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Chapter 16

Endocrine System notes ch. 16

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Boston College
ENGL 1010
Laura Sterrett

• Endocrine system- interacts with nervous system to coordinate and integrate activity of body cells, influences metabolic activity using hormones • Hormones- chemical influences that affect metabolic activity, released into blood, initiate responses seconds to days after being released, responses are also prolonged, most are amino acid based, some are steroids • Endocrine glands- ductless glands, produce hormones and release them to surrounding tissue fluid, arranged in branching networks to maximize contact between them and surrounding capillaries • Endocrine glands include pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands • The hypothalamus is a neuroendocrine organ • Autocrines- chemicals that exert their effects on the same cells that secrete them • Paracrines- act locally but affect cell types other than those releasing the paracrine chemicals • Steroids- synthesized from cholesterol, very few hormones are steroidal • Eicosanoids- lipds released by nearly all cell membranes, their effects are typically highly localized, aren’t really hormones because they don’t affect distant targets • Target cells- certain tissue cells whose activity is affected by a given hormone, cell activity is altered • Hormonal stimulus produces one of more of the following changes: 1. Alters plasma membrane permeability or membrane potential by opening or closing ion channels, 2. Stimulates synthesis of proteins within the cell. 3. Activates or deactivates enzymes, 4. Induces secretory activity, 5. Stimulates mitosis • Water-soluble hormones interact with receptors in the plasma membrane via G proteins then use intracellular second messengers • Lipid-soluble hormones act on intracellular receptors by directly activating genes • Second messengers- generated when a hormone binds to a receptor on the plasma membrane, they are the effects of amino acid-based hormones • Cyclic AMP- second messenger used by neurotransmitters 1. First messenger is a hormone that binds to its receptor 2. Hormone binding causes receptor to change shape to bind to an inactive G protein (GDP replaced by GTP) 3. Active G protein binds to effector enzyme adenlate cyclase to either stimulate or inhibit until the G protein becomes inactive again 4. If activated, adenylate cyclase generates second-messenger camp from ATP 5. camp free to diffuse through cell triggering cascade to activate protein kinases • Phosphodiesterase- intracellular enzyme that rapidly degrades camp • PIP-calcium signal mechanism- 1. Hormone binding to receptor changes shape of receptor 2. Receptor binds to G protein and activates it 3. Activated G protein binds to and activates membrane-bound phospholipase C (effector enzyme) 4. Phospholipase C splits plasmamembrane phospholipids PIP2 into two different second messengers DAG and IP3 5. DAG activates protein kinase C and IP3 release calcium ions 6. Calcium ions become second messengers • Up-regulation target cells form more receptors in response to rising blood levels of the specific hormones to which they respond • Down-regulation- prolonged exposure to high hormone concentrations desensitizes target cells to hormonal stimulation • Half-life length of time for a hormone’s blood level to be decreased by half, shortest for water-soluble hormones • Permissiveness- one hormone cannot exert full effect without another hormone being present • Synergism- more than one hormone produces same effects at target cell and combined effect is amplified • Antagonism- one hormone opposes the action of another hormone • Negative feedback system- hormone secretion is triggered by some internal or extermal stimulus, rising levels cause target organ effects and inhibit further hormone release • Humoral stimuli- endocrine glands that secrete hormones in direct response to changing blood levels of certain critical ions and nutrients • Neural stimuli- nerve fibers stimulate hormone releases • Hormonal stimuli- hormones released in response to hormones produced by other endocrine organs • Nervous system overrides normal endocrine controls to maintain homeostasis • Pituitary gland- otherwise known as the hypophysis, secretes more than nine hormones, size of a pea, one neural lobe and one glandular • Infudibulum- funnel-shape connecting pituitary gland to hypothalamus superiorly • Posterior pituitary- composed of mostly glia-like supporting cells called pituicytes and nerve fibers, releases neurohormones, hormone-storage area, not a true endocrine gland because it receives ready-made hormones from the hypothalamus • Neurohypophysis- consists of posterior pituitary lobe and infundibulum • Anterior pituitary- adenohypophysis, manufactures and releases a number of hormones • Posterior pituitary is a part of the brain that is a downgrowth of hypothalamic tissue • Hypothalamic-hypophyseal tract- nerve bundle connecting hypothalamus to posterior pituitary lobe, runs through infundibulum • Hypophyseal portal system- primary and secondary capillary plexuses and intervening hypophyseal portal veins, route for releasing and inhibiting hormones secreted by neurons in hypothalamus • Tropins- hormones that regulate the secretory action of other endocrine glands • Growth hormone- produced by cells called somatotrophs of the anterior pituitary lobe, stimulates body cells to increase in size and divide, promotes protein synthesis • Insulin-like growth factors- somatomedins, mediate growth-promoting effects of GH, family of growth-promoting proteins • Growth hormone-releasing hormone- stimulates GH release • Growth hormone-inhibiting hormone – somatostatin inhibits release of GH • Gigantism- result of hypersecretion of GH • Acromegaly- excessive amount of GH secreted after epiphyseal plates have
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