1、第四章 神經(jīng)與感官生物物理本章授課內(nèi)容安排 Electrophysiology of cell membraneIon channelsNeurophysiology (I) Information transmission in NeuronsNeurophysiology (II) Sensory transductionHormone SecretionSynaptic transmission1 細(xì)胞電活動(dòng)解釋各種生物電、生物磁現(xiàn)象和效應(yīng)的基礎(chǔ) 生物組織可以對(duì)外界刺激發(fā)生反應(yīng)，當(dāng)刺激達(dá)到一定閾值時(shí)，生物組織發(fā)生反應(yīng)，稱為興奮。 在神經(jīng)和肌肉中，這種能力高度發(fā)展，主要表現(xiàn)是細(xì)胞膜的電位發(fā)

2、生快速的改變。 近年來研究發(fā)現(xiàn)，腺體細(xì)胞的分泌活動(dòng)，卵和精子的受精過程，免疫細(xì)胞的吞噬功能都與細(xì)胞膜的電位變化有關(guān)。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假設(shè)胞內(nèi)有兩種離子K+、P-組成， K+可透過細(xì)胞膜， P-不能透過細(xì)胞膜。1.1.1 細(xì)胞跨膜電位的產(chǎn)生1.1.2 離子平衡電位理解生物膜的所有電現(xiàn)象的基礎(chǔ)定義：恰好對(duì)抗任何特定的X離子跨膜流動(dòng)所需的電位差稱為該離子的平衡電位Ex，完全取決于該離子的跨膜濃度差要建立細(xì)胞膜上某離子的平衡電位，要求：存在跨膜的離子濃度梯度；存在對(duì)某些離子選擇性通透的膜。離子通道的存在使得膜可以選擇性地通透不同的離子。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 膜電位 當(dāng)只有單一離子X+能流動(dòng)時(shí)，Nernst方程可描述膜電位，Vm=Ex。 現(xiàn)實(shí)情況是，存在具有不同

5、跨膜濃度梯度的若干種離子，和對(duì)這些離子具有不同離子選擇性通透的若干種離子通道共存。 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 方程當(dāng)細(xì)胞膜上存在能對(duì)K+、Na+、Cl-選擇性通過的離子通道時(shí)，得到P（通透系數(shù)）: 衡量離子跨膜擴(kuò)散能力的參數(shù)。靜息狀態(tài)下，PK : PNa : PCl=1.0 : 0

7、.04 : 0.45通透性最高的離子的跨膜濃度對(duì)跨膜電壓的產(chǎn)生影響最大。細(xì)胞膜對(duì)K+、Na+不同通透性對(duì)靜息電位產(chǎn)生的影響凈驅(qū)動(dòng)力K+ Na+凈電流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+細(xì)胞膜對(duì)K+、Na+不同通透性對(duì)靜息電位產(chǎn)生的影響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靜息膜電位靜息電位是指，凈電流=0時(shí)的膜電位，所以 iNa+iK+iCl+其他離子流=0 gNa（V-ENa）+gK（V-EK）+gCl（V-ECl）=0歐姆定律 Ix=(Vm-Ex)gx電化學(xué)驅(qū)動(dòng)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.神經(jīng)細(xì)胞肌細(xì)胞腺細(xì)胞前提 外在表現(xiàn) 本質(zhì)表現(xiàn) 刺激反應(yīng)(興奮和抑制)動(dòng)作電位可興奮細(xì)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有關(guān)概念示意圖去極化 復(fù)極化 90mv (Rp)超極化 極化 反極化 80mv100mv0mv+30mv動(dòng)作電位的Na+假說 記錄動(dòng)作電位的同時(shí)，測(cè)量膜電阻，發(fā)現(xiàn)膜電阻大大減小，說明膜對(duì)某種離子通透性增加。 靜息時(shí)，PK:PNa:PCl=1.0:0.04:0.45去極化時(shí)， PK:PNa:PCl=1.0:20:0.45PKE E K; PNa E E Na; PCl E E Cl 只有E Na0，故去極化極有可能是Na+通透性增加引起的。這種觀點(diǎn)叫動(dòng)作電位的Na+學(xué)說.根據(jù)Goldman-Hodgkin-Kalz方程： Hodgkin和Huxley用

21、電壓箝位(Voltage Clamp)技術(shù)發(fā)現(xiàn)了神經(jīng)動(dòng)作電位的離子機(jī)制，從而獲得1963年Nobel醫(yī)學(xué)和生理學(xué)獎(jiǎng)(一)Ap的產(chǎn)生機(jī)制 1. 上升支：主要是Na內(nèi)流形成。前提細(xì)胞受刺激后，PNaPK；Na細(xì)胞外為細(xì)胞內(nèi)12倍。 結(jié)果：膜內(nèi)電位升高 內(nèi)正外負(fù)。過程： 細(xì)胞外Na 順電化學(xué)梯度 細(xì)胞內(nèi)；Na+ Channel function in action potential膜內(nèi)電位漸 AP Na迅速大量?jī)?nèi)流臨界值Na通道突然大量開放Na通道：電壓門控式通道。正反饋機(jī)制閾電位 使膜對(duì)Na通道突然大量開放而觸發(fā) Ap的臨界膜電位數(shù)值。 AP上升支最大電位略小于Na的平衡電位 2.下降支主要是由

22、K外流形成。結(jié)果：升高的膜內(nèi)電位下降 內(nèi)負(fù)外正。過程： 細(xì)胞內(nèi)K 順電化學(xué)梯度 細(xì)胞外；前提Ap上升支達(dá)最大電位后K細(xì)胞內(nèi)為細(xì)胞外39倍。 Na通道關(guān)閉，K通道開放；K+ Channel function In action potential3.Ap后：離子泵運(yùn)轉(zhuǎn)，把內(nèi)流的 Na泵出去， 把外流的K泵進(jìn)來。逆濃度差“排Na吸K”注AP的上升支和下降支形成：被動(dòng)過程； 復(fù)極化后的電位恢復(fù)過程：主動(dòng)過程。動(dòng)作電位產(chǎn)生的離子機(jī)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)閾電位(局部電位)與動(dòng)作電位的關(guān)系水平比Rp絕對(duì)值小1020mV (-70-50mV)。閾電位產(chǎn)生Ap的必要條件意義 興奮性1RP與閾電位之差值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-附錄：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方程的推導(dǎo)過程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