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Endocrine System Lecture # 1 Prof Kumlesh K. Dev Department of Physiology Chapter 4 Principles of Neural and Hormonal Communication Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Endocrine Lectures Thu 16th Apr 10-11 hr Hormones – basics on hormone signaling Fri 17th Apr 14-15 hr Hormones II – hypothalamus, pituitary, adrenal glands Mon 20th Apr 09-10 hr Cancelled – no lecture Thu 23rd Apr 10-11 hr Hormones III – thyroid, parathyroid, pineal glands Chapter 4 Principles of Neural and Hormonal Communication Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 1 Types of cellular communication Classification Communication is critical for cell survival 2 major regulatory systems: – Nervous System – Endocrine System Chapter 4 Principles of Neural and Hormonal Communication Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Four types of chemical messengers Classification 1) Autocrine/Paracrine • Local chemical messengers • Exert effect on neighbouring cells (paracrine) or on same cell (autocrine) 2) Neurotransmitter • Short-range chemical messengers • Diffuse across narrow space (synapse) to act on adjoining target cell (another neuron, a muscle, or a gland) 2 Four types of chemical messengers Classification 3) Neurohormones • Hormones released into blood by neurosecretory neurons • Distributed through blood to distant target cells 4) Hormones • Long-range messengers • Secreted into blood by endocrine glands in response to appropriate signal and act on distant target cells 1. Autocrine/Paracrine Classification Secretory Cell – chemical messenger released from a cell : – can act on itself (autocrine) – can act on another cell close-by (paracrine) Cell #2 3 2. Neurotransmitters Classification – Neurotransmitters released from synaptic vesicles – Diffuse across synapse between two cells synapse with muscles synapse with glands – Transmitter binds a receptor – Receptor activation stimulates the cell synapse with another neuron 3. Neurohormones Classification – Released from synaptic vesicles by neuron – Released into blood – Acts in manner similar to hormones 4 4. Hormones Classification – Messengers of endocrine system – Released from endocrine gland into blood – Transported in blood – Target cell:expresses specific receptors Nervous and Endocrine Systems Classification Property Nervous System Endocrine System Structure Wired system of neurons Wireless system of glands Chemical Messenger Neurotransmitter Hormones Target site Very close Far away Distance of Action Across synaptic cleft Carried by blood Speed of Response milliseconds mins to hours Duration of Action milliseconds mins to days Major Function fast precise responses long duration responses 5 Neuroendocrine cells Classification Neuroendocrine cells release neurohormones and are found in, for example: • • • • • • • • adrenal medulla thyroid islet of Langerhangs pituitary cells renin-secreting cells mucosa of the gastro-intestinal tract pancreas lower respiratory tract Types of hormone Classification • Two hormone categories based on solubility – Hydrophilic (lipophobic, water soluble) • Peptide hormones • Catecholamines – Lipophilic (hydrophobic, water insoluble) • Thyroid hormone • Steroid hormones 6 Types of hormone Classification Property Peptide Catecholamines Thyroid Steriods Structure amino acids tyrosine derivative iodinated tyrosine cholesterol derivative Solubility hydrophilic lipophobic hydrophilic lipophobic lipophilic hydrophobic lipophilic hydrophobic Synthesis in ER in cytosol organelles intracell Storage granules granules colloid lipd droplets Secretion exocytosis of granules exocytosis of granules endocytosis of colloid diffusion Blood Transport free hormone plasma bound plasma bound plasma bound Receptors on cell surface on cell surface inside cell inside cell Hormone hypothalamic, pituitary, pineal, pancreas, parathyroid, GIT, kidney, liver, heart adrenal medulla thyroid follicular adrenal cortex gonads, Vit D Steroid hormone Synthesis female sex hormone male sex hormone adrenal cortex hormone - adrenal cortex hormone female sex hormone steroid hormones derived from cholesterol are small lipid-soluble molecules diffuse throw cell membrane receptor is intracellular 7 Steroid hormone Synthesis All steroid hormones are produced via cholesterol modification Extracellular Receptors Receptors • all cells maybe exposed to hormones via circulation ion channel • only cells with receptors for hormone can respond enzyme • extracellular chemical (hormone) binds to receptor • receptor activation alters – channel function – second-messenger systems change in ion flow (e.g. Na, Ca, K, Cl) Increased product signal (eg cAMP) 8 Intracellular/Nuclear Receptors Receptors • steroid receptors bind steroid hormone • hormone-receptor complex becomes a transcription factor (alters gene transcription) • each steroid receptor binds a unique DNA sequence (response element within an enhancer region) plasma protein carrier oestrogen oestrogen receptor protein synthesis mRNA transcription trans loca tion • this alters the rate of transcription Receptor Regulation Receptors • target cell may be unresponsive due to lack of receptors (physiological, genetic, disease) • one hormone can influence activity of another hormone by regulation of its receptor: Permissiveness – one hormone is required for another hormone to work Synergism – actions of several hormones are complimentary – combined effect is greater than sum of separate effects Antagonism – one hormone inhibits another hormone’s receptors – reduces effectiveness of second hormone 9 Steroid hormone receptors Receptors • Type I receptors – Sex hormone receptors • Androgen receptor • Estrogen receptor • Progesterone receptor – Glucocorticoid receptor – Mineralocorticoid receptor • Type II receptors (No HSP association) – Vitamin A receptor (vitamin A) – Vitamin D receptor (vitamin D) – Retinoid receptor – Thyroid hormone receptor Types of Endocrine Glands Endocrine System – Derived from epithelial tissue – Primary endocrine organs – Secondary endocrine organs 10 Primary & Secondary Endocrine Glands Endocrine System endocrine glands are composed of clumps of secretory cells of epithelial origin surrounded by capillaries (fenestrated) unlike exocrine glands, endocrine glands have no duct system endocrine glands may exist in discrete organs - pituitary, thyroid, parathyroid, adrenal and pineal glands ENDOCRINE SECRETION AS PRIMARY FUNCTION - Pituitary gland - Adrenal glands - Thyroid gland - Parathyroid glands - Pineal gland • • • • • • • Hypothalamus Thymus Pancreas Gonads Kidney Heart Adipose tissue Endocrine System Endocrine System Overall functions – regulate organic metabolism – controls water and electrolyte balance – induces adaptive changes to help cope with stress – promotes smooth, growth and development – controls reproduction – regulates red blood cell production – controls circulation, digestion, absorption of food 11 Hormone Signalling Endocrine System Hormones secreted by endocrine gland cells • enter blood and distributed throughout body • can act at distant target sites Hormone secretion by endocrine glands triggered by many signals: • Insulin secretion regulated by blood glucose conc • Secretion of sex hormones from ovaries and testes controlled by anterior pituitary hormones • Oxytocin secretion from posterior pituitary regulated by neurotransmitter release Hormone Concentrations Endocrine System • plasma conc. of hormones is controlled by rate of secretion • secretory output of endocrine cells controlled by – neural input – another hormone • plasma conc. also influenced by – rate of removal from blood by metabolism and excretion – rate of activation or its extent of binding to plasma proteins 12 Endocrine Dysfunction: Hyposecretion Endocrine System Primary hyposecretion • due to abnormality within gland • causes – – – – – – – genetic dietary (eg lack of iodine) chemical or toxic immunologic (autoimmune diseases) cancer Iatrogenic (eg surgical removal) idiopathic (unknown) Secondary hyposecretion • deficiency of tropic hormone Endocrine Dysfunction: Hypersecretion Endocrine System • Causes – Tumours continuously secrete hormone – Immunologic factors • Primary hypersecretion – too much hormone secreted due to abnormality within gland • Secondary hypersecretion – excessive stimulation of gland causes oversecretion 13 Tropic Hormones Endocrine System • regulates secretion of other hormones • stimulates/maintains endocrine target tissues • example – Thyroid-stimulating hormone (TSH) – secreted from anterior pituitary – stimulates thyroid hormone secretion from thyroid gland – maintains structural integrity of thyroid gland Control by Feedback Mechanisms Endocrine System Gland 1 - Positive Feedback e.g. Hormone 3 activates Gland 2 Hormone 1 Gland 2 Negative Feedback Hormone 2 e.g. Hormone 3 inhibits Gland 1 Gland 3 + Hormone 3 14