Action potential, its phases. Modern idea of ​​the mechanism of its generation. Membrane potential and action potential and its phases. Difference between excitation phases

The shape of the action potential allows us to divide the process of its generation into several phases: pre-spike, fast depolarization, repolarization and trace potentials (Fig. 2.3).

Rice. 2.3.

Prespike - This is a process of slow depolarization of the membrane, which begins with the first deviation from the resting potential and ends with the achievement of KUD. The prespike involves passive membrane depolarization and an active local response. An active response occurs when passive depolarization of the membrane reaches 70-80% of the ADC values ​​and is the first manifestation of the incipient active state of the membrane - the beginning of its excitation. Thanks to passive depolarization and a local active response, the potential shift on the membrane reaches a critical level of depolarization, at which the AP itself develops.

Phase fast(avalanche-like) depolarization membrane is the first phase of PD. At this stage, the membrane potential quickly shifts from the critical level of depolarization to zero and continues to shift until the G1D peak, recharging the membrane. During the first phase of AP, the potential on the membrane is “perverted”, i.e. the membrane is discharged to zero and recharged with the opposite sign. The PD section with values ​​from zero to the recharge peak is called overshot(English, overshoot) potential. Instead of negative values, the potential on the membrane becomes positive. In the giant squid axon, the AP peak reaches values ​​of the order of +50 mV, and the depolarization phase with overshoot lasts about 0.5 ms.

Phase repolarization is the second phase of PD. During this phase, the membrane potential returns to its original value, i.e. to resting potential. This phase can be subdivided into a fast repolarization from +50 mV to 0 V and a slower repolarization from 0 V to KUD and further to the resting potential. The repolarization phase takes 1-2 ms.

Trace potentials may in some cases develop at the end of AP in the form of slow depolarization or even slow hyperpolarization. Trace hyperpolarization is observed, in particular, on the membrane of the squid giant axon.

Ionic nature of action potential phases was studied in experiments on giant squid axons by Hodgkin and Huxley. It turned out that at the moment of AP generation, the electrical resistance of the axon membrane for a period of 1-2 ms decreases by 20-30 times, i.e. The conductivity of the membrane increases sharply, and current begins to flow through the membrane. But what current is this? It turned out that if Na + cations are removed from the external solution and replaced with sucrose, the amplitude of the action potential decreases sharply or AP does not occur at all. This allowed us to conclude that the main reason for the generation of AP and recharging of the membrane to positive values ​​is the occurrence of high permeability of the membrane to sodium cations and the rapid entry of these cations into the cell.

The inward movement of sodium occurs under the influence of two forces. The first force is associated with the presence of a transmembrane concentration gradient of sodium cations. The sodium concentration in the external solution is 20-30 times higher than inside, i.e. the concentration gradient for Na+ is directed into the cell, and if there is sufficient permeability, sodium cations will quickly enter the cell. The second force is associated with the presence of a large negative charge on the inner side of the membrane (about -70 mV). The negative charge on the inside of the membrane will allow positively charged sodium cations to enter the cell. Entering, sodium cations will first rapidly reduce the negative charge of the membrane to zero, and then recharge the membrane to positive values, bringing the membrane potential closer to the equilibrium potential for Na +. Let us recall that the equilibrium potential for Na" cations can be calculated using the Nernst equation and is +55 mV for the squid giant axon.

The participation of the incoming sodium current in the creation of the depolarization phase of the AP is supported by the results of experiments with tetrodotoxin, a blocker of voltage-dependent sodium permeability. Tetrodotoxin is able to completely block the development of G1D (Fig. 2.4, A).

Rice. 2.4. Changes in PD arising from the action of selective blockers of sodium permeability - tetrodotoxin (I) or potassium permeability - tetraegilammonium on the membrane (b)

Thus, the sodium hypothesis satisfactorily explains the development of the depolarization phase of AP, but leaves open the question of the causes of rciolarization, i.e. AP phase, leading to a return of the membrane potential to the resting potential level. It was suggested that another process develops on the membrane - its permeability to potassium ions increases. It was clear that this was a special active potassium permeability, different from the passive potassium permeability that exists in the membrane at rest (passive potassium leak). Additional potassium permeability of the membrane occurs only in response to depolarization of the membrane to a critical level, and with a slight delay compared to the increase in sodium permeability. In the event of such additional active permeability to potassium, K* cations begin to leave the cell under the influence of a concentration gradient and a charge on the membrane created by the advanced entry of sodium cations. Incoming Na + cations charge the inner side of the membrane positively and the outer side negatively. The additional outgoing current of potassium cations will reduce the positive charge created by the sodium current inside the cell and return the electrical charge on the membrane to its original values, i.e. to resting potential.

The participation of the outgoing potassium current in the creation of the repolarization phase of AP was supported by the results of experiments using a blocker of active potassium permeability - tetraethylammonium. Tetraethylammonium sharply slows down the repolarization phase of AP (Fig. 2.4, b).

If AP is the result of the appearance and development of two new ion currents on the membrane that were not present at rest, namely sodium and potassium currents, then, consequently, upon depolarization, new voltage-activated ion channels open on the membrane. These channels conduct sodium first and then potassium. The properties of such channels can be understood by analyzing the development of currents that arise during their operation. But these currents must be recorded “in their pure form,” i.e. not complicated by simultaneous changes in membrane potential and capacitive membrane currents. For this purpose, Hodgkin and Huxley, in their experiments on giant squid axons, first used the method of fixing potential on the membrane (eng. voltage-clamp).

Membrane potential fixation method consists of connecting two amplifiers to the axon membrane of the system. One amplifier is designed to record shifts in membrane potential, the second operates on the principle of negative feedback. Two wire microelectrodes are inserted into the axon. One of them measures shifts in membrane potential and transmits them to a negative feedback amplifier. This amplifier (which monitors potential shifts on the membrane and generates currents) is connected at the output to the second intracellular microelectrode - the current one. A current will be supplied through this microelectrode, which can be measured in the external circuit of an indifferent electrode located outside the axon.

If you now artificially depolarize the membrane to CUD, then in response, voltage-activated currents begin to flow through the excited membrane: sodium and potassium. The shifts in membrane potential created by these currents are instantly monitored using a feedback amplifier, which sends currents of equal amplitude but opposite directions through the current microelectrode - feedback occurs. Such “clamping currents” keep (fix) the membrane from potential shifts and are essentially a mirror image of the Na + - and K + -currents. Clamping currents can be easily measured in the external circuit of the circuit (Fig. 2.5).

Rice. 2.5.

(voltage-clamp):

Using a feedback amplifier, the current electrode passes a clamping current, which is a mirror image of transmembrane currents

In Fig. Figure 2.6 shows the data obtained using the potential fixation method. When the membrane is depolarized from -65 to -9 mV, the membrane is excited, which is accompanied by the generation of a biphasic current. It can be seen that first a fast incoming current appears, which fades and is replaced by a more slowly developing outgoing current. It turned out that the incoming current can be completely blocked using tetrodotoxin, a selective blocker of voltage-gated sodium channels. It follows that the incoming current is a sodium current.

The outgoing current, which also arose in response to depolarization, is preserved and detected in its pure form. This current develops with a slight delay, increases more slowly, but does not fade and persists throughout the entire depolarization time. It is completely blocked by the voltage-activated potassium channel blocker tetraethylammonium and, therefore, represents a voltage-activated K + current. Thus, using the potential clamp method and the use of selective blockers of sodium and potassium currents, it was possible to separate and identify separately two currents that arise during the generation of AP, show their independence from each other, and analyze each of them.

Rice. 2.6.

A - shifting the membrane potential by 56 mV and fixing it at -9 mV;

6 - biphasic (early in and late out) current in response to potential clamping at -9 mV; V- pharmacological separation of two currents using sodium (tetrodotoxin) and potassium (tetraethylammonium) blockers

Action potential - an electrical impulse that occurs between the inner and outer sides of the membrane and is caused by changes in the ionic permeability of the membrane.

PD phases:

Prespike is the process of slow depolarization of the membrane to a critical level of depolarization.

Spike (peak potential) - consisting of an ascending part (membrane depolarization) and a descending part (repolarization)

Negative trace potential - from the critical level of depolarization to the initial level of membrane polarization.

Positive trace potential is an increase in membrane potential and its gradual return to its original value.

First period- local response is an active local depolarization resulting from an increase in sodium permeability of the cell membrane. However, with a subthreshold stimulus, the initial increase in sodium permeability is not large enough to cause rapid membrane depolarization. The local response occurs not only with subthreshold, but also with suprathreshold stimulation and is an integral component of the action potential. Thus, the local response is the initial and universal form of tissue response to stimulation of varying strengths. The biological meaning of the local response is that if the irritation is small, then the tissue reacts to it with minimal energy expenditure, without turning on the mechanisms of specific activity. In the same case, when the stimulation is suprathreshold, the local response turns into an action potential. The period from the beginning of stimulation to the beginning of the depolarization phase, when the local response, increasing, reduces the membrane potential to a critical level, is called latent period or hidden period. The duration of the latent period depends on the nature of the irritation (Fig. 3.5.).

Second period- depolarization phase. This part of the action potential is characterized by a rapid decrease in membrane potential and even recharging of the membrane: its inner part becomes positively charged for some time, and the outer part negatively. Unlike the local response, the speed and magnitude of depolarization does not depend on the strength of the stimulus. The duration of the depolarization phase in a frog nerve fiber is about 0.2 - 0.5 ms.

Third period action potential - repolarization phase, its duration is 0.5-0.8 ms. During this time, the membrane potential is gradually restored and reaches 75 - 85% of the resting potential. In the literature, the second and third periods are often called the peak of the action potential.

The oscillations in membrane potential following the peak of the action potential are called trace potentials. There are two types of trace potentials - subsequent depolarization And subsequent hyperpolarization, which correspond to the fourth and fifth phases of the action potential. The trace depolarization is a continuation of the repolarization phase and is characterized by a slower (compared to the repolarization phase) restoration of the resting potential. The trace depolarization turns into a trace hyperpolarization, which is a temporary increase in the membrane potential above the initial level. In myelinated nerve fibers, trace potentials are more complex. A trace depolarization can turn into a trace hyperpolarization, then sometimes a new depolarization occurs, only after which the resting potential is completely restored.

Ionic mechanism of action potential occurrence

The action potential is based on changes in the ionic permeability of the cell membrane that develop sequentially over time. When a cell is exposed to an irritant, the permeability of the membrane for Na + ions sharply increases due to the activation (opening) of sodium channels (Fig. 3.6.). In this case, Na + ions vary in concentration.

In this case, Na + ions intensively move along the concentration gradient from outside to intracellular space. The entry of Na + ions into the cell is also facilitated by electrostatic interaction. As a result, the permeability of the membrane for Na + becomes 20 times greater than the permeability for K + ions.

Since the flow of Na + into the cell begins to exceed the potassium current from the cell, a gradual decrease in the resting potential occurs, leading to a reversion - a change in the sign of the membrane potential. In this case, the inner surface of the membrane becomes positive with respect to its outer surface. The indicated changes in membrane potential correspond to the ascending phase of the action potential (depolarization phase)

The membrane is characterized by increased permeability to Na + ions only for a very short time of 0.2 - 0.5 ms. After this, the permeability of the membrane for Na + ions decreases again, and for K + it increases. As a result, the flow of Na + into the cell is sharply weakened, and the flow of K + from the cell increases (Fig. 3.7.).

During an action potential, a significant amount of Na + enters the cell, and K + ions leave the cell. Restoration of the cellular ionic balance is carried out thanks to the work of the Na + ,K + - ATPase pump, the activity of which increases with an increase in the internal concentration of Na + ions and an increase in the external concentration of K + ions. Thanks to the operation of the ion pump and the change in membrane permeability for Na + and K +, their initial concentration in the intra- and extracellular space is gradually restored.

The result of these processes is membrane repolarization: the internal contents of the cell again acquire a negative charge in relation to the outer surface of the membrane.

1. Prespike- the process of slow depolarization of the membrane to a critical level of depolarization (local excitation, local response).
2. Peak potential, or spike, consisting of an ascending part (membrane depolarization) and a descending part (membrane repolarization).
3. Negative trace potential- from the critical level of depolarization to the initial level of membrane polarization (trace depolarization).
4. Positive trace potential- an increase in membrane potential and its gradual return to its original value (trace hyperpolarization).

General provisions

The polarization of the membrane of a living cell is due to the difference in the ionic composition on its inner and outer sides. When the cell is in a quiet (unexcited) state, ions on opposite sides of the membrane create a relatively stable potential difference, called the resting potential. If you insert an electrode into a living cell and measure the resting membrane potential, it will have a negative value (about −70 - −90 mV). This is explained by the fact that the total charge on the inner side of the membrane is significantly less than on the outer side, although both sides contain cations and anions. Outside there are an order of magnitude more sodium, calcium and chlorine ions, inside there are potassium ions and negatively charged protein molecules, amino acids, organic acids, phosphates, sulfates. We must understand that we are talking specifically about the charge of the membrane surface - in general, the environment both inside and outside the cell is neutrally charged.

The membrane potential can change under the influence of various stimuli. An artificial stimulus can be an electric current applied to the outer or inner side of the membrane through an electrode. Under natural conditions, the stimulus is often a chemical signal from neighboring cells, arriving through a synapse or through diffuse transmission through the intercellular medium. The membrane potential can shift to negative ( hyperpolarization) or positive ( depolarization) side.

In nervous tissue, an action potential usually occurs during depolarization - if the depolarization of the neuron membrane reaches or exceeds a certain threshold level, the cell is excited, and a wave of electrical signal propagates from its body to the axons and dendrites. (In real conditions, postsynaptic potentials usually appear on the body of a neuron, which are very different in nature from the action potential - for example, they do not obey the all-or-nothing principle. These potentials are converted into an action potential at a special part of the membrane - the axon hillock, so action potential does not propagate to dendrites).

This is due to the fact that the cell membrane contains ion channels - protein molecules that form pores in the membrane through which ions can pass from the inside to the outside of the membrane and vice versa. Most channels are ion-specific - the sodium channel allows almost only sodium ions to pass through and does not allow others to pass through (this phenomenon is called selectivity). The cell membrane of excitable tissues (nervous and muscle) contains a large amount voltage-dependent ion channels that can quickly respond to a shift in membrane potential. Membrane depolarization primarily causes the opening of voltage-gated sodium channels. When enough sodium channels open at the same time, positively charged sodium ions rush through them to the inside of the membrane. The driving force in this case is provided by the concentration gradient (there are many more positively charged sodium ions on the outside of the membrane than inside the cell) and the negative charge on the inside of the membrane (see Fig. 2). The flow of sodium ions causes an even greater and very rapid change in membrane potential, which is called action potential(in specialized literature it is designated as PD).

Rice. 3. A simple diagram showing a membrane with two sodium channels in the open and closed states, respectively.

According to the all-or-nothing law the cell membrane of excitable tissue either does not respond to the stimulus at all, or responds with the maximum strength possible for it at the moment. That is, if the stimulus is too weak and the threshold is not reached, the action potential does not occur at all; at the same time, a threshold stimulus will cause an action potential of the same amplitude as a stimulus exceeding the threshold. This does not mean that the amplitude of the action potential is always the same - the same section of the membrane, being in different states, can generate action potentials of different amplitudes.

After excitation, the neuron finds itself in a state of absolute refractoriness for some time, when no signals can excite it again, then enters a phase of relative refractoriness, when it can only be excited by strong signals (in this case, the AP amplitude will be lower than usual). The refractory period occurs due to inactivation of the fast sodium current, that is, inactivation of sodium channels.

Action potential- an excitation wave moving along the membrane of a living cell during the transmission of a nerve signal. In essence, it is an electrical discharge - a rapid short-term change in potential in a small area of ​​the membrane of an excitable cell (neuron, muscle fiber or glandular cell), as a result of which the outer surface of this area becomes negatively charged in relation to neighboring areas of the membrane, while its inner surface becomes positively charged charged in relation to neighboring areas of the membrane. The action potential is the physical basis of a nerve or muscle impulse that plays a signaling (regulatory) role.

The action potential develops on the membrane as a result of its excitation and is accompanied by a sharp change in membrane potential.

There are several phases in an action potential:

Depolarization phase;

Rapid repolarization phase;

Slow repolarization phase (negative trace potential);

Hyperpolarization phase (positive trace potential).

Depolarization phase. The development of AP is possible only under the influence of stimuli that cause depolarization of the cell membrane. When the cell membrane is depolarized to a critical depolarization level (CDL), an avalanche-like opening of the potential of sensitive Na+ channels occurs. Positively charged Na+ ions enter the cell along a concentration gradient (sodium current), as a result of which the membrane potential very quickly decreases to 0 and then becomes positive. The phenomenon of changing the sign of the membrane potential is called membrane charge reversal.

Phase of fast and slow repolarization. As a result of membrane depolarization, voltage-sensitive K+ channels open. Positively charged K+ ions leave the cell along a concentration gradient (potassium current), which leads to restoration of the membrane potential. At the beginning of the phase, the intensity of the potassium current is high and repolarization occurs quickly; towards the end of the phase, the intensity of the potassium current decreases and repolarization slows down. Repolarization is enhanced by the entry of Ca2+ into the cell. The hyperpolarization phase develops due to the residual potassium current and due to the direct electrogenic effect of the activated Na+/K+ pump. The entry of Cl– into the cell additionally hyperpolarizes the membrane. The change in the value of the membrane potential during the development of the action potential is associated primarily with a change in the permeability of the membrane for sodium and potassium ions.

Modern ideas about the mechanism of its generation

Using the method of fixing the membrane potential, it was possible to measure the currents flowing through the plasma membrane of the squid axon (axolemma) and make sure that at rest the current of cations (K +) is directed from the cytoplasm to the interstitium, and during excitation the current of cations (Na +) into the cell dominates. In a state of "rest" plasmalemma almost impermeable to ions located in the intercellular space (Na + C1 - and HCO3 - ,).

When excited, the permeability to sodium ions increases sharply for a period of several milliseconds and then decreases again.

5. Laws of irritation: Law of force. All or nothing law

1. The “all or nothing” law: With subthreshold stimulation of the cell or tissue, no response occurs. At the threshold strength of the stimulus, a maximum response develops, so an increase in the strength of stimulation above the threshold is not accompanied by its intensification. In accordance with this law, a single nerve and muscle fiber, the heart muscle, reacts to irritation.

2.Law of force: The greater the strength of the stimulus, the stronger the response. However, the severity of the response increases only to a certain maximum. Integral skeletal, smooth muscle is subject to the law of force, since they consist of numerous muscle cells with different excitability.

3.Law of force-duration. There is a certain relationship between the strength and duration of the stimulus. The stronger the stimulus, the less time it takes for a response to occur. The relationship between the threshold strength and the required duration of stimulation is reflected in the strength-duration curve. From this curve, a number of excitability parameters can be determined.

An action potential is a rapid change in membrane potential that occurs when nerve, muscle, and some glandular cells are excited. Its occurrence is based on changes in the ionic permeability of the membrane. There are four consecutive periods in the development of an action potential: 1) local response; 2) depolarization; 3) repolarization and 4) trace potentials (Fig. 2.11).

Local response is an active local depolarization resulting from an increase in sodium permeability of the cell membrane. A decrease in membrane potential is called depolarization. However, with a subthreshold stimulus, the initial increase in sodium permeability is not large enough to cause rapid membrane depolarization. A local response occurs not only at subthreshold, but also at suprathreshold

Rice. 2.11.

1 - local response; 2 - depolarization phase; 3 - repolarization phase; 4 - negative trace potential; 5 - positive (hyperpolarization) trace potential

stimulation and is a component of the action potential. Thus, the local response is the initial and universal form of tissue response to stimulation of varying strengths. The biological meaning of the local response is that if the stimulus is small in strength, then the tissue reacts to it with minimal energy expenditure, without turning on the mechanisms of specific activity. In the same case, when the stimulation is suprathreshold, the local response turns into an action potential. The period from the beginning of stimulation to the beginning of the depolarization phase, when the local response, increasing, reduces the membrane potential to a critical level (CLP), is called the latent or latent period, the duration of which depends on the strength of stimulation (Fig. 2.12).

Depolarization phase characterized by a rapid decrease in membrane potential and even recharging of the membrane: its inner part becomes positively charged for some time, and the outer part becomes negatively charged. The change in the sign of charge on the membrane is called perversion - reversion of potential. Unlike the local response, the speed and magnitude of depolarization do not depend on the strength of the stimulus. The duration of the depolarization phase in a frog nerve fiber is about 0.2-0.5 ms.

Duration repolarization phases is 0.5-0.8 ms. Restoring the original value of membrane polarization is called repolarization. During this time, the membrane potential

Rice. 2.12. Action potentials arising in response to threshold stimulation with short (A) and long-term (B) stimuli. Irritating stimuli, under the influence of which responses A and B are obtained: PP - resting potential; Ekud. - critical level of membrane depolarization (according to A.L. Katalymov)

cial is gradually restored and reaches 75-85% of the resting potential. In the literature, the second and third periods are often called peak of the action potential.

The fluctuations in membrane potential following the peak of the action potential are called trace potentials. There are two types of trace potentials - trace depolarization and trace hyperpolarization, which correspond to the fourth and fifth phases of the action potential. Trace depolarization (negative trace potential) is a continuation of the repolarization phase and is characterized by a slower (compared to the repolarization phase) restoration of the resting potential. The trace depolarization turns into a trace hyperpolarization (positive trace potential), which is a temporary increase in the membrane potential above the initial level. An increase in membrane potential is called hyperpolarization. In myelinated nerve fibers, trace potentials are more complex: trace depolarization can turn into trace hyperpolarization, then sometimes a new depolarization occurs, only after which the resting potential is completely restored.

Ionic mechanism of action potential occurrence. The basis of the action potential is the changes in the ionic permeability of the cell membrane that develop sequentially over time.

When a cell is exposed to an irritant, the permeability of the membrane for Na + ions sharply increases due to the activation (opening) of sodium channels.

In this case, Na + ions intensively move along the concentration gradient from outside to intracellular space. The entry of Na + ions into the cell is also facilitated by electrostatic interaction. As a result, the permeability of the membrane for Na + becomes 20 times greater than the permeability for K + ions.

At first, depolarization occurs relatively slowly. When the membrane potential decreases by 10-40 mV, the rate of depolarization increases sharply and the action potential curve rises steeply. The level of membrane potential at which the rate of membrane depolarization sharply increases due to the fact that the flow of Na + ions into the cell is greater than the flow of K + ions outward is called critical level of depolarization.

As the flow of Na + into the cell begins to exceed the potassium current from the cell, a gradual decrease in the resting potential occurs, leading to a reversion - a change in the sign of the membrane potential. In this case, the inner surface of the membrane becomes electropositive with respect to its outer electronegative surface. These changes in membrane potential correspond to the ascending phase of the action potential (depolarization phase).

The membrane is characterized by increased permeability to Na + ions only for a very short time (0.2-0.5 ms). After this, the permeability of the membrane for Na + ions decreases again, and for K + it increases. As a result, the flow of Na + into the cell is sharply weakened, and the flow of K + from the cell increases.

During an action potential, a significant amount of Na + enters the cell, and K + ions leave the cell. Restoration of the cellular ionic balance is carried out thanks to the work of the sodium-potassium pump, the activity of which increases with an increase in the internal concentration of Na + ions and an increase in the external concentration of K + ions. Thanks to the operation of the ion pump and the change in membrane permeability for Na + and K +, their concentration in the intra- and extracellular space is gradually restored.

The result of these processes is membrane repolarization: the internal contents of the cell again acquire a negative charge in relation to the outer surface of the membrane.

Trace negative potential is recorded during the period when NO + channels are inactivated and repolarization associated with the release of K + ions from the cell occurs more slowly than during the descending part of the action potential peak. This long-term preservation of the negativity of the outer surface of the excited area in relation to the non-excited one is called trace depolarization. Trace depolarization means that during this period the outer surface of the excitable formation has a less positive charge than at rest.

Trace positive potential corresponds to the period of increasing resting membrane potential, i.e. membrane hyperpolarization. During a trace positive potential, the outer surface of the cell is more positively charged than at rest. The trace positive potential is often called the trace hyperpolarization. It is explained by the long-term preservation of increased permeability for K + ions. As a result, a potential is established on the membrane equal to the equilibrium potential (for K + - 90 mV).

Changes in excitability during the development of excitation. By influencing stimuli of different strengths in different phases of the action potential, it is possible to trace how excitability changes during excitation. In Fig. 2.13" it is clear that the period of the local response is characterized by increased excitability (the membrane potential approaches the critical level of depolarization); during the depolarization phase, the membrane loses excitability (the cell becomes refractory), which is gradually restored during repolarization.

Highlight absolute refractory period, which lasts about 1 ms in nerve cells and is characterized by their complete inexcitability. The period of absolute refractoriness occurs as a result of almost complete inactivation (impermeability) of sodium channels and an increase in potassium conductance of the membrane. Even at rest, not all membrane channels are activated; 40% of them are in a state of inactivation. During depolarization, the number of inactivated channels increases and the peak of the action potential corresponds to the inactivation of all sodium channels.

As the membrane repolarizes, sodium channels are reactivated. This relative refractory period: an action potential can only occur when exposed to stronger (suprathreshold) stimuli.

IN period of negative trace potential the phase of relative refractoriness is replaced by a phase of increased (supernormal) excitability. During this period, the threshold of irritation is reduced compared to the initial value, since the membrane potential is closer to the critical value than at rest (Fig. 2.14).

The phase of trace hyperpolarization, caused by the residual release of potassium from the cell, on the contrary, is characterized by a decrease

Rice. 2.13.

A - components of the excitation wave: 1 - depolarization; 2 - repolarization; MP - membrane potential; mV - microvolt; MK - critical level of depolarization: a - duration of the threshold potential; b - action potential duration; c - trace negativity; r - trace positivity; B - changes in excitability in different phases of the excitation wave; EF - level of excitability at rest: a - increase in excitability during the period of threshold potential; b - drop in excitability to zero during the occurrence of an action potential (absolute refractoriness); c, - return of excitability to the initial level during trace negativity (relative refractoriness); c 2 - increase in excitability during the period of the end of trace negativity (exaltation or supernormality); c - the entire period of trace negativity; d - drop in excitability during the period of hyperpolarization (subnormality)

excitability. Since the membrane potential is greater than at rest, a stronger stimulus is required to “shift” it to the level of critical depolarization.

Thus, in the dynamics of the excitatory process, the ability of the cell to respond to stimuli changes, i.e. excitability.

Rice. 2.14.

The magnitude of the membrane potential: E 0 - at rest; - in the exaltation phase; E 2 - in the hyperpolarization phase. The threshold potential value: e 0 - at rest; e, - in the exaltation phase; e 2 - in the hyperpolarization phase

This is of great importance because at the moment of greatest excitation (the peak of the action potential) the cell becomes completely inexcitable, which protects it from death and damage.

• See: Leontyeva N.N., Marinova K.V. Decree. op.
• Right there.