动作电位指的是处于静息电位状态的细胞膜受到适当刺激而产生的，短暂而有特殊波形的，跨膜电位搏动。细胞产生动作电位的能力被称为兴奋性，有这种能力的细胞如神经细胞和肌细胞。动作电位是实现神经传导和肌肉收缩的生理基础。

动作电位的形成与细胞膜上的离子通道开关相联系。一个初始刺激，只要达到了阈电位（而不论超过了多少，这就是全或无定理），就能引起一系列离子通道的开放和关闭，而形成离子的流动，改变跨膜电位。而这个跨膜电位的改变尤能引起临近位置上细胞膜电位的改变，这就使得兴奋能沿着一定的路径传导下去。

首先细胞膜处于静息电位状态，大概在-90到-70 mV水平。动作电位可分为四个相位:

1.在初始相膜电位去极至阈电位，这是动作电位产生的前提。刺激可以是旁边已兴奋的细胞膜电位改变，或者是化学突触上离子通道中的离子内流。
2.去极化过程中，膜电位陡峭上升至正值水平。这个锋电位中去极化部分被称为升支，而正的电位值则被称为超射。（注意：去极化指的是整个上升过程。）
3.接下来朝静息电位方向的下降过程被称为复极化。
4.在下降过程中，电位会短时间下降到低于静息电位水平，然后再上升达到静息电位，这种静息电位的增大（{jd1}值）被称为超极化（而下降部分被称为负后电位，上升部分则是正后电位）。
动作电位持续约1-2 ms（神经元），但也可达几百毫秒（心脏）。

动作电位后是不应期，这又分为0.5 ms的{jd1}不应期和3.5 ms的相对不应期。前者无论刺激多频繁多强都不能引起动作电位，而后者则要更强的刺激（阈电位提高了）才能引起动作电位。

离子通道的特征

细胞膜上有多种离子通道。而动作电位的产生，则与钠和钾离子通道有关。这些离子通道的开关状态与膜电位有关，即是所谓的电压门控通道。

例如钠离子通道，在静息时它是关闭并且是可xx的。当去极化到一特定值时就会引起其构象的改变，成为打开状态。但是离子通道却不会持续停留在开放状态，它会在几毫秒内关闭。这是通过膜上一蛋白质的失活域的活动实现的，这个失活域会像塞子一样堵住离子通道。离子通道这种状态被称为关闭并失活的。过渡状态关闭但可xx的只有在xx复极化后才可能出现，而开放可xx的状态是在简单模型中不可能实现的。(文献中也写道，一个关闭并失活的通道在复极过程中首先短时间内还是开放状态，然后才改变构象直接成为关闭但可xx的。再次xx只能发生在xx复极之后，在去极化的细胞膜中不可能存在着过渡状态开放并失活的)。

当然，并不是所有的通道在电位到达一定值之时全部打开。更可能的是，通道的处于某种状态的概率是与电压相关的。而当阈电位出现时，大部分的通道便会开放，上述的模型便能很好的描述这种状态。而状态之间过渡所需的时间也是因通道而异的。钠通道从关到开发生在1毫秒之内，而钾通道则要10毫秒。除了电压外，还有其他开关通道的机制。对动作电位来说，有两种值得一提。一种是与内向整流性钾通道 Kir有关，这种通道是不可调控的。但却有一些带正电的小分子如精素，能够在去极化到一定程度时堵塞通道孔。另一种机制与钾通道有关，当细胞间的钙离子与它结合后会开放。

初始状态
初始时，细胞膜电位为静息电位。钠通道关闭，只有部分钾通道(主要是Kir) 开放，钾离子决定了静息电位的大小。而各种离子运动的方向和强度为电化学梯度所决定。例如钠离子就有向细胞内扩散的趋向，因为其膜外浓度远较膜内高。

起始相
起始相中，要有一个刺激使得膜电位去极化到达阈值。刺激可由于突触后离子通道的开放(Na+, Ca2+)或是旁边区域产生的动作电位而形成。

当电位到达- 60 mV时，Kir-通道会被精素阻断，继而导致快速的去极化并达到钠通道开放的阈值。但是外流的钾离子却会稍微阻挡这一进程(例如钙离子依赖的钾离子通道，由于在去极化过程中钙离子内流而被xx，因而会稍微阻挡这一进程)。

升支和超射
但钠通道起始要到-60 mV以上电位时才会大量开放。钠离子悬殊的浓度差，使得钠离子很快进入细胞内，造成细胞膜的去极化，更多的钠通道被xx，就有更多钠离子进入，形成了升支和正位的超射。

复极开始
但在电位到达{zd0}正值之前，NaV-通道已经开始失活。然后电压门控钾通道开始被xxKV。两者虽然阈值相近，但后者开放所需的时间更长，直到去极化达到一定程度才开始显示其作用。当钠内流达到{zd0}值时，钾通道才有一半开放，而当所有钠通道失活时，钾通道的开放才到了{zd0}值。所以钠通道开放最多的时候，是电压{zd0}值之时，而钾通道的则位于复极开始后。

复极
当复极快至静息电位时，KV关闭，Kir (对静息电位很重要)的堵塞会被去除，NaV会慢慢xx。

后超极化
在很多细胞中(主要是神经元)还可以看到后超极化。这是因为钾离子的电导性仍然高于静息值，钾离子持续流出，而钠离子已停止流入，这时电位更接近钾的平衡电位（见能斯特方程）。在动作电位时，流入的钙离子会打开钾通道，这是导致钾电导性的增高的原因。当钙水平下降时，电位才会慢慢恢复正常。工作效率增高的钠钾泵对后超极化也有影响。

不应期
动作电位后，轴突在一段时间内不能再响应外界刺激。这个时间段，不应期的长短，是由NaV活性恢复所需的时间所决定的。不应期分为“{jd1}不应期”和“相对不应期” 在动作电位后紧接的{jd1}不应期，当时复极化仍在继续，通道不能再打开。可以说，此时阈值无限大。在相对不应期，提高刺激强度可以获得一个弱的动作电位。阈值由无限大向正常值靠近。

阈值
动作电位需要电位到达阈值才能发起，此时钠通道开放，钠离子内流。科学家尝试找出这一个阈值，但都以失败告终。神经元可以在一个范围的刺激内被xx。因此神经科学家放弃了寻找一特定阈值的努力。从系统理论来说，动作电位的产生是通过被动电位动力学和动作电位动力学的分叉实现的。但在很多教科书中，当讲解到阈值时，只会将它描述为静息电位和动作电位间的灰色地带。

特殊的动作电位
除了钠电压门控通道外，电压门控钙通道也可以引起动作电位(如心肌和浦肯野细胞).
植物细胞也能产生动作电位，例如在植物运动之时。
Action potential refers at resting potential state membrane are properly stimulated generated short while special waveform, transmembrane potential pulsatility. Ability of cells called the action potential excitability, have the ability of cells such as nerve cells and muscle cells. Action potential is a nerve conduction and muscle contraction to achieve the physiological basis of.

Action potential formation and membrane associated ion channel switch. An initial stimulus, as long as the reached the threshold potential (regardless of how much more than that is all or none theorem), can lead to a series of ion channels open and close, and the formation of ion flow and change the transmembrane potential. Change in membrane potential and this can cause particular location near the plasma membrane potential changes, which makes the excitement to go along a certain path of conduction.
Description of the process of action potential

First of all, in the resting membrane potential of the state, probably in the -90 to -70 mV level. Action potential can be divided into four phases:

1. In the initial phase of membrane potential to the extreme threshold potential, which is the premise of the action potential. Stimulation can be next to the exciting changes in membrane potential or chemical synaptic ion channels in the plasma flow.
2. To the polarization process, the membrane potential to a time when the steep rise in the level. The spike in the depolarization part of the ascending branch is called, and is the potential value is called overshoot. (Note: to increase the polarization refers to the whole process.)
3. Next, the direction towards a decline in resting potential process is called repolarization.
4. In the fall the process, potential will be a short time fell below the level of resting potential, and then rise to resting potential, resting potential of this increase (absolute value) is called hyperpolarization (decline was partly after referred to as a negative potential, while others are after rising potential).
Action potential lasted about 1-2 ms (neurons), but up to several hundred milliseconds (heart).

After the action potential is a refractory period, which is divided into the absolute refractory period of 0.5 ms and 3.5 ms relative refractory period. The former matter more frequent and more powerful stimulus can lead to action potential, which will have to stronger stimulation (threshold potential increase) to rise to action potentials.
Causes of action potentials

Characteristics of ion channels

There are a variety of membrane ion channels. The action potential generation, then with sodium and potassium ion channels. These ion channels switch state and the membrane potential, that is, the so-called voltage-gated channel.

For example, sodium channel, in the rest when it is closed and is activatable. When the depolarization to a certain value will lead to changes in their conformation and become open. But ion channels have not continued to stay in the open state, it will close within a few milliseconds. This is a membrane protein through the inactivation domain of activities to achieve, the inactivation of the same domain will be blocked as the plug ion channels. This condition is known as ion channels close and deactivation. Transition state close but can be activated only after the complete repolarization potential, and open to activate the state is in the simple model can not be realized. (Literature, also wrote that a closed and inactivated channels during repolarization the first short period of time, or open, and then change directly to a closed conformation but can be activated. Re-activation occurs only after the complete repolarization, in the cell membrane depolarization can not exist in the transition state of open and inactivated).

Of course, not all of the channels in the potential to reach all the open when a certain value. More likely, the channel is in a state of probability is voltage dependent. When the threshold potential there, most of the channel will open, the model can well describe this state.

The time required for the transition between states is a result of channel-specific. Sodium channel related to the development from students in a matter of milliseconds, while the potassium channel will have to 10 milliseconds.

In addition to voltage, there are other mechanisms to switch channels. On the action potential, there are two worth mentioning. One is with the inward rectifier potassium channel Kir the, this channel is not regulated. But there are some small positively charged molecules such as precision elements, can be depolarized to a certain extent blocked access hole. Another mechanism of potassium channels, when intracellular calcium ions combine with it will open.

The process of action potential

Initial state
Initially, the cell membrane potential to resting potential. Sodium channels close, only some of the potassium channel (mainly Kir) open, potassium ions determines the size of the resting potential. And a variety of ion movement in the direction and strength determined by the electrochemical gradient. For example, there are sodium ions tend to spread to the cells, because of its high extracellular concentrations than membrane.

Initial phase
Initial phase, there must be a stimulus makes the membrane potential reaches threshold. Stimulation can be as open postsynaptic ion channels (Na +, Ca2 +) or next to the zone, resulting in the formation of action potentials.

When the potential reaches – 60 mV when, Kir-channel blocking factor will be fine, and then lead to rapid depolarization and reached the threshold of sodium channel opening. But the outflow of potassium ions was slightly blocked the process (such as calcium dependent potassium channels, depolarization due to calcium influx in the process of being activated, which will slightly block the process).

Ramus and the overshoot
But the sodium channel to -60 mV to be above the start only when a large number of potential open. The concentration of sodium ion gap difference, so sodium ions into the cells very quickly, resulting in membrane depolarization, more sodium channels are activated, there is more sodium ions into the formed ramus and orthotopic overshoot.

Repolarization started
However, most positive potential reached before, NaV-channel inactivation has already begun. And voltage-gated potassium channels begin to be activated KV. Although the threshold between the two similar, but the latter required longer open until the depolarization reaches a certain level before starting to show its effect. When the flow reaches the maximum sodium and potassium channels open only half, and when all the sodium channel inactivation, the potassium channels open only to the maximum. Therefore, sodium channels open up to the time when a voltage is maximum, while the potassium channel is located at the start of repolarization.

Repolarization
When the fast recovery time to resting potential, KV closed, Kir (very important on the resting potential) of the plug will be removed, NaV will gradually activated.

After hyperpolarization
In many cells (mainly neurons) can also be seen after hyperpolarization. This is because the potassium ion conductivity is still higher than the resting value, continuous outflow of potassium ions, while the inflow of sodium ions is stopped, then potential closer to the K equilibrium potential (see Nernst equation). In the action potential, influx of calcium ions to open potassium channels, which is leading to increased potassium conductance of the reason. When calcium levels drop, the potential will gradually return to normal. Increase the efficiency of the sodium-potassium pump also have an impact on the after hyperpolarization.

Refractory period
Action potential, the axon can not be a period of time to respond to external stimuli. This time, not duration, is the time for recovery NaV activity determined. Refractory period is divided into “absolute refractory period” and the “relative refractory period” in the action potential immediately after the absolute refractory period, when repolarization is continuing, the channel can no longer open. It can be said at this time threshold infinity.

In the relative refractory period, increased intensity of stimulation can get a weak action potential. Threshold from the infinite to near normal.
Threshold
Potential to reach action potential threshold required to initiate, then sodium channel opening, Na + influx. Scientists try to find this threshold, but all failed. Neurons can be within a range of stimulus are activated.Therefore,neuroscientists gave up looking for a specific threshold efforts. From system theory, the action potential is generated by passive potential dynamics and bifurcation dynamics of action potential realized. But in many textbooks, when explaining to the threshold value, will only describe it as the resting potential and action potential gray area.

Specific action potential
In addition to voltage-gated sodium channel, voltage-gated calcium channel can also lead to action potential (such as myocardial and Purkinje cells). Plant cells also can produce action potentials, such as when the movement in plants.