single amino acid molecule Nonsteroid Hormone Structure A. Protein hormone B. Peptide hormone C. Amino acid derivative How Hormones Work Hormones signal a cell by binding to specific receptors on or in the target cell. (Like a “lock-and-key”) Only bind to the receptors that “fit” them exactly. Cells can have many different type of receptors Each hormone-receptor interaction produces different regulatory changes Target Cell Concept Synergism – when hormones work together to enhance each other’s influence on a target cell. Permissiveness – when a small amount of one hormone allows a second hormone to have its full effect on a target cell. Antagonism – one hormone produces the opposite effect of another hormone. Mechanisms of Steroid Hormone Action Can’t travel in plasma because they are lipids Attach to soluble plasma proteins Most steroid receptors are found inside cells Formation of hormone-receptor complex and activates mRNA synthesis Steroid Hormone Mechanism Mechanisms of Nonsteroid Hormone Action Second messenger hypothesis – theory of hormone action in which the hormone binds to receptors of the target cell, which then triggers a second molecule within the cell to accomplish its function. Example of Second Messenger Mechanism 1. 2. 3. 4. 5. 6. 7. Hormone-receptor complex G-protein reacts with GTP Activation of adenyl cyclases Phosphates removed from ATP, converting it to cAMP Activates protein kinase Activation of specific intracellular enzyme Enzymes influence specific cellular reactions Regulation of Hormone Secretion Usually part of a negative feedback loop Responses that result from the operation of feedback loops within the endocrine system are called endocrine reflexes CalciumCalmodulin as a 2nd Messenger 1. 2. 3. 4. 1st messenger binds to a receptor in plasma membrane Activation of membrane bound proteins that trigger the opening of Ca++ channels Ca++-calmoduln complex formed (2nd messenger) Activates or inactivates enzyme Endocrine Feedback Loop Prostaglandins (PG’s) Unique and diverse group that doesn’t meet the usual definition of a hormone. Rapidly metabolized so circulating levels are very low Produced in a tissue and diffuse a short distance to other cells in the tissue Pituitary Gland Structure Very small, but very important— sometimes called the “master gland” Found on the ventral surface of the brain Actually two glands Anterior pituitary Posterior pituitary Structure and Location of the Pituitary Gland Anterior Pituitary (Adenohypophysis) Composed of several cell types that stain differently from one another Hormones include: Growth hormone— growth and blood glucose levels Prolactin—milk secretion Tropic hormones— stimulate other glands Growth Hormone Abnormalities Posterior Pituitary (Neurohypophysis) Storage and release site for two hormones: Antidiuretic hormone—prevents the formation of large amounts of urine Oxytocin—stimulates contraction of uterine muscles and causes milk ejection from the breasts (positive feedback mechanisms) Pineal Gland Tiny pine cone shaped structure Nervous and endocrine system Functions to support the body’s biological clock Melatonin—thought to induce sleep Thyroid Gland Located in the neck Thyroid hormone (TH) – helps to regulate metabolic rate as well as cell growth and differentiation (general target) Calcitonin – influences tends to decrease blood calcium levels and promote conservation of bone matrix Thyroid and Parathyroid Glands Thyroid Abnormalities Parathyroid Embeded within the thyroid Secretes Parathyroid Hormone (PTH) – acts as an antagonist to calcitonin by increasing release of calcium into the blood Adrenal Glands Located atop the kidneys Adrenal cortex Aldosterone – sodium homeostasis Cortisol – many functions Adrenal medulla Epinephrine Norepinephrine Adrenal Gland Structure Pancreatic Islets Edocrine tissue (Islets) account for about 2% or 3% of the total mass of the pancreas Important pancreatic hormones: Glucagon – increases blood glucose levels by converting glycogen to glucose Insulin – promotes movement of glucose out of the blood into the cells Pancreas Regulation of Blood Glucose Levels Gonads Testes produce androgens (male sex hormones – mainly testosterone which is responsible for male sex characteristics and sperm production Ovaries Estrogens – female sex characteristics and reproductive cycle Progesterone – maintains lining of the uterus for pregnancy Placenta Interface between the circulatory systems of the mother and developing fetus Produces human chorionic gonadotropin (hCG) which serves to signal the retention of the uterine lining