1、Neuronal Electric Activities神經(jīng)元的電活動,主講教師 劉風(fēng)雨 萬有 神經(jīng)科學(xué)研究所、神經(jīng)生物學(xué)系,Neuronal Electric Activities Include:,Rest Potential (Chapter 3) Action Potential (Chapter 4) Local Potentials Post-Synaptic Potential Excitatory Post-Synaptic Potential Inhibitory Post-Synaptic Potential End-plate Potential Receptor Pot

2、ential,Chapter 3The Neuronal Membrane at Rest,The CAST OF CHEMICALS Cytosol and Extracellular Fluid The Phospholipid Membrane Protein The MOVEMENT OF IONS Diffusion Electricity The IONIC BASIS OF RESTING MEMBRANE POTENTIAL Equilibrium Potential The Distribution of Ions Across the Membrane Relative I

3、on Permeabilities of Membrane at Rest The Importance of Regulating the External Potassium Concentration CONCLUDING REMARKS,Cytosol and Extracellular Fluid,Water: Its uneven distribution of electrical charge, so H2O is a polar molecule Ions: Salt dissolves readily in water because the charged portion

4、s of the water molecule have a stronger attraction for the ions than they have for each other,The Phospholipid Membrane (磷脂膜),The lipids of the neuronal membrane forming: a barrier to water-soluble ions a barrier to water,頭端-極性磷酸鹽-親水,尾端-非極性碳氫化合物 -疏水,5,Protein,These proteins provide routes for ions t

5、o cross the neuronal membrane. The resting and action potentials depend on special proteins that span the phospholipid bilayer.,Protein Amino Acids,The Peptide Bond (肽鍵) and a Polypeptide (多肽),Figure 3.6 Protein Structure,The primary structure,The secondary structure,The tertiary structure,The quate

6、rnary structure,Each of the different polypeptides contributing to a protein with quaternary structure is called a subunit (亞基).,Channel Proteins,Channel protein is suspended in a phospholipid bilayer, with its hydrophobic (疏水的) portion inside the membrane hydrophilic (親水的) ends exposed to the water

7、y environments on either side,Figure 3.7 A Membrane Ion Channel,10,Two Properties of Ion Channels,Ion selectivity (離子選擇性) The diameter of the pore The nature of the R groups lining it Gating (門控特性) Channels with this property can be opened and closed-gated by changes in the local microenvironment of

8、 the membrane,Ion Pumps (離子泵),Ion pumps are enzymes that use the energy released by the breakdown of ATP to transport certain ions across the membrane,Chapter 3The Neuronal Membrane at Rest,THE CAST OF CHEMICALS Cytosol and Extracellular Fluid The Phospholipid Membrane Protein THE MOVEMENT OF IONS D

9、iffusion Electricity THE IONIC BASIS OF RESTING MEMBRANE POTENTIAL Equilibrium Potential The Distribution of Ions Across the Membrane Relative Ion Permeabilities of Membrane at Rest The Importance of Regulating the External Potassium Concentration CONCLUDING REMARKS,THE MOVEMENT OF IONS,A channel ac

10、ross a membrane is like a bridge across a river. An open channel A net movement of ions across the membrane. Ion movement requires that external forces be applied to drive ions across. Two factors influence ion movement through channels: Diffusion (擴散) Electricity (電勢差),Diffusion,Temperature-depende

11、nt random movement of ions and molecules tends to distribute the ions evenly throughout the solution so that there is a net movement of ions from regions of high concentration to regions of low concentration. This movement is called diffusion (擴散). A difference in concentration is called a concentra

12、tion gradient (濃度梯度).,15,Figure 3.8 Diffusion,Driving ions across the membrane by diffusion happens when The membrane possesses channels permeable to the ions There is a concentration gradient across the membrane,Electricity,Another way to induce a net movement of ions in a solution is to use an ele

13、ctrical field (電場), because ions are electrically charged particles. Opposite charges attract and like charges repel.,Figure 3.9 The movement of ions influenced by an electrical field,Opposite charges attract and like charges repel,Electricity,Two important factors determine how much current (I) wil

14、l flow: Electrical potential (V, 電勢) Electrical conductance (g, 電導(dǎo)) Electrical conductance Electrical resistance (電阻, R=1/g) Ohms law: I = gV,Figure 3.10 Electrical current flow across a membrane,Driving an ion across the membrane electrically requires The membrane possesses channels permeable to th

15、e ions There is a electrical potential difference across the membrane,20,Diffusion and Electricity,Electrical charged ions in solution on either side of the neuronal membrane. (帶電離子溶解在細胞膜兩側(cè)的溶液中) Ions can cross the membrane only by protein channel. (離子必須通過離子通道實現(xiàn)跨膜運動) The protein channels can be highl

16、y selective for specific ions. (離子通道對離子具有高度的選擇性) The movement of any ion through channel depends on the concentration gradient and the difference in electrical potential across the membrane. (離子的跨膜運動依賴于膜兩側(cè)的濃度梯度和電位差),Chapter 3The Neuronal Membrane at Rest,The CAST OF CHEMICALS Cytosol and Extracellul

17、ar Fluid The Phospholipid Membrane Protein The MOVEMENT OF IONS Diffusion Electricity The IONIC BASIS OF RESTING MEMBRANE POTENTIAL Equilibrium Potential The Distribution of Ions Across the Membrane Relative Ion Permeabilities of Membrane at Rest The Importance of Regulating the External Potassium C

18、oncentration CONCLUDING REMARKS,The membrane potential (膜電位) is the voltage across the neuronal membrane at any moment, represented by the symbol mV. Microelectrode (微電極) and mV measurement,THE IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL (靜息電位),Establishing Equilibrium Potential (平衡電位),Figure 3.12

19、 Establishing equilibrium in a selectively permeable membrane,No potential difference Vm = 0 mV,The diffusional force = The electrical force Vm = - 80 mV,20,:1,Equilibrium potentials,The electrical potential difference that exactly balances an ionic concentration gradient is called an ionic equilibr

20、ium potential, or simply equilibrium potential (當(dāng)離子移動所產(chǎn)生的電位差和離子移動所造成的濃度勢能差平衡時，不再有離子的凈移動，這時膜兩側(cè)的電位差稱為離子的平衡電位) Generating a steady electrical potential difference across a membrane requires An ionic concentration gradient Selective ionic permeability,25,Before moving on to the situation in real neurons

21、, four important points should be made:,Large changes in membrane potential are caused by minuscule changes in ionic concentrations (僅需要微小的離子濃度改變就可以引起膜電位大幅度的變化),100 mM,99.99999 mM,Vm = - 80 mV,Vm = 0 mV,Before moving on to the situation in real neurons, four important points should be made:,2. The n

22、et difference in electrical charge occurs at the inside and outside surfaces of the membrane (膜內(nèi)外兩側(cè)電荷的不同僅僅分布于膜的內(nèi)外側(cè)面，而不是分布于整個細胞的內(nèi)外液),Figure 3.13,(5 nm),Before moving on to the situation in real neurons, four important points should be made:,Ions are driven across the membrane at a rate proportional t

23、o the difference between the membrane potential and the equilibrium potential (離子的跨膜速率與膜電位和平衡電位的差值成正比). Net movement of K+ occurs as the membrane potential differed from the equilibrium potential. This difference (Vm - Eion) is called the ionic driving force (離子驅(qū)動力). If the concentration difference

24、across the membrane is known for an ion, an equilibrium potential can be calculated for that ion (根據(jù)某離子膜兩側(cè)濃度的差值可以計算該離子的平衡電位).,Na+ Equilibrium Potential,Figure 3.14 Another example establishing equilibrium in a selectively permeable membrane,The Nernst Equation,The exact value of an equilibrium poten

25、tial in mV can be calculated using the Nernst equation, which takes into consideration: The charge of the ion The temperature The ratio of the external and internal ion concentrations Page 64. Box 3.2. Mark F. Bear, et al. ed. Neuroscience: Exploring the Brain. 2nd edition.,EK = 2.303 log,30,Figure

26、3.15,Figure 3.15 Approximate ion concentrations on either side of a neuronal membrane.,Relative Ion Permeabilities of Membrane at Rest,The resting membrane permeability is forty times greater to K+ than to Na+ The resting membrane potential is 65mV,The Distribution of Ions Across the Membrane,Ionic

27、concentration gradients are established by the actions of ions pumps in the neuronal membrane (膜內(nèi)外兩側(cè)的離子濃度梯度的形成依賴于 離子泵的活動) Two important ion pumps: The sodium-potassium pump (鈉鉀泵) is an enzyme that breaks down ATP in the presence of internal Na+. The calcium pump (鈣泵) is an enzyme that actively trans

28、ports Ca2+ out of the cytosol across the cell membrane.,Figure 3.16,Figure 3.16 The sodium-potassium pump.,K+,K+,Na+,Na+,Figure 4.4,Membrane currents and conductances,35,The most potassium channels have four subunits that are arranged like the staves of a barrel to form a pore Of particular interest

29、 is a region called the pore loop (孔袢), which contributes to the selectivity filter that makes the channel permeable mostly to K+ ions.,The wide world of potassium channels,Figure 3.18,Figure 3.18 A view of the atomic structure of the potassium channel pore,The importance of regulating the external

30、potassium concentration,Increasing extracellular potassium depolarizes neurons,Figure 3.19 The dependence of membrane potential on external potassium concentration.,Two protective mechanisms in the brain,Blood-brain barrier (血腦屏障) limits the movement of potassium (and other blood-borne substances) i

31、nto the extracellular fluid of the brain Glia, particularly astrocytes, take up extracellular K+ whenever concentrations rise, as they normally do during periods of neural activity.,Figure 3.20,Figure 3.20 Potassium spatial buffering by astrocytes. When brain K+o increases as a result of local neura

32、l activity, K+ enters astrocytes via membrane channels. The extensive network of astrocytic processes helps dissipate the K+ over a large area.,40,Chapter 3The Neuronal Membrane at Rest,The CAST OF CHEMICALS Cytosol and Extracellular Fluid The Phospholipid Membrane Protein The MOVEMENT OF IONS Diffu

33、sion Electricity The IONIC BASIS OF RESTING MEMBRANE POTENTIAL Equilibrium Potential The Distribution of Ions Across the Membrane Relative Ion Permeabilities of Membrane at Rest The Importance of Regulating the External Potassium Concentration CONCLUDING REMARKS,Neuronal Electric Activities Include:

34、,Rest Potential (Chapter 3) Action Potential (Chapter 4) Local Potentials Post-Synaptic Potential Excitatory Post-Synaptic Potential Inhibitory Post-Synaptic Potential End-plate Potential Receptor Potential,Chapter 4 The Action Potential,PROPERTIES OF THE ACTION POTENTIAL The Ups and Downs of an Act

35、ion Potentials Generation of an Action Potential The Generation of Multiple Action Potentials THE ACTION POTENTIAL IN THEORY Membrane Currents and Conductances The Ins and Outs of Action Potential THE ACTION POTENTIAL IN REALITY The Voltage-Gated Sodium Channel Voltage-Gated Potassium Channels Putti

36、ng the Pieces Together ACTION POTENTIAL CONDUCTION Factor influencing conduction velocity ACTION POTENTIALS, AXONS, AND DENDRITES CONCLUDING REMARKS,Methods of Recording Action Potentials,細胞內(nèi)記錄,細胞外記錄,示波器,The Ups and Downs of an Action Potentials,- 65 mV,45,Generation of an action potential,The perce

37、ption of sharp pain when a thumbtack enters your foot is caused by the generation of action potentials in certain nerve fibers in the skin: The thumbtack enters the skin (圖釘扎入皮膚) The membrane of the nerve fibers in the skin is stretched (感覺神經(jīng)纖維的細胞膜被牽拉) Na+-permeable channels open. The entry of Na+ d

38、epolarizes the membrane (Na+通道打開，細胞膜產(chǎn)生去極化) The critical level of depolarization that must be crossed in order to trigger an action potential is called threshold (閾電位). Action potential are caused by depolarization of the membrane beyond threshold.,The depolarization that causes action potential aris

39、es in different ways in different neurons (引起去極化的不同方式):,Caused by the entry of Na+ through specialized ion channels that sensitive to membrane stretching (膜的牽拉) In interneurons, depolarization is usually caused by Na+ entry through channels that are sensitive to neurotransmitters (神經(jīng)遞質(zhì)的釋放) released

40、by other neurons 3. In addition to these natural routes, neurons can be depolarized by injecting electrical current (注入電流) through a microelectrode, a method commonly used by neuroscientists to study action potentials in different cells. Applying increasing depolarization to a neuron has no effect u

41、ntil it crosses threshold, and then “pop” one action potential. For this reason, action potentials are said to be “all-or-none” (全或無現(xiàn)象).,The generation of multiple action potentials,Continuous depolarizing current Many action potentials in succession,注入電流,The firing frequency of action potentials re

42、flects the magnitude of the depolarizing current (頻率反應(yīng)去極化電流的大小),This is one way that stimulation intensity is encoded in the nervous system (中樞神經(jīng)系統(tǒng)編碼刺激強度的一種方式),Though firing frequency increases with the amount of depolarizing current, there is a limit to the rate at which a neuron can generate actio

43、n potentials.,Absolute refractory period (絕對不應(yīng)期) Once an action potential is initiated, it is impossible to initiate another for about 1 ms (動作電位產(chǎn)生后1 ms, 不可能產(chǎn)生別的動作電位) Relative refractory period (相對不應(yīng)期) The amount of current required to depolarize the neuron to action potential threshold is elevated

44、above normal (絕對不應(yīng)期之后的幾個ms, 需要比正常更大的閾電流才能爆發(fā)動作電位),50,Chapter 4 The Action Potential,PROPERTIES OF THE ACTION POTENTIAL The Ups and Downs of an Action Potentials Generation of an Action Potential The Generation of Multiple Action Potentials THE ACTION POTENTIAL IN THEORY Membrane Currents and Conducta

45、nces The Ins and Outs of Action Potential THE ACTION POTENTIAL IN REALITY The Voltage-Gated Sodium Channel Voltage-Gated Potassium Channels Putting the Pieces Together ACTION POTENTIAL CONDUCTION Factor influencing conduction velocity ACTION POTENTIALS, AXONS, AND DENDRITES CONCLUDING REMARKS,THE AC

46、TION POTENTIAL IN THEORY,Depolarization of the cell during the action potential is caused by the influx of sodium ions across the membrane (去極化是鈉離子內(nèi)流造成的) Repolarization is caused by the efflux of potassium ions (復(fù)極化是鉀離子外流造成的),The Ins and Outs of Action Potential,The rising phase A very large driving

47、 force on Na+ (- 80 - 62) mV = - 142mV The membrane permeability to Na+ K+ Depolarization of the membrane beyond threshold, membrane sodium channels opened. This would allow Na+ to enter the neuron, causing a massive depolarization until the membrane potential approached ENa. The falling phase The d

48、ominant membrane ion permeability to K+ K+ flow out of the cell until the membrane potential approached EK.,The ins and outs and ups and downs of the action potential in an ideal neuron is shown as below: (Fig 4.5),55,Chapter 4 The Action Potential,PROPERTIES OF THE ACTION POTENTIAL The Ups and Down

49、s of an Action Potentials Generation of an Action Potential The Generation of Multiple Action Potentials THE ACTION POTENTIAL IN THEORY Membrane Currents and Conductances The Ins and Outs of Action Potential THE ACTION POTENTIAL IN REALITY The Voltage-Gated Sodium Channel Voltage-Gated Potassium Cha

50、nnels Putting the Pieces Together ACTION POTENTIAL CONDUCTION Factor influencing conduction velocity ACTION POTENTIALS, AXONS, AND DENDRITES CONCLUDING REMARKS,Voltage clamp (電壓鉗) proves the above theory:,The Voltage-Gated Sodium Channel(電壓門控的鈉離子通道),The protein forms a pore in the membrane that is h

51、ighly selective to Na+ ions (對Na+具有高度的選擇性). The pore is opened and closed by changes in the electrical potential of the membrane (Na+通道的開放和關(guān)閉具有電壓依從性).,Sodium channel structure(Na+ 通道的結(jié)構(gòu)),Created from a single long polypeptide Has 4 distinct domains, numbered I-IV. The four domains are believed to cl

52、ump together to form a pore between them Each domain consists of 6 transmembrane alpha helices, numbered S1-S6 The channel has pore loops that are assembled into a selectivity filter,60,Figure 4.6 Structure of the voltage-gated sodium channel (a) How the sodium channel polypeptide chain is believed

53、to be woven into the membrane. The molecule consists of four domains, I-IV. Each domain consists of 6 alpha helices, which pass back and forth across the membrane,Figure 4.6,(b) An expanded view of one domain showing the voltage sensor of alpha helix S4 and the pore loop (red), which contributes to

54、the selectivity filter (c) A view of the molecule showing how the domains may arrange themselves to form a pore between them.,電壓感受器,Figure 4.7,When the membrane is depolarized to threshold, the molecule twists into a configuration that allows the passage of Na+ through the pore.The voltage sensor re

55、sides in segment S4 of the molecule. In this segment, positively charged amino acid residues are regularly spaced along the coils of the helix. Thus, the entire segment can be forced to move by changing the membrane potential. Depolarization pushes S4 away from the inside of the membrane, and this c

56、onformational change in the molecule causes the gate to open.,The patch-clamp (膜片鉗) Method,- 40 mV,65,Functional properties of the sodium channel (Na+ 通道的功能),They open with little delay They stay open for about 1 ms and then close (inactivate) They cannot be opened again by depolarization until the

57、membrane potential returns to 65 mV,關(guān)閉,開放,失活,去失活,Functional properties of the sodium channel,Figure 4.9 (c) A model for how changes in the conformation of the sodium channel protein might yield its functional properties. The closed (關(guān)閉) channel; Opens (開放) upon membrane depolarization; Inactivation

58、(失活) occurs when a globular portion of the protein swings up and occludes the pore; Deinactivation (去失活) occurs when the globular portion swings away and the pore closes by movement of the transmembrane domains,關(guān)閉,開放,失活,去失活,Toxins on the sodium channel,Tetrodotoxin (TTX, 河豚毒素) and saxitoxin Channel-

59、blocking toxin Batrachotoxin, veratridine and aconitine Open the channels inappropriately Open at more negative potentials Open much longer than usual,Putting the Pieces Together (page 89),Threshold Rising phase Overshoot Falling phase Undershoot Absolute refractory period Relative refractory period,Figure 4.10 The molecular basis of the action potential,70,Chapter 4 The Action Potential,PROPERTIES OF THE ACTION POTENTIAL The Ups and Downs of an Action Potentials Generation of an Action Potential The Generation of Multiple Action Potentials THE ACTION POTENTIAL