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Transcript
Lecture 20 Coordination and Control In Animals - 2 Coordination and Control Systems • The nervous system coordinates rapid and precise responses to stimuli using action potentials. • The endocrine system maintains homeostasis and long-term control using chemical signals. • The endocrine system works in parallel with the nervous system to control growth and maturation along with homeostasis. The Endocrine System • Is a collection of glands that secrete chemical messages we call hormones. • These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. • Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands. Function of Endocrine System • • • • • controls numerous body processes growth and development metabolism reproduction homeostasis Structure of The Endocrine System • 1. 2. 3. 4. 5. 6. • Comprises a number of separate organs and tissues scattered throughout the body. These include: The pituitary (hypophysis) attached to the underside of the brain. The thyroid, situated in the neck close to the larynx. The parathyroids, associated with the thyroid but comprising separate structures. The adrenals, situated on top of each kidney. The endocrine portion of the pancreas. The ovaries or testes (The endocrine role of the ovaries and testes will be dealt with later in the course, in association with the reproductive system. Produce hormones. Hormones • PROTEINS (eg. insulin). • PEPTIDE (at least 3 amino acids eg. oxytocin). most abundant, cell surface receptors. • AMINO ACID DERIVATIVES (eg. thyroxine). • AMINE (eg adrenaline) cell surface or intracellular receptors modified amino acid residues. • STEROID (eg. cortisol) intracellular receptors. Steroids • Are lipids derived from cholesterol. • Testosterone is the male sex hormone. • Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. • Specific protein carrier molecules. • Slow acting - genomic actions. Glucocorticoids • • • • The glucocorticoids get their name from their effect of raising the level of blood sugar (glucose). One way they do this is by stimulating gluconeogenesis in the liver: the conversion of fat and protein into intermediate metabolites that are ultimately converted into glucose. The most abundant glucocorticoid is cortisol (also called hydrocortisone). Cortisol and the other glucocorticoids also have a potent antiinflammatory effect on the body. They depress the immune response, especially cell-mediated immune responses. Widely used in therapy: 1. to reduce the inflammatory destruction of rheumatoid arthritis and other autoimmune diseases. 2. to prevent the rejection of transplanted organs. 3. to control asthma. Mineralocorticoids • The mineralocorticoids get their name from their effect on mineral metabolism. • The most important of them is the steroid aldosterone. • Aldosterone acts on the kidney promoting the reabsorption of sodium ions (Na+) into the blood. Water follows the salt and this helps maintain normal blood pressure. • Aldosterone also : - acts on sweat glands to reduce the loss of sodium in perspiration. - acts on taste cells to increase the sensitivity of the taste buds to sources of sodium. • The secretion of aldosterone is stimulated by: - a drop in the level of sodium ions in the blood. • - a rise in the level of potassium ions in the blood. Androgens • The adrenal cortex secretes precursors to androgens such as testosterone. • In sexually-mature males, this source is so much lower than that of the testes that it is probably of little physiological significance. However, excessive production of adrenal androgens can cause premature puberty in young boys. • In females, the adrenal cortex is a major source of androgens. Their hypersecretion may cause some masculinization in adult females, producing a masculine pattern of body hair and cessation of menstruation. Synthesis of Sex Hormones Peptide Hormones • Water soluble, • Stored in vesicles, • Fast acting, • Short half-life • They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys Amine Hormones • Derivatives of single amino acid residues, primarily TYROSINE • Secreted by thyroid and adrenal medulla • Can behave either like peptides or steroids • Key characteristics: - Carrier molecules required - Surface and intracellular receptors - Fast and slow acting Blood Levels of Hormones • The concentration of a hormone in the blood stream will depend on rate of secretion vs rate of removal. • The rate of secretion, which may be divided into: I) the rate of synthesis, dependent on chemical precursors from diet or from other cells in the body. Some hormones are stored, others undergo continuous turnover. II) the rate of release of preformed hormone into the blood, dependent on the stimulus for release. Blood Levels of Hormones • The rate of removal, which may be divided into: I) the rate of inactivation, dependent on the rate at which the hormone is used up at its site of action or rendered inactive during passage through the liver or kidney. II) the rate of excretion, dependent upon renal handling of either the inactivated hormone or its metabolic breakdown products (often used as a basis for "drug" testing). Mechanism of Action of Hormones • Proteins, peptides and amines are not lipid soluble, so they cannot cross the plasma membranes of cells. • Hormones classified as these types of compound have their receptor on the outside of the cell membrane. • Combination of the hormone with its membrane receptor causes the production within the cell of a second messenger compound which then exerts the hormone's actions within the cell. • The second messenger activates, accelerates or inhibits preprogrammed activities within the cell. These activities usually involve changing enzyme action, altering protein synthesis (through messenger RNA) or opening or closing membrane channels. E.g cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), inositol triphosphate (IP3) and calcium ions. . Action of Secondary Messengers Mechanism of Hormone Action • The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process. • Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. • The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins. Action of Steroids Control of Hormone Release • There are three ways in which the release of a hormone from an endocrine gland may be controlled. 1. 2. 3. Substrate control. This sets up a simple control loop in which the substrate is controlling release of the hormone, which by its action(s) is altering the level of the substrate. E.g calcitonin (substrate is calcium), aldosterone (substrate is potassium) and insulin (substrate is glucose). Nervous control. Some hormones are released directly in response to nervous stimuli. Endocrine glands which are most influenced by nerve inputs include the pituitary, where the nervous control arises in the hypothalamus, and the adrenal medulla, where the nervous control arises from the sympathetic nervous system. Trophic hormones. In some cases, the endocrine gland is itself a target organ for another hormone. Hormones of this type are termed trophic hormones, and they are all released from the anterior pituitary gland (adenohypophysis). Endocrine glands controlled principally by trophic hormones include the thyroid gland and the adrenal cortex. The Adrenal Glands • Each kidney has an adrenal gland located above it. • The adrenal gland is divided into an inner medulla and an outer cortex. • The medulla synthesizes amine hormones, the cortex secretes steroid hormones. • The adrenal medulla consists of modified neurons that secrete two hormones: epinephrine and norepinephrine. • Stimulation of the cortex by the sympathetic nervous system causes release of hormones into the blood to initiate the "fight or flight" response. • The adrenal cortex produces several steroid hormones in three classes: mineralocorticoids, glucocorticoids, and sex hormones. • Mineralocorticoids maintain electrolyte balance. • Glucocorticoids produce a long-term, slow response to stress by raising blood glucose levels through the breakdown of fats and proteins; they also suppress the immune response and inhibit the inflammatory response. The Adrenal Medulla • The adrenal medulla consists of masses of neurons. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. • The adrenal medulla releases: - adrenaline (also called epinephrine) and - noradrenaline (also called norepinephrine) • Both are derived from the amino acid tyrosine. • Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors - transmembrane proteins in the plasma membrane of many cell types. Adrenaline • Some of the effects are: a) increase in the rate and strength of the heartbeat resulting in increased blood pressure, b)blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain, rise in blood sugar, increased metabolic rate, c) bronchi dilate d) pupils dilate, e) hair stands on end ("gooseflesh" in humans), f) clotting time of the blood is reduced, increased ACTH secretion from the anterior lobe of the pituitary. • All of these effects prepare the body to take immediate and vigorous action. Adrenal Gland The Thyroid Gland • The thyroid gland is located in the neck. Follicles in the thyroid secrete thyroglobulin, a storage form of thyroid hormone. Thyroid stimulating hormone (TSH) from the anterior pituitary causes conversion of thyroglobulin into thyroid hormones T4 and T3. Almost all body cells are targets of thyroid hormones. • Thyroid hormone increases the overall metabolic rate, regulates growth and development as well as the onset of sexual maturity. Calcitonin is also secreted by large cells in the thyroid; it plays a role in regulation of calcium. The Pancreas • The pancreas contains exocrine cells that secrete digestive enzymes into the small intestine and clusters of endocrine cells (the pancreatic islets). • The islets secrete the hormones insulin and glucagon, which regulate blood glucose levels. • After a meal, blood glucose levels rise, prompting the release of insulin, which causes cells to take up glucose, and liver and skeletal muscle cells to form the carbohydrate glycogen. • As glucose levels in the blood fall, further insulin production is inhibited. • Glucagon causes the breakdown of glycogen into glucose, which in turn is released into the blood to maintain glucose levels within a homeostatic range. Glucagon production is stimulated when blood glucose levels fall, and inhibited when they rise. Diabetes • Results from inadequate levels of insulin. • Type I diabetes is characterized by inadequate levels of insulin secretion. • Type II usually develops in adults. Loss of response of targets to insulin rather than lack of insulin causes this type of diabetes. • Diabetes causes impairment in the functioning of the eyes, circulatory system, nervous system, and failure of the kidneys. • Diabetes is the second leading cause of blindness in the US. Treatments involve daily injections of insulin, monitoring of blood glucose levels and a controlled diet. The Pituitary Gland • Often called the master gland. • Is located in a small bony cavity at the base of the brain. • A stalk links the pituitary to the hypothalamus, which controls release of pituitary hormones. • The pituitary gland has two lobes: the anterior and posterior lobes. The anterior pituitary is glandular. Anterior Lobe • Growth hormone is a peptide anterior pituitary hormone essential for growth. • Cells under the action of GH increase in size (hypertrophy) and number (hyperplasia). GH also causes increase in bone length and thickness by deposition of cartilage at the ends of bones. • Thyroid-stimulating hormone (TSH) travels to the thyroid where it promotes production of thyroid hormones, which in turn regulate metabolic rates and body temperatures. • Gonadotropins and prolactin are also secreted by the anterior pituitary. Gonadotropins (which include follicle-stimulating hormone, FSH, and luteinizing hormone, LH) affect the gonads by stimulating gamete formation and production of sex hormones. Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production. . Posterior Lobe • The posterior lobe of the pituitary releases two hormones. • Antidiuretic Hormone (ADH), a peptide of 9 amino acids. It is also known as vasopressin. • ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood. • This it acts to reduce the volume of urine formed (giving it its name of antidiuretic hormone). • Oxytocin is a peptide of 9 amino acids. • Its principal actions are, stimulating contractions of the uterus at the time of birth, and stimulating release of milk when the baby begins to suckle.