Endocine System - Lecture notes provided by Professor Childress PDF

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Lecture notes provided by Professor Childress...

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Endocrine System The Two Control Systems of the Body -

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The nervous system and endocrine system serve as the two complementary systems of control Nervous System - Nerve signals cause neurotransmitter release - Effect is localized to specific area of the body - Target is neurons, muscles, and glands - Provides a rapid reaction time - Response is short-term Endocrine System - Secrete hormones that transport within blood - Effect is wide-spread - Target is any cell with the hormone receptor - Relatively slow reaction time - Response is long-lasting

General Function of Endocrine System 1. 2. 3. 4.

Regulating development, growth, and metabolism Maintaining homeostasis of blood composition and volume Controlling digestive processes Controlling reproductive activities Endocrine system glands synthesize and secrete hormones that communicate with and control other body cells Cells that have specific receptors for a hormone are called target cells How can an endocrine gland control an organ located a significant distance from the gland? Endocrine glands lack ducts. Hormones are released from the gland, through the interstitial fluid, and enter the blood. Hormones are transported through the blood vessels to the body tissues, a process that is relatively slow. Hormones randomly leave the blood from capillaries providing body cells with access to the hormone (effects widespread) Hormone binds to target cells’ receptors, initiating or inhabiting metabolic activities within the cell

Location of Major Endocrine Glands -

Endocrine cells (synthesize hormones) are organized as a single organ with only endocrine functions, or as cells clustered within organs that have other primary functions

Stimulation of Hormone Synthesis -

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The secretion of a hormone is controlled through a reflex Endocrine reflexes are initiated by one of three types of stimulation Hormonal Stimulation - The release of a hormone from an endocrine gland is stimulated by the binding of another hormone Humoral Stimulation - The release of a hormone from an endocrine gland is stimulated by changing levels of nutrient molecules in the blood Nervous Stimulation - The release of a hormone from an endocrine gland is stimulated by direct stimulation from the nervous system

Categories of Hormones -

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Hormones can circulate in the blood (circulating hormones) or be local hormones that influence cells in the local tissue Circulating Hormones - Steroid Hormones - Lipid-soluble molecules synthesized from cholesterol - Produced within the gonads and by the adrenal cortex - Biogenic Amines - Water-soluble molecules (except thyroid hormone - lipid-soluble) - Synthesized from modified amino acids - Protein Hormones - Water-soluble molecules - Synthesized from amino acids - Most hormones are in this category - Includes: insulin, glucagon, growth hormone, erythropoietin, antidiuretic hormone Local Hormones (Eicosanoids) - Short-lived signaling molecules - Do not circulate in blood - Bind to the cell that produced them (Autocrine Stimulation) - Bind to neighboring cells (Paracrine Stimulation)

Transport in the Blood -

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Lipid-soluble Hormones - Do not dissolve readily in blood - Require carrier proteins - Water-soluble proteins synthesized by the liver - Carrier protein also protects the hormone from early destruction - Binding between the hormone and a carrier is temporary - Any hormone that is attached to a carrier is a bound hormone (majority of hormones in the blood) - An unattached hormone is an unbound (free) hormone - Only unbound hormones can leave blood and attach to a target cell Water-soluble Hormones - Readily dissolve in blood - Do not require carrier proteins - Most will use carrier proteins as protection to prolong the life of the hormone

Levels of Circulating Hormone -

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The amount of a hormone is the blood must be tightly regulated to prevent potential clinical consequences There are two factors that influence hormone concentration: hormone release and hormone elimination Hormone Release/Synthesis - An increase in release of the hormone results in a higher hormone concentration within the blood - A decrease in release of the hormone results in a lower hormone concentration within the blood Hormone Elimination - Hormones are typically eliminated in three ways - Through enzymatic degradation - Through removal from the body in urine - Through uptake into a target cell Half-life: the amount of time necessary to reduce the hormone concentration within the blood to one-half of what had been secreted originally - Generally, water-soluble hormones have a relatively short half-life - Steroids generally have the longest half-life, since their carrier proteins protects them - The shorter the half-life of a hormone, the more frequently it must be replaced to maintain normal blood concentrations

Target Cells: Interaction with Hormones -

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Hormones interact only with target cells (cells with receptors for those hormones) to initiate a specific cellular response Lipid-soluble Hormones (Steroid hormones & Thyroid hormone) - Small, nonpolar, lipophilic (lipid-loving) molecules - Plasma membrane of a cell is not an effective barrier - Unbound lipid-soluble hormones are able to diffuse into the cell - Once in the cell, the hormone binds to intracellular receptors in the cytosol or nucleus to form a hormone-receptor complex - The hormone-receptor complex then binds to a particular DNA sequence called hormone-response elements - Binding to a DNA sequence results in transcription of messenger ribonucleic acid (mRNA) - Subsequent translation of the mRNA by ribosomes synthesize a specific protein - The change in protein synthesis within the cell may result in: - An alteration in cell structure - Shift in target cells’ metabolic activity Water-soluble Hormones (Proteins and Biogenic Amines, except TH) - Polar molecules - Denied entry into the cell - Stimulation of the target cell is initiated when the hormone binds to a plasma membrane receptor - Binding of a water-soluble hormone to a plasma membrane receptor initiates a series of biochemical events across the membrane called a signal transduction pathway - Signal Transduction Pathway - In this pathway the hormone is the signaling molecule, or first messenger - The binding of a hormone on the plasma membrane receptor causes an internal molecule called a G protein to become activated - Activated G protein moves along the inside of the plasma membrane, where it binds to the plasma membrane protein adenylate cyclase - Activated adenylate cyclase increases the formation of the second messenger, cAMP from an ATP molecule - The cAMP then activates a protein kinase, an enzyme that phosphorylates (adds phosphate to) other molecules within the cell - Phosphorylation of other molecules in the cell results in - Activation or inhibition of these molecules - Increases in cellular division - Release of cellular secretion - Changes in membrane permeability

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Muscle contraction or relaxation

Target Cells: Degree of Cellular Response -

The response of a target cell to a hormone depends upon the number of receptors displayed and the interaction when different hormones bind simultaneously to one cell 1. Number of Receptors - Up-Regulation: number of receptors on the cell increases - Increase the cell sensitivity to a hormone - Occurs when blood levels of a hormone are low - Down-regulation: number of receptors on the cell decreases - Decrease the cell sensitivity to a hormone - Occurs when blood levels of the hormones are high 2. Hormone Interactions - Different hormones can bind simultaneously to a target cell 1. Synergistic interactions - Occurs when the activity of one hormone reinforces the activity of another hormone - Ex. estrogen and progesterone effects on a target cell 2. Permissive interactions - Occurs when one hormone requires a second hormone for the first to function - Ex. oxytocin’s milk ejection effect requires prolactin’s milk generating effect 3. Antagonistic interactions - Occurs when the activity of one hormone opposes the effects of another hormone - Ex. glucagon increases blood glucose while insulin lowers blood glucose levels - Anatomic Relationship of the Hypothalamus and the Pituitary Gland - Hypothalamus is the “control center” 1. Has direct control of hormone release from the pituitary gland 2. Also influences the release of hormone from several other endocrine organs (thyroid gland, adrenal gland, liver, testes, & ovaries) - Pituitary gland (hypophysis) 1. Inferior to the hypothalamus (pea-sized) 2. Connected to hypothalamus by a stalk called the infundibulum 3. Partitioned into anterior and posterior pituitary (lobes) - Posterior pituitary (neurohypophysis) 1. Composed of neurons (Approximately 10,000 neurons extended from the hypothalamus to the posterior pituitary)

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Dendrites and cell body are located in the hypothalamus Unmyelinated axons extend through the infundibulum as the hypothalamo-hypophyseal tract - Synaptic knobs are within the posterior pituitary 2. Posterior pituitary stores two hormones that were synthesized in the hypothalamus - The hypothalamus synthesizes the two hormones - The hormones are packaged and transports through the unmyelinated axons - Hormones are released from the synaptic knobs within the posterior pituitary 3. Posterior pituitary does not produce (synthesize) hormones Interactions Between the Hypothalamus and the Posterior Gland - The two hormones produced in the hypothalamus and stored in the posterior pituitary are: oxytocin and antidiuretic hormone (vasopressin) - Oxytocin - Stimulates smooth muscle contractions of the uterus with increasing force until the baby is delivered - Stimulates smooth muscle contractions within the breast to cause ejection of breast milk - Antidiuretic hormone (vasopressin) - Stimulates kidneys to decrease urine output - Increases consciousness of being thirsty and the need to increase fluid intake - In high doses, causes vasoconstriction - Anterior pituitary (adenohypophysis) - Anterior pituitary gland is the endocrine portion of the pituitary gland - The connection between the hypothalamus and the anterior pituitary involves two capillary plexuses - The blood vessel network is called the hypothalamo-hypophyseal portal system - Provides a direct blood pathway between the hypothalamus and the anterior pituitary - Regulatory hormones are released from the hypothalamus into the hypothalamo-hypophyseal portal system - The regulatory hormones cause hormonal stimulation at the anterior pituitary, triggering the release of its hormones into the general circulation

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Interactions Between the Hypothalamus and the Anterior Pituitary Gland - Regulatory hormones produced and released from the hypothalamus falls into one of two groups: 1. Releasing hormones: stimulate the production and secretion of specific anterior pituitary hormones 2. Inhibiting hormones: decrease the production and secretion of specific anterior pituitary hormones Hormones Secreted by the Anterior Pituitary - Thyroid-stimulating hormone (TSH): stimulates growth of the thyroid gland and the thyroid gland’s release of hormone - Prolactin: regulates mammary gland growth and the breast milk production - Gonadotropins (follicle-stimulating hormone & luteinizing hormone): - In females, act on ovaries to control development of oocyte and follicle, ovulation, and release of estrogen and progesterone - In males, act on testes to regulate the development of sperm and release of testosterone - Adrenocorticotropic hormone: stimulates the adrenal cortex to produce and secrete glucocorticoids - Growth hormone: stimulates the liver to release both insulin-like growth factor 1 and 2 - GH and IFGs function synergistically to stimulate cell growth and division - Melanocyte-stimulating hormone: little effect in humans and secretion ceases prior to adulthood

Thyroid Gland -

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Largest structure devoted to endocrine activities Inferior to the thyroid cartilage Anterior to the trachea Composed of right and left lobe - Connected by right and left lobes - Connected by isthmus Highly vascularized

Microscopic Structures -

Composed of microscopic follicles - Follicular cells form the wall of the follicles - The lumen of each follicle houses protein-rich fluid called colloid - Function in producing thyroid hormone - Parafollicular cells are located around the follicular cells - Functioning in producing calcitonin

Production of Thyroid Hormone -

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Inside the Follicular Cell - Iodide ion is moved from the blood across the follicular cell into the follicular lumen - Two iodide ions joined to form iodine at the plasma membrane of the follicular cell - Concurrently, thyroglobulin protein is released from the follicular cell into the colloid Inside the Colloid - Iodine attaches to the thyroglobulin and produces: - MIT (monoiodotyrosine) - DIT (diiodotyrosine) - The MIT and DIT pair up to form complex structures - DIT + MIT = Pre-T3 - DIT + DIT = Pre-T4 Inside the Follicular Cell - Pre-T3 and Pre-T4 endocytosis into a follicular cell - Lysosomal enzymes cleave pre-T3 and pre-T4 from the thyroglobulin, forming T3 and T4 (collectively called thyroid hormone) - T3 and T4 (thyroid hormone) move from the follicular cell into the blood Effects of Thyroid Hormone - Thyroid hormone increases protein synthesis on all cells, especially neurons - Stimulates synthesis of sodium-potassium pumps in neurons, which generates heat (calorigenic effect) - Thyroid hormone increases amino acid uptake and glucose uptake - Respiration rate increases in response to TH to meet additional demands for oxygen - Heart rate and force of heart contraction increase blood flow to tissue

Production of Calcitonin -

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Synthesizes and releases calcitonin from the parafollicular cells - Located around the follicular cells Calcitonin is released if blood calcium levels are high - Inhibits osteoclast activity in bone - Increases excretion of calcium in urine The net effect is a reduction in blood calcium levels

Parathyroid Glands -

Located on the posterior surface of the thyroid gland Two different cells make up the parathyroid gland: chief cells and oxyphil cells (function unknown) - Chief cells: source of parathyroid hormone which functions to increase blood calcium levels - Released in response to a decrease in blood calcium levels

Adrenal Glands -

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Pyramid-shaped endocrine glands anchored on the superior surface of each kidney The adrenal glands are composed of both nervous tissue and hormone-producing cells - The adrenal medulla (deep portion) is composed of nervous tissue - The adrenal cortex (superficial portion) is composed of hormone-producing cells Adrenal Medulla - Releases catecholamines: epinephrine and norepinephrine from chromaffin cells - Fight-or-flight hormone - Release of these hormones is stimulated by the sympathetic division of the nervous system Adrenal Cortex - Synthesize more than 25 different lipid-soluble corticosteroids - Partitioned into three separate regions: - Zona glomerulosa - Zona fasciculata - Zona reticularis

Hormones of the Adrenal Cortex -

Zona Glomerulosa - Superficial layer of the adrenal cortex - Synthesizes mineralocorticoids - Group of hormones that help regulate the concentration of electrolytes (ions) in body fluids - Principal mineralocorticoid is aldosterone - Regulates the ratio of sodium and potassium in our blood and body fluids by altering the amounts excreted by the kidneys into urine

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Zona Fasciculata - Middle layer of the adrenal cortex - Synthesizes glucocorticoids - Group of hormones that help regulate blood sugar

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Principal glucocorticoid is cortisol - Increases nutrient levels in the blood, especially in an attempt to resist stress and help repair injured or damaged tissues

Zona Reticularis - Deep layer of the adrenal cortex - Synthesizes gonadocorticoids - Minor amounts of sex hormones - Principal gonadocorticoid is androgens - Male sex hormone - Secondary sites in males - Primary site in females

Cortisol: Its Regulations and Effects -

Both emotional stress (ex. anxiety, and ger, fear) and physical stress (ex. fever, trauma, intense exercise) increase the release of cortisol The body’s response to stress is initiated by the hypothalamus and involves both the nervous system and endocrine system and is described in three stages - The Alarm Reaction - Initial reaction to stress and is regulated primarily by the sympathetic division of the autonomic nervous system - The sympathetic division stimulates the adrenal medulla to release epinephrine and norepinephrine into the blood - Some changes to the body are: increases respiration rate, increases blood pressure, increase in glucose levels in blood, sweating - The Stage of Resistance - Occurs after a few hours as glycogen stores in liver are depleted - Regulated by the endocrine system - Release of glucocorticoids (cortisol) to provide glucose to meet the increased energy demands - Net result is elevated blood glucose levels - The Stage of Exhaustion - Occurs after weeks or months, as fat stores in adipose connective tissue are depleted - Structural proteins of body cells break down - Body becomes progressively weaker - Electrolyte imbalance occurs - Ultimately cause organ failure and death

Pancreas -

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Posterior to the stomach Vast majority of cells of the pancreas serve as exocrine gland function - Acinar cells: produce digestive enzymes which are released into the duodenum of the small intestine The endocrine cells of the pancreas are located within small clusters called pancreatic (islets of Langerhans) - A pancreatic islet is composed of two primary types of cells: alpha cells and beta cells

Pancreatic Hormones -

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Alpha cells: secrete glucagon - Released in response to low blood glucose levels - The release of glucagon results in an increase in glucose, glycerol, and fatty acids in the blood and decrease in storing these molecules within body tissue - Glucose is important for cellular respiration in nervous tissue and blood glucose levels must be prevented from dropping to low Beta cells: secrete insulin - Released in response to high blood glucose levels - The release of insulin results in both a decrease in all nutrients in the blood and an increase in storage of these molecules within body tissue - Chronically high blood glucose levels can damage blood vessels and the kidneys

Pineal Gland -

Located in the posterior region of the epithalamus of the brain Secretes melatonin - Causes drowsiness Melatonin production tends to be cyclic - Increases at night - Decreases during the day - Helps regulate the circadian rhythm

Structures with an Endocrine Function -

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Thymus - Anterior to the heart - Growth continues until puberty, then it is mostly replaced by adipose tissue - Secrete thymic hormones which participate in the maturation of T-lymphocytes Heart - Endocrine tissue within the atria synthesizes and releases the hormone atrial

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natriuretic peptide (ANP) in response to increased stretch of the atrial wall - This indicated increase in blood volume and blood pressure - ANP stimulates the kidneys to increase urine output and the blood vessels to dilate Kidneys - Endocrine tissue within the kidneys release erythropoietin (EPO) when receptors detect low blood oxygen levels - EPO stimulates the red bone marrow to increase the production rate of (erythrocytes) red blood cells Liver - Recall, releases insulin-like g...