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Because the electric field is the gradient of the voltage distribution, rapid changes in voltage within a small region imply a strong electric field; on the converse, if the voltage remains approximately the same over a large region, the electric fields in that region must be weak. A strong electric field, equivalent to a strong voltage gradient, implies that a strong force is exerted on any charged particles that lie within the region.

Because the electric field is the gradient of the voltage distribution, rapid changes in voltage within a small region imply a strong electric field; on the converse, if the voltage remains approximately the same over a large region, the electric fields in that region must be weak. A strong electric field, equivalent to a strong voltage gradient, implies that a strong force is exerted on any charged particles that lie within the region.

===离子和驱动其运动的力===

===离子和驱动离子运动的力===

[[File:Diffusion.en.svg|thumb|right|250px|离子(粉红色圆)会从较高的浓度流向较低的浓度（顺着浓度梯度），产生电流。然而，这会产生一个跨膜电压，阻止离子的运动。当这个电压达到平衡值时，两者相抵，离子流停止。<ref name=":2">Campbell Biology, 6th edition</ref>|链接=Special:FilePath/Diffusion.en.svg]]

[[File:Diffusion.en.svg|thumb|right|250px|离子(粉红色圆)会从较高的浓度流向较低的浓度（顺着浓度梯度），产生电流。然而，这会产生一个跨膜电压，阻止离子的运动。当这个电压达到平衡值时，两者相抵，离子流停止。<ref name=":2">Campbell Biology, 6th edition</ref>|链接=Special:FilePath/Diffusion.en.svg]]

生物体的电信号通常是离子驱动的。<ref name=":3">Johnston and Wu, p. 9.</ref> 动作电位最重要的阳离子是钠离子（Na⁺）和钾离子（K⁺）。<ref name="bullock_140_141">[[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, pp. 140–41.</ref> 两者都是带一个正电荷的单价阳离子。动作电位也可能需要钙离子（Ca²⁺），<ref name=":4">[[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, pp. 153–54.</ref> 一种带有两个正电荷的二价阳离子。氯离子（Cl⁻）在某些藻类的动作电位中起主要作用，<ref name="mummert_1991">{{cite journal |vauthors=Mummert H, Gradmann D | year = 1991 | title = Action potentials in Acetabularia: measurement and simulation of voltage-gated fluxes | journal = Journal of Membrane Biology | volume = 124 | pages = 265–73 | pmid = 1664861 | doi = 10.1007/BF01994359 | issue = 3| s2cid = 22063907 }}</ref> 然而在大多数动物的动作电位中起微不足道的作用。<ref name=":5">[[Knut Schmidt-Nielsen|Schmidt-Nielsen]], p. 483.</ref>

生物体的电信号通常是离子驱动的。<ref name=":3">Johnston and Wu, p. 9.</ref> 动作电位最重要的阳离子是钠离子（Na⁺）和钾离子（K⁺）。<ref name="bullock_140_141">[[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, pp. 140–41.</ref> 两者都是带一个正电荷的单价阳离子。动作电位也可能需要钙离子（Ca²⁺），<ref name=":4">[[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, pp. 153–54.</ref> 一种带有两个正电荷的二价阳离子。氯离子（Cl⁻）在某些藻类的动作电位中起主要作用，<ref name="mummert_1991">{{cite journal |vauthors=Mummert H, Gradmann D | year = 1991 | title = Action potentials in Acetabularia: measurement and simulation of voltage-gated fluxes | journal = Journal of Membrane Biology | volume = 124 | pages = 265–73 | pmid = 1664861 | doi = 10.1007/BF01994359 | issue = 3| s2cid = 22063907 }}</ref> 然而在大多数动物的动作电位中的作用微不足道。<ref name=":5">[[Knut Schmidt-Nielsen|Schmidt-Nielsen]], p. 483.</ref>

Ions cross the cell membrane under two influences: [[diffusion]] and [[electric field]]s. A simple example wherein two solutions—A and B—are separated by a porous barrier illustrates that diffusion will ensure that they will eventually mix into equal solutions. This mixing occurs because of the difference in their concentrations. The region with high concentration will diffuse out toward the region with low concentration. To extend the example, let solution A have 30 sodium ions and 30 chloride ions. Also, let solution B have only 20 sodium ions and 20 chloride ions. Assuming the barrier allows both types of ions to travel through it, then a steady state will be reached whereby both solutions have 25 sodium ions and 25 chloride ions. If, however, the porous barrier is selective to which ions are let through, then diffusion alone will not determine the resulting solution. Returning to the previous example, let's now construct a barrier that is permeable only to sodium ions. Now, only sodium is allowed to diffuse cross the barrier from its higher concentration in solution A to the lower concentration in solution B. This will result in a greater accumulation of sodium ions than chloride ions in solution B and a lesser number of sodium ions than chloride ions in solution A.

Ions cross the cell membrane under two influences: [[diffusion]] and [[electric field]]s. A simple example wherein two solutions—A and B—are separated by a porous barrier illustrates that diffusion will ensure that they will eventually mix into equal solutions. This mixing occurs because of the difference in their concentrations. The region with high concentration will diffuse out toward the region with low concentration. To extend the example, let solution A have 30 sodium ions and 30 chloride ions. Also, let solution B have only 20 sodium ions and 20 chloride ions. Assuming the barrier allows both types of ions to travel through it, then a steady state will be reached whereby both solutions have 25 sodium ions and 25 chloride ions. If, however, the porous barrier is selective to which ions are let through, then diffusion alone will not determine the resulting solution. Returning to the previous example, let's now construct a barrier that is permeable only to sodium ions. Now, only sodium is allowed to diffuse cross the barrier from its higher concentration in solution A to the lower concentration in solution B. This will result in a greater accumulation of sodium ions than chloride ions in solution B and a lesser number of sodium ions than chloride ions in solution A.

离子的跨膜运动受两个因素的影响：扩散和电场。可以简单举个例子说明扩散作用，如图 A 和 B 两种溶液被带孔的隔板隔开，扩散作用最终会使两者混合成等浓度的溶液。这种混合是因两者存在浓度差异而发生的。高浓度区域向低浓度区域扩散。扩展这个例子，让溶液 A 有 30 个钠离子和 30 个氯离子；另外让溶液 B 只含有 20 个钠离子和 20 个氯离子。假设隔板允许两种类型的离子通过它，那么两种溶液都将达到一个稳定状态，即都有 25 个钠离子和 25 个氯离子。然而，如果带孔隔板是选择性的让离子通过，那么最终的溶液将不会仅由扩散作用决定。回到前面的例子，让我们现在构造一个只能透过钠离子的隔板。现在，只有钠可以从溶液 A 中较高的浓度扩散到溶液 B 中较低的浓度。这将导致钠离子在 B 溶液中的积累大于氯离子，钠离子的数量少于在 A 溶液中的氯离子。

离子的跨膜运动受两个因素的影响：扩散作用和电场。举个简单的例子，带孔的隔板隔开两种溶液 A 和 B，扩散作用会使两者最终混合成等浓度的溶液。这种混合是因两者存在浓度差而发生的，高浓度区域向低浓度区域扩散。扩展这个例子，让溶液 A 有 30 个钠离子和 30 个氯离子；另外让溶液 B 只含有 20 个钠离子和 20 个氯离子。假设隔板允许两种类型的离子通过，那么将达到一个稳态，即两种溶液均有 25 个钠离子和 25 个氯离子。然而，如果带孔隔板是选择性的让离子通过，那么最终的溶液将不会仅由扩散作用决定。回到前面的例子，让我们现在构造一个只能透过钠离子的隔板。现在，只有钠可以从溶液 A 中较高的浓度扩散到溶液 B 中较低的浓度。这将导致 B 溶液中积聚多于氯离子的钠离子，而 A 溶液中钠离子少于氯离子。

This means that there is a net positive charge in solution B from the higher concentration of positively charged sodium ions than negatively charged chloride ions. Likewise, there is a net negative charge in solution A from the greater concentration of negative chloride ions than positive sodium ions. Since opposite charges attract and like charges repel, the ions are now also influenced by electrical fields as well as forces of diffusion. Therefore, positive sodium ions will be less likely to travel to the now-more-positive B solution and remain in the now-more-negative A solution. The point at which the forces of the electric fields completely counteract the force due to diffusion is called the equilibrium potential. At this point, the net flow of the specific ion (in this case sodium) is zero.

This means that there is a net positive charge in solution B from the higher concentration of positively charged sodium ions than negatively charged chloride ions. Likewise, there is a net negative charge in solution A from the greater concentration of negative chloride ions than positive sodium ions. Since opposite charges attract and like charges repel, the ions are now also influenced by electrical fields as well as forces of diffusion. Therefore, positive sodium ions will be less likely to travel to the now-more-positive B solution and remain in the now-more-negative A solution. The point at which the forces of the electric fields completely counteract the force due to diffusion is called the equilibrium potential. At this point, the net flow of the specific ion (in this case sodium) is zero.

这意味着在溶液 B 中存在净正电荷，这是由于带正电荷的钠离子比带负电荷的氯离子浓度高。同样，在溶液 A 中，由于带负电荷的氯离子的浓度比带正电荷的钠离子的浓度大，所以存在净负电荷。由于异性电荷相互吸引，同性电荷相互排斥，因此离子现在除了扩散作用也受到电场力的影响。因此，带正电的钠离子将不太能移到现在带更多正电的溶液 B ，而留在现在带更多负电的溶液 A。电场的力完全抵消扩散作用的点称为平衡电位。此时，特定离子（本例中是钠离子）净流量为零。

这意味着在溶液 B 中存在净正电荷，这是由于带正电荷的钠离子比带负电荷的氯离子浓度高。同样，在溶液 A 中，由于带负电荷的氯离子的浓度比带正电荷的钠离子的浓度大，所以存在净负电荷。由于异性电荷相互吸引，同性电荷相互排斥，因此离子现在除了扩散作用也受到电场力的影响。因此，带正电的钠离子较不可能移到现在带更多正电的溶液 B，而留在现在带更多负电的溶液 A。电场完全抵消扩散作用的点称为平衡电位。此时，特定离子（本例中是钠离子）净流量为零。

===Plasma membranes 细胞质膜===

===Plasma membranes 细胞质膜===

Every cell is enclosed in a [[plasma membrane]], which has the structure of a [[lipid bilayer]] with many types of large molecules embedded in it.  Because it is made of lipid molecules, the plasma membrane intrinsically has a high electrical resistivity, in other words a low intrinsic permeability to ions. However, some of the molecules embedded in the membrane are capable either of actively transporting ions from one side of the membrane to the other or of providing channels through which they can move.<ref name="lieb_1986">{{cite book |vauthors=Lieb WR, Stein WD | year = 1986 | chapter = Chapter 2. Simple Diffusion across the Membrane Barrier | title = Transport and Diffusion across Cell Membranes | publisher = Academic Press | location = San Diego | isbn = 978-0-12-664661-0 | pages = 69–112}}</ref>

Every cell is enclosed in a [[plasma membrane]], which has the structure of a [[lipid bilayer]] with many types of large molecules embedded in it.  Because it is made of lipid molecules, the plasma membrane intrinsically has a high electrical resistivity, in other words a low intrinsic permeability to ions. However, some of the molecules embedded in the membrane are capable either of actively transporting ions from one side of the membrane to the other or of providing channels through which they can move.<ref name="lieb_1986">{{cite book |vauthors=Lieb WR, Stein WD | year = 1986 | chapter = Chapter 2. Simple Diffusion across the Membrane Barrier | title = Transport and Diffusion across Cell Membranes | publisher = Academic Press | location = San Diego | isbn = 978-0-12-664661-0 | pages = 69–112}}</ref>

每个细胞都被包裹在质膜中，质膜具有脂双分子层的结构，其中包含许多类型的大分子。因为它是由脂分子组成的，所以质膜本身具有很高的电阻率，换句话说，对离子的固有渗透性很低。然而，嵌入在膜中的一些分子能够将离子从膜的一侧有效地转运到另一侧，或者提供离子可以移动的通道。<ref name="lieb_1986" />

每个细胞都被包裹在质膜中，质膜具有脂双分子层的结构，其中包含很多种类的大分子。因为它是由脂分子组成的，所以质膜本身具有很高的电阻率，换句话说，对离子的固有渗透性很低。然而，嵌入膜中的一些分子能够将离子从膜的一侧有效地转运到另一侧，或者提供离子移动的通道。<ref name="lieb_1986" />

In electrical terminology, the plasma membrane functions as a combined [[resistor]] and [[capacitor]]. Resistance arises from the fact that the membrane impedes the movement of charges across it. Capacitance arises from the fact that the lipid bilayer is so thin that an accumulation of charged particles on one side gives rise to an electrical force that pulls oppositely charged particles toward the other side. The capacitance of the membrane is relatively unaffected by the molecules that are embedded in it, so it has a more or less invariant value estimated at about 2 μF/cm² (the total capacitance of a patch of membrane is proportional to its area).  The conductance of a pure lipid bilayer is so low, on the other hand, that in biological situations it is always dominated by the conductance of alternative pathways provided by embedded molecules.  Thus, the capacitance of the membrane is more or less fixed, but the resistance is highly variable.

In electrical terminology, the plasma membrane functions as a combined [[resistor]] and [[capacitor]]. Resistance arises from the fact that the membrane impedes the movement of charges across it. Capacitance arises from the fact that the lipid bilayer is so thin that an accumulation of charged particles on one side gives rise to an electrical force that pulls oppositely charged particles toward the other side. The capacitance of the membrane is relatively unaffected by the molecules that are embedded in it, so it has a more or less invariant value estimated at about 2 μF/cm² (the total capacitance of a patch of membrane is proportional to its area).  The conductance of a pure lipid bilayer is so low, on the other hand, that in biological situations it is always dominated by the conductance of alternative pathways provided by embedded molecules.  Thus, the capacitance of the membrane is more or less fixed, but the resistance is highly variable.

在电学术语中，质膜功能上是电阻和电容的组合。电阻的产生是因为质膜阻碍电荷的跨膜运动。电容产生于这样一个事实，即脂双分子层是如此之薄，以至于一侧带电粒子的积聚产生了一个电场力，将带相反电荷的粒子拉向另一侧。膜的电容相对而言不受嵌入其中的分子的影响，因此它有一个或多或少的不变值，估计在 2 μF/cm² 左右（膜片的总电容与其面积成正比）。另一方面，纯的脂双分子层的电导率非常低，在生物学情况下，它总是由嵌入分子提供的支路的电导率决定。因此，膜的电容或多或少是固定的，但电阻是高度可变的。

电学上讲，质膜在功能上是电阻和电容的组合。电阻的产生是因为质膜阻碍电荷的跨膜运动。电容是这样产生的：脂双分子层非常之薄，以至于膜的一侧积聚的粒子产生电场力，将带相反电荷的粒子拉向膜的另一侧。膜的电容相对而言不受嵌入其中的分子的影响，因此它的数值相对恒定，约为 2 μF/cm² 左右（膜片的总电容与其面积成正比）。另一方面，纯的脂双分子层的电导率非常之低，在生物体中通常由嵌入分子提供的支路的电导率决定。因此，膜的电容相对是固定的，但电阻是高度可变的。

The thickness of a plasma membrane is estimated to be about 7-8 nanometers. Because the membrane is so thin, it does not take a very large transmembrane voltage to create a strong electric field within it. Typical membrane potentials in animal cells are on the order of 100 millivolts (that is, one tenth of a volt), but calculations show that this generates an electric field close to the maximum that the membrane can sustain—it has been calculated that a voltage difference much larger than 200 millivolts could cause [[dielectric breakdown]], that is, arcing across the membrane.

The thickness of a plasma membrane is estimated to be about 7-8 nanometers. Because the membrane is so thin, it does not take a very large transmembrane voltage to create a strong electric field within it. Typical membrane potentials in animal cells are on the order of 100 millivolts (that is, one tenth of a volt), but calculations show that this generates an electric field close to the maximum that the membrane can sustain—it has been calculated that a voltage difference much larger than 200 millivolts could cause [[dielectric breakdown]], that is, arcing across the membrane.

质膜的厚度估计约为 7-8 纳米。因为膜非常薄，它不需要很大的跨膜电压就可以在其中产生强电场。动物细胞中典型的膜电位大约为 100 毫伏（即十分之一伏特），但计算表明，这种电位产生的电场接近膜所能承受的最大电场——据计算，大于 200 毫伏的电压差可能导致介质击穿，也就是跨越膜的电弧。

质膜的厚度估计约为 7-8 纳米。膜非常薄，因此不需要很大的跨膜电压就以在其中产生强电场。动物细胞中典型的膜电位大约为 100 毫伏（即十分之一伏特），但计算表明，这种电位产生的电场接近膜所能承受的最大电场——据计算，大于 200 毫伏的电压差可能导致介质击穿，即产生跨膜电弧。

===Facilitated diffusion and transport易化扩散和转运 ===

===Facilitated diffusion and transport易化扩散和转运 ===

The resistance of a pure lipid bilayer to the passage of ions across it is very high, but structures embedded in the membrane can greatly enhance ion movement, either [[active transport|actively]] or [[passive transport|passively]], via mechanisms called [[facilitated transport]] and [[facilitated diffusion]]. The two types of structure that play the largest roles are ion channels and [[ion transporter|ion pump]]s, both usually formed from assemblages of protein molecules. Ion channels provide passageways through which ions can move. In most cases, an ion channel is permeable only to specific types of ions (for example, sodium and potassium but not chloride or calcium), and sometimes the permeability varies depending on the direction of ion movement. Ion pumps, also known as ion transporters or carrier proteins, actively transport specific types of ions from one side of the membrane to the other, sometimes using energy derived from metabolic processes to do so.

The resistance of a pure lipid bilayer to the passage of ions across it is very high, but structures embedded in the membrane can greatly enhance ion movement, either [[active transport|actively]] or [[passive transport|passively]], via mechanisms called [[facilitated transport]] and [[facilitated diffusion]]. The two types of structure that play the largest roles are ion channels and [[ion transporter|ion pump]]s, both usually formed from assemblages of protein molecules. Ion channels provide passageways through which ions can move. In most cases, an ion channel is permeable only to specific types of ions (for example, sodium and potassium but not chloride or calcium), and sometimes the permeability varies depending on the direction of ion movement. Ion pumps, also known as ion transporters or carrier proteins, actively transport specific types of ions from one side of the membrane to the other, sometimes using energy derived from metabolic processes to do so.

===Ion pumps===

===Ion pumps 离子泵===

[[File:Scheme sodium-potassium pump-en.svg|thumb|right|350px|钠-钾泵利用 ATP 产生的能量，将钠离子与钾离子跨膜交换。|链接=Special:FilePath/Scheme_sodium-potassium_pump-en.svg]][[Ion transporter|Ion pumps]] are [[integral membrane protein]]s that carry out [[active transport]], i.e., use cellular energy (ATP) to "pump" the ions against their concentration gradient.<ref name="hodgkin_1955">{{cite journal | author = [[Alan Lloyd Hodgkin|Hodgkin AL]], [[Richard Keynes|Keynes RD]] | year = 1955 | title = Active transport of cations in giant axons from ''Sepia'' and ''Loligo'' | journal = J. Physiol. | volume = 128 | pages = 28–60 | pmid = 14368574 | issue = 1 | pmc = 1365754 | doi=10.1113/jphysiol.1955.sp005290}}</ref> Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there).

[[File:Scheme sodium-potassium pump-en.svg|thumb|right|350px|钠-钾泵利用 ATP 产生的能量，将钠离子与钾离子跨膜交换。|链接=Special:FilePath/Scheme_sodium-potassium_pump-en.svg]][[Ion transporter|Ion pumps]] are [[integral membrane protein]]s that carry out [[active transport]], i.e., use cellular energy (ATP) to "pump" the ions against their concentration gradient.<ref name="hodgkin_1955">{{cite journal | author = [[Alan Lloyd Hodgkin|Hodgkin AL]], [[Richard Keynes|Keynes RD]] | year = 1955 | title = Active transport of cations in giant axons from ''Sepia'' and ''Loligo'' | journal = J. Physiol. | volume = 128 | pages = 28–60 | pmid = 14368574 | issue = 1 | pmc = 1365754 | doi=10.1113/jphysiol.1955.sp005290}}</ref> Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there).

离子泵是内在膜蛋白，进行主动运输，也就是说，使用细胞能量（ATP）“泵”离子逆浓度梯度.<ref name="hodgkin_1955" />。这种离子泵从膜的一边吸收离子（顺浓度梯度），然后从另一侧释放离子（增加那边的浓度）。

离子泵是一类内在膜蛋白，其执行主动运输，即利用细胞能量（ATP）将离子逆浓度梯度用“泵”输送<ref name="hodgkin_1955" />。这种离子泵将离子从膜的一侧摄入（而减少浓度），释放到膜的另一侧（增加浓度）。

The ion pump most relevant to the action potential is the [[Na+/K+-ATPase|sodium–potassium pump]], which transports three sodium ions out of the cell and two potassium ions in.<ref name="caldwell_1960">{{cite journal | vauthors = Caldwell PC, [[Alan Lloyd Hodgkin|Hodgkin AL]], [[Richard Keynes|Keynes RD]], Shaw TI | year = 1960 | title = The effects of injecting energy-rich phosphate compounds on the active transport of ions in the giant axons of ''Loligo'' | journal = J. Physiol. | volume = 152 | issue = 3 | pages = 561–90 | pmid = 13806926 | pmc = 1363339 | doi = 10.1113/jphysiol.1960.sp006509 }}</ref>  As a consequence, the concentration of [[potassium]] ions K⁺ inside the neuron is roughly 20-fold larger than the outside concentration, whereas the sodium concentration outside is roughly ninefold larger than inside.<ref name="steinbach_1943">{{cite journal |vauthors=Steinbach HB, Spiegelman S | year = 1943 | title = The sodium and potassium balance in squid nerve axoplasm | journal = J. Cell. Comp. Physiol. | volume = 22 | issue = 2 | pages = 187–96 | doi = 10.1002/jcp.1030220209}}</ref><ref name="hodgkin_1951">{{cite journal | author = Hodgkin AL | year = 1951 | title = The ionic basis of electrical activity in nerve and muscle | journal = Biol. Rev. | volume = 26 | issue = 4 | pages = 339–409 | doi = 10.1111/j.1469-185X.1951.tb01204.x| s2cid = 86282580 | author-link = Alan Lloyd Hodgkin }}</ref> In a similar manner, other ions have different concentrations inside and outside the neuron, such as [[calcium]], [[chloride]] and [[magnesium]].<ref name="hodgkin_1951" />

The ion pump most relevant to the action potential is the [[Na+/K+-ATPase|sodium–potassium pump]], which transports three sodium ions out of the cell and two potassium ions in.<ref name="caldwell_1960">{{cite journal | vauthors = Caldwell PC, [[Alan Lloyd Hodgkin|Hodgkin AL]], [[Richard Keynes|Keynes RD]], Shaw TI | year = 1960 | title = The effects of injecting energy-rich phosphate compounds on the active transport of ions in the giant axons of ''Loligo'' | journal = J. Physiol. | volume = 152 | issue = 3 | pages = 561–90 | pmid = 13806926 | pmc = 1363339 | doi = 10.1113/jphysiol.1960.sp006509 }}</ref>  As a consequence, the concentration of [[potassium]] ions K⁺ inside the neuron is roughly 20-fold larger than the outside concentration, whereas the sodium concentration outside is roughly ninefold larger than inside.<ref name="steinbach_1943">{{cite journal |vauthors=Steinbach HB, Spiegelman S | year = 1943 | title = The sodium and potassium balance in squid nerve axoplasm | journal = J. Cell. Comp. Physiol. | volume = 22 | issue = 2 | pages = 187–96 | doi = 10.1002/jcp.1030220209}}</ref><ref name="hodgkin_1951">{{cite journal | author = Hodgkin AL | year = 1951 | title = The ionic basis of electrical activity in nerve and muscle | journal = Biol. Rev. | volume = 26 | issue = 4 | pages = 339–409 | doi = 10.1111/j.1469-185X.1951.tb01204.x| s2cid = 86282580 | author-link = Alan Lloyd Hodgkin }}</ref> In a similar manner, other ions have different concentrations inside and outside the neuron, such as [[calcium]], [[chloride]] and [[magnesium]].<ref name="hodgkin_1951" />

与动作电位最相关的离子泵是钠钾泵，它将三个钠离子输出细胞，两个钾离子输入细胞。<ref name="caldwell_1960" /> 因此，钾离子在神经元内的浓度大约是外部浓度的20倍，而钠离子在神经元外的浓度大约是内部浓度的9倍.<ref name="steinbach_1943" /><ref name="hodgkin_1951" />。类似地，其他离子在神经元内外有不同的浓度，如钙、氯和镁.<ref name="hodgkin_1951" />。

对于动作电位最重要的离子泵是钠钾泵（sodium–potassium pump），其一次可将三个钠离子输出细胞，两个钾离子输入细胞。<ref name="caldwell_1960" /> 结果就是，钾离子浓度在神经元胞内约是胞外的 20 倍，而钠离子浓度在神经元胞外大约是胞内的 9 倍。<ref name="steinbach_1943" /><ref name="hodgkin_1951" /> 类似地，钙离子、氯离子和镁离子等离子也在神经元胞内和胞外有不同的浓度。<ref name="hodgkin_1951" />

If the numbers of each type of ion were equal, the sodium–potassium pump would be electrically neutral, but, because of the three-for-two exchange, it gives a net movement of one positive charge from intracellular to extracellular for each cycle, thereby contributing to a positive voltage difference.  The pump has three effects:  (1) it makes the sodium concentration high in the extracellular space and low in the intracellular space; (2) it makes the potassium concentration high in the intracellular space and low in the extracellular space; (3) it gives the intracellular space a negative voltage with respect to the extracellular space.

If the numbers of each type of ion were equal, the sodium–potassium pump would be electrically neutral, but, because of the three-for-two exchange, it gives a net movement of one positive charge from intracellular to extracellular for each cycle, thereby contributing to a positive voltage difference.  The pump has three effects:  (1) it makes the sodium concentration high in the extracellular space and low in the intracellular space; (2) it makes the potassium concentration high in the intracellular space and low in the extracellular space; (3) it gives the intracellular space a negative voltage with respect to the extracellular space.

The sodium-potassium pump is relatively slow in operation. If a cell were initialized with equal concentrations of sodium and potassium everywhere, it would take hours for the pump to establish equilibrium. The pump operates constantly, but becomes progressively less efficient as the concentrations of sodium and potassium available for pumping are reduced.

The sodium-potassium pump is relatively slow in operation. If a cell were initialized with equal concentrations of sodium and potassium everywhere, it would take hours for the pump to establish equilibrium. The pump operates constantly, but becomes progressively less efficient as the concentrations of sodium and potassium available for pumping are reduced.

Ion pumps influence the action potential only by establishing the relative ratio of intracellular and extracellular ion concentrations.  The action potential involves mainly the opening and closing of ion channels not ion pumps.  If the ion pumps are turned off by removing their energy source, or by adding an inhibitor such as [[ouabain]], the axon can still fire hundreds of thousands of action potentials before their amplitudes begin to decay significantly.<ref name="hodgkin_1955" />  In particular, ion pumps play no significant role in the repolarization of the membrane after an action potential.<ref name="bullock_140_141" />

Ion pumps influence the action potential only by establishing the relative ratio of intracellular and extracellular ion concentrations.  The action potential involves mainly the opening and closing of ion channels not ion pumps.  If the ion pumps are turned off by removing their energy source, or by adding an inhibitor such as [[ouabain]], the axon can still fire hundreds of thousands of action potentials before their amplitudes begin to decay significantly.<ref name="hodgkin_1955" />  In particular, ion pumps play no significant role in the repolarization of the membrane after an action potential.<ref name="bullock_140_141" />

离子泵仅通过建立细胞内和细胞外离子浓度的相对比例来影响动作电位。动作电位主要涉及离子通道的开闭，而非离子泵。如果通过移除离子泵的能量源或者加入 ouabain 这样的抑制剂来关闭离子泵，轴突仍然可以在其振幅开始明显衰减之前激发数十万个动作电位.<ref name="hodgkin_1955" /> 。特别是，离子泵在动作电位后细胞膜的复极化过程中没有发挥重要作用.<ref name="bullock_140_141" />。

离子泵仅通过建立细胞内外的离子浓度对来影响动作电位。动作电位主要是离子通道的开关，而非离子泵。若去除其能量来源，或加入哇巴因（ouabain）之类的抑制剂来关闭离子泵，轴突仍然可以在其幅值开始明显衰减前发放数十万动作电位。<ref name="hodgkin_1955" /> 特别是，离子泵在动作电位后细胞膜的复极化过程中未有明显作用。<ref name="bullock_140_141" />

Another functionally important ion pump is the [[sodium-calcium exchanger]]. This pump operates in a conceptually similar way to the sodium-potassium pump, except that in each cycle it exchanges three Na⁺ from the extracellular space for one Ca⁺⁺ from the intracellular space. Because the net flow of charge is inward, this pump runs "downhill", in effect, and therefore does not require any energy source except the membrane voltage. Its most important effect is to pump calcium outward—it also allows an inward flow of sodium, thereby counteracting the sodium-potassium pump, but, because overall sodium and potassium concentrations are much higher than calcium concentrations, this effect is relatively unimportant. The net result of the sodium-calcium exchanger is that in the resting state, intracellular calcium concentrations become very low.

Another functionally important ion pump is the [[sodium-calcium exchanger]]. This pump operates in a conceptually similar way to the sodium-potassium pump, except that in each cycle it exchanges three Na⁺ from the extracellular space for one Ca⁺⁺ from the intracellular space. Because the net flow of charge is inward, this pump runs "downhill", in effect, and therefore does not require any energy source except the membrane voltage. Its most important effect is to pump calcium outward—it also allows an inward flow of sodium, thereby counteracting the sodium-potassium pump, but, because overall sodium and potassium concentrations are much higher than calcium concentrations, this effect is relatively unimportant. The net result of the sodium-calcium exchanger is that in the resting state, intracellular calcium concentrations become very low.

另一个重要的功能离子泵是钠钙交换器。这种泵的工作原理与钠-钾泵相似，只是在每个循环中，它从细胞内空间从细胞外液中交换3个钠离子来换取1个钙离子。因为电荷净流量是向内的，这个泵实际上是“下坡”的，因此除了膜电位之外不需要任何能源。它最重要的作用是将钙向外泵出ーー它还允许钠向内流动，从而抵消了钠钾泵，但是，由于总的钠钾浓度远高于钙浓度，这种作用相对来说并不重要。钠钙交换的最终结果是在静息状态下，细胞内钙浓度变得非常低。

另一个有重要功能的离子泵是钠钙交换体（sodium-calcium exchanger）。这种泵的工作原理与钠钾泵相似，只是在每个循环中，它将 3 个胞内钠离子与 1 个胞外钙离子交换。因为净电流是向胞内的，这个泵实际上是走“下坡”，因而除了膜电位之外不需要任何能量来源。其最重要的作用是将钙离子泵出胞外ーー还允许钠离子内流从而抵消钠钾泵产生的钠外流，但由于总的钠钾浓度远高于钙浓度，这种作用相对来说并不重要。钠钙交换体的最终结果是在静息状态下，胞内钙离子浓度变得非常低。

===Ion channels===

===Ion channels离子通道===

[[File:Action potential ion sizes.svg.png|thumb|Despite the small differences in their radii,<ref name=":6">''CRC Handbook of Chemistry and Physics'', 83rd edition, {{ISBN|0-8493-0483-0}}, pp. 12–14 to 12–16.</ref> ions rarely go through the "wrong" channel. For example, sodium or calcium ions rarely pass through a potassium channel.

[[File:Action potential ion sizes.svg.png|thumb|Despite the small differences in their radii,<ref name=":6">''CRC Handbook of Chemistry and Physics'', 83rd edition, {{ISBN|0-8493-0483-0}}, pp. 12–14 to 12–16.</ref> ions rarely go through the "wrong" channel. For example, sodium or calcium ions rarely pass through a potassium channel.

[[Ion channel]]s are [[integral membrane protein]]s with a pore through which ions can travel between extracellular space and cell interior. Most channels are specific (selective) for one ion; for example, most potassium channels are characterized by 1000:1 selectivity ratio for potassium over sodium, though potassium and sodium ions have the same charge and differ only slightly in their radius. The channel pore is typically so small that ions must pass through it in single-file order.<ref name="eisenman_theory">{{cite book | author = Eisenman G | year = 1961 | chapter = On the elementary atomic origin of equilibrium ionic specificity | title = Symposium on Membrane Transport and Metabolism | editor = A Kleinzeller |editor2=A Kotyk | publisher = Academic Press | location = New York | pages = 163–79}}{{cite book | author = Eisenman G | year = 1965 | chapter = Some elementary factors involved in specific ion permeation | title = Proc. 23rd Int. Congr. Physiol. Sci., Tokyo | publisher = Excerta Med. Found. | location = Amsterdam | pages = 489–506}}<br />* {{cite journal |vauthors=Diamond JM, Wright EM | year = 1969 | title = Biological membranes: the physical basis of ion and nonekectrolyte selectivity | journal = Annual Review of Physiology | volume = 31 | pages = 581–646 | doi = 10.1146/annurev.ph.31.030169.003053 | pmid = 4885777}}</ref>    Channel pores can be either open or closed for ion passage, although a number of channels demonstrate various sub-conductance levels. When a channel is open, ions permeate through the channel pore down the transmembrane concentration gradient for that particular ion. Rate of ionic flow through the channel, i.e. single-channel current amplitude, is determined by the maximum channel conductance and electrochemical driving force for that ion, which is the difference between the instantaneous value of the membrane potential and the value of the [[reversal potential]].<ref name="junge_33_37">Junge, pp. 33–37.</ref>

[[Ion channel]]s are [[integral membrane protein]]s with a pore through which ions can travel between extracellular space and cell interior. Most channels are specific (selective) for one ion; for example, most potassium channels are characterized by 1000:1 selectivity ratio for potassium over sodium, though potassium and sodium ions have the same charge and differ only slightly in their radius. The channel pore is typically so small that ions must pass through it in single-file order.<ref name="eisenman_theory">{{cite book | author = Eisenman G | year = 1961 | chapter = On the elementary atomic origin of equilibrium ionic specificity | title = Symposium on Membrane Transport and Metabolism | editor = A Kleinzeller |editor2=A Kotyk | publisher = Academic Press | location = New York | pages = 163–79}}{{cite book | author = Eisenman G | year = 1965 | chapter = Some elementary factors involved in specific ion permeation | title = Proc. 23rd Int. Congr. Physiol. Sci., Tokyo | publisher = Excerta Med. Found. | location = Amsterdam | pages = 489–506}}<br />* {{cite journal |vauthors=Diamond JM, Wright EM | year = 1969 | title = Biological membranes: the physical basis of ion and nonekectrolyte selectivity | journal = Annual Review of Physiology | volume = 31 | pages = 581–646 | doi = 10.1146/annurev.ph.31.030169.003053 | pmid = 4885777}}</ref>    Channel pores can be either open or closed for ion passage, although a number of channels demonstrate various sub-conductance levels. When a channel is open, ions permeate through the channel pore down the transmembrane concentration gradient for that particular ion. Rate of ionic flow through the channel, i.e. single-channel current amplitude, is determined by the maximum channel conductance and electrochemical driving force for that ion, which is the difference between the instantaneous value of the membrane potential and the value of the [[reversal potential]].<ref name="junge_33_37">Junge, pp. 33–37.</ref>

离子通道是一种完整的膜蛋白，它有一个孔，离子可以通过这个孔在细胞外液和细胞内部之间穿梭。大多数钾离子通道对单个离子具有特异性(选择性) ; 例如，大多数钾离子通道对钾的选择性比为1000:1，而钾离子和钠离子的拥有属性相同，只是半径略有不同。通道孔通常非常小，以至于离子必须以单列顺序通过.<ref name="eisenman_theory" />。虽然一些通道表现出不同的次电导水平，但是通道孔可以为离子通过而打开或关闭。当通道打开时，离子通过通道孔，沿着该特定离子的跨膜浓度梯度向下渗透。离子通过通道的速率，即。单通道电流的幅度，是由该离子的最大通道电导和电化学驱动力决定的，即膜电位的瞬时值与翻转电位值之间的差值.<ref name="junge_33_37" />。朱格，页。33–37.

离子通道是一种内在膜蛋白，它有一个孔，离子可以通过这个孔在细胞外液和细胞内部之间穿梭。大多数钾离子通道对单个离子具有特异性（选择性） ; 例如，大多数钾离子通道对钾与钠的选择性比为 1000:1，尽管钾离子和钠离子带同样的电荷，只是半径略有不同。通道孔通常非常小，以至于离子必须以单列顺序通过。<ref name="eisenman_theory" /> 虽然一些通道表现出不同的亚电导水平，但是通道孔可以为离子通过而打开或关闭。当通道打开时，离子通过通道孔，顺着该离子种类的跨膜浓度梯度渗透。离子通过通道的速率，即单通道电流的幅度，是由该离子的最大通道电导和电化学驱动力决定的，即膜电位的瞬时值与逆转电位（reversal potential）值之间的差值 <ref name="junge_33_37" />

[[File:Potassium channel1.png|thumb|left|200px|Depiction of the open potassium channel, with the potassium ion shown in purple in the middle, and hydrogen atoms omitted. When the channel is closed, the passage is blocked.<nowiki>左图 | 200px | 描绘开放的钾离子通道，中间以紫色显示的钾离子，省略了氢原子。当通道关闭时，通道就被堵塞了。</nowiki>|链接=Special:FilePath/Potassium_channel1.png]]

[[File:Potassium channel1.png|thumb|left|200px|<nowiki>Depiction of the open potassium channel, with the potassium ion shown in purple in the middle, and hydrogen atoms omitted. When the channel is closed, the passage is blocked.左图 | 200px | </nowiki>描绘开放的钾离子通道，中间以紫色显示的钾离子，省略了氢原子。当通道关闭时，通道就被堵塞了。|链接=Special:FilePath/Potassium_channel1.png]]

Schematic stick diagram of a tetrameric potassium channel where each of the monomeric subunits is symmetrically arranged around a central ion conduction pore. The pore axis is displayed perpendicular to the screen. Carbon, oxygen, and nitrogen atom are represented by grey, red, and blue spheres, respectively. A single potassium cation is depicted as a purple sphere in the center of the channel.

Schematic stick diagram of a tetrameric potassium channel where each of the monomeric subunits is symmetrically arranged around a central ion conduction pore. The pore axis is displayed perpendicular to the screen. Carbon, oxygen, and nitrogen atom are represented by grey, red, and blue spheres, respectively. A single potassium cation is depicted as a purple sphere in the center of the channel.