Human Anatomy & Physiology II (PSIO202) Endocrine System Introduction PDF

Human Anatomy & Physiology II (PSIO202) Spring 2025 Endocrine System: Introduction 1. List the major organs of the endocrine system, and describe how endocrine functions are distributed through the body....

Human Anatomy & Physiology II (PSIO202) Spring 2025 Endocrine System: Introduction 1. List the major organs of the endocrine system, and describe how endocrine functions are distributed through the body. 2. Compare and contrast the endocrine and nervous systems, including the role of negative feedback regulation in maintenance of homeostasis. 3. Distinguish between peptide and steroid hormones in terms of structure, transport through the body, and mechanisms of action. 4. Describe the role of hormones in second messenger activation. Endocrine System vs. Nervous System Similarities Differences Control and Nature of message coordination of – nervous system: APs & NTs body activities. (action potentials & neurotransmitters) – endocrine system: hormones Use of chemical Message speed & duration messengers for – nervous system: milliseconds to seconds intercellular – endocrine system: seconds to minutes; communication. days to weeks Homeostasis = the ability to maintain a “relatively stable internal state” that persists despite changes in the “world outside”  Mechanisms of Homeostasis: When a change in a variable occurs in a closed system with feedback circuits, the system responds. Two types of feedback circuits: 1. Negative feedback loops changes in monitored parameter cause control mechanism to work to counteract further changes of the parameter in the same direction. Result: changes are triggered to reverse the detected change in the Result: change triggers reversal of detected change parameter. 2. Positive feedback loops AKA “cumulative causation” changes in monitored parameter cause control mechanism to work to cause/amplify changes of the parameter in the same direction. Result: changes are triggered to amplify the detected change in the parameter. Result: change triggers amplification of detected change  Mechanisms of Homeostasis: Homeostatic control systems have 3 basic components 1. Receptors - sense changes in the internal or external environment 2. Control center - processes information gathered from receptors & sends a response to the effectors 3. Effector - adjusts the regulated parameter  NOTE: The endocrine system does not include any exocrine glands. Exocrine Glands vs. Endocrine Glands Exocrine glands – Endocrine glands – secrete products into secrete products into ducts or lumens or to the interstitial fluid* or blood outer surface of the body *which diffuse into blood EXAMPLES?? NOTE: Some organs have both exocrine and endocrine portions. Only the endocrine portion of an organ functions as part of the endocrine system.  Anatomy of the Endocrine System Endocrine glands: – Pituitary – Thyroid – Parathyroid – Adrenal – Pineal Other organs/tissues with endocrine cells: – Hypothalamus – Thymus – Pancreas – Ovaries/Testes – Kidneys, Liver, Stomach, SI, Heart, Skin – Adipose Tissue NOTE: The endocrine system is a very diffuse system: any tissue/organ containing endocrine cells is part of the endocrine system.  Hormones = chemical messengers (aka mediator molecules) released in one part of the body regulate activity in other parts of the body Local hormones v.s. Circulating hormones Local Hormones NOTE: Some local hormones can act as autocrine and paracrine signals at the same time (e.g. IL‐2). Act locally on neighboring cells (paracrine) or on the same cell that secreted them (autocrine) without first entering the blood stream. Examples: – IL‐2 released from helper T cells stimulates proliferation of helper T cells and activation of cytotoxic T cells – Histamine released by mast cells stimulates HCl secretion from parietal cells in stomach  Circulating Hormones Enter interstitial fluid and then the bloodstream Lipid soluble hormones are bound to transport proteins for transport in body fluids. Water soluble hormones are freely dissolved in body fluids.  Lipid‐Soluble Hormones Steroid hormones derived from cholesterol chemical groups attached to structure’s core make it unique e.g. cortisol, testosterone, estrogens, progesterone, aldosterone…. Thyroid Hormones Thyroid hormones tyrosine ring with attached iodines very lipid soluble steroid & thyroid hormones can be ingested!  Water‐Soluble Hormones Peptide & protein hormones chains of amino acids hypothalamic releasing and inhibiting hormones ADH, oxytocin, hGH, TSH, ACTH insulin, glucagon, EPO Biogenic amines Biogenic amines modified amino acids catecholamines: NE, epinephrine, dopamine serotonin, melatonin Epinephrine Serotonin histamine from tyrosine, tryptophan, histidine…  Functions of Hormones Hormones are: released from glands in response to internal or external changes. produce wide‐reaching coordinated effects on multiple target tissues. help maintain homeostasis. regulate many aspects of organ system function. – ion and nutrient levels in extracellular fluid and blood – metabolic pathways – biological clock EXAMPLES? – contraction of cardiac & smooth muscle – glandular secretion Next: compare the mechanism – some immune functions – growth & development of action for water vs lipid – reproduction soluble hormones… NOTE: This list is by no means exhaustive. There is practically no aspect of organ system function which is not influenced by local and circulating hormones.  General Mechanism for Circulating Hormones Endocrine cell Blood capillary Circulating hormone Hormone receptor Hormones only affect target cells with specific membrane or intracellular proteins called hormone receptors. Distant target cells CRITICAL CONCEPT: All hormones have 1 or more types of target cells. To be a target cell for a hormone, all a cell must do is have a receptor for that hormone.  Mechanisms of Transport & Action for Hormones Hormones leave secretory cell by exocytosis or diffusion. Hormones travel freely in the blood or bound to a transport protein. Hormones bind to cell surface receptors or receptors inside target cell. Cell responds via: – synthesis of new molecules – alteration of existing molecules – change in membrane permeability – altered rates of reaction Different target cells may respond to the same hormone differently. – Hepatocytes ‐ insulin stimulates glycogen synthesis – Adipocytes ‐ insulin stimulates triglyceride synthesis  Steroid Transport protein Free hormone Blood capillary Hormone Action 1 Lipid‐soluble hormone Plasmalemma diffuses into cell ‐Leave secretory cell by diffusion Nucleus 2 Activated ‐Require transport receptor–hormone Receptor proteins to travel in complex alters gene blood expression DNA ‐Diffuse freely into Cytosol mRNA target cells Ribosome ‐Receptors are typically 3 Newly formed intracellular New mRNA directs protein ‐Work by changing level synthesis of specific proteins 4 New proteins alter of specific gene on ribosomes cell's activity expression NOTE: The response by target cells to Target cell steroid hormones is relatively slow!  Peptide Hormone Action ‐Leave secretory cell by exocytosis ‐Don’t require transport proteins to travel in blood ‐Bind to cell surface receptors ‐Typically evoke changes in activity of existing proteins via second messengers e.g. c‐AMP enzyme cascade ‐Second messengers alter phosphorylation state of existing proteins alteration of phosphorylation state of existing proteins http://encyclopedia.lubopitko‐bg.com/images/ The%20binding%20of%20a%20peptide%20hormone.jpg NOTE: The response by target cells to water‐soluble hormones is very, very fast!  More About Second Messengers… Water soluble (peptide) hormones (“first messenger”) bind to cell surface receptors  produce intracellular “second messengers” note: can or 2nd messenger levels Examples of second messengers: cAMP calcium ions cGMP The same hormone may use different second messengers in different target cells (for example: norepinephrine) Second messengers initiate a cascade of biochemical reactions (often involving phosphorylation or dephosphorylation) within target cells. kinases phosphatases  Blood capillary 1 Binding of hormone Adenylate cyclase (1 messenger) to its Water‐soluble hormone 2 st Activated adenylate cyclase receptor activates a Receptor converts ATP to cAMP specific G protein, which activates adenylate cyclase G protein ATP 2nd messenger cAMP Protein kinases 6 Phospho‐ 3 cAMP serves as a Activated diesterase protein inactivates 2nd messenger kinases to activate cAMP Kinases add Protein protein phosphates, ATP kinases phosphatases 4 Activated remove them. 5 Millions of ADP protein kinases phosphorylated Protein ‐ P phosphorylate proteins cause cellular proteins reactions that produce physiological responses Target cell CONSIDER: How does a hormone cause one type of response in one target cell but then also cause a completely different response in another target cell? ANSWER: Different types of receptors generate different second messengers! Slide from http://ourphysiology.blogspot.com/2010/03/35‐second‐messengers.html CONSIDER: How does a hormone cause one type of response in one target cell but then also cause a completely different response in another target cell? ANSWER: Different types of receptors generate different second messengers! Slide from: http://ourphysiology.blogspot.com/2010/03/35‐ second‐messengers.html