Chapter 18: Endocrine System
Chapter 18: Endocrine System BIOL 224
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Date Created: 09/02/16
Chapter 18 – Endocrine System 08/30 & 09/01 The Endocrine System • System of glands and hormones • Broad reaching, can almost reach every cell What the system does: • Regulate processes that go on for decades • Long term development and control • Slow response • Reproduction Hormones • Chemical messengers secreted into the bloodstream Found in: • Hypothalamus • Pituitary gland • Thyroid gland • Parathyroid gland • Adrenal gland • Pineal gland • Pancreas *All organs have some secondary hormonal function (involved in digestion in some way) • Unlike the nervous system, which is directed to a specific location, hormones are secreted into the bloodstream and travel throughout the entire body. - Can stay there for long periods of time - Since they all travel through the bloodstream, different hormones interact and perform different responses in the body depending on location and which hormones they are On Exam Glands & their hormones Gland Hormones Hypothalamus Production of ADH, OXT, & regulatory hormones Pineal gland Melatonin Parathyroid gland Parathyroid hormone (PTH) Pituitary gland Anterior: MSH, FSH, LH, ACTH, TH, PRL, GH Posterior: release OXT & ADH Thyroid gland Thyroxine (T4), Triiodothyronine (T3), Calcitonin Adrenal gland Medulla: epinephrine, norepinephrine Cortex: cortisol, corticosterone, aldosterone, androgens Pancreatic islet Insulin, glucagon On Exam Organs with secondary endocrine function & hormones secreted Organ Hormones Heart Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) Thymus Thymosins Adipose tissue Leptin Kidneys Erythropoietin (EPO), calcitriol Digestive tract Numerous hormones involved in the coordination of system functions, glucose, metabolism, appetite Gonads Testes: androgens (testosterone), inhibin Ovaries: estrogen, progesterone, inhibin * Most if not all organs have secondary function that is similar to primary function Intercellular Communication • 4 types 1. Direct communication • Two cells of same type - Not common, fairly rare • Ions and molecules across gap junctions - Share cytoplasm across a pore and share whatever is inside of it • Cells directly next to each other 2. Paracrine communication • Chemical signals among cells within a tissue - Multicellular labor at tissue level - Cell to cell communication within tissue and coordinates things within those sets of cells • Most common form of communication • Cells are close to each other but not directly next to each other 3. Synaptic communication • Occurs across synaptic clefts • Chemical message = neurotransmitter • Ideal for crisis management • Fast response • Limited to specific area • Neuron travels down neurotransmitters to the cell • Single neuron can only innervate to a single cell 4. Endocrine communication • Chemical signal = hormones - Hormones are the form in which messages travel - Hormone because it is secreted into the bloodstream • Broad reaching - Coordinate activity across organ systems a. possible to do so with one message sent out - long-term regulation ex. Fertilization of ovum to day you die hormones throughout lifetime controls what occurs in your body • Response - Specific for a given target cell - May be highly variable among different target cells a. Cells may respond differently - Hormone response dependent on receptors and type they are in a cell * Target cell • Has receptors needed to bind and read hormonal messages • Binding of hormones (to target cell) a. Stimulate transcription/translation - synthesis of enzymes/structural proteins b. Increase or decrease rate of synthesis c. Activate/inactivate enzyme - hormone receptive complex can turn enzyme on or off d. Open/close a membrane channel Classes of hormones (3) • Functionally different (split up by basic structure) 1. Amino acid derivatives • single amino acids • small • commonly derived from tyrosine and tryptophan ex. of tyrosine: thyroid hormone, catecholamine such as epinephrine, norepinephrine, dopamine ex. of tryptophan: serotonin, melatonin, fried turkey 2. Peptide hormones • longer chain of amino acids (shortest is 9) • synthesized as prohormones - amplifies affects of existing hormones • associated with sugars and proteins • glycoproteins ex. TSH (release of thyroid hormone; affects metabolism), LH & FSH (release of sex hormones) • lengths a. Shorter - antidiuretic hormone (ADH) & oxytocin (OXT) - 9 amino acids long b. Larger - growth hormone (GH) – 191 amino acids & prolactin (PRL) – 198 amino acids * Both water-soluble and don’t easily cross cell membranes 3. Lipid derivatives • steroids - cholesterol (derived from) - nonpolar • eicosanoids (not a major focus) - extracellular paracrine factors - some secondary roles as hormones 3a. gonads • secondary function - androgens - estrogens - progestins (sex hormones) •primary function is to make gametes 3b. adrenal glands (corticosteroids) • ex. aldosterone, cortisol • good example of endocrine glands as primary function 3c. kidneys (calcitriol) • secondary function • Lipid soluble, don’t like water 4. Free hormones • Most hormones aren’t secreted in this form • Remain functional for less than 1 hour in 3 ways a. diffuse out of bloodstream and bind to receptors on target cells b. are broken down and absorbed by cells of liver or kidneys - filter out of blood into digestive or urinary tracts c. are broken down by enzymes in plasma or interstitial fluids Secretion and Distribution of hormones 1. Thyroid and steroid hormones • 99% bound to transport proteins - bounded to carrier proteins and keeps it in bloodstream as a reserve • Why? - bunch of hormones allows for constant, steady supply of hormone - can be described as “big bank with little trickle” * Important with metabolism, which you don’t want to fluctuate Mechanisms of hormone action 1. Hormone receptor • Located on membrane or inside cell • Lock and key fit with hormone - specific target cell will bind to specific hormone • Presence/absence = hormone sensitivity - determines if target cell responds • Different tissues = different receptor combinations - can alter entire system by sending same signal and having different responses by different tissues/cells 2. Hormone regulation • Ability of cell to alter how they respond by presence or absence of hormones 2a. down-regulation - chronic elevated hormone levels triggers decrease receptors - cells become less sensitive to it - fewer receptors take higher concentration of hormone to activate response 2b. up-regulation - absence of a hormone triggers increase in number of receptors - cells become more sensitive to it - greater amount of receptors takes a lower concentration of hormone to activate response First and second messengers 1. The first messenger is the hormone - message trying to send to cell • Some hormones initiate target cell response on their own - go directly into the cell and bind * which ones? - steroid hormones ex. anabolic steroids • Slower *why? - transcription/translation are slow and takes time to process and create protein product - single message across membrane lipid soluble and goes into a cell and initiates a response 2. Second messenger • Amplify information within cell - first message binding cause 1000’s of messages in cell • May act as enzyme activator (turn on & off), inhibitor, or cofactor 2a. Action • Hormone binds to receptor outside cell – why? - because hormones are water soluble • Activates G-protein - embedded in cell membrane - enzyme complex with membrane receptor • G-protein activates adenylate cyclase - good example: binding causes in protein 2 messenger action to occur and for the process to start • Converts ATP to cyclic AMP (cAMP) = common secondary messenger - ~80% of prescription drugs target receptors that work with G-proteins - The first messenger binds to a membrane receptor and activates a G-protein - Alpha receptors (epinephrine or norepinephrine) decrease levels of cAMP in the cytoplasm - Beta receptors (epinephrine or norepinephrine) increase levels of cAMP, enzymes may be activated or ion channels may be opened, accelerating the metabolic activity of the cell - Opposite reaction with the same hormone - G-proteins and calcium ions activated G-proteins could also trigger • Opening of calcium ion channels in membrane • Release of calcium ions from intracellular stores activation of enzymes * First messenger: binds outside * Second messenger: binds inside & does work * Effects of intercellular hormone binding process 1. Diffusion through membrane lipids 2. binding of hormone to cytoplasmic or nuclear receptors 3. Binding of hormone – receptor complex to DNA 4. Gene activation 5. Transcription and mRNA production 6. Translation and protein synthesis 7. Target cell response Hormones and Plasma Membrane Receptors 1. Catecholamines and Peptide hormones • Hydrophilic (water soluble) - can’t pass through cell membrane • Binds to extracellular receptors - faces outward so it can bind to receptor outside of cell nd - binding to cell allows for arrival of 2 messenger iClicker question: Which of the following is a catecholamine? A. testosterone B. all amino acid hormones C. antidiuretic hormone D. epinephrine E. none of the above Control of Endocrine Activity by Endocrine Reflexes Endocrine reflexes • Stimulus triggers production of hormones • 3 types 1. Humoral stimuli • Ions • Changes in composition of extracellular fluid - something in fluid causing hormone imbalance ex. too much sodium, pee a lot of sodium out 2. Hormonal stimuli • Thing in fluid is hormones • Arrival or removal of specific hormone • Tropic hormones - cause another hormone to be released 3. Neural stimuli • Arrival of neurotransmitters at neuroglandular junctions Negative feedback typically controls reflex - reduce levels to normal - control through reflex (response to stimulus) 3 types of Endocrine reflexes 1. Simple endocrine reflex • Involves only one hormone 2. Complex endocrine reflex • Involves intermediary steps & 2 or more hormones 3. Neuroendocrine reflexes • Pathways include both neural and endocrine components • Change in frequency in action potential • Send a burst of hormones with a big increase and drop - if frequent enough it can cause one response, if not then a different response *Figure 18-5 The Mechanisms of Hypothalamic Control over Endocrine function - 3 mechanisms 1. Hypothalamus is the main connection between the endocrine and nervous system. It’s the nervous tissue acting like typical nervous tissue where it sends a neuron out and synapses with a gland and the release of a neurotransmitter. In this case it’s a sympathetic stimulus, so it is norepinephrine that’s being secreted. Norepinephrine binds to the glandular cells and the adrenal medulla. Lead to the release of the hormones norepinephrine and epinephrine. In this case because it is sympathetic, there is a neurotransmitter norepinephrine because of where it ends up being secreted. Gets dumped into the blood, it becomes a hormone. 2. Hypothalamus acting like an endocrine structure. Secretes a messenger into the bloodstream so it secretes a hormone. The hormone enters the blood, travels through the blood and then finds a target cell somewhere else. The target cell it finds is the anterior lobe of the pituitary and it passes through the portal system, the hypophyseal portal system (infundibulum). In the end it is secreting hormones into the bloodstream, so it is acting like an endocrine structure. 3. Grey area between the 2 other mechanisms to where it acts like both. The hypothalamus has neurons that synthesize hormones and the body of the neuron is in the hypothalamus. The axon extends down into the posterior lobe of the pituitary and the terminal ends end at the capillary. So the hormones are secreted in the blood (their messengers), making it a hormone, but it comes out of a neuron. Play as both systems because it comes out of a neuron (kind of nervous), and secretes into the bloodstream (kind of endocrine). Complex commands • Issued by changing - amount of hormone secreted - pattern of hormone release • Hypothalamic and pituitary hormones released in sudden bursts • Frequency changes response of target cells - big amount = big response - small amount = small response (same with frequency) The Pituitary Gland (aka hypophysis) “master gland” • Directly contacts hypothalamus through infundibulum • Lies within sella turcica • Releases 9 important peptide hormones - tell different endocrine structures to secrete their hormone • Hypothalamic neurons release regulatory hormones into median eminence • Neurosecretory neurons - involved in connection to posterior lobe of pituitary gland • Portal vessels link two capillary networks collectively = portal system • Hypophyseal portal system - Importance? - to give a high concentration direct blood connection to the 2 tissues directly without diluting the message capillary network à portal system à capillary network (different tissue) The Anterior Lobe of the Pituitary Gland (aka adenohypophysis) • Hormones “turn on” endocrine glands or support other organs • Has 3 regions - Pars distalis - Pars tuberalis - Pars intermedia Hypothalamic Control of the Anterior Lobe 1. Releasing hormones (RH) • Tell anterior lobe to release hormone • Stimulate synthesis and secretion by anterior lobe 2. Inhibiting hormone (IH) • Inhibits the release • Prevent synthesis and secretion by anterior lobe • Gives fine control of secretion of anterior lobe • Negative feedback controls rate of secretion • Anterior pituitary secretes tropic hormones - regulate other endocrine glands • Maintain very minimal fluctuation - 2 areas of control • More complicated feedback loop, the more stable it is *