Endocrine System Intro Lecture 2023 PDF

1 College of Medicine and Dentistry Endocrine system Intro lecture Dr Doris Pierce 2 Learning Objectives After completing this topic, you should be able to https://staffresources.uhi.ac.uk/aph/endocrinesystem/index.html# • describe the key components and structures of the endocrine system, in...

1 College of Medicine and Dentistry Endocrine system Intro lecture Dr Doris Pierce 2 Learning Objectives After completing this topic, you should be able to https://staffresources.uhi.ac.uk/aph/endocrinesystem/index.html# • describe the key components and structures of the endocrine system, including the major glands and their locations within the body. • define the functions of hormones and their role in regulating physiological processes, such as growth, metabolism, reproduction, and stress response. • identify the interplay between the endocrine and other physiological systems, particularly the nervous system. • explain the mechanisms of hormone secretion, transportation in the bloodstream, and interaction with target tissues or organs. • recognise the importance of feedback mechanisms in maintaining hormonal balance and homeostasis within the body. 3 Endocrine versus nervous system Endocrine system Nervous system Signalling mechanism(s) Chemical Chemical/electrical Primary chemical signal Hormones Neurotransmitters Control Involuntary Voluntary and involuntary Response time Fast or slow Always fast (1 – 10 ms) Responses Widespread Targeted and specific Together, the autonomic nervous and endocrine systems maintain homeostasis. 4 Endocrine system development Arises from all three embryonic germ layers • Mesoderm - glands that produce steroid hormones (gonads, adrenal cortex) • Glands that produce the amine, peptide, and protein hormones from endoderm and ectoderm • Ectoderm - pituitary and pineal gland, adrenal medulla (neuroectoderm) • Endoderm - thyroid & parathyroid glands, pancreas, and thymus 5 Endocrine glands • The same in males and females except for testes (males only) and ovaries (females only) • Primary function of these ductless glands secrete hormones directly into the surrounding fluid • Some glands have both endocrine and exocrine function, e. g. pancreas • Other tissues, e. g. adipose tissue, also have endocrine functions Mariana Ruiz Villarreal. https://commons.wikimedia.org/wiki/File:Endocrine_system_glands.png . Creative Commons Attribution-Share Alike 3.0 6 Classes of hormones Three classes of hormones based on their chemical structure Amine hormones Peptide & protein hormones • synthesised from a single amino acid (tryptophan or tyrosine) • e. g. melatonin, adrenaline, noradrenaline, and dopamine • peptide hormones - multiple amino acids that link to form a short amino acid chain, e. g. growth hormone • protein hormones – longer polypeptides, e. g. insulin Water-soluble and insoluble in lipids cannot pass through cell membranes Lipid-derived steroid hormones • mostly derived from cholesterol • e. g. oestrogen and testosterone Lipid-soluble and non-polar - can diffuse across cell membranes 7 Two key pathways of hormone action Pathways involving intracellular hormone receptors • Receptors inside cell → hormones must be able to cross the cell membrane • Lipid-derived hormones readily diffuse through cell membrane and bind to receptor in cytosol • Thyroid hormones contain benzene rings studded with iodine - also lipid-soluble and can enter the cell. • Receptor-hormone complex enters nucleus and binds to target gene on DNA • Gene transcription creates mRNA → translated into desired protein Marieb & Hoehn (2019). https://ebookcentral.proquest.com/lib/jcu/reader.action?docID=5573766&ppg=634 8 Two key pathways of hormone action Pathways involving cell membrane hormone receptors • Receptor on cell surface - hydrophilic (water-soluble) hormones unable to diffuse through the lipid bilayer • All amino acid-derived hormones except thyroid hormones • Do not directly affect transcription of target genes but initiate signaling cascade via a second messenger • Cyclic adenosine monophosphate (cAMP) most used second messenger • Hormone is first messenger 9 Pathway involving cell membrane hormone receptors 1 1. Epinephrine binds to beta-adrenergic receptors on plasma membrane. 2. Hormone binding to receptor activates Gprotein. 3. G-protein activates adenylyl cyclase. 2 3 4. Adenylate cyclase converts ATP to cAMP (second messenger) 5 5. cAMP activates protein kinases – triggers target cell response (activates enzymes, stimulates cellular secretion etc.) 6. The enzyme phosphodiesterase breaks down cAMP, terminating the signal. 4 6 Biology 2e. Provided by: OpenStax. Located at: http://cnx.org/contents/[email protected]. License: CC BY: Attribution. 10 Endocrine gland stimuli Chemical and neural stimuli control endocrine activity. Marieb & Hoehn (2019). https://ebookcentral.proquest.com/lib/jcu/reader.action?docID=5573766&ppg=634 11 Regulation of hormone secretion – Feedback loops Positive feedback loop – release of additional hormone in response to original release, e. g. oxytocin release during childbirth OpenStax. https://commons.wikimedia.org/wiki/File:106_PregnancyPositive_Feedback.jpg. This file is licensed under the Creative Commons Attribution 4.0 International license. Negative feedback loop - most common method of hormone regulation - inhibition of further secretion of a hormone in response to adequate levels of that hormone CNX OpenStax. . This file is licensed under the Creative Commons Attribution 4.0 International license. https://commons.wikimedia.org/wiki/File:Figure_33_03_01.jpg 12 Hypothalamus-pituitary complex “Command centre” of the endocrine system Hypothalamus • Structure of diencephalon located anterior and inferior to thalamus • Both neural and endocrine functions • Anatomically and functionally related to the pituitary gland (or hypophysis) Pituitary gland • Bean-sized, two-lobed organ suspended from hypothalamus by the infundibulum (or pituitary stalk) • Posterior pituitary (neurohypophysis) is neural tissue derived from hypothalamic tissue. • Anterior pituitary (adenohypophysis) is glandular tissue developed from the primitive digestive tract. Richard Bowen (2018). http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/hypopit/histo_pit.html 13 Hypothalamic hormones 6 regulate anterior pituitary 2 released into capillaries in posterior pituitary when hypothalamic neurons are stimulated LadyofHats. https://commons.wikimedia.org/wiki/File:Endocrine_central_nervous_en.svg. This work has been released into the public domain by its author. 14 Anterior pituitary • Hypothalamic hormones can stimulate or inhibit AP hormone secretion • Secreted by neurons but enter AP through hypophyseal portal system • 7 AP hormones: follicle-stimulating hormone (FSH), luteinising hormone (LH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), prolactin (PRL), beta-endorphin, and growth hormone (GH) – FLAT PEG • FSH, LH, ACTH, and TSH (FLAT) referred to as tropic hormones – “turn on or off” other endocrine glands OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/17-3-the-pituitary-gland-and-hypothalamus. Creative Commons Attribution 4.0 International License. 15 Anterior pituitary hormones – Gonadotropins • FSH and LH – regulate function of ovaries and testes • Release triggered by GnRH during and after puberty • FSH o Stimulates production and maturation of gametes, including ova in women and sperm (Sertoli cells) in men o Also promotes follicular growth → release of oestrogen in ovaries • LH o Triggers ovulation and production of oestrogens and progesterone by ovaries o Stimulates production of testosterone by male testes (Leydig cells) • Negative feedback loop - high levels of reproductive hormones inhibit release of GnRH Artoria2e5. https://commons.wikimedia.org/wiki/File:Hypothalamic%E2%80%93pituitary%E2% 80%93gonadal_axis.svg. This file is licensed under the Creative Commons Attribution 3.0 Unported license. 16 Anterior pituitary hormones – Adrenocorticotropic hormone • Also called corticotropin • Release triggered by corticotropinreleasing hormone (CRH) – naturally occurs in a daily rhythm • Internal and external factors such as fever, hypoglycemia, and stressors can also trigger the release of CRH, and hence ACTH. • Stimulates adrenal cortex to synthesise and release corticosteroid hormones – cortisol, aldosterone and androgens Ross AP, Ben-Zacharia A, Harris C and Smrtka J. https://upload.wikimedia.org/wikipedia/commons/6/66/Hypothalamic-pituitaryadrenal_axis_diagram.jpg. This file is licensed under the Creative Commons Attribution 3.0 Unported license. 17 Adrenal glands 18 Adrenal gland hormones - Overview • Wedges of glandular and neuroendocrine tissue adhering to the top of the kidneys by a fibrous capsule • Have a rich blood and nerve supply 19 Adrenal glands – Adrenal cortex HPA axis • Component of hypothalamic-pituitary-adrenal (HPA) axis - mesoderm-derived • Hypothalamic stimulation of ACTH release via CRH • ACTH stimulates adrenal cortex to produce mineralocorticoids, glucocorticoids, and androgens - important for regulation of long-term stress response, blood pressure and blood volume, nutrient uptake and storage, fluid and electrolyte balance, and inflammation Biolinkage Project. https://commons.wikimedia.org/wiki/File:HPAaxis_-_Hypothalamus,_Pituitary_and_Adrenal_cortex_Axis.svg. This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. 20 Adrenal cortex hormones - Mineralocorticoids • Produced in zona glomerulosa • Affect body minerals, especially Na+ and K+ - essential for fluid and electrolyte balance • Aldosterone - major mineralocorticoid. • Important in regulating Na+ and K+ ion concentrations in urine, sweat, and saliva • For example, in response to low blood Na+, blood pressure, or blood volume, aldosterone increases Na+ retention, blood volume and blood pressure. (More about this in renal physiology) OpenStax College. https://commons.wikimedia.org/wiki/File:2711_Aldosterone _Feedback_Loop-01.jpg, This file is licensed under the Creative Commons Attribution 3.0 Unported license. 21 Adrenal cortex hormones - Glucocorticoids • Produced in zona fasciculata • Major role in glucose metabolism – cortisol most important • Inhibits tissue building while stimulating breakdown of stored nutrients to maintain adequate fuel supplies • Long-term stress - promotes catabolism of glycogen to glucose, stored triglyceride into fatty acids and glycerol, and muscle proteins into amino acids • Raw materials used to synthesise additional glucose and ketones for fuel • Medications containing glucocorticoids - downregulation of immune system, which inhibits the inflammatory response https://ilovebicycling.com/cortisol-and-cycling-the-stresshormone/ 22 Adrenal cortex hormones - Androgens • Produced in the gonads during puberty and most of adulthood • Supplemented by small amounts of androgens produced in zona reticularis • Converted to testosterone or oestrogens in tissues • May contribute to sex drive in adult women but function in adult men not well understood • Androgens produced by the zona reticularis become main source of oestrogens in postmenopausal women as ovarian functions decline. 23 Adrenal glands – Adrenal medulla Adrenal medulla - Sympathoadrenal system • Neuroectoderm-derived • Neuroendocrine tissue and an extension of the autonomic nervous system – part of sympathoadrenal system • Stimulated to secrete adrenaline and noradrenaline Chinski72. https://commons.wikimedia.org/wiki/File:Sympathoadrenal_System.jpg. This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. 24 Adrenal medulla hormones • Released in response to acute, short-term stress • Unique postganglionic SNS neurons (chromaffin cells) produce and release catecholamines adrenaline and noradrenaline (4 to 1 ratio) into systemic circulation –→ hormones • Secretion controlled by a neural pathway that originates from hypothalamus in response to danger or stress • Stimulate fight-or-flight response: increased blood glucose, heart rate, pulse, and blood pressure; dilation of airways and vasodilation of lungs, brain, heart, and skeletal muscle but vasoconstriction in liver, kidneys, GI tract, and skin • Other effects: dry mouth, loss of appetite, pupil dilation, and loss of peripheral vision 25 Adrenal gland – Acute vs chronic stress response Chronic Stressor: HypothalamicPituitary-Adrenal (HPA) axis Acute Stressor: Autonomic Nervous System Activation https://www.unm.edu/~lkravitz/Article%20folder/stressphysiology21.html 26 Adrenal gland – Chronic stress response Elevated stress hormones linked to higher risk of high blood pressure and heart events Brgfx. https://www.freepik.com/free-vector/stress-cortisol-systemscheme_41982300.htm#query=cortisol&position=0&from_view=keyword&track=sph. Free license 27 Anterior pituitary hormones – Thyroid-stimulating hormone • Also called thyrotropin • Released in response to TRH from hypothalamus • Stimulates normal development and secretory activity of thyroid gland • Triggers secretion of thyroid hormones by the thyroid gland – triiodothyronine (T3) and thyroxine (T4) • Negative feedback loop - elevated levels of thyroid hormones in the bloodstream trigger a drop in production of TRH and, subsequently TSH Alan Sved. https://commons.wikimedia.org/wiki/File:Thyroid_hormone_and_basal_metabolic_rate.png. This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. 28 Thyroid and parathyroid glands 29 The thyroid gland • Each lobe embedded with parathyroid glands, primarily on posterior surfaces • Thyroid hormones produced in colloid when mineral iodine atoms attach to the glycoprotein thyroglobulin - diffuse across the follicle cell membrane and enter the bloodstream • Triiodothyronine (T3), with three iodine and thyroxine (T4) with four iodine atoms • T4 converted into the more active T3 with a shorter half-life OpenStax College. https://openstax.org/books/anatomy-and-physiology/pages/17-4-the-thyroid-gland. Creative Commons Attribution 4.0 International License. 30 Thyroid hormone functions • Influence the body’s basal metabolic rate (metabolic hormones) • Metabolic actions o Increased nutrient breakdown and use of oxygen to produce ATP o Increased glucose oxidation with high level of heat as byproduct → raised body temperature • Essential for foetal and childhood tissue development and growth, especially nervous system • Complex interrelationship with reproductive hormones - deficiencies can influence libido, fertility, and other aspects of reproductive function • Increase body’s sensitivity to adrenaline and noradrenaline by upregulation of receptors in blood vessels → excessive T3 and T4 accelerate heart rate, strengthen heartbeat, and raise blood pressure 31 Thyroid hormone regulation Negative feedback loop OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/17-4-the-thyroid-gland. Creative Commons Attribution 4.0 International License. 32 Thyroid hormone - Calcitonin • Produced by parafollicular cells in tissue between thyroid follicles • Released in response to a rise in blood calcium levels - appears to decrease blood calcium concentrations by: o Inhibiting osteoclast activity o Increasing osteoblast activity o Decreasing calcium absorption in intestines o Increasing calcium loss in the urine • Functions usually not significant in maintaining calcium homeostasis - importance of calcitonin is not entirely understood https://myendoconsult.com/learn/calcitonin/ 33 Parathyroid glands • Embedded in the posterior surface of the thyroid gland • Chief cells produce and secrete parathyroid hormone (PTH), in response to low blood calcium levels. • PTH o causes calcium release from bones by stimulating osteoclasts and inhibiting osteoblasts. o causes increased reabsorption of calcium in kidney tubules from urine filtrate. o initiates production of calcitriol (active form of vitamin D3) in kidneys → increased absorption of dietary calcium by intestines BodyParts3D is made by DBCLS . https://commons.wikimedia.org/wiki/File:Parathyroid_med_ani.gif. This file is licensed under the Creative Commons Attribution-Share Alike 2.1 Japan license. • Negative feedback loop - rising blood calcium levels inhibit further release of PTH 34 Anterior pituitary hormones - Prolactin • In females, stimulates breast development and milk production (lactation) • Normally inhibited by prolactin-inhibiting factor (PIF; dopamine) • Levels rise during pregnancy in response to prolactin-releasing factors (e. g. oxytocin and TRH) from hypothalamus • Suckling most powerful stimulus for PRL release • No negative feedback system - without inhibition by dopamine, prolactin would be secreted indefinitely 35 Anterior pituitary hormones – Growth hormone • Also called somatotropin • GH levels controlled by release of GHRH and GHIH (aka somatostatin) • Primary function is anabolic - mediates growth and protein synthesis by triggering the liver and other tissues to produce insulin-like growth factors (IGFs) GHIH (GH-inhibiting hormone) inhibits the release of GH • IGFs enhance cellular proliferation and inhibit apoptosis - stimulate cells to increase amino acid uptake from blood for protein synthesis, particularly skeletal muscle and cartilage cells • Also promotes lipolysis and inhibits glucose uptake CNX OpenStax. https://commons.wikimedia.org/wiki/File:Figure_37_03_07.png. This file is licensed under the Creative Commons Attribution 4.0 International license. 36 Posterior pituitary • Actually, an extension of the neurons of the hypothalamus • Cell bodies rest in hypothalamus but axons descend as hypothalamic–hypophyseal tract to posterior pituitary • Does not produce hormones but stores and secretes hormones produced by hypothalamus • Oxytocin from paraventricular nuclei and ADH from supraoptic nuclei • Signals from the hypothalamic neurons initiate hormone release into blood. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/17-3-the-pituitary-gland-and-hypothalamus. Creative Commons Attribution 4.0 International License. 37 Posterior pituitary – Antidiuretic hormone • Also called vasopressin • Released in response to increase in plasma osmolarity (i. e. water loss or reduced intake) • Acts on kidneys to promote water reabsorption from forming urine – conserves water • Can also cause vasoconstriction (hence vasopressin name) and raise blood pressure in high concentrations • Negative feedback loop - as blood osmolarity decreases, hypothalamic osmoreceptors prompt decreased ADH secretion • Drugs, e. g., alcohol, can affect ADH secretion OpenStax College. https://commons.wikimedia.org/wiki/File:2709_ADH.jpg. This file is licensed under the Creative Commons Attribution 3.0 Unported license. 38 Posterior pituitary – Oxytocin • Stimulates uterine contractions and dilation of the cervix • Continually released throughout childbirth through a positive feedback mechanism • Continues to play a role in maternal and newborn health - necessary for milk ejection reflex ( “letdown”) and contributes to parent–newborn bonding (attachment) • Also thought to be involved in feelings of love and closeness, as well as sexual response OpenStax. https://openstax.org/books/anatomy-and-physiology-2e/pages/1-5-homeostasis . Creative Commons Attribution 4.0 International License.