静息电位及动作电位的形成原理

静息电位及动作电位的形成原理 百度一下 查看文章 相关文章 静息电位和动作及其形成原理 2009-09-16 16:19 静息电位及其形成原理 细胞膜的生物电现象主要有两种现形式，即安静时的静息电位和受刺激时产生的膜电位的改变(包括局部电位和动作电位)。生物电现象是以细胞为单位产生的，以细胞膜两侧带电离子的不均衡分布和离子的选择性跨膜转运为基础。 1.静息电位(resting potential，RP):指细胞未受刺激时存在于细胞膜内外两侧的电位差。将一对测量电极中的一个放在细胞的外表面，另一个与微电极相连，准备刺入细胞膜内。当两个电极都位于膜外时，电极之间不存在电位差。在微电极尖端刺入膜内的一瞬间，示波器上显示一突然的电位跃变，表明两个电极间出现电位差，膜内侧的电位低于膜外侧电位。该电位差是细胞安静时到的，因此称为静息电位。几乎所有的动植物细胞的静息电位都表现为膜内电位值较膜外为负，如规定膜外电位为0，膜内电位可以负值表示，即大多数细胞的静息电位在-10,-100mV之间。神经细胞的静息电位约为-70mV，红细 胞的约为-10mV。 细胞膜两侧存在电位差，以及此电位差在某种条件下会发生波动，使细胞膜处于不同的电学状态。人们将细胞安静时膜两侧保持的内负外正的的状态称为膜的极化;当膜电位向膜内负值加大的方向变化时，称为膜的超极化;相反，膜电位向膜内负值减小的方向变化，称为膜的去极化;细胞受刺激后先发生去极化，再向膜内为负的静息电位水平恢复，称为膜的复极化。 2.静息电位形成的原理 (1)细胞膜内、外的离子浓度差 RP的形成与细胞膜两侧的离子有关。下表显示枪乌贼巨轴突细胞膜两侧主要离子浓度。由表可见，细胞膜内外的离子呈不均衡分布，膜内K+多于膜外，Na+和Cl-低于膜外，即细胞内为高钾低钠低氯的状态。此外，A-表示带负电的蛋白质基团，仅存在于膜内。 枪乌贼巨轴突细胞膜两侧主要离子浓度 离子 胞浆(mmol/L) 细胞外液(mmol/L) 平衡电位(mV) K+ 400 20 -75 Na+50 440 +55 investigators carefully to verify that such credit party authenticity, when necessary, to the borrower unit verification and should strictly distinguish between regular employees, temporary employees, categories and on the job, retirement, early retirement, etc. Purpose to verify investigators should require borrowers to provide proof of use data and verified to ensure that lending purposes should be consistent with the relevant laws and regulations and national policies. Bank ban on issuing personal loans without specified uses. Personal loans should be a sharp distinction between consumer and business uses, management personal loans should refer to working capital loans required to verify authenticity, consumer loans should ensure a clear, true and reasonable. Strictly guard against borrowers will loan money to the stock market, real estate, loan-sharking, private fund raising and other high risk areas or used for illegal business strictly preventing borrowers loans capital private public or misappropriated ... Understand borrowers ' credit business credit situation of the parties. Strict precautions against Cl-52 560 -60 A- 385 - - 2) 细胞膜对离子的选择通透性和K+平衡电位 Hodgkin和Huxley推测:由于细胞内外存在K+的浓度差(细胞内高钾)， K+具有从膜内侧向膜外侧扩散的趋势。如果细胞膜在安静时只能允许K+自由通透(K+通道开放)，K+即可顺浓度差外流到细胞外。虽然胞内A-的浓度也很高，但细胞膜对A-不能通透，它只能因正负电荷的相互吸引作用，排列于细胞的内侧面。而扩散出细胞的K+也不能远离膜，而排列在膜的外侧面。这样在膜的内外两侧就形成了内负外正的电位差。 K+的这种外向扩散不能无限制的进行，因为K+外流造成的外正内负的电场力，将阻碍带正电的K+继续外流，而且K+外流愈多，这种电势的阻碍就会愈大。当促使K+外流的膜两侧K+浓度差势能， 与阻碍K+外流的电位差势能相等时，即膜两侧电-化学势的代数和为零时，K+外流量与回收(回到胞内)的量达到了动态平衡，K+的跨膜净移动为零，此时膜两侧电位差就稳定在某一不再增大的数值，即静息电位。因其是K+移动达到平衡时的膜电位，又可称作K+平衡电位(EK)。 上述理论是否就是事实，还需要进一步的证实。根据物理化学原理，已知枪乌贼巨轴突膜内外的K+浓度， 通过Nernst方程(见下)可计算出此时的K+平衡电位(-87mV)。这一数值与Hodgkin用微电极实际测得的RP值(-77mV)十分接近。这部分证明了上述假说的合理性。 Nerst方程 Hodgkin又通过人工改变细胞外液中的K+浓度，进一步观察到RP值随胞外[K+]的改变而改变，且改变后的RP值仍与Nernst方程计算出的K+平衡电位相一致。但改变细胞外液中的Na+浓度则对RP没有影响。这说明RP的产生确实与K+密切相关。后人通过采用带有放射性的K+，发现安静时细胞膜确实对K+具有通透性，而对其它离子的通透性不明显 综上所述，细胞内[K+]>细胞外[K+]，和细胞膜在安静时对K+选择性通透是RP产生的根本原因。 (3)维持细胞膜内外离子浓度差的机制 RP不等于K+平衡电位的原因:RP的实测值总是比计算值稍大一些。 用标有放射活性的离子仔细观察时，发现细胞安静时膜不仅对K+通透，对Na+也有通透性，只是与K+的通透性相比，Na+的通透性要小得多(约为K+通透性的1/50,1/100)，即静息时也有极少量的Na+从膜外通透 investigators carefully to verify that such credit party authenticity, when necessary, to the borrower unit verification and should strictly distinguish between regular employees, temporary employees, categories and on the job, retirement, early retirement, etc. Purpose to verify investigators should require borrowers to provide proof of use data and verified to ensure that lending purposes should be consistent with the relevant laws and regulations and national policies. Bank ban on issuing personal loans without specified uses. Personal loans should be a sharp distinction between consumer and business uses, management personal loans should refer to working capital loans required to verify authenticity, consumer loans should ensure a clear, true and reasonable. Strictly guard against borrowers will loan money to the stock market, real estate, loan-sharking, private fund raising and other high risk areas or used for illegal business strictly preventing borrowers loans capital private public or misappropriated ... Understand borrowers ' credit business credit situation of the parties. Strict precautions against 到膜内(Na+的浓度差和电位差均驱使其内流)，部分抵消K+外流造成负电位，导致RP的实测值比Nernst方程计算的EK值稍大。 RP较EK稍大的后果:由于未达到K+平衡电位，K+仍然会不断少量外流，而RP与Na+平衡电位相差甚远，也会使Na+不断地内流，如此下去，细胞安静时膜内外稳定的离子浓度差将遭到破坏。 钠泵(sodium pump)的活动维持了安静时细胞内外的离子平衡:引起细胞膜上钠泵活动的因素是细胞内Na+的增加和细胞外K+的升高。只要细胞内外的Na+、K+平衡稍有变化，Na+泵就被激活，在泵出胞内多余的Na+的同时，将胞外 多余 的K+泵回，从而维持了细胞内外正常的离子浓度梯度。 (4)静息电位小结: ?几乎所有的细胞均有膜内较膜外为负的RP。细胞的RP是由膜内外的K+离子浓度差及安静时膜对K+有通透性形成的。RP的值接近于K+平衡电位。 ?由于RP不等于K+平衡电位(因为在静息时也有少量Na+内流)，经常有少量的K+外流。然而，细胞膜上Na+泵的经常性活动，将胞内多余的Na+泵出，将胞外多余的K+泵回，从而维持了细胞内、外的正常离子浓度。可兴奋细胞的RP及膜两侧的离子浓度差(势能)是产生兴奋的基础。 ?近年来已认识到， 安静时K+离子通过细胞膜弥散的实质是因为膜上有非门控的K+离子通道。这种离子通道没有门，总是开着的。K+离子是否通过和通过多少是由膜两侧的离子浓度差和电位差决定的。 二、动作电位及其产生机制 (一)动作电位 细胞受刺激时，在静息电位的基础上发生一次短暂的扩布性的电位变化，这种电位变化称为动作电位。 实验观察，动作电位包括一个上升相和一个下降相。上升相代表膜的去极化过程。以 0mv电位为界，上升相的下半部分为膜的去极化，是膜内负电位减小，由-70,-90mv.变为0mv;上升相的上半部分是膜的反极化(超射)，是膜电位的极性发生倒转即膜外变负，膜内变正，由0mv上升到+20,40mv。上升相膜内电位上升幅度约为90,130mv。下降相代表膜的复极化过程。它是膜内电位从上升相顶端下降到静息电位水平的过程。由于动作电位幅度大、时间短不超过2ms，波形很象一个尖峰，故又称峰电位。在峰电位完全恢复到静息电位水平之前，膜两侧还有微小的连续缓慢的电变化，称为后电位。 (二)动作电位产生的机制 动作电位产生的机制与静息电位相似，都与细胞膜的通透 investigators carefully to verify that such credit party authenticity, when necessary, to the borrower unit verification and should strictly distinguish between regular employees, temporary employees, categories and on the job, retirement, early retirement, etc. Purpose to verify investigators should require borrowers to provide proof of use data and verified to ensure that lending purposes should be consistent with the relevant laws and regulations and national policies. Bank ban on issuing personal loans without specified uses. Personal loans should be a sharp distinction between consumer and business uses, management personal loans should refer to working capital loans required to verify authenticity, consumer loans should ensure a clear, true and reasonable. Strictly guard against borrowers will loan money to the stock market, real estate, loan-sharking, private fund raising and other high risk areas or used for illegal business strictly preventing borrowers loans capital private public or misappropriated ... Understand borrowers ' credit business credit situation of the parties. Strict precautions against 性及离子转运有关。 l.去极化过程 当细胞受刺激而兴奋时，膜对Na+通透性增大，对K+通透性减小，于是细胞外的Na+便会顺其波度梯度和电梯度向胞内扩散，导致膜内负电位减小，直至膜内电位比膜外高，形成内正外负的反极化状态。当促使Na+内流的浓度梯度和阻止Na+内流的电梯度，这两种拮抗力量相等时，Na+的净内流停止。因此，可以说动作电位的去极化过程相当于Na+内流所形成的电一化学平衡电位。 2(复极化过程 当细胞膜除极到峰值时，细胞膜的Na+通道迅速关闭，而对K+的通透性增大，于是细胞内的K+便顺其浓度梯度向细胞外扩散，导致膜内负电位增大，直至恢复到静息时的数值。 可兴奋细胞每发生一次动作电位，总会有一部分Na+在去极化中扩散到细胞内，并有一部分K+在复极过程中扩散到细胞外。这样就激活了Na+,K+依赖式 ATP酶即Na+,K+泵，于是钠泵加速运转，将胞内多余的Na+泵出胞外，同时把胞外增多的K+泵进胞内，以恢复静息状态的离子分布，保持细胞的正常兴奋性。如果说静息电位是兴奋性的基础，那么，动作电位是可兴奋细胞兴奋的标志。 三、动作电位的引起和传导 (一)动作电位的引起 1(阈电位可兴奋细胞(如神经细胞)受刺激后，首先是膜上Na+通道少量开放，出现Na+少量内流，使膜内负电位减小。当膜电位减小到某一临界值时，受刺激部分的 Na+通道大量开放，使Na+快速大量内流，表现为扩布性电位，即动作电位。这个引起膜对Na+通透性突然增大的临界电位值，称为阈电位。阈电位是可兴奋细胞的重要生理参数之一。一般它与静息电位相差约20毫伏。如果两者差距减小，则可兴奋细胞的兴奋性升高。反之，则降低。 2(局部电位可兴奋细胞在受阈下刺激时细胞膜对Na+的通透性轻度增加，使膜内负电位减小，发生去极化但达不到阈电位，所以不产生动作电位。这种去极产生的电位称为局部电位或局部反应。其特点:?刺激越强，局部电位的幅度越大。?随扩布距离的增加而减小，不能远传。?局部反应可以总合，即多个局部电位可叠加起来达到阈电位而引起动作电位。局部电位除了上述的去极化形式外，还可表现为超极化的形式。 (二)动作电位的传导 细胞膜某一点受刺激产生兴奋时，其兴奋部位膜电位由极化状态(内负外正)变为反极化状态(内正外负)，于是兴奋部位和静息部位之间出现了电位差，导致局部的电荷移动，即产生局部电流。此电流的方向是膜外电流由静息部位流向兴奋部位，膜内电流由兴奋部位流向静息部位，这就造成静息部位膜内电位升高，膜外电位降低(去极化)。 investigators carefully to verify that such credit party authenticity, when necessary, to the borrower unit verification and should strictly distinguish between regular employees, temporary employees, categories and on the job, retirement, early retirement, etc. Purpose to verify investigators should require borrowers to provide proof of use data and verified to ensure that lending purposes should be consistent with the relevant laws and regulations and national policies. Bank ban on issuing personal loans without specified uses. Personal loans should be a sharp distinction between consumer and business uses, management personal loans should refer to working capital loans required to verify authenticity, consumer loans should ensure a clear, true and reasonable. Strictly guard against borrowers will loan money to the stock market, real estate, loan-sharking, private fund raising and other high risk areas or used for illegal business strictly preventing borrowers loans capital private public or misappropriated ... Understand borrowers ' credit business credit situation of the parties. Strict precautions against 当这种变化达到阈电位时，便产生动作电位。新产生的动作电位又会以同样方式作用于它的邻点。这个过程此起彼伏地逐点传下去，就使兴奋传至整个细胞。 不论在哪一点上，动作电位峰值都是由离子流决定的。而同一细胞的离子成分及其电化学梯度都是一致的。所以动作电位传导时，绝不会因距离增大而幅度减小。因此，动作电位传导的特点是不衰减的。由于具备不衰减传导的特性，动作电位在远程快速信息传递中就可发挥其特长。所谓神经冲动，就是在上传导的动作电位。 1.静息电位及其产生原理 静息电位是指细胞在安静时，存在于膜内外的电位差。生物电产生的原理可用"离子学说"解释。该学说认为:膜电位的产生是由于膜内外各种离子的分布不均衡，以及膜在不同情况下，对各种离子的通透性不同所造成的。在静息状态下，细胞膜对K+有较高的通透性，而膜内K+又高于膜外，K+顺浓度差向膜外扩散;细胞膜对蛋白质负离子(A-)无通透性，膜内大分子A-被阻止在膜的内侧，从而形成膜内为负、膜外为正的电位差。这种电位差产生后，可阻止K+的进一步向外扩散，使膜内外电位差达到一个稳定的数值，即静息电位。因此，静息电位主要是K+外流所形成的电-化学平衡电位。 2.动作电位及其产生原理 细胞膜受刺激而兴奋时，在静息电位的基础上，发生一次扩布性的电位变化，称为动作电位。动作电位是一个连续的膜电位变化过程，波形分为上升相和下降相。细胞 +膜受刺激而兴奋时，膜上Na通道迅速开放，由于膜外Na+浓度高于膜内，电位比膜内正，所以，Na+顺浓度差和电位差内流，使膜内的负电位迅速消失，并进而转为正电位。这种膜内为正、膜外为负的电位梯度，阻止Na+继续内流。当促使Na+内流的浓度梯度与阻止Na+内流的电位梯度相等时，Na+内流停止。因此，动作电位的上升相的顶点是Na+内流所形成的电-化学平衡电位。 在动作电位上升相达到最高值时，膜上Na+通道迅速关闭，膜对Na+的通透性迅速下降，Na+内流停止。此时，膜对K+的通透性增大，K+外流使膜内电位迅速下降，直到恢复静息时的电位水平，形成动作电位的下降相。 可兴奋细胞每发生一次动作电位，膜内外的Na+、K+比例都会发生变化，于是钠-钾泵加速转运，将进入膜内的Na+泵出，同时将逸出膜外的K+泵入，从而恢复静息时膜 investigators carefully to verify that such credit party authenticity, when necessary, to the borrower unit verification and should strictly distinguish between regular employees, temporary employees, categories and on the job, retirement, early retirement, etc. Purpose to verify investigators should require borrowers to provide proof of use data and verified to ensure that lending purposes should be consistent with the relevant laws and regulations and national policies. Bank ban on issuing personal loans without specified uses. Personal loans should be a sharp distinction between consumer and business uses, management personal loans should refer to working capital loans required to verify authenticity, consumer loans should ensure a clear, true and reasonable. Strictly guard against borrowers will loan money to the stock market, real estate, loan-sharking, private fund raising and other high risk areas or used for illegal business strictly preventing borrowers loans capital private public or misappropriated ... Understand borrowers ' credit business credit situation of the parties. Strict precautions against 内外的离子分布，维持细胞的兴奋性。 investigators carefully to verify that such credit party authenticity, when necessary, to the borrower unit verification and should strictly distinguish between regular employees, temporary employees, categories and on the job, retirement, early retirement, etc. Purpose to verify investigators should require borrowers to provide proof of use data and verified to ensure that lending purposes should be consistent with the relevant laws and regulations and national policies. Bank ban on issuing personal loans without specified uses. Personal loans should be a sharp distinction between consumer and business uses, management personal loans should refer to working capital loans required to verify authenticity, consumer loans should ensure a clear, true and reasonable. Strictly guard against borrowers will loan money to the stock market, real estate, loan-sharking, private fund raising and other high risk areas or used for illegal business strictly preventing borrowers loans capital private public or misappropriated ... Understand borrowers ' credit business credit situation of the parties. Strict precautions against