class 10-第四章 神经生物物理学-1课件

1、第四章 神经与感官生物物理本章授课内容安排 Electrophysiology of cell membraneIon channelsNeurophysiology (I) Information transmission in NeuronsNeurophysiology (II) Sensory transductionHormone SecretionSynaptic transmission1 细胞电活动解释各种生物电、生物磁现象和效应的基础 生物组织可以对外界刺激发生反应，当刺激达到一定阈值时，生物组织发生反应，称为兴奋。 在神经和肌肉中，这种能力高度发展，主要表现是细胞膜的电位发

2、生快速的改变。 近年来研究发现，腺体细胞的分泌活动，卵和精子的受精过程，免疫细胞的吞噬功能都与细胞膜的电位变化有关。Outline1.1 Ionic Basis of generation of membrane potential & Nernst Equation1.2 Goldman-Hodgkin-Katz (GHK) equation1.3 Electrical model of cell membrane1.4 Electrical excitability and mechanism of generation of action potential1.5 Voltage cla

3、mp1.6 Patch clampK+P-P-P-P-P-P-P-P-P-P-K+K+K+K+K+K+K+K+K+K+P-P-P-P-P-P-P-P-P-P-K+K+K+K+K+K+K+K+K+E假设胞内有两种离子K+、P-组成， K+可透过细胞膜， P-不能透过细胞膜。1.1.1 细胞跨膜电位的产生1.1.2 离子平衡电位理解生物膜的所有电现象的基础定义：恰好对抗任何特定的X离子跨膜流动所需的电位差称为该离子的平衡电位Ex，完全取决于该离子的跨膜浓度差要建立细胞膜上某离子的平衡电位，要求：存在跨膜的离子浓度梯度；存在对某些离子选择性通透的膜。离子通道的存在使得膜可以选择性地通透不同的离子。I

4、onic Basis of generation of membrane potential & Nernst EquationGoldman-Hodgkin-Katz (GHK) equationElectrical model of cell membraneElectrical excitability and mechanism of generation of action potentialVoltage clamppatch clamp techniqueOutline1.2 膜电位 当只有单一离子X+能流动时，Nernst方程可描述膜电位，Vm=Ex。 现实情况是，存在具有不同

5、跨膜浓度梯度的若干种离子，和对这些离子具有不同离子选择性通透的若干种离子通道共存。 Membrane potential versus extracellular K+Note that the curve bends at low potassium concentrationsThree facts we learned from the previous experiments(1) Vm (EK) roughly follows the equation.(2) Vm is largely linear to the extracellular K+ concentration. It

6、 indicates that the membrane potential is largely determined by the ratio of intracellular K+ concentration to the extracellular K+ concentration.(3) Vm bends at very low K+ concentration. Why?Goldman-Hodgkin-Katz 方程当细胞膜上存在能对K+、Na+、Cl-选择性通过的离子通道时，得到P（通透系数）: 衡量离子跨膜扩散能力的参数。静息状态下，PK : PNa : PCl=1.0 : 0

7、.04 : 0.45通透性最高的离子的跨膜浓度对跨膜电压的产生影响最大。细胞膜对K+、Na+不同通透性对静息电位产生的影响净驱动力K+ Na+净电流K+ Na+Only K+ channel contributes to the resting channelA few resting Na+ channels are added to the membraneNew balanced resting potential, outward movement of K+ balance inward movement of Na+细胞膜对K+、Na+不同通透性对静息电位产生的影响The rest

8、ing membrane potential is determined by resting ion channelResting channels in Glial cells are selective for potassium onlyResting channels in nerve cells are selective for several ion speciesPassive flux of sodium and potassium is balanced by active pumping of the ionsChloride ions may be passively

9、 distributed静息膜电位静息电位是指，净电流=0时的膜电位，所以 iNa+iK+iCl+其他离子流=0 gNa（V-ENa）+gK（V-EK）+gCl（V-ECl）=0欧姆定律 Ix=(Vm-Ex)gx电化学驱动力Ionic Basis of generation of membrane potential & Nernst EquationGoldman-Hodgkin-Katz (GHK) equationElectrical model of cell membraneElectrical excitability and mechanism of generation of

10、action potentialVoltage clampPatch clamp techniqueOutlineEE1R1E2R2Electrical Model of Cell MembraneK+Na+Ca2+Cl-VmExtracellularIntracellularEKENaGKEClECaGNaGCaGClEquivalent circuit modelModel of a cell membrane with four different ion channelsElectrical Model of Cell MembraneMembrane potential Vm is

11、the sum of equilibrium potentials (EX), each weighted by the ions fractional conductance.Ion with higher permeability (larger conductance G) contributes more to the membrane potential. Biological meaning of the equationThe property of cell membrane as capacitatorCapacitance=Q/V+Q QVmThe unit capacit

12、ance of most cell membrane is 1 mF/cm2.+Q QdVCapacitor model of cell membraneV0V=V0e(-t/RC)t=RC37% V0ICTimeTimeTime Course of VoltageTime Course of CurrentCmRm+Switch closeSwitch closeCapacitative Current0Basic electrical property of a capacitatorStimulusTimeResponse ImCell membraneImpose a stimulus

13、 (a Voltage) on a cell. What will we see?CurrentCapacitator model of cell membraneCmRm+Due to the existence of membrane capacitance, total membrane current has two components: IX: ionic current, carried by ions flowing through the membrane (via ion channels).IC: capacitattive current, carried by ion

14、s as they charge or discharge the membrane capacitance.Components of currents through the cell membraneIonic Basis of generation of membrane potential & Nernst EquationGoldman-Hodgkin-Katz (GHK) equationElectrical model of cell membraneElectrical excitability and mechanism of generation of action po

15、tentialVoltage clampPatch clamp techniqueOutlineCellular communication in the nervous system is based on electrical and chemical signaling events.1.4 Electrical excitability and action potentialCertain types of cells, including neurons, myocytes, can generate action potential when the membrane is de

16、polarized above a threshold. This property of neurons and myocytes is called electrical excitability.Action potential is a transient, regenerative electrical impulse, in which the membrane potential rapidly rise to a peak ( 100 mV more positive than the resting Vm). Action potential can propagate fo

17、r long distances along the nerve of muscle fibers.神经细胞肌细胞腺细胞前提 外在表现 本质表现 刺激反应(兴奋和抑制)动作电位可兴奋细胞InitiationOvershootHyperpolarization phasePeakThresholdDepolarization phaseRepolarization phaseGeneral phases of a typical action potential0+Stimulus currentTime courseMembrane potential (Vm)What determines

18、the rate of change in potential?Will a brief synaptic current always produce a similar potential change, regardless of the size of the postsynaptic cell?What determines whether a stimulus will or will not produce an action potential?However, Goldman equation can not provide any information about cha

19、nges in the membrane potential in response to stimulus, since it applies only to the steady state when the voltage does not change.InitiationOvershootHyperpolarization phasePeakThresholdDepolarization phaseRepolarization phase0+Membrane potential (Vm)Stimulus currentTime courseGeneral phases of a ty

20、pical action potential有关概念示意图去极化 复极化 90mv (Rp)超极化 极化 反极化 80mv100mv0mv+30mv动作电位的Na+假说 记录动作电位的同时，测量膜电阻，发现膜电阻大大减小，说明膜对某种离子通透性增加。 静息时，PK:PNa:PCl=1.0:0.04:0.45去极化时， PK:PNa:PCl=1.0:20:0.45PKE E K; PNa E E Na; PCl E E Cl 只有E Na0，故去极化极有可能是Na+通透性增加引起的。这种观点叫动作电位的Na+学说.根据Goldman-Hodgkin-Kalz方程： Hodgkin和Huxley用

21、电压箝位(Voltage Clamp)技术发现了神经动作电位的离子机制，从而获得1963年Nobel医学和生理学奖(一)Ap的产生机制 1. 上升支：主要是Na内流形成。前提细胞受刺激后，PNaPK；Na细胞外为细胞内12倍。 结果：膜内电位升高 内正外负。过程： 细胞外Na 顺电化学梯度 细胞内；Na+ Channel function in action potential膜内电位渐 AP Na迅速大量内流临界值Na通道突然大量开放Na通道：电压门控式通道。正反馈机制阈电位 使膜对Na通道突然大量开放而触发 Ap的临界膜电位数值。 AP上升支最大电位略小于Na的平衡电位 2.下降支主要是由

22、K外流形成。结果：升高的膜内电位下降 内负外正。过程： 细胞内K 顺电化学梯度 细胞外；前提Ap上升支达最大电位后K细胞内为细胞外39倍。 Na通道关闭，K通道开放；K+ Channel function In action potential3.Ap后：离子泵运转，把内流的 Na泵出去， 把外流的K泵进来。逆浓度差“排Na吸K”注AP的上升支和下降支形成：被动过程； 复极化后的电位恢复过程：主动过程。动作电位产生的离子机制：Changes in Membrane Conductance to Na+ and K+ Underlying Action PotentialMembrane pot

23、ential (Vm)Conductance (G)Vm (Action potential)Time-60+300-30Changes in Membrane Conductance to Na+ and K+ Underlying Action PotentialMembrane potential (Vm)Conductance (G)GNaGKVm (Action potential)EK= 95 mVENa=+67 mVTime-60+300-30Changes in Membrane Conductance to Na+ and K+ Underlying Action Poten

24、tialDuring an action potentialDepolarization cause membrane potential reaches the thresholdVoltage-gated Na+ channels activated, (Na+ conductance GNa+ increased), Na+ ions flux into the cell (inward current).Membrane potential shifts towards the Nernst Potential of Na+ and gets depolarized further.K

25、+ channels activated, (conductance GK+ increased). K+ ions flux out of the cell (outward current). Na+ channels become inactivatedMembrane potential repolarizedK+ channels inactivated.(二)阈电位(局部电位)与动作电位的关系水平比Rp绝对值小1020mV (-70-50mV)。阈电位产生Ap的必要条件意义 兴奋性1RP与阈电位之差值Dissection of Na+ and K+ currents by spec

26、ific channel inhibitorsTTX, Na+ channel inhibitorTEA, K+ channel inhibitorSTX, Na+ channel inhibitorTotal membrane ionic currentA stepwise depolarization stimulus imposed on axonThus the total membrane ionic current is the summation of Na+ and K+ currentQ: What if a hyperpolarization was imposed on

27、axon?This is “delayed rectifier” K+ current! During an action potential, Na+ current is initiated at negative voltage (-30 mV) when membrane is depolarized. Because Na+oNa+i, opening of Na+ channels cause inward current, which causes further depolarization in membrane potential.K+ current is initiat

28、ed at positive voltage when membrane is further depolarized due to Na+ influx during an action potential. Because K+o 0I 0Na+Cl-Na+Cl-附录：Vm, the membrane potential, is defined as Vin Vout. Thus, when we say Vm0, that implies there is a bit more negative charge on the inside.When positive charges com

29、e into the cell, the cell becomes more positive. Thus, Vm increases.The term “depolarization” means Vm becomes more positive. The term “hyperpolarization” means Vm becomes more negative.Some RecapThe Nernst potential (AKA reversal potential, equilibrium potential) is the voltage at which a certain i

30、on would have no net flow. If Vm is not equal to the Nernst potential, the ion will flow in the direction as to bring Vm towards it.As voltage changes, currents change; as currents change, voltage changes. Its very often a cyclical process in reality.Many experiments are done with non-physiological

31、concentrations of ions or voltages; thus, you have to be careful with graphs and the directions they show.GHK方程的推导过程Factors affect the currents carried by ion X across the membrane:(1) Concentration gradient of ion X across the membrane(2) Membrane potential Vm(3) Permeability of the membrane to ion

32、 X (PX).XoXiPartition coefficientbVmLElectrodiffusion Model of Cell MembraneIntracellularExtracellularDiffusion coefficientDAssumptions about how ion X behaves in the membrane:1. The membrane is a homogenous medium with a thickness of L.2. The voltage difference varies linearly with distance across

33、the membrane (Constant-field assumption)3. The movement of ion X through the membrane is independent of any other ions (Independence Principle).4. The permeability coefficient PX is constant.XoXiPartition coefficientbVmLElectrodiffusion Model of Cell MembraneIntracellularExtracellularDiffusion coeff

34、icientDAssumptions about how ion X behaves in the membrane:Px=Db/L. D is the diffusion coefficient for ion X in the membraneb is the membrane/water partition coefficientL is the thickness of the membrane.Px describes the ability of ion X to dissolve in the membrane (b) and diffuse from one side of t

35、he membrane to the other side (Dx) over the distance L.XoXiPartition coefficientbVmLElectrodiffusion Model of Cell MembraneIntracellularExtracellularDiffusion coefficientDBased on the above assumptions, using basic physical laws below, one can derive the GHK current equation: (1) Ficks law of diffusion Jx=Px(Xo-Xi)(2) Electrophoresis theory(3) Ohms lawDe