10/7/2011. 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 (Az ideg-és izom-rendszerelektrofiziológiai vizsgálómódszerei) RICHÁRD CSERCSA, ISTVÁN ULBERTand GYÖRGY KARMOS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMS LECTURE4 ACTION POTENTIAL (Akciós potenciál) ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) 10/7/2011 TÁMOP –4.1.2-08/2/A/KMR-2009-0006 3 AIMS: In this lecture, the student will become familiar with the generation and propagation of the action potential, the basic element of information processing in the brain. www.itk.ppke.hu ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 4 DEFINITION: When the membrane potential of a neuron depolarizes and reaches a threshold, the neuron will generate an action potential, in other words, it will ‘fire’. The action potential, also known as a ‘spike’ is a short-lasting event in which the membrane potential of thecell rapidly rises and falls, following a stereotyped trajectory. In neurons, they play a central role in cell-to-cell communication. The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Therefore action potentials are said to be all-or-none, since they either occur fully or they do not occur at all.(wikipedia) An action potential is typically generated in the initial segment of the axon (hillock) of the neuron and propagates along the axon, and at the axon terminals it is transmitted to other neurons through synapses. www.itk.ppke.hu ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 5 rising phase overshoot falling phase undershoot resting potential resting potential threshold potential Vm[mV] time 2ms SCHEMATIC ACTION POTENTIAL www.itk.ppke.hu ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 6 When the depolarizing membrane potential reaches a threshold, a stereotypic process is initiated. The depolarization opens Na+channels and Na+ions flow into the cell. This further depolarizes the cell (rising phase). Na+channels are inactivated after a small delay, and K+channels open. Thus, Na+flow into the cell stops, and K+flow out of the cell starts. This hyperpolarizes the membrane (falling phase).Themembrane potentialgoes below resting potential (undershoot), then finally returns to rest. After initiated, the action potential always takes place the same way, its amplitude is independent from the amount of current that produced it (all-or-nothing). It propagates along the axon and launches synaptic events at the axon terminals. Hence, it is the basis of neuronal communication and information processing. www.itk.ppke.hu SCHEMATIC ACTION POTENTIAL 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 7 ION CHANNEL ACTIVITY ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) A: Resting phase • Channels and membrane are at rest • Vm = Vr B: Depolarizing phase • Vm exceeds opening threshold of Na+channel • Na+channel opens, Na+flows into the cell • Vm depolarizes • K+channel still closed, no K+current flow www.itk.ppke.hu A extra intra Na+ K+ Na+ K+ B extra intra 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 8 ION CHANNEL ACTIVITY ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) C: Repolarizing phase • Na+channel inactivates, Na+current stops • K+channel opens, K+flows out of the cell • Vm repolarizes D: Afterhyperpolarizing phase (AHP) • Na+channel closed, close to rest, no Na+current flow • K+channel still active, K+flows out of the cell • Vm hyperpolarizes www.itk.ppke.hu C extra intra Na+ K+ Na+ K+ D extra intra CHANGES IN CHANNEL CONDUCTANCES ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu time (ms) time (ms) Membrane pot. (mV) Conductance gNa gK Membrane potential Ion channel conductances ARP RRP time REFRACTORY PERIOD ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) Refractory period starts at the rising phase of an action potential, during this period the cell cannot be stimulated, no action potential can be generated. Absolute refractory period (ARP):no stimulus can evoke a response Relative refractory period (RRP):only strong stimulus can generate an action potential www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 11 REFRACTORY PERIOD ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) Causes of ARP: 1. During the depolarizing phase no more Na+channels can activate. 2. During repolarization, Na+channels become inactive, time is needed for reaching resting state, the channel won’t open during this time. Causes of RRP: 1. Na+channels do not return from inactivity in the same time, stronger stimulus is necessary to activate enough Na+channels to generate an action potential. 2. K+channels are still open, hence the cell is hyperpolarized, stronger stimulus is necessary for reaching the opening threshold of Na+channels. www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 12 EVOKING AND MEASURING ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) hyperpolarization depolarization Vrest Vthreshold Vm [mV] action potential Is Vm stimulating current Is excitation inhibition www.itk.ppke.hu Time constant: rm: membrane resistance (depends on the number of open ion channels) cm: membrane capacitance (depends on properties of lipid bilayer) The time when the 63% (1-1/e) of the maximal action potential amplitude is reached. The bigger the time constant, the slower the rising phase of AP. Length constant: ri: intrinsic resistance (depends on the diameter of axon) The distance where the amplitude of the electrotonicallyspread depolarization decreases to 37% of its initial value. The greater the length constant, the farther the AP has effect. MEMBRANE CONSTANTS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 If the time constant is big enough, consecutive postsynaptic potentials are summed. Summation: Summation in time: Summation in space: Linear summation of postsynaptic potentials that can pull the membrane potential closer or farther from the AP threshold. If the length constant is big enough, postsynaptic potentials from different locations are summed. SUMMATION ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 15 http://upload.wikimedia.org/wikipedia/commons/0/0c/AP_propagation_membrane_model_view.jpg MODEL OF PROPAGATION ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu (commons.wikimedia.org) 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 16 MODEL OF PROPAGATION ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu A local depolarization opens local sodium channels. Sodium current causes membrane depolarization (T1) which spreads to adjacent membrane, depolarizing it as well (T2). Sodium channels then open in this adjacent membrane, and the depolarization spreads further down the membrane (T4). Meanwhile, delayed rectifierpotassium current flows in the original membrane patch, causing the falling phase of the AP. Note that in T4, depolarization spreads in both directions down the membrane (i.e. also toward the place where the AP originated. A second action potential is not fired in that membrane because it is refractory, thus allowing for one-way AP conduction. (commons.wikimedia.org) PROPAGATION OF ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) Axons of neurons are protected by a myelin sheath thatareformedbyglialcells: Schwann cellsintheperiphericnervoussystem, and oligodendrogliacellsinthecentralnervoussystem. They help the action potential (and the information) spread faster. Action potential propagation on myelinated axons can be 10 times faster than on unmyelinated axons. The reason is that the myelin sheath does not allow current flow, so it can take place only on nodes of Ranvier, where no myelin sheath protects the cell (called saltatory propagation). Thusthepresenceof myelinsheatincreasesthe propagationvelocity, whichis, ingeneral, proportional with the thickness of theaxon. Losingthemyelinsheathcancauseseriousdysfunctioninthenervoussystem, likemultiplesclerosis. www.itk.ppke.hu PROPAGATION OF ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu (commons.wikimedia.org) 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 19 Action potentials generated on the axon hillock propagate through the axon in a wave. The inward current at one node of the axon depolarizes the surroundings of the node, activating the neighbouring voltage-gated ion channels. This resultsinan inward current at the neighbouring node, causing the spread of depolarization on the axon. PROPAGATION OF ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu http://upload.wikimedia.org/wikipedia/commons/c/c6/Action_potential_seq.png (commons.wikimedia.org) Electrochemical basis of action potentials is the ion concentration difference between the two sides of the membrane Molecular basis is the voltage-gated ion channel (Na, K) Action potential is triggered by membrane depolarization Phases: depolarization, repolarization, afterhyperpolarization (AHP) Depolarizing phase: membrane depolarization opens Na channel > Na flows in > depolarization > Na channel inactivation Repolarizing phase: K channel opens > K flows out > repolarization AHP phase: K channel still open > K flows out > afterhyperpolarization SUMMARY ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 21 SUMMARY ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) All-or-nothing (digital) Propagates without amplitude loss No summation Absolute and relative refractory period Can be evoked on axon hillock and along the axon, because voltage-gated channels are located there Can not be evoked on the soma and dendrites www.itk.ppke.hu SYNAPTIC COMMUNICATION ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu (commons.wikimedia.org) 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 http://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Synapse_Illustration2_tweaked.svg/1000px-Synapse_Illustration2_tweaked.svg.png Synaptic vesiculesin the presynaptic terminal Action potential arrives to the terminal Caflows in through the presynaptic membrane Initiates exocytosis Synaptic vesicle dump neurotransmitters into the synaptic cleft Neurotransmitters diffuse to the receptors on the postsynaptic cell Ion channel opens Postsynaptic potential is generated on the postsynaptic membrane Neurotransmitter is broken down or re-uptaken FUNCTIONING OF SYNAPSES ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 EXCITATORY PSP (EPSP): Vm depolarizes AP chance increases INHIBITORY PSP (IPSP): Vm hyperpolarizes AP chance decreases EXCITATORY NEUROTRANSMITTERS: Depolarizes OpensNa, Ca(K) channel Glutamate, Ach, etc... INHIBITORY NEUROTRANSMITTERS: Hyperpolarizes OpensCl orK channel GABA (gamma-amino butyric acid) POSTSYNAPTIC RECEPTORS: AMPA, kainate POSTSYNAPTIC RECEPTORS: GABAA, GABAB TYPES OF POSTSYNAPTIC POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 Vm pre Vm post Vm pre Vm post Vm pre Vm post Vm pre Vm post intracellular electrode presynaptic cell postsynaptic cell synapse EPSP IPSP dt dt: synapticdelay summation MEASURING POSTSYNAPTIC POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 axon axon hillock AP initiation zone excitatory synapse soma, dendrite depolarizing current intracell current POSTSYNAPTIC POTENTIALS AND ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 27 Synapses are usually located on dendrites or on the soma They do not have voltage-gated ion channels, can not initiate AP AP is initiated on axon hillock or on the axon Depolarizing or hyperpolarizing current flows in intracell space One synapse has small effect in initiation zone AP generation usually requires several EPSPs IPSP sums in time and space with EPSP IPSP decreaseschance of AP generation POSTSYNAPTIC POTENTIALS AND ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu Postsynaptic pot. Action potential Generated on soma and dendrites Generated on axon hillock Propagates with loss Propagates without loss Summation No summation Analog Digital (all-or-nothing) Propagation speed: 50000 m/s Propagation speed: 0.1 –100 m/s POSTSYNAPTIC POTENTIALS AND ACTION POTENTIALS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 Chemical synapses Presynaptic device (axon terminal): AP > Ca flows in > neurotransmitter release Postsynaptic membrane: neurotransmitter > receptor > PSP Excitatory PSP (EPSP) depolarizes Inhibitory PSP (IPSP) hyperpolarizes EPSP > Na/Cachannel (glutamate) > Na/Caflows in > depolarization > increases chance of AP IPSP > Cl channel (GABA) > Cl flows in > hyperpolarization > decreases chance of AP SUMMARY ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 30 SUMMARY ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) Most synapses on soma and dendrites AP initiation on axon hillock PSPs propagate electrotonically (analog, fast, with loss) Summation in time and space The potential on the axon hillock (created by summed EPSPs and IPSPs) decides AP generation Creates basis of neuronal information processing www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 31 LINKS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) AP propagation & Synapse: http://www.youtube.com/watch?v=90cj4NX87Yk Action potential: http://mcb.berkeley.edu/courses/mcb64/action_potential.html Resting potential: http://bcs.whfreeman.com/thelifewire/content/chp44/4402001.html Action potential: http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html http://trc.ucdavis.edu/biosci10v/bis10v/week10/06potential.html http://www.blackwellpublishing.com/matthews/channel.html http://www.ywpw.com/cai/software/hhsimu/ Synaptic transmission: http://bcs.whfreeman.com/thelifewire/content/chp44/4402003.html http://trc.ucdavis.edu/biosci10v/bis10v/week10/06synapse.html www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 32 REFERENCES ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) Don L. Jewett, Martin D. Rayner: Basic Concepts of Neuronal Function, Little, Brown, and Company, Boston, 1984. Michael J. Zigmond, Floyd E. Bloom, Story C. Landis, James L. Roberts, Larry R. Squire: Fundamental Neuroscience, Academic Press, 1999. www.itk.ppke.hu 10/7/2011. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 33 REVIEW QUESTIONS ELECTROPHYSIOLOGICAL METHODS FOR THE STUDY OF THE NERVOUS-AND MUSCULAR-SYSTEMELECTROENCEPHALOGRAPHY (EEG) • What phases does an action potential have? • What is the ion channel activity in each phase? • What is the change of conductance of channels during the action potential? • What is the refractory period? • How do action potentials propagate? • How do chemical synapses function? • What are the types of postsynaptic potentials? • What are the membrane constants? • What are the differences between action potentials and postsynaptic potentials? www.itk.ppke.hu