9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 1 Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework** Consortium leader PETER PAZMANY CATHOLIC UNIVERSITY Consortium members SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund *** **Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben ***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg. PETER PAZMANY CATHOLIC UNIVERSITY SEMMELWEIS UNIVERSITY sote_logo.jpg dk_fejlec.gif INFOBLOKK Peter Pazmany Catholic University Faculty of Information Technology www.itk.ppke.hu (Neurális interfészek és protézisek ) RICHÁRD CSERCSA and GYÖRGY KARMOS BASICSOF ELECTRICALSTIMULATION LECTURE2 NEURALINTERFACESAND PROSTHESES (Elektromos ingerlés alapjai) NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011 TÁMOP –4.1.2-08/2/A/KMR-2009-0006 3 AIMS: Inthislecture, thestudentwillbecomefamiliarwiththenecessaryelementsof electricstimulation. Oneimportantelementis an excitablenervecell. The studentwilllearnaboutthesemi-permeablemembrane, theionsproducingthemembranepotential, and thegenerationof theresting potential. Theywillalsolearnaboutelectricfields, currents, and theeffecttheyhaveonneurons. Thislecturementionsthetypesof electrodesand stimulationtechniques, introducingconceptsessentialinpractice, suchasnon-polarizableelectrodes, biphasicstimulation, chronaxy, andrheobase. www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 4 http://upload.wikimedia.org/wikipedia/commons/b/b0/Duchenne_Mecanisme_de_la_physionomie_humaine%2C_fig._78_.jpg Duchenne, 1862 www.itk.ppke.hu (commons.wikimedia.org) NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 5 Electric stimulation means a functional change in a nerve (muscle) cell due to electric current. Needed: • Excitable cell • Electric current • Stimulating electrode Excitable cell: • Cell membrane • Membrane proteins • Ion concentration difference between the two sides of the membrane www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 6 (commons.wikimedia.org) STRUCTURE OF NEURON MEMBRANE www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 7 The neuron membraneis a phospholipidbilayerthatseparatestheintracellularand theextracellularfluids. Eachphospholipidmoleculeconsistsof a hydrophilicand a hydrophobicpart. Moleculesareorganizedintolayerssuchthathydrophobicpartsareinsidethemembrane, and hydrophilicpartscontactwiththeexternal/internalworld. Thisstructuremakesthemembranenotpermeableforionsand chargedmolecules, thusa gooddielectric. Certainproteinsmaybe embeddedinthemembrane. Theycanbe peripheral, iftheydonotspanthroughthewholemembrane, ortransmembrane, iftheyreachbothsidesof themembrane. Theseproteinsarecalledion channelsortransportersiftheycantransportionsfromonesideof themembranetotheother. www.itk.ppke.hu STRUCTURE OF NEURON MEMBRANE NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 8 www.itk.ppke.hu phospholipidmolecule hydrophile hydrophobe phospholipd bilayer (membrane) in water good dielectric not permeable for ions, charged molecules not permeable for big molecules permeable for water and small, uncharged molecules molecules soluble in fat may dissolve in the membrane BUILDING BLOCKS OF NEURON MEMBRANE (commons.wikimedia.org) NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 9 Ions can be transported through the cell membrane passively, without energy investment, or actively, when energy is needed for the transport. The necessary energy comes from adenosine triphosphate (ATP) dephosphorylation. Ion channels can be closed, when they are in a conformation that they cannot transport ions, or open. Ion channels can be ligand gated or voltage gated, depending on the way they can become open. Ligand gated channels require certain molecules attached to them in order to open, while for voltage gated channels, a certain potential difference between the two sides of the membrane is necessary. www.itk.ppke.hu ION CHANNELS NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 10 www.itk.ppke.hu PROTEINS OF NEURON MEMBRANE phospholipidmolecule hydrophile hydrophobe (commons.wikimedia.org) NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 11 VOLTAGE-GATED NA+CHANNEL At rest (Vm= -75 mV) Immediately after depolarization (Vm= -50 mV) 5 msec after depolarization (Vm= -50 mV) extra intra Plasma membrane mgate hgate Na+ www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 12 Atresting potential, them gateof thechannelis closed, thereforeNa+ ionsarenotabletoflow throughthemembrane. Whenthemembraneis depolarized(thepotentialdifferencebetweenthetwosidesof themembraneis smaller), bothgatesof thevoltage-gatedNa+ channelopen, allowingtheNa+ ionstoflow intothecell, furtherdepolarizingthemembrane. Soonafterthedepolarization( ~5 msec) theh gateof thechannelcloses, stoppingtheinwardflow of Na+. www.itk.ppke.hu VOLTAGE-GATED NA+CHANNEL NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 13 www.itk.ppke.hu low concentration HIGH CONCENTRATION DIFFUSION BETWEEN SPACES WITHDIFFERENT CONCENTRATION NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 14 www.itk.ppke.hu positive potential negative potential ION MOVEMENT IN ELECTRIC SPACE ions NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 15 The movement of ions through the membrane depends on • the ion concentration gradient, • electric charges. If the concentration of a certain molecule is higher in one compartment than the other, they will diffuse to the compartment with lower concentration (diffusion force). If the electric field is positive in one compartment, negative ions will tend to move there, while positive ions will move away and vice versa (electrostatic force). Furthermore, ion movement is determined also by the type of open channels. Some channels are selective for ions (e.g. only cations, or only K+ions). www.itk.ppke.hu ION MOVEMENT NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 16 Concentration difference Electric field Ion flow Charge distribution difference Resting potential In a living cell [K+]intracell> [K+]extracelland [Na+]i< [Na+]e In a living cell at rest K+flows through the membrane much more easily than any other ions. If pK=1thenpNa=0.1. Very few ions are transported, only small changes in concentration take place. www.itk.ppke.hu ION MOVEMENT NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 17 www.itk.ppke.hu more positive charges + POTENTIAL less positive charges -POTENTIAL ION MOVEMENT THROUGH SELECTIVE CHANNEL K-channel diffusion electric force intracell extracell NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 18 Diffusion Electric field diffusion flux diffusivity concentration drift flux valence concentration velocity mobility www.itk.ppke.hu http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Membrane_potential_ions_en.svg/1000px-Membrane_potential_ions_en.svg.png (commons.wikimedia.org) ION MOVEMENT NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 19 Ion diffusion: Electric field: No net current flow in equilibrium: Nernst equation www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 20 www.itk.ppke.hu Vm = Vk= Vi-Ve Nernst equation T = 273 + 37 z = 1 F =Faradayconstant[9.649 ×104 C/mol] T = absolute temperature [K] R = gas constant[8.314 J/(mol·K)] zk= valence ci,k= intracell concentration co,k= extracell concentracion Vk=equilibriumpotential Equilibrium Vk= RT zkF ln ci,k ce,k - Vk= 61 log10 ci,k ce,k - . [mV] IDIFF + IE = Inet = 0 ion selective membrane ion concentration difference mobile +ion(K, intracell) non mobile –ion(A, intracell) - . Vk= 61 log10(24) ci,K = 120mmol/l ce,K = 5mmol/l VK = -84.19mV Vm: membrane potential NERNST EQUILIBRIUM diffusion electric force intracell extracell Ve Vi NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 21 For an excitable cell, a dielectric membrane and ion concentration difference on its two sides are essential. Resting membrane potential is the membrane potential (the potential difference between the two sides of the membrane) when there is no net ion movement. It is an equilibrium state when the sum of diffusion and electrostatic forces is zero. It also means there is no net current flow through the membrane. The Nernst equation gives the equilibrium potential of an ion, given its intra-and extracellular concentrations. This is the potential when there is no net movement of that ion. This value is proportional to the logarithm of the quotient of concentrations. www.itk.ppke.hu NERNST EQUILIBRIUM NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 22 www.itk.ppke.hu OSMOTIC CATASTROPHE Due to high concentration of ions inside the cell, water would diffuse into the cell untilitdisrupts. NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 23 www.itk.ppke.hu NaCl in the extracell space! ion selective membrane ion concentration difference mobile +ion(K, intracell) non mobile –ion(A, intracell) mobile –ion(Cl, extracell) non mobile +ion(Na, extracell) compensated for water diffusion: ci=ce ci,K=ce,Clce,K=ci,Cl VD = VCl= VK= Vm= Vi-Ve VD= 61 log10 ci,K+ ce,Cl ce,K + ci,Cl - . [mV] VD= VK = VCl= Vm = -84.19mV Equilibrium IDIFF + IE = Inet = 0 COMPENSATE FOR THE DIFFUSION OF WATER NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 24 www.itk.ppke.hu ion selective membrane ion concentration difference mobile +ion(K, intracell) non mobile –ion(A, intracell) mobile –ion(Cl, extracell) non mobile +ion(Na, extracell) compensated for water diffusion different permeability of ion channels active transport for maintaining ion gradient (Na/K pump) no equilibriumon ion channels (leak) REALISTIC CELL MODEL NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION TÁMOP –4.1.2-08/2/A/KMR-2009-0006 25 www.itk.ppke.hu ion intra [mmol/l] extra [mmol/l] Vk Na+ 15 150 +61 mV K+ 120 5 -84.19 mV Cl- 7.5 125 -74.53 mV ion permeability, P [cm/sec] Na+ 0.05 x 10-7 K+ 1 x 10-7 Cl- 0.1 x 10-7 Vr= -61.15 mV Goldman-Hodgkin-Katz equation PK> PCl> PNa Vr= 61 log10 PK.ci,K+ PNa.ci,Na + PCl.ce,Cl - . [mV] PK.ce,K+ PNa.ce,Na + PCl.ci,Cl Depolarization: Vm >Vr Hyperpolarization: VmVr Hyperpolarization: Vm 0 Vm=0 V = +V0 V = -V0 hyper- polarized depolarized Vm= Vr=R–Vr=R+. Vm~ E0· cos(.) .= 0 › Vm >0 › depolarized . = ./2› Vm =0 › depolarized . = .› Vm < 0 › depolarized www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 37 Strong field Weak field www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 38 Electrical stimulation means a functional change in a nerve (muscle) cell due to electric current. Needed: • Excitable cell • Electric current • Stimulating electrode Stimulating electrode: • Types of stimulation • Electrode properties • Stimulation effects www.itk.ppke.hu Direct control over all ion currents. Technical implemetation is hard. INTRACELLULAR STIMULATION WITH ELECTRIC CURRENT 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu Current Cell Electrode NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION EXTRACELLULAR STIMULATION WITH ELECTRIC CURRENT monopolar bipolar Indirect control over all ion currents. Technical implemetation is easier. 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu -V -V +V Cell Electrode Current Ground (Return electrode) NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 41 CHARACTERISTICS OF VARIOUS ELECTRODES Current Ag/AgCl Platinum Copper Stainless steel www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 42 - - - - - + + + + + - - - - - - - - - + + + + + + + + + - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - + + + + + + + + + ELECTRICAL DOUBLE LAYER Ion movementduringmetal-liquidcontact› polarization› equilibrium 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu (a)Ionic flow into solution immediately after immersion, (b) accumulation of ions in solution, (c) ionic flow into and out of solution at different rates, (d) equilibrium when rates are equal. (Cooper, 1962) a b c d Polarizable electrodes: • amount of current depends on properties of double layer (C, R) • current charges or discharges the double layer • current vs. voltage is non-linear • DC impedance is big, acts as capacitance Metals: Ag, Pt, Au, etc. but can be decreased by increasing the surface. POLARIZABLE ELECTRODES 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu Non-polarizable electrodes: • current does not influence properties of double layer • current flows freely on the double layer • current vs. voltage is linear • acts as resistance on DC Ag/AgCl NON-POLARIZABLE ELECTRODES 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION GALVANIC SERIES In case of metal-liquid-metal contact they form a galvanic cell. Electrode potential is the potential difference relative to the standard hydrogen electrode. Metals with positive potential to hydrogen are called noble metals. 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu Potassium K Sodium Na Calcium Ca Magnesium Mg Aluminium Al Zinc Zn Iron Fe Hydrogen H Copper Cu Silver Ag Platinum Pt Gold Au Metals more reactive than hydrogen Metals less reactive than hydrogen Decreasing chemical reactivity 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 46 MODEL OF ELECTRODE-ELECTROLYTE INTERFACE www.itk.ppke.hu Rd Rs Cd Ehc + - Ehc: half-cell potential Rs: electrolyte resistance Rd: interface resistance ZC: electrode impedance C: capacitance NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 47 0 1 2 3 4 5 6 7 8 9 10 300 250 200 150 100 50 0 Ag/AgCl Platinum alloy Stainless steel FREQUENCY-DEPENDENCE OF ELECTRODE IMPEDANCE www.itk.ppke.hu Frequency (Hz) Impedance (k.) NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION Monophasic repetitive stimulation High current density Hydrolysis Thermic effect Charge injection Tissue damage Biphasic repetitive stimulation STIMULATION WITH REPETITIVE IMPULSES 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION Stimulus intensity(mA) Muscle contraction torque (ft*lbs) STIMULUS VS. RESPONSE 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 www.itk.ppke.hu NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION http://upload.wikimedia.org/wikipedia/commons/f/fb/Rheobase_chronaxie.png Response depends on: stimulus strength (Is) stimulus duration (t) Rheobase (Irh): the minimal current amplitude of infinite duration that results inan action potential. e0358 STIMULUS STRENGTH-DURATION CURVE 9/14/2011. www.itk.ppke.hu TÁMOP –4.1.2-08/2/A/KMR-2009-0006 Chronaxy (Chr): the minimum time over which an electric current double the strength of the rheobaseneeds to be applied, in order togenerate an action potential. NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 51 www.itk.ppke.hu ionselective(semipermeable) membrane membrane permeable for water, not permeable for ions condition for excitation and inhibition: ion concentration difference between the two sides of the membrane voltage on membrane depends on diffusion and electrostatic forces in Nernst equilibrium the voltage is proportional to the logarithm of the quotient of concentrations in Nernst equilibrium there is no net current flow on ion channels resting potential is determined by ion concentration differences and ion channel permeabilities the most important mobile ions are Na, K, and Cl intracellularlymany negatively charged proteins and K, extracellularlymany Na and Clions PK> PCl> PNa SUMMARY NEURALINTERFACESAND PROSTHESESBASICSOF ELECTRICALSTIMULATION 9/14/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 52 www.itk.ppke.hu ions in dynamic balance, no real equilibrium, leaky channels leak compensated by energy demanding pumps (Na-K pump) Goldman equation gives good approximation for resting potential depolarization Vm>Vr, hyperpolarization Vm