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VT 106 Comparative Anatomy and Physiology Endocrine System ENDOCRINE SYSTEM – regulates body functions endocrine glands – ductless glands; secrete products into interstitial fluid hormones – products of endocrine glands and cells usually diffuse into capillaries and circulate in blood major endocrine glands – pituitary, thyroid, parathyroid, adrenals, pineal other organs containing endocrine tissues hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, small intestine, placenta, others COMPARISON OF NERVOUS AND ENDOCRINE REGULATION Nervous System synaptic communication - neurotransmitter released at a specific site targets a specific muscle, gland, or neuron rapid onset of response (milliseconds) short duration – local inactivation of neurotransmitter Endocrine System endocrine communication - hormones released in bloodstream targets any tissues in the body with the correct receptors onset of response may take seconds to hours longer duration – hormone inactivated by liver or excreted by kidneys paracrine communication (local hormones) – hormones released in interstitial fluid target neighboring cells with correct receptors HORMONE FUNCTION hormone receptors – receptor proteins on or in specific target cells bind specific hormones and alter cell activity in some way Chemical Classes of Hormones steroid hormones – derived from cholesterol sex hormones – androgens, estrogens adrenal cortex hormones – mineralocorticoids, glucocorticoids calcitriol amino acid derivatives thyroid hormones, epinephrine, norepinephrine, melatonin peptide or protein hormones insulin, growth hormone, oxytocin, many others fatty acid derivatives (usually act as local hormones) prostaglandins 1 Mechanisms of Hormone Action – various target cells can respond differently to the same hormone depending on the receptor present turn genes on or off – alters gene expression (synthesis of a protein) enzyme – alters target cell function structural protein – alters target cell structure turn enzymes on or off = alters cell function Target Cell Response – depends on 3 factors: 1) hormone concentration 2) number of hormone receptors on target cell down-regulation - decrease number of receptors; decreases sensitivity up-regulation – increase number of receptors; increases sensitivity 3) influence of other hormones antagonistic effect – one hormone opposes the action of another net result depends on balance of opposing hormones synergistic effect – effect of 2 hormones is greater than sum of those hormones acting alone permissive effect – the action of one hormone is required for the proper function of another hormone Regulation of Hormone Secretion neural stimuli –endocrine cell stimulated by a neuron (neuroglandular junction) humoral stimuli – endocrine cell stimulated by changes in composition of blood or interstitial fluid hormonal stimuli – presence or absence of another hormone stimulates endocrine cell negative feedback – most endocrine cells are inhibited as their products or effects increase (eg. high thyroid hormone levels inhibit thyroid gland secretion) HYPOTHALAMUS AND PITUITARY GLAND – function together to regulate most aspects of growth, development, metabolism, and homeostasis HYPOTHALAMUS – integrates nervous and endocrine systems Neurosecretory cells – specialized neurons which produce hormones hormones stored by posterior pituitary antidiuretic hormone (ADH) oxytocin produced by neuron cell bodies in hypothalamus hormone vesicles travel down axons to posterior pituitary regulatory hormones – regulate anterior pituitary releasing hormones – stimulate anterior pituitary inhibiting hormones – inhibit anterior pituitary produced in hypothalamus and released in interstitial space 2 pituitary portal system – system of 2 capillary networks delivers hormones secreted by the hypothalamus to the anterior pituitary Autonomic motor neurons sympathetic stimulation of adrenal medulla secretes epinephrine and norepinephrine as hormones PITUITARY GLAND (HYPOPHYSIS) – ventral to hypothalamus in pituitary fossa of sphenoid bone infundibulum – stalk attaching pituitary to hypothalamus Hormones of Anterior Pituitary (adenohypophysis) tropins – hormones that regulate other endocrine glands or tissues 1) thyroid-stimulating hormone (TSH) – (thyrotropin) stimulates thyroid to produce and secrete thyroid hormones (T3 and T4) promoted by a releasing hormone from hypothalamus inhibited by negative feedback of T3 and T4 2) adrenocorticotropic hormone (ACTH) - (corticotropin) stimulates adrenal cortex to produce and secrete glucocorticoids promoted by a releasing hormone (during stress) inhibited by negative feedback of glucocorticoids 3) follicle-stimulating hormone (FSH) – (gonadotropin) female – stimulates ovaries to produce egg follicles and estrogen male – stimulates testes to produce sperm promoted by a releasing hormone inhibited by negative feedback of sex hormones 4) luteinizing hormone (LH) - (gonadotropin) female – stimulates ovaries to ovulate and maintain pregnancy male – stimulates testes to secrete testosterone promoted by gonadotropin-releasing hormone (GnRH) inhibited by negative feedback of sex hormones 5) prolactin (PRL) – (mammotropin) stimulates mammary glands to produce and secrete milk permissive and synergistic effects of other hormones required: estrogens, progestins, glucocorticoids, GH, T4, insulin, oxytocin promoted by a releasing hormone inhibited by an inhibiting hormone (hypothalamus) 3 6) growth hormone (GH) – (somatotropin) stimulates cells in most body tissues and organs causes cell growth and division, mobilization of energy stores, increased protein synthesis promoted by a releasing hormone inhibited by an inhibiting hormone gigantism – hypersecretion of GH during youth pituitary dwarfism – hyposecretion of GH during youth acromegally – hypersecretion of GH during adulthood 7) melanocyte-stimulating hormone (MSH) associated with color changes in reptiles and amphibians function unknown in mammals Hormones of Posterior Pituitary – produced by hypothalamus neuroendocrine reflexes – hormones are released due to neural impulses 1) oxytocin stimulates uterine wall muscles to contract during parturition stimulates mammary glands to eject milk (milk let-down) promoted by sensory stimuli: stretch of the cervix suckling of young positive feedback loop – hormone release continues until sensory stimulus stops (birth, or young stop suckling) 2) antidiuretic hormone (ADH) – prevents diuresis (increased urine output) promoted by: high blood osmotic pressure (high solute concentration) low blood pressure or volume (baroreceptors) inhibited by negative feedback target organs: kidneys – less urine produced sweat glands – decrease sweating arterioles constrict = increases blood pressure diabetes insipidus – hyposecretion of ADH polyuria – excess urine output polydipsia – excess drinking THYROID GLAND Anatomy of the Thyroid adjacent to trachea, caudal to larynx thyroid follicles – contain colloid (large quantity of thyroid hormones, T4 & T3, precursor molecule) follicular cells – simple cuboidal epithelium that secrete T3 and T4) parafollicular cells (C cells) – produce calcitonin 4 Thyroid Hormones (T4 and T3) T4 has 4 iodines (tetraiodothyronine) [thyroxine] T3 has 3 iodines (triiodothyronine) (thyroid hormones cannot be synthesized without adequate dietary iodine) target – most cells of body increases basal metabolic rate increase cellular metabolism – more ATP production, protein synthesis, more sodium/potassium pumps, mobilization of energy stores calorigenic effect – body temperature rises participate in growth – especially nervous tissue Regulation of Thyroid Hormones promoted by TSH from anterior pituitary inhibited by negative feedback of T3 Thyroid Disorders hypothyroidism – reduced secretion in young – small size and mental retardation in adult – low heart rate and body temperature, weakness, weight gain, alopecia (hair loss) hyperthyroidism – increased secretion high heart rate and blood pressure, elevated body temperature, nervousness, weight loss, increased appetite signs of increased GI and urinary activity goiter – enlarged thyroid dietary iodine deficiency – hyperplasia of follicular cells CALCITONIN – from parafollicular (C) cells of thyroid lowers blood calcium (increases bone density) targets osteoclasts – inhibits; decreases bone resorption kidneys – increases excretion of calcium ions in urine promoted by high blood calcium inhibited by low blood calcium 5 PARATHYROID GLANDS Anatomy of the Parathyroids 2-4 small glands attached to or within thyroid gland produce parathyroid hormone (PTH) [parathormone] Parathyroid Hormone – major regulator of calcium level in blood increases blood calcium targets: osteoclasts – stimulates resorption of bone, which releases calcium into blood kidneys – decreases calcium excretion in urine stimulates production of calcitriol – increases absorption of dietary calcium promoted by low blood calcium inhibited by high blood calcium hypocalcemia – low blood calcium; causes spontaneous nervous depolarization and muscle weakness hypoparathyroidism – insufficient PTH milk fever (dairy cattle) – excess calcium loss in milk eclampsia (cats and dogs) – high fetal calcium demands or milk production hypercalcemia – high blood calcium hyperparathyroidism – excess PTH primary – tumor secondary – insufficeint dietary calcium or calcitriol (Vit. D) when chronic, can cause fragile bones ADRENAL GLANDS Anatomy of Adrenals – cranial and medial to each kidney 2 functional regions of adrenal gland: adrenal cortex – outer region produces 3 types of hormones: mineralocorticoids glucocorticoids sex hormones adrenal medulla – central region modified sympathetic ganglion neuroendocrine cells – produce epinephrine and norepinephrine promoted by sympathetic ANS (fight-or-flight effects) (review effects of sympathetic nervous system) 6 Adrenal Cortical Hormones 1) Mineralocorticoids – regulate electrolyte and water homeostasis aldosterone – main mineralocorticoid produced targets kidneys – decreases excretion of Na+ and water in urine increases excretion of K+ in urine (increases blood pressure and blood Na+, decreases blood K+ ) promoted low blood Na+ or high blood K+ Also regulated by renin-angiotensin-aldosterone (RAA) pathway 1) decreased blood volume and pressure 2) kidneys secrete renin (enzyme) into blood 3) renin converts angiotensinogen (plasma protein from liver) into angiotensin I 4) enzyme in lung capillaries converts angiotensin I to angiotensin II 5) angiotensin II promotes aldosterone secretion by adrenal cortex 6) aldosterone promotes Na+ and water reabsorption in kidneys 7) blood volume and pressure increase 2) Glucocorticoids – increases blood glucose and helps resist stress cortisol (hydrocortisone) – main glucocorticoid target – most cells of the body promoted by ACTH from anterior pituitary (stress) inhibited by negative feedback of glucocorticoids Effects of Glucocorticoids (stress hormones) resistance to stress – mobilize energy sources; increase blood pressure, increase appetite protein breakdown (muscles) mobilizes amino acids lipolysis – breakdown and metabolism of fats glucose synthesis and storage in liver anti-inflammatory effects – inhibit white blood cells, reduce swelling and pain, slows healing depression of immune responses Cushing’s syndrome – hyperadrenocorticism loss of muscle, redistribution of fat (pendulous abdomen), poor healing, infections, weakness, polyphagia (increased appetite) high blood pressure – polyuria/polydipsia Addison’s disease – hypoadrenocorticism glucocorticoid drugs – frequently given to decrease inflammation and reduce immune responses may cause iatrogenic Cushing’s 7 3) Sex hormones little effect as amount is low compared to that produced in gonads THE STRESS RESPONSE stressor – physical or emotional disturbance that threatens equilibrium regulated mainly by the hypothalamus alarm phase – immediate sympathetic nervous response initiated by hypothalamus rapid energy mobilization to brain, heart, skeletal muscles reduced non-essential functions resistance phase – longer-lasting hormonal response initiated by hypothalamic releasing hormones CRH – ACTH – cortisol GHRH – GH TRH – TSH – T4 & T3 1) mobilizes lipids and proteins for energy 2) glucose-sparing for use by nervous tissues 3) synthesis of glucose from other molecules 4) maintains blood volume and pressure – supplies nutrients to tissue PANCREATIC ISLETS pancreas – between stomach and sm. intestine 99% exocrine glands producing digestive enzymes 1% pancreatic islets (islets of Langerhans) – produce hormones alpha (A) cells – secrete glucagon (increases blood glucose) beta (B) cells – secrete insulin (decreases blood glucose) delta (D) cells – secrete somatostatin inhibits glucagon and insulin release; slows nutrient absorption from GI tract Glucagon – increases blood glucose main target - liver glycogenolysis (glycogen stores broken down to glucose) gluconeogenesis (lactic acid and amino acids converted to glucose) other target – adipocytes stimulates lipolysis (fat breakdown for energy) decreases glucose consumption promoted by hypoglycemia (low blood glucose) and sympathetic impulses inhibited by glucose feedback, insulin, somatostatin from D cells 8 Insulin – decreases blood glucose targets – most body cells causes uptake of glucose by body cells by stimulating synthesis of glucose transporter proteins stimulates use of glucose for ATP synthesis also stimulates cellular synthesis and energy storage promoted by hyperglycemia (high blood glucose) and parasympathetic n.s. inhibited by negative feedback Pancreatic Islet Disorders Diabetes mellitus – inability to produce or respond to insulin cells can’t take in or use glucose for energy Signs and symptoms: hyperglycemia glucosuria – glucose in urine polyuria / polydipsia – water follows glucose into urine polyphagia – animal is starving lipolysis – break down fat stores for energy ketoacidosis – high blood ketones from fatty acids insulin shock – hypoglycemia due to overdose of insulin or not eating lack of glucose to brain causes convulsions, loss of consciousness, and death Pineal Gland – part of diencephalon; regulated by light and darkness melatonin – secreted mainly in darkness functions: circadian rhythm – sleep and awakening cycles timing of seasonal reproductive cycles Thymus – lies over base of heart and distal trachea participates in immune system hormones promote maturation of lymphocytes atrophies with age 9 Other Endocrine Tissues Stomach – gastrin – stimulates gastric functions Kidneys erythropoietin (EPO) – stimulates red blood cell formation calcitriol (vitamin D) – stimulates dietary calcium absorption Small intestine – secretin and cholecystokinin – stimulate exocrine pancreatic functions and inhibit gastric functions Placenta – various hormones that support pregnancy and parturition Ovaries and Testes – sex hormones – androgens, estrogens, progestins, others stimulate sexual characteristics, breeding behaviors, and pregnancy prostaglandins (PGs) – various local hormones in may tissues derived from membrane fatty acids PGEs – released by damaged cells and inflammatory cells stimulate inflammation, fever, and pain NSAIDS (non-steroidal anti-inflammatory drugs) – drugs that inhibit PGE synthesis 10