What is Action Potential?

Action potential refers to an expandable potential change process based on resting potentials when excitable cells are stimulated. Action potential is composed of peak potential (general name of rapid depolarization rising branch and rapid repolarization falling branch) and post potential (slow potential change, including negative post potential and positive post potential). The peak potential is the main component of the action potential, so the action potential in the general sense mainly refers to the peak potential. The amplitude of the action potential is about 90 ~ 130mV, and the action potential exceeds the zero potential level by about 35mV. This segment is called overshoot. The action potential of nerve fibers generally lasts about 0.5 ~ 2.0ms, and can be transmitted along the membrane, also known as nerve impulses, that is, excitation and nerve impulses have the same meaning as action potentials.

Chinese name: Action potential
Foreign name: action potential
component: Peak potential, post potential

Amplitude: About 90 ~ 130mV
Formation principle: Difference in ion concentration on both sides of the cell membrane
Features: "All or None" No superposition and no attenuation conduction
Affecting transmission factors: Axon diameter myelin sheath

Action potential formation conditions

Potassium rush out of cell membrane

There is a difference in ion concentration on both sides of the cell membrane. The potassium ion concentration in the cell membrane is higher than that outside the cell membrane, and the extracellular sodium, calcium, and chloride ions are higher than in the cell. The maintenance of this concentration difference depends on the active transport of the ion pump. (Mainly the sodium-potassium pump (for every 3 Na + flowing out of the cell, 2 K + flows into the cell. Na +: K + = 3: 2)

Cell membranes have different permeability to different ions in different states. For example, potassium ions are mainly allowed to permeate when quiet, while sodium ions are mainly allowed to be depolarized to a threshold potential level.

Excitable tissues or cells are stimulated by threshold or above threshold.

Action potential formation

Action potential ascending branch

Greater than or equal to threshold stimulus Partial depolarization of cells A small amount of influx of sodium ions Depolarization to a threshold potential level Positive feedback of sodium ions inflow and depolarization (sodium ions influx) Basically reach sodium ions Equilibrium potential (the inside of the membrane is positive and the outside is negative, and the maximum value is just close to the sodium ion equilibrium potential due to a small amount of potassium ion outflow).

Decreasing branch of action potential

Membrane depolarization reaches a certain potential level Sodium ion inflow stops and potassium ion quickly flows out.

Action potential formation principle

The concentration of extracellular sodium ions is much higher than that inside the cell. It has a tendency to diffuse from the outside to the inside of the cell, but whether sodium ions can enter the cell is determined by the state of sodium channels on the cell membrane. When cells are stimulated to excite,

Experimental mode diagram for measuring single nerve fiber resting and action potential The first is that a small number of highly excited sodium channels are opened, and a small amount of sodium ions enter the cell with a low concentration, resulting in a decrease in the potential difference between the two sides of the membrane and a degree of depolarization. When the membrane potential decreases to a certain value (threshold potential), a large number of sodium channels on the cell membrane will be opened at the same time. At this time, under the action of the sodium ion concentration difference on both sides of the membrane and the potential difference (internal negative external positive), The extracellular sodium ions flow in rapidly and in large quantities, resulting in a rapid increase in the positive charge in the cell and a sharp rise in the potential, forming an ascending branch of the action potential, that is, depolarization. When the positive potential inside the membrane increases enough to prevent further inward flow of sodium ions, that is, the equilibrium potential of sodium ions, the sodium ions stop inward flow and the sodium channel is deactivated and closed. During the influx of sodium ions, potassium channels are activated and opened, and potassium ions flow from the inside of the cell to the outside of the cell along a concentration gradient. When the inflow velocity of sodium ions and the outflow velocity of potassium ions are balanced, a peak potential is generated. Subsequently, the outflow velocity of potassium ions is greater than the inflow velocity of sodium ions, and a large amount of cation outflow causes a rapid decrease in the potential in the cell membrane, forming a descending branch of the action potential, that is, repolarization. At this time, although the cell membrane potential has basically recovered to the resting potential level, the sodium ions flowing in by depolarization and the potassium ions flowing out by repolarization have not been reset. At this time, the sodium ions pumped in by the activity of the sodium and potassium pumps Pump out the potassium ions that flow out, and restore the uneven distribution of these two ions on both sides of the cell membrane before the action potential is restored to prepare for the next excitement. ^[1]

In short, the depolarization of the action potential is due to the large and rapid inflow of sodium ions caused by the opening of a large number of sodium channels; the repolarization is the result of the rapid outflow of potassium ions caused by the opening of a large number of potassium channels. ^[1]

The amplitude of the action potential is determined by the difference in sodium ion concentration between the inside and outside of the cell. The decrease of the sodium ion concentration in the extracellular fluid also reduces the amplitude of the action potential, while blocking the sodium ion channel (tetrodotoxin) can hinder the generation of action potential.

Action potential characteristics

"All or None"

Only threshold or suprathreshold stimuli can cause action potentials. The depolarization of the membrane potential during the action potential is caused by the opening of the sodium channel, so the stimulus causes the membrane to depolarize, but only makes the membrane potential from the resting potential to the threshold potential level, and has nothing to do with the final level of the action potential. Therefore, the level of action potential caused by the threshold stimulus is equal to that of any intensity above the threshold, which is called "all or nothing". ^[1]

Cannot stack

Because the action potential has the characteristics of "all or nothing", it is impossible for the action potential to produce any superposition or sum in any sense. ^[1]

Non-attenuating conduction

If an action potential is generated at any point on the cell membrane, the entire cell membrane will experience exactly the same action potential, and its shape and amplitude will not change. ^[1]

Action potential conduction principle

The action potential generated at any point on the cell membrane will spread to the entire cell membrane without attenuation. This is called conduction of action potential. If it occurs on a nerve fiber, the conducted action potential is also called a nerve impulse. ^[1]

Taking neurons as an example, the conduction of action potentials along the axons is achieved through local currents across the membrane.

Conduction of action potentials on nerve fibers Now. Stimulating axonal sites sufficiently strong can cause them to generate action potentials. At this time, the potential difference between the inner and outer sides of the membrane temporarily reverses, that is, the state of the membrane is negative and the membrane is positive when it is quiet. For excitement membrane. There is a potential difference between the excitatory membrane and the surrounding resting membrane (non-excited membrane) both inside and outside the membrane. At the same time, the solution on both sides of the cell membrane is electrically conductive. Charge movement, this charge movement is the local current. On the outside of the membrane, current flows from the resting membrane to the excited membrane; on the inside of the membrane, current flows from the excited membrane to the resting membrane. As a result, the inner potential of the resting membrane increased and the outer side of the membrane decreased, that is, depolarization occurred. When the membrane potential of the resting membrane reaches the threshold potential level, a large number of sodium channels are activated, causing an action potential. At this time, the original resting membrane turns into an excitatory membrane and continues to conduct to the surrounding resting membrane. Therefore, the so-called action potential conduction is actually the process in which the excitation membrane moves forward. Local current can be generated between the axons excited by stimulation and the surrounding resting membrane, so they can conduct in two directions, which is called bidirectional conduction of action potential. ^[1]

The action potential is not attenuated during the conduction process. The reason is that when the action potential is conducting, it is actually the movement of the depolarized area and the successive generation of the action potential. It can be seen that the conduction distance is not related to the amplitude, so the action potential amplitude will not change due to the increase of the conduction distance. ^[1]

Nerve fiber conducts extremely fast, but the conductance of different nerve fibers varies greatly. For example, the transmission speed of some thicker bone marrow fibers in the human body can reach 100 m / s, while the transmission speed of some thinner myelinated fibers is even lower than 1 m / s. ^[1]

Factors affecting action potential

Action potential axon diameter

The conduction of the action potential is achieved by local current. When the axon is thick, the resistance is significantly reduced, so the local current intensity is large, and the potential difference between it and the neighboring parts is large, so that the surrounding parts can reach the threshold value quickly. Fast conduction speed. In addition, the number of sodium channels on nerve fibers of different diameters is different. The thicker the nerve fibers, the greater the number of sodium channels. Therefore, the stronger the inward current of the sodium ion formed, the faster the action potential is formed. ^[1]

Action potential myelin

Many vertebrates have myelin sheaths around the nerve fibers, which is an important reason for the accelerated action potential conduction speed and is more effective than simply increasing the diameter. Myelin sheaths are intermittently arranged along the axon, and every other segment has an unmyelinated area called a Langfie knot. Due to the characteristics of high resistance and low capacitance of the myelin sheath, the generated action potential can only form a local current in the adjacent Langfie junction region. In addition, there are dense sodium channels in the junction region, so the action potential can be formed only in this region. . Action potential conduction seems to jump from one node region to another. Therefore, the conduction of action potentials on myelinated nerve fibers is skipped. Jumping conduction is a very economical conduction method. On the one hand, the conduction speed is greatly improved, and on the other hand, energy is saved (the total number of ions involved in the transmembrane motion involved in each action potential is much less per unit length). ^[1]

Action potential channel connection

Link to voltage-gated ion channels

Schematic diagram of a sodium pump that controls the movement of sodium ions There are at least two voltage-gated ion channels on excitable cells, sodium channels and potassium channels. Voltage-gated sodium channels have two gates and three functional states. When closed, it is in the standby state. At this time, the activation door is closed and the deactivated door is opened. When activated, it is open. At this time, both doors are in the open state. Sometimes the activation door opens and the deactivation door closes.

After activation of the sodium channel, it must first enter the inactive state, and then gradually return from the inactive state to the closed state for the next activation. It cannot enter the closed state directly from the activated state. During the generation of action potentials, sodium channels are activated by sodium channel activation leading to influx of sodium ions. Therefore, the state of sodium channels must affect the response of cells to new stimuli.

After the sodium channel is inactivated from the activated state, no matter how powerful the stimulation is, it cannot cause it to open again, that is, it causes a new action potential. This is the absolute refractory period. Voltage-gated potassium channels have only one gate and two functional states. When it is quiet, it is closed and the door is closed; when it is activated, it is open, at this time the door is open. ^[1]

Intrinsic connection of action potential

The intrinsic connection of action potentials and excitability

During the peak potential, the cell is in an absolute refractory period, at which time no stimulus of any intensity can cause new

Time relationship between action potentials and excitability changes The action potential is generated. This is because during the depolarization phase of the action potential, all sodium channels have been opened. In the early stage of repolarization, that is, most of the time in the descending branch, the sodium channel is inactive, and at this time, the sodium channel cannot be activated again. In the later period of repolarization and hyperpolarization of action potential, the cells are in a relatively refractory period. At this time, suprathreshold stimulation may trigger action potential. This is because the sodium channel is partially or completely restored to the closed state at this time, and the stimulus can be opened again. However, since the potassium channel is still open, potassium outflow can counteract the depolarization caused by sodium inflow. Therefore, the stimulus intensity must be stronger than the threshold stimulus in order to depolarize the membrane potential to the threshold potential level, thereby inducing the action potential. At this time, the excitability of the membrane is lower than normal. ^[1]

Action potential local potential

Action potential definition

Although sub-threshold stimulation cannot trigger action potentials, it also causes a small amount of sodium ions to flow in, resulting in a small degree of depolarization, but this depolarization is not sufficient to induce action potentials, and is limited to the stimulated site. . This smaller degree of depolarization, which occurs at the site of stimulation, is called local potential. ^[1]

Action potential characteristics

The amplitude of the potential is small and is attenuated, which decreases rapidly with the increase of the propagation distance;

It is not "all or nothing", the local potential increases with the increase of stimulation intensity;

There is a summing effect. Multiple subthreshold stimuli can be superimposed in time (give multiple stimuli in the same part continuously) or spatially (give multiple stimuli in adjacent parts). If the sum of the depolarization intensity is generated, Exceeding the threshold potential can induce action potential. ^[1]

Measurement process and result analysis of action potential resting potential and action potential

The neural stem compound action potential is the sum of many nerve fiber activities. To reveal the mechanism of nerve impulse generation and conduction, it is best to record the potential change on a single nerve fiber, but the axon diameter of a person is very thin, only about 0.01mm. Until the 1930s, researchers discovered that giant axons, the giant nerve fibers of calamari, have diameters of up to 1 mm, which can be distinguished by the naked eye. In addition, the development of microelectrode technology has made it possible to directly measure the change in transmembrane potential of a single nerve fiber. become possible. If an electrode with a diameter of about 100um or a thinner electrode is inserted into the axon, it will generally not cause significant damage. Modern microelectrode technology can pull glass microelectrodes into a tip diameter of less than 0.5um ", which basically solves the problem of recording membrane potentials of human coarse nerve fibers ^[2] .

Difference between action potential and compound action potential

There is a big difference between the action potential and the compound action potential in terms of concept, measurement method, and generation principle. The characteristics of action potential are: 1) It has the characteristics of "all or nothing". Only threshold or suprathreshold stimuli can cause action potentials. The depolarization of the membrane potential during the action potential is caused by the opening of the sodium channel, so the stimulus causes the membrane to depolarize, but only brings the membrane potential from the resting potential to the threshold potential level, and has nothing to do with the final level of the action potential. Therefore, Threshold stimulus and the level of action potential caused by suprathreshold stimulus of any intensity are the same, called "all or nothing"; 2) Cannot be superimposed: action potentials have the characteristics of "all or nothing", so action potentials cannot produce any Superposition or sum in the sense; 3) Non-decaying conduction: Generate an action potential at any point on the cell membrane, and form a potential difference with the surrounding unexcited area. Under the stimulation of local current, the Na channels in the surrounding unexcited area are opened, and the entire cell membrane Will experience exactly the same action potential once, and its shape and amplitude will not change ^[2] .

The difference between the neural stem composite action potential and the single nerve fiber action potential is mainly reflected in two aspects: First, it does not have the "all or nothing" characteristic. This is because the neural stem is composed of many nerve fibers, although each nerve Fibrous action potentials have "all or nothing" characteristics, but because the excitability of each nerve fiber in the neural stem is different, its thresholds are also different. When the neural stem is stimulated and its strength is below the threshold of any fiber, no action potential is generated. When the stimulus intensity reaches the threshold of a few fibers, a small compound action potential may appear. As the stimulus is strengthened, the number of fibers involved in excitement increases, and the amplitude of the compound action potential also increases. When the stimulation intensity is increased so that all the fibers are excited, the amplitude of the composite action potential reaches a maximum value. Even if the stimulation intensity is further increased, the amplitude of the composite action potential will not increase with the enhancement of the stimulation intensity. In other words, the composite action potential of the neural stem is the superposition of the action potential of a single nerve fiber. (Second, when biphase action potentials are recorded with a two-electrode, the following characteristics are observed: the peak of the first phase is always higher than that of the second phase; the second phase continues Time is always greater than Phase 1; the ascending and descending branches of each phase are not symmetrical. This indicates that the action potential of the neural stem is not "non-fading conduction". The reason for this phenomenon is that the nerve fibers of the first recording point are synchronized The number of excitations is higher, so the peak value of the first phase of the action potential recorded is higher. Due to the different conduction velocity of each fiber, the number of synchronized excitations of the nerve fibers at the second recording point is less, so the second phase peak of the recorded action potential is higher Low but long duration, and the distance between the peak and trough of the biphasic action potential is related to the distance between the two electrodes A and B ^[2] .