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1,LIU Chuan Yong 刘传勇Institute of PhysiologyMedical School of SDUTel 88381175(lab)88382098(office)Email:Website:,2,Section 2,Bioelectrical Phenomena of the Cell,3,Basic Concepts,VoltA charge difference between two points in space,4,Basic Concepts,Ions charged particlesAnions Negatively charged particlesCations Positively charged particles,5,Basic ConceptsForces that determine ionic movement,Electrostatic forcesOpposite charges attractIdentical charges repelConcentration forcesDiffusion movement of ions through semipermeable membraneOsmosis movement of water from region of high concentration to low,6,Selective Permeability of Membranes,Some ions permitted to cross more easily than othersNeuronal membranes contain ion channelsProtein tubes that span the membraneSome stay open all the time(nongated)Some open on the occasion of an action potential,causing a change in the permeability of the membrane(gated),7,I.Membrane Resting Potential,A constant potential difference across the resting cell membraneCells ability to fire an action potential is due to the cells ability to maintain the cellular resting potential at approximately 70 mV(-.07 volt)The basic signaling properties of neurons are determined by changes in the resting potential,8,Membrane Resting Potential,Every neuron has a separation of electrical charge across its cell membrane.The membrane potential results from a separation of positive and negative charges across the cell membrane.,9,Membrane Resting Potential,excess of positive charges outside and negative charges inside the membrane maintained because the lipid bilayer acts as a barrier to the diffusion of ions,gives rise to an electrical potential difference,which ranges from about 60 to 70 mV.(Microelectrode),10,Concept of Resting Potential(RP),A potential difference across the cell membrane at the rest stage or when the cell is not stimulated.Property:It is constant or stableIt is negative inside relative to the outsideResting potentials are different in different cells.,11,Ion Channels,Two Types of Ion ChannelsGatedNon-Gated,12,Resting Membrane Potential,Na+and Cl-are more concentrated outside the cell K+and organic anions(organic acids and proteins)are more concentrated inside.,13,Intracellular vs extracellular ion concentrations,Ion Intracellular ExtracellularNa+5-15 mM 145 mMK+140 mM 5 mMMg2+0.5 mM 1-2 mMCa2+10-7 mM 1-2 mMH+10-7.2 M(pH 7.2)10-7.4 M(pH 7.4)Cl-5-15 mM 110 mM,14,Resting Membrane Potential,Potassium ions,concentrated inside the cell tend to move outward down their concentration gradient through nongated potassium channels But the relative excess of negative charge inside the membrane tend to push potassium ions out of the cell,15,Potassium equilibrium,-90 mV,16,Resting Membrane Potential,But what about sodium?Electrostatic and Chemical forces act together on Na+ions to drive them into the cell The Na+channel close during the resting state,Na+is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient,Na+is driven into the cell by the electrical potential difference across the membrane.,17,Na+electrochemical gradient,18,Equilibrium Potentials,Theoretical voltage produced across the membrane if only 1 ion could diffuse through the membrane.If membrane only permeable to K+,K+diffuses until K+is at equilibrium.Force of electrical attraction and diffusion are=opposite.,19,Calculating equilibrium potentialNernst Equation,Allows theoretical membrane potential to be calculated for particular ion.Membrane potential that would exactly balance the diffusion gradient and prevent the net movement of a particular ion.Value depends on the ratio of ion on the 2 sides of the membrane.,20,where,Co and Ci=extra and intracellular ion R=Universal gas constant(8.3 joules.K-1.mol-1)T=Absolute temperature(K)F=Faraday constant(96,500 coulombs.mol-1)z=Charge of ion(Na+=+1,Ca2+=+2,Cl-=-1),For K+,with K+o=4 mmol.l-1 and K+i=150 mmol.l-1At 37C,EK=-97mV ENa=+60mv,21,K+o=4 mmol.l-1,22,Resting Membrane Potential,Resting membrane potential is less than Ek because some Na+can also enter the cell.The slow rate of Na+influx is accompanied by slow rate of K+outflux.Depends upon 2 factors:Ratio of the concentrations of each ion on the 2 sides of the plasma membrane.Specific permeability of membrane to each different ion.Resting membrane potential of most cells ranges from-65 to 85 mV.,23,The Sodium-Potassium Pump,Dissipation of ionic gradients is ultimately prevented by Na+-K+pumps,extrudes Na+from the cell while taking in K,24,Resting Potential,25,The formation of resting potential depends on:,Concentration difference of K+across the membranePermeability of Na+and K+during the resting stateNa+-K+pump,26,Factors that affect resting potential,Difference of K+ion concentration across the membranePermeability of the membrane to Na+and K+.Action of Na+pump,27,Basic Electrophysiological Terms I:,Polarization:a state in which membrane is polarized at rest,negative inside and positive outside.Depolarization:the membrane potential becomes less negative than the resting potential(close to zero).Hyperpolarization:the membrane potential is more negative than the resting level.,28,Basic Electrophysiological Terms I:,Reverspolarization:a reversal of membrane potential polarity.The inside of a cell becomes positive relative to the outside.Repolarization:restoration of normal polarization state of membrane.a process in which the membrane potential returns toward from depolarized level to the normal resting membrane value.,29,II Action Potential,Successive Stages:Resting StageDepolarization stageRepolarization stageAfter-potential stage,(1),(2),(3),(4),30,Concept,Action potential is a rapid,reversible,and conductive change of the membrane potential after the cell is stimulated.Nerve signals are transmitted by action potentials.,31,Action Potential Sequence,Voltage-gated Na+Channels open and Na+rushes into the cell,32,Action Potential Sequence,At about+30 mV,Sodium channels close,but now,voltage-gated potassium channels open,causing an outflow of potassium,down its electrochemical gradient,33,Action Potential Sequence,equilibrium potential of the cell is restored,34,Action Potential Sequence,The Sodium Potassium Pump is left to clean up the mess,35,Ion Permeability during the AP,Figure 8-12:Refractory periods,36,Basic Electrophysiological Terms II(1),Excitability:The ability of the cell to generate the action potentialExcitable cells:Cells that generate action potential during excitation.in excitable cells(muscle,nerve,secretery cells),the action potential is the marker of excitation.Some scholars even suggest that in excitable cells,action potential is identical to the excitation.,37,Basic Electrophysiological Terms II(2),Stimulus:a sudden change of the(internal or external)environmental condition of the cell.includes physical and chemical stimulus.The electrical stimulus is often used for the physiological research.Threshold(intensity):the lowest or minimal intensity of stimulus to elicit an action potential(Three factors of the stimulation:intensity,duration,rate of intensity change),38,Basic Electrophysiological Terms II(3),Types of stimulus:Threshold stimulus:The stimulus with the intensity equal to thresholdSubthreshold stimulus:The stimulus with the intensity weaker than the thresholdSuprathreshold stimulus:The stimulus with the intensity greater than the threshold.,39,Action Potential Summary,Reduction in membrane potential(depolarization)to threshold level leads to opening of Na+channels,allowing Na+to enter the cellInterior becomes positiveThe Na+channels then close automatically followed by a period of inactivation.K+channels open,K+leaves the cell and the interior again becomes negative.Process lasts about 1/1000th of a second.,40,Properties of the Action Potential,“All or none”phenomenonA threshold or suprathreshold stimulus applied to a single nerve fiber always initiate the same action potential with constant amplitude,time course and propagation velocity.PropagationTransmitted in both direction in a nerve fiber,41,III Initiation of Action Potential,42,Squid giant axon,43,Gated channel states,44,Na+Channel a1-Subunit Structure,I,II,III,IV,-Inactivation“Gate”,IVS4 Voltage Sensor,45,Voltage gated,But“ready”,Not“ready”,46,Activation&Fast Inactivation,47,Sodium Activation and Inactivation Variable vs Voltage,Activation Gate,Inactivation gate,If resting potential depolarized by 15 20 mV,then activation gate opened with 5000 x increase in Na+permeability followed by inactivation gate close 1 ms later,48,Positive feedback loop,Na+enters(depolarization),V-gate Na+channels open,Stimulation,49,Action potential initiation,S.I.Z.,50,Action potential termination,51,52,Threshold Potential,Threshold potential plays a key role in the genesis of action potential.Threshold potential is a critical membrane potential level at which an action potential can occur.Why can all the Na+channel open at the threshold potential?It is dependent on the gating property of the voltage-gated Na+channels.The value of threshold potential of most excitable cell membrane is about 15 to 20 mV less negative than the resting potential.The threshold stimulus is just strong enough to depolarize the membrane to the threshold potential level,therefore it can cause an action potential.,53,Electrophysiological Method to Record Membrane Potential I,Voltage Clamp,54,55,Cole and colleagues developed a method for maintaining Vm at any desired voltage level(FBA,Feedback Amplifier)Required monitoring voltage changes,feeding it through an amplifier to drive current into or out of the cell to dynamically maintain the voltage while recording the current required to do so,The voltage clamp,56,The Hodgkin-Huxley Model of Action Potential Generation,57,58,Triphasic response,59,Evidence for a Sodium Current,Remove extracellular sodium,60,Modern proof of nature of currents,Use ion selective agents,61,Removing Na+from the bathing medium,INa becomes negligible so IK can be measured directly.Subtracting this current from the total current yielded INa.,62,63,Conductance of Na+and K+channels,64,Voltage-Dependence of Conductance,65,An action potential,gNa increases quickly,but then inactivation kicks in and it decreases again.gK increases more slowly,and only decreases once the voltage has decreased.The Na+current is autocatalytic.An increase in V increases gNa,which increases the Na+current,and increases V,etc.Hence,the threshold for action potential initiation is where the inward Na+current exactly balances the outward K+current.,66,67,Cytoplasm,Ion channels,Giga-seal,Glass,microelectrode,Suction,1 m,Patch clamp recording,Cell Membrane,68,69,100 ms,4 pA,Closed,Open,Single channel record,70,One result from patch clampstudies was the finding that ion channels conduct currentsin an all or nothing fashion,71,Voltage-dependent Channel Conductance,72,How channel conductances accumulate,Next page shows an idealized version,73,74,Inactivating Na+channel currents,75,IV Local Response,76,Graded(local)potential changes,2 x more chemical=2 x more potential change,77,Local Response,Definition:Local response is a small change in membrane potential caused by a subthreshold stimulusProperties:It s a graded potentialIts propagation is electronic conductionIt can be summed by two ways Spatial summation Temporal summation,78,Excitatory,Excitatory,Inhibitory,Time,Membrane Potential(mV),Spatial Summation,Spatial Summation,a,b,c,d,a,b,c,d,79,Excitatory,Excitatory,Inhibitory,Time,Membrane Potential(mV),Temporal&Spatial Summation,Temporal Summation,a,b,c,d,a,b,c,d,80,Distribution of channels,Leak channels everywhere,Axon Hillock(Trigger Zone),81,Role of the Local Potential,Facilitate the cell.This means it increase excitability of the stimulated cellCause the cell to excite once it is summed to reach the threshold potential,82,83,V.Propagation of the Action Potential,84,85,86,Myelinated neuron of the central nervous system,87,Saltatory conduction:The action potential jumps from node to node,88,89,Saltatory Conduction,90,Saltatory Conduction,The pattern of conduction in the myelinated nerve fiber from node to node It is of value for two reasons:very fastconserves energy.,91,92,Factors that affect the propagation,Bioelectric properties of the membraneVelocity and amplitude of membrane depolarization,93,V Excitation and Excitability of the Tissue,94,Excitation and Excitability of the Tissue,Review:Excitation and Excitable CellReview:Threshold Stimulation and ExcitabilityChange of Excitability after the Excitation,95,96,Slide 3 of 28,97,4.Factors that Determine the Excitability,Resting potentialThreshold potentialConcentration of extracellular Ca2+,98,If resting potential depolarized by 15 20 mV,then activation gate opened with 5000 x increase in Na+permeability followed by inactivation gate close 1 ms later,Activation Gate,Inactivation gate,99,the threshold for action potential initiation is where the inward Na+current exactly balances the outward K+current.,100,Concentration of extracellular Ca2+,101,