BMF_Membrane Potentials_2023 PW.pptx

RCSI Royal College of Surgeons in Ireland Medical University of Bahrain Membrane Potentials Class Course Code Title Lecturer Date Year 1 The Body: Movement & Function MED 1 - 102 Membrane Potentials Dr. Patrick Walsh 07.11.23 Learning Objectives  Describe the physicochemical properties of nerv...

RCSI Royal College of Surgeons in Ireland Medical University of Bahrain Membrane Potentials Class Course Code Title Lecturer Date Year 1 The Body: Movement & Function MED 1 - 102 Membrane Potentials Dr. Patrick Walsh 07.11.23 Learning Objectives  Describe the physicochemical properties of nerve cell membranes  Differentiate between active and passive transport mechanisms  Describe, both qualitatively and quantitatively, the transmembrane gradients for major ions  Describe the Na+-K+ pump as a driving force for membrane potential, transporters, and exchangers Cell (“plasma”) membrane • 3 – 10 nm Cell (“plasma”) membrane Sherwood, Human Physiology Cell (“plasma”) membrane Sherwood, Human Physiology PLASMA MEMBRANE • All cells surrounded by fluid lipid-protein bi-layer • Functions o Forms basic structural barrier enclosing cell o Serves as barrier to free passage of substances o Cell can maintain different mixtures of substances inside and outside (nutrients, waste products) o Fluidity of membrane/cell o Communication between cells o Response to external stimuli • Membrane proteins o Integral / transmembrane o Extrinsic / peripheral TRANSPORT ACROSS MEMBRANES • Diffusion (e.g., O2, CO2, lipid (fat)-soluble) • Protein-mediated membrane transport – Channel or Carrier proteins Guyton & Hall: Textbook of Medical Physiology TRANSPORT ACROSS MEMBRANES • Diffusion (e.g., O2, CO2, lipid (fat)-soluble) • Protein-mediated membrane transport – Channel or Carrier proteins • Endocytosis – Phagocytosis – Pinocytosis • Exocytosis Guyton & Hall: Textbook of Medical Physiology extracellular Plasma membrane - Phospholipid bilayer that encloses ce - Embedded with proteins that act as receptors/channels intracellular + + Head (hydrophilic) choline, phosphate & glycerol Tails (hydrophobic) fatty acid nucleus MOVEMENT ACROSS PM MEMBRANE • Lipid soluble & small uncharged molecules (e.g. O2) – Freely diffusible – Movement driven by concentration gradient • Charged (ions) / water soluble – Hydrophobic interior of PM prevents free movement nonpolar polar MOVEMENT ACROSS PM MEMBRANE • Lipid soluble & small uncharged molecules (e.g. O2) – Freely diffusible – Movement driven by concentration gradient • Charged (ions) / water soluble – Hydrophobic interior of PM prevents free movement • PM is said to be “selectively permeable” MEMBRANE POTENTIAL • Separation of opposite charges gives rise to “membrane potential” as opposite charges attract – Difference in cations (+) and anions (-) across PM extracellular + + + + + + intracellular - - MEMBRANE POTENTIAL • Separation of opposite charges gives rise to “membrane potential” as opposite charges attract – Difference in cations (+) and anions (-) across PM • All cells have a membrane potential o o o o Electrical difference across membrane Excitable cells actively induce changes Basis for electrical excitability of nerve and muscle Harnessed for transporting substances MEMBRANE POTENTIAL • Separation of opposite charges gives rise to “membrane potential” as opposite charges attract – Difference in cations (+) and anions (-) across PM • All cells have a membrane potential o o o o Electrical difference across membrane Excitable cells actively induce changes Basis for electrical excitability of nerve and muscle Harnessed for transporting substances • How? o PM “barrier” allows cells to establish differences in concentrations of key charged ions across PM o Magnitude of potential depends on degree of charge separation ABSENCE OF A MEMBRANE POTENTIAL PM Extracellular + - + - + + - + - + + + + Intracellular - + + - + - - + + - - - + + + + + - + - Electrically neutral: Despite membrane barrier, no charge difference across membrane: – therefore, no membrane potential PRESENCE OF A MEMBRANE POTENTIAL PM Extracellular + - + + - + - - + + + - + - Remainder of fluid electrically neutral Intracellular + + + + + + + + + + - + + - - - + + - - + + - Remainder of fluid electrically neutral Separated charges responsible for potential [RESTING] MEMBRANE POTENTIAL - RMP • Cell membrane is more negative inside than outside – neuron is about -70 mV ECF ICF [RESTING] MEMBRANE POTENTIAL - RMP • Cell membrane is more negative inside than outside – neuron is about -70 mV What is responsible? • 1st: Unequal distribution of K+, Na+ and A(large protein anions) between inside and outside of cell ECF ICF [RESTING] MEMBRANE POTENTIAL - RMP • Cell membrane is more negative inside than outside – neuron is about -70 mV What is responsible? • 1st: Unequal distribution of K+, Na+ and A(large protein anions) between inside and outside of cell • 2nd: “selective” leakiness of PM to K+ ECF ICF DISTRIBUTION OF IONS RESPONSIBLE FOR THE RMP ion ECF ICF K+ 5 150 Na+ 150 15 A- 0 65 Values in mM Na+ / K+ ATPase (pump) establishes K+ & Na+ concentrations across PM • Membrane-spanning enzyme • Transports Na+ out, K+ in • 3 Na+ out for 2 K+ in – uses energy (ATP) – 200 million ions/sec • Establishes concentration gradients for Na+ and K+ 3 Na+ ECF ICF 2 K+ Na+ / K+ ATPase Na-K pump RMP CONT. • Unequal transport of positive ions by Na+/K+ATPase also generates a small potential – inside becomes a little negatively charged with regards outside because of 3Na+ out for 2K+ in RMP CONT. • Unequal transport of positive ions by Na+/K+ATPase also generates a small potential – inside becomes a little negatively charged with regards outside because of 3Na+ out for 2K+ in But: What is responsible for most of the potential? • Diffusion of K+ out of cell down concentration gradient established by pump – Leaves inside more negative ECF ICF + + - K+ + - Concentration gradient for K+ K+ + + + - A- PM IS LEAKY TO K+ • But doesn’t PM stop ion movement? ion ECF ICF Relative permeability K+ 5 150 50-75 Na+ 150 15 1 A- 0 65 0 • Despite “barrier”, PM contains many K+ leak channels, • Allows diffusion of K+ “at rest” out of cell down its concentration gradient ECF ICF + + - K+ + - Concentration gradient for K+ K+ + + + - A- EFFECT OF K+ MOVEMENT ON MP • K+ leaks out because of large concentration gradient, making inside negative • A- (cannot pass) are left behind – further increases inside negativity • Resting membrane potential is -70 mV ADDITIONAL CONTRIBUTORS TO NEGATIVE MEMBRANE POTENTIAL* PS - - PIP 2 – Phosphatidylinositol (PIP2) – Phosphatidylserine (PS) + • Membrane constituents also contribute to negative inner membrane potential - • Both provide negative charge to inner portion of plasma membrane *not essential to know this NERNST EQUILIBRIUM • Chemical gradient acts as a driving force for diffusion of K+ out of the cell ECF ICF + + - K+ + + - Concentration gradient for K+ K+ NERNST EQUILIBRIUM • Chemical gradient acts as a driving force for diffusion of K+ out of the cell • The residual negative charge acts as a electrical driving force drawing K+ back into the cell ECF Electrical gradient K+ ICF + + + + - Concentration gradient for K+ K+ NERNST EQUILIBRIUM • Chemical gradient acts as a driving force for diffusion of K+ out of the cell • The residual negative charge acts as a electrical driving force drawing K+ back into the cell • Equilibrium: electrical force balances chemical force, no net transport NERNST EQUATION E = + 61 x log Co Ci E = Equilibrium potential for ion in mV 61 = constant: incorporating Gas constant R; absolute temperature (T), ions valence when the valence is +1: as we have Na+/K+(z), the Faraday constant and logarithmic conversion (natural to base 10) 61 = (RT/zF) Co = conc. ion outside Ci = conc. ion inside EK+ = 61 x log 5mM 150mM NERNST EQUATION E = + 61 x log Co Ci E = Equilibrium potential for ion in mV 61 = constant: incorporating Gas constant R; absolute temperature (T), ions valence when the valence is +1: as we have Na+/K+(z), the Faraday constant and logarithmic conversion (natural to base 10) 61 = (RT/zF) Co = conc. ion outside Ci = conc. ion inside EK+ = 61 x log 5mM 150mM EK+ = - 90 mV WHAT ABOUT Na ? + • Na+ is concentrated outside cell • Concentration and electrical gradient would “drive” Na+ into cell ENa+ = 61 x log 150mM 15mM WHAT ABOUT Na ? + • Na+ is concentrated outside cell • Concentration and electrical gradient would “drive” Na+ into cell ENa+ = 61 x log 150mM 15mM ENa+ = 60 mV WHAT ABOUT Na ? + • Na+ is concentrated outside cell • Concentration and electrical gradient would “drive” Na+ into cell • But, PM is very little permeable to Na+ – Very few leak channels for sodium! ECF Electrochemical gradient for Na+ Na+ ICF + + + + + + - Na+ NERNST EQUATION • Why is the K+ equilibrium potential important? • The resting membrane potential is always close to the potassium equilibrium potential • If the membrane becomes permeable to another ion, it will „move toward“ its equilibrium potential • The membrane potential will change as a result • So, if you open a channel for Na+ it will „drive“ the membrane potential toward the ENa+ (i.e. membrane potential will go from negative to positive) The sodium-potassium ATPase is a plasma membrane-spanning enzyme and is essential to preserve the ionic gradients across the cell membrane. What ions are transported via the sodium-potassium ATPase and in what direction? A. Three Ca2+ ions out of the cell and two K+ ions into the cell B. Three Cl- ions out of the cell and two Na+ ions into the cell C. Three Na+ ions out of the cell and two K+ ions into the cell B. Two Ca2+ ions out of the cell and two K+ ions into the cell E. Two Na+ ions out of the cell and three K+ ions into the cell The sodium-potassium ATPase is a plasma membrane-spanning enzyme and is essential to preserve the ionic gradients across the cell membrane. What ions are transported via the sodium-potassium ATPase and in what direction? A. Three Ca2+ ions out of the cell and two K+ ions into the cell B. Three Cl- ions out of the cell and two Na+ ions into the cell C. Three Na+ ions out of the cell and two K+ ions into the cell B. Two Ca2+ ions out of the cell and two K+ ions into the cell E. Two Na+ ions out of the cell and three K+ ions into the cell MEMBRANE POTENTIAL CHANGES • Depolarisation – more positive membrane potential • Hyperpolarisation – more negative membrane potential • Repolarisation – Restores the potential +60 Membrane potential +50 +40 +30 +20 +10 0 -10 -20 -30 -40 Resting Membrane Potential -50 +- -60 -70 -80 -90 -+ +60 Membrane potential +50 +40 +30 +20 +10 0 -10 -20 -30 -40 Resting Membrane Potential -50 - + -60 -70 -+ -80 -90 Hyperpolarization +60 Depolarization Membrane potential +50 +40 +30 +20 +10 0 -10 -20 -30 -40 Resting Membrane Potential -50 +- -60 -70 -+ -80 -90 Hyperpolarization NA-K PUMP IS THE MAJOR DRIVING FORCE FOR TRANSPORT PROCESSES ACROSS MEMBANES • Cell „cashes in“ chemical gradient of Na+ to transport substances into cell • Cotransporters – e.g. Na+-Glucose cotransporter Na+-glucose co-transporter (SGLT) Na+-amino acid co-transporter NA-K PUMP IS THE MAJOR DRIVING FORCE FOR TRANSPORT PROCESSES ACROSS MEMBANES • Cell „cashes in“ chemical gradient of Na+ to transport substances into cell • Exchangers – e.g. Na+-Ca2+ exchanger Na+-Ca2+ exchanger (NCX) Na+-H+ exchanger (NHE) Classes of ion channels • Leak channel – e.g. K+ leak channel K+ Na+ • Voltage-gated channel – e.g. Voltage-gated sodiumchannel (VGSC) • Ligand-gated channel – e.g. Acetylcholine receptor NT Na+ EXAMPLES OF DRUGS MODIFYING VOLTAGESENSITIVE ION CHANNELS • Local anaesthetics (inhibit voltage-gated sodium (Na+ channels, i.e. Xylocaine/Lidocaine) • Ca2+-channel blockers (antihypertensive drugs, i.e. nifedipine) RECOMMENDED READING MATERIALS FUN1: PHYSIOLOGY • Ganong, Ganong’s review of medical physiology, Chapter 4 – http://accessmedicine.mhmedical.com.proxy.library.rcsi.ie/content.aspx?bookid= 393&sectionid=39736740 • Human Physiology by Lauralee Sherwood, Brooks/Cole-Cengage Learning Ch. 4 & 5 – https://www-dawsonera-com.proxy.library.rcsi.ie/readonline/9781408088838/startPage/62 • Guyton & Hall, Medical Physiology, Chapter 4 & 5, in-depth reading – – https://www-clinicalkey-com.proxy.library.rcsi.ie/#!/content/book/3-s2.0-B9781455770052000 056 (click-through library-proxy: https://www.rcsi.ie/index.jsp?p=108&n=429 : Search term:‘Guyton’) Online resources: Resting membrane potential (disregard the concentration gradients as illustrated here for the squid): http://sites.sinauer.com/neuroscience5e/animations02.01.html Ion channels and excitable cells: http://www.nature.com/scitable/topicpage/ion-channels-and-excitable-cells-14406097