action potential. The action potential of a nerve cell

Action potential (AP) called the rapid fluctuation of the membrane potential that occurs when the excitation of nerve, muscle and some other cells. It is based on changes in the ionic permeability of the membrane. The AP amplitude depends little on the strength of the stimulus that causes it, it is only important that this strength be not less than a certain critical value, which is called irritation threshold. Having arisen at the site of irritation, AP propagates along the nerve or muscle fiber without changing its amplitude.

Under natural conditions, APs are generated in nerve fibers upon stimulation of receptors or excitation of nerve cells. The distribution of AP along nerve fibers ensures the transmission of information in the nervous system. Upon reaching the nerve endings, PD cause secretion chemical substances(mediators) that provide signal transmission to muscle or nerve cells. In muscle cells, AP initiate a chain of processes that cause a contractile act. Ions penetrating into the cytoplasm during AP generation have a regulatory effect on cell metabolism and, in particular, on the processes of protein synthesis that make up ion channels and ion pumps.

Rice. 3. Skeletal muscle fiber action potential , registered with an intracellular microelectrode: a – depolarization phase, b – repolarization phase, c – trace depolarization phase (negative trace potential). The moment of application of irritation is shown by an arrow.

It has been established that during the ascending phase (the phase of depolarization), not only does the resting potential disappear (as was originally assumed), but a potential difference arises opposite sign: the internal content of the cell becomes positively charged with respect to the external environment, in other words, the membrane potential is reversed . During the descending phase (repolarization phase), the membrane potential returns to its original value. If we consider an example of AP recording in a skeletal muscle fiber of a frog (see Fig. 3), it can be seen that at the moment the peak is reached, the membrane potential is +30–+40 mV. The duration of the AP peak in various nerve and muscle fibers varies from 0.5 to 3 ms, and the repolarization phase is longer than the depolarization phase.

Changes in membrane potential following the peak of an action potential are called trace potentials. . There are two types of trace potentials - trace depolarization and trace hyperpolarization.

The ionic mechanism of PD occurrence . As noted, at rest, the permeability of the membrane to potassium exceeds its permeability to sodium. As a result, the flow of K + from the cytoplasm into the external solution exceeds the oppositely directed flow of Na + . Therefore, the outer side of the membrane at rest has a positive potential relative to the inner.

Under the action of an irritant on the cell, the permeability of the membrane for Na + increases sharply and becomes approximately 20 times greater than the permeability for K +. Therefore, the flow of Na + from the external solution into the cytoplasm begins to exceed the outward potassium current. This leads to a sign change (reversion) of the membrane potential: inner side membrane at the site of excitation becomes positively charged with respect to its outer surface. This change in the membrane potential corresponds to the ascending phase of AP (depolarization phase).

The increase in membrane permeability for Na + continues only for a very a short time. Following this, the permeability of the membrane for Na + again decreases, and for K + increases. The process leading to a decrease in the previously increased sodium permeability of the membrane is called sodium inactivation. . As a result of inactivation, the flow of Na + into the cytoplasm is sharply weakened. An increase in potassium permeability causes an increase in the flow of K + from the cytoplasm into the external solution. As a result of these two processes, membrane repolarization occurs: the inner contents of the cell again acquire a negative charge in relation to the outer side of the membrane. This potential change corresponds to the descending phase of AP (repolarization phase). Experiments on giant nerve fibers of the squid made it possible to confirm the correctness of the sodium theory of the occurrence of AP.

AP occurs when the surface membrane is depolarized . Small amounts of depolarization lead to the opening of part of the sodium channels and a slight penetration of Na ions into the cell. These reactions are subthreshold and cause only local changes on the membrane. ( local response ). With an increase in the strength of stimulation, when the threshold of excitability is reached, changes in the membrane potential reach a critical level of depolarization (CUD). For example, the value of the resting potential is -70 mV, KUD = -50 mV. To cause excitation, it is necessary to depolarize the membrane to -50 mV, i.e. by -20 mV to reduce its initial resting potential. Only upon reaching the KUD is observed abrupt change membrane potential, which is recorded in the form of PD. Thus, the main condition for the occurrence of an action potential is a decrease in the membrane potential to a critical level of depolarization.

The considered changes in the ionic permeability of the membrane during AP generation are based on the processes of opening and closing of specialized ion channels in the membrane, which have two important properties:

■ selectivity (selectivity) in relation to certain ions;

■ electrical excitability, i.е. the ability to open and close in response to changes in membrane potential.

Like ion pumps, ion channels are formed by protein macromolecules penetrating the lipid bilayer of the membrane.

Active and passive ion transport. In the process of recovery after PD, the operation of the sodium-potassium pump ensures that excess sodium ions are "pumped out" and the lost potassium ions are "pumped" inward, due to which the inequality of Na + and K + concentrations on both sides of the membrane, disturbed during excitation, is restored. About 70% of the energy needed by the cell is spent on the operation of this mechanism.

Thus, in a living cell, there are two systems for the movement of ions through the membrane.

One of them is carried out along the ion concentration gradient and does not require energy (passive ion transport). It is responsible for the occurrence of the resting potential and AP and ultimately leads to an equalization of the concentration of ions on both sides of the cell membrane.

The second is carried out against the concentration gradient. It consists in "pumping out" sodium ions from the cytoplasm and "forcing" potassium ions into the cell. This type of ion transport is possible only if the energy of metabolism is consumed. He's called active ion transport. It is responsible for maintaining the constancy of the difference in ion concentrations between the cytoplasm and the fluid surrounding the cell. Active transport is the result of the work of the sodium pump, due to which the initial difference in ionic concentrations, which is violated with each burst of excitation, is restored.

Carrying out excitation

A nerve impulse (action potential) has the ability to propagate along the nerve and muscle fibers.

In a nerve fiber, the action potential is a very strong stimulus for neighboring sections of the fiber. The amplitude of the action potential is usually 5 to 6 times the depolarization threshold. This provides high speed and reliability of implementation.

Between the excitation zone (which has a negative charge on the fiber surface and a positive charge on the inner side of the membrane) and the adjacent unexcited section of the nerve fiber membrane (with an inverse charge ratio), electric currents arise - the so-called local currents . As a result, depolarization of the neighboring area develops, an increase in its ion permeability and the appearance of an action potential. In the original zone of excitation, the resting potential is restored. Then the next section of the membrane is covered by excitation, and so on. Thus, with the help of local currents, excitation spreads to neighboring sections of the nerve fiber, i.e. conduction of a nerve impulse . The amplitude of the action potential does not decrease as the conduction progresses. , those. excitation does not fade even with a large length of the nerve.

In the process of evolution, with the transition from non-fleshy nerve fibers to pulpy ones (covered with a myelin sheath), there was a significant increase in the speed of nerve impulse conduction. The non-medullary fibers are characterized by continuous conduction of excitation, which sequentially covers each adjacent section of the nerve. The fleshy nerves are almost completely covered by an insulating myelin sheath. Ionic currents in them can pass only in the bare sections of the membrane - the intercepts of Ranvier, devoid of this shell. When conducting a nerve impulse, the action potential jumps from one interception to another and can even cover several intercepts. Such a conduct taught the name of somersault (Latin somersault - jump). This increases not only the speed, but also the cost-effectiveness of the implementation. Excitation captures not the entire surface of the fiber membrane, but only a small part of it. Hence, less energy is spent on the active transport of ions across the membrane during excitation and in the process of recovery.

The speed of conduction in different fibers is different. Thicker nerve fibers conduct excitation at a greater speed: they have greater distances between the nodes of Ranvier and longer jumps. Top speed conduction have motor and proprioceptive afferent nerve fibers - up to 100 m / s. In thin sympathetic nerve fibers (especially in unmyelinated fibers), the conduction velocity is low - on the order of 0.5 - 15 m/s.

During the development of the action potential, the membrane completely loses excitability. This state is called complete non-excitability, or absolute refractoriness. . It is followed by relative refractoriness, when the action potential can only occur with very strong irritation. Gradually, excitability is restored to its original level.

Laws of conducting excitation in nerves :

1. Conduction of impulses is possible only under the condition of anatomical and physiological integrity of the fiber.

2. Bilateral conduction: when a nerve fiber is irritated, excitation spreads along it in both centrifugal and centripetal directions.

3. Isolated conduction: in the peripheral nerve, impulses propagate along each fiber in isolation, i.e. without passing from one fiber to another and exerting an effect only on those cells with which the endings of this nerve fiber come into contact.

13. Define homeostasis.

14. Name the main ways of regulation of various functions in highly organized animals and humans.

15. By whom and when was "animal electricity" discovered?

16. What tissues are excitable? Why are they called that?

17. Name the main functional characteristics of excitable tissues.

18. What is called the threshold of excitability?

19. On what factors does the threshold value depend?

20. What is lability? Who put forward the concept of lability, what properties of excitable tissues does it characterize?

21. What is called the membrane potential (resting potential)?

22. What causes the presence of electrical potentials in living cells?

23. In what cases is it said about depolarization (or hyperpolarization) of the cell membrane?

24. What role does the potassium-sodium pump of the membrane play in the formation of the resting potential?

25. What is called an action potential? What is its role in the nervous system?

26. What underlies the emergence of an action potential?

27. Describe the phases of the action potential.

28. What is called membrane potential reversion?

29. Describe the ionic mechanism of action potential occurrence.

30. What is meant by sodium inactivation?

31. What is the critical level of depolarization?

32. What are the properties of the ion channels of the cell membrane?

33. Describe two types of ion transport in a cell:

■ passive;

■ active.

Module 1 GENERAL CNS PHYSIOLOGY

action potential

The physical basis of excitation is the action potential. In essence, the action potential is an electrical discharge - a quick 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, the outer surface of this region becomes negatively charged with respect to neighboring regions of the membrane, while its inner surface becomes positively charged with respect to neighboring regions of the membrane. The action potential is physical basis nerve or muscle impulse.

If an electrode is inserted into a living cell and the resting membrane potential is measured, it will have negative meaning(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 one, although both sides contain both cations and anions. Outside - an order of magnitude more sodium, calcium and chlorine ions, inside - potassium ions and negatively charged protein molecules, amino acids, organic acids, phosphates, sulfates. It must be understood that we are talking 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 electricity applied to the outer or inner side of the membrane through the electrode. Under natural conditions, the stimulus is often a chemical signal from neighboring cells, coming through the synapse or by diffuse transmission through the intercellular medium. The shift of the membrane potential can occur in a negative (hyperpolarization) or positive (depolarization) direction.

For specifics, consider nerve cells. In the nervous tissue, the action potential, as a rule, occurs during depolarization. According to the degree of depolarization, stimuli can be subthreshold, threshold and suprathreshold. When exposed to subthreshold stimuli, the so-called local response occurs - a local slight depolarization of the membrane, characterized by such properties as decrement, summation and graduality.

If the depolarization of the neuron membrane reaches a certain threshold level or exceeds it (threshold and superthreshold stimuli), the cell is excited, and an electrical signal wave, an action potential, propagates from its body to axons and dendrites (Fig. 3). This is due to the presence of ion channels on the cell membrane. The cell membrane of excitable tissues (nervous, secretory and muscle) contains a large number of voltage-gated ion channels capable of rapidly responding to shifts in membrane potential. Membrane depolarization primarily causes voltage-gated sodium channels to open. When enough sodium channels open at the same time, positively charged sodium ions rush through them to the inside of the membrane.

Rice. 3.

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. The flow of sodium ions causes an even greater and very rapid change in the membrane potential, which is called the action potential (in the special literature it is denoted as AP).

Upon reaching the value of the membrane potential of 0 mV, depolarization continues, passing into the stage of reversion (recharging). At this moment, potassium potential - dependent channels (slow relative to sodium channels) are included in the formation of AP, and sodium channels pass into an inactivated state (close). Upon reaching the membrane potential peak value- about 30 mV - there is an increase in the restoration of its value - repolarization due to the current of K ions in the opposite direction relative to Na (from the cell along the concentration gradient to the intercellular medium). When the initial value of the membrane potential is reached, a short hyperpolarization occurs due to the flow of Cl ions into the cell (Fig. 4).

Rice. 4.

According to the “all-or-nothing” law, the cell membrane of an excitable tissue either does not respond to the stimulus at all, or responds with the maximum possible for it to this moment by force. That is, if the stimulus is too weak and the threshold is not reached, the action potential does not arise at all; at the same time, a threshold stimulus will elicit an action potential of the same amplitude as a stimulus above 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.

A detailed consideration of AP can distinguish 6 phases of its development (Fig. 5).

1. Slow depolarization - from MP to the critical level of depolarization (CDL), in fact, is a local response to a threshold stimulus.

2. Rapid depolarization - from FCA to 0 mV, caused by an avalanche-like flow of Na ions into the cell.

3. Reversion (overshoot, overlap) - from 0 mV to the peak of depolarization, K channels open, Na channels are inactivated.

4. Rapid repolarization - from the peak of depolarization to AUD, caused by the current of K ions from the cell.

5. Slow repolarization - from AC to MP.

6. Hyperpolarization - overlap through the MP with the restoration of its value, caused by the current of Cl ions into the cell.

Rice. 5.

refractoriness and excitability

Inactivation of the sodium system during the generation of the action potential leads to the fact that the cell cannot be re-excited during this period, i.e., a state of absolute refractoriness is observed. The gradual recovery of the resting potential during repolarization makes it possible to evoke a repeated action potential, but this requires a suprathreshold stimulus, since the cell is in a state of relative refractoriness.

The study of cell excitability during a local response or during a negative trace potential showed that the generation of an action potential is possible when the stimulus is below the threshold value. This is a state of supernormality (in the phase of slow repolarization), or exaltation (in the phase of slow depolarization). Finally, the hyperpolarization phase reduces excitability and manifests itself as a subnormal period.

The duration of the period of absolute refractory limits the maximum frequency of generation of action potentials by this cell type. For example, with an absolute refractory period of 4 ms, the maximum frequency is 250 Hz.

Rice. 6.

N. E. Vvedensky introduced the concept of lability, or functional mobility, of excitable tissues. A measure of lability is the number of action potentials that an excitable tissue can generate per unit time. Obviously, the lability of excitable tissue is primarily determined by the duration of the refractory period. The most labile are the fibers of the auditory nerve, in which the frequency of action potential generation reaches 1000 Hz.

An action potential is a rapid change in the 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. In the development of the action potential, four successive periods are distinguished: 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 the sodium permeability of the cell membrane. The 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 - phase of repolarization; 4 - negative trace potential; 5 - positive (hyperpolarization) trace potential

stimulus and is an integral component of the action potential. Thus, the local response is the initial and universal form of tissue response to stimuli of various strengths. The biological meaning of the local response is that if the stimulus is small in strength, then the tissue reacts to it with a minimum expenditure of energy, not including the mechanisms of specific activity. In the same case, when the irritation is suprathreshold, the local response turns into an action potential. The period from the onset of stimulation to the onset of the depolarization phase, when the local response, increasing, reduces the membrane potential to a critical level (KUD), is called the latent or latent period, the duration of which depends on the strength of the stimulation (Fig. 2.12).

Depolarization phase characterized by a rapid decrease in the 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 the charge on the membrane is called perversion - reversion of potential. Unlike a local response, the rate and magnitude of depolarization do not depend on the strength of the stimulus. The duration of the depolarization phase in the frog nerve fiber is about 0.2-0.5 ms.

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

Rice. 2.12. Action potentials arising in response to threshold stimulation by short (A) and long-term (B) stimuli Annoying stimuli, under the influence of which responses A and B were 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 action potential peak.

The fluctuations in the 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. The trace depolarization (negative trace potential) is a continuation of the repolarization phase and is characterized by a slower (compared to the repolarization phase) recovery of the resting potential. The trace depolarization turns into 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 that the resting potential is fully restored.

The ionic mechanism of the action potential. The action potential is based on changes in the ionic permeability of the cell membrane that develop sequentially over time.

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

At the same time, Na + ions intensively move along the concentration gradient from outside to the 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 proceeds 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 increases sharply, 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.

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 electropositive with respect to its outer electronegative surface. These changes in the membrane potential correspond to the ascending phase of the action potential (depolarization phase).

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

During the action potential, a significant amount of Na + enters the cell, and K + ions leave the cell. Restoration of cellular ion balance is carried out due 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. Due to the work of the ion pump and the change in the permeability of the membrane for Na + and K +, their concentration in the intra- and extracellular space is gradually restored.

The result of these processes is the repolarization of the membrane: the inner 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 the Na + 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 unexcited one is called after depolarization. Trace depolarization means that during this period the outer surface of the excitable formation has a lower positive charge than at rest.

Trace positive potential corresponds to the period of increase in the 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 referred to as the trace hyperpolarization. It is explained by the long-term preservation of increased permeability for K + ions. As a result, a potential equal to the equilibrium potential is established on the membrane (for K + - 90 mV).

Changes in excitability in the process of development of excitation. Influencing stimuli of different strengths in different phases of the action potential, one can trace how excitability changes during excitation. On fig. 2.13" it can be seen that the period of the local response is characterized by increased excitability (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.

Allocate absolute refractory period, which lasts about 1 ms in nerve cells and is characterized by their complete non-excitability. The period of absolute refractoriness occurs as a result of the almost complete inactivation (impermeability) of sodium channels and an increase in the potassium conductivity 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 peak corresponds to the inactivation of all sodium channels.

As the membrane repolarizes, sodium channels are reactivated. This is relative refractory period: an action potential can only arise under the action of stronger (above-threshold) stimuli.

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

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

Rice. 2.13.

A - components of the excitation wave: 1 - depolarization; 2 - repolarization; MP - membrane potential; mV - microvolt; MC - critical level of depolarization: a - threshold potential duration; b - duration of the action potential; c - trace negativity; d - trace positivity; B - changes in excitability in different phases of the excitation wave; HC - the level of excitability at rest: a - an increase in excitability during the period of threshold potential; b - drop in excitability to zero during the course of the action potential (absolute refractoriness); c, - return of excitability to the initial level during trace negativity (relative refractoriness); in 2 - an increase in excitability during the period of the end of trace negativity (exaltation "or supernormality); in - the entire period of trace negativity; d - a decrease 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 value of the membrane potential: E 0 - at rest; - in the exaltation phase; E 2 - in the phase of hyperpolarization. The value of the threshold potential: e 0 - at rest; e, - in the phase of exaltation; e 2 - in the phase of hyperpolarization

It has great importance, since at the moment of greatest excitation (peak of the action potential) the cell becomes absolutely non-excitable, which protects it from death and damage.

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

The work of the organs and tissues of our body depends on many factors. Some cells (cardiomyocytes and nerves) depend on the transmission of nerve impulses generated in special cell components or nodes. It is based on the formation of a specific wave of excitation, called the action potential.

What it is?

An action potential is called a wave of excitation that moves from cell to cell. Due to its formation and passage through, a short-term change in their charge occurs (normally, the inner side of the membrane is negatively charged, and the outer side is positively charged). The generated wave contributes to a change in the properties of the ion channels of the cell, which leads to the recharging of the membrane. At the moment when the action potential passes through the membrane, there is a short-term change in its charge, which leads to a change in the properties of the cell.

The formation of this wave underlies the functioning of the system of pathways of the heart.

If its formation is disturbed, many diseases develop, which makes the determination of the action potential necessary in the complex of diagnostic and treatment measures.

How is the action potential formed and what is characteristic of it?

Research History

The study of the occurrence of excitation in cells and fibers was begun quite a long time ago. The first to notice its occurrence were biologists who studied the effects of various stimuli on the frog's exposed tibial nerve. They noticed that when exposed to a concentrated solution of edible salt, muscle contraction was observed.

In the future, research was continued by neurologists, but the main science after physics that studies the action potential is physiology. It was physiologists who proved the presence of an action potential in heart cells and nerves.

As we delved deeper into the study of potentials, the presence of the resting potential was also proved.

From the beginning of the 19th century, methods began to be created that made it possible to fix the presence of these potentials and measure their magnitude. Currently, the fixation and study of action potentials is carried out in two instrumental studies - the removal of electrocardiograms and electroencephalograms.

Action potential mechanism

The formation of excitation occurs due to changes in the intracellular concentration of sodium and potassium ions. Normally, the cell contains more potassium than sodium. The extracellular concentration of sodium ions is much higher than in the cytoplasm. Changes caused by the action potential contribute to a change in the charge on the membrane, resulting in the flow of sodium ions into the cell. Because of this, the charges outside change and inside it is charged positively, and external environment- negatively.

This is done to facilitate the passage of the wave through the cell.

After the wave has been transmitted through the synapse, the charge is reversed due to the current inside the cell of negatively charged chloride ions. The initial charge levels outside and inside the cell are restored, which leads to the formation of a resting potential.

Periods of rest and excitation alternate. In a pathological cell, everything can happen differently, and the formation of AP there will obey somewhat different laws.

PD phases

The course of an action potential can be divided into several phases.

The first phase proceeds until formation (a slow discharge of the membrane is stimulated by the passing action potential, which reaches maximum level, it is usually around -90 meV). This phase is called the prespike. It is carried out due to the entry of sodium ions into the cell.

The next phase - the peak potential (or spike), forms a parabola with an acute angle, where the ascending part of the potential means membrane depolarization (fast), and the descending part - repolarization.

The third phase - negative trace potential - shows trace depolarization (transition from the peak of depolarization to the state of rest). Caused by the entry of chloride ions into the cell.

At the fourth stage, the phase of the positive trace potential, the charge levels of the membrane return to the initial level.

These phases, determined by the action potential, strictly follow one after the other.

Action potential functions

Undoubtedly, the development of the action potential is important in the functioning of certain cells. Excitation plays a major role in the work of the heart. Without it, the heart would simply be an inactive organ, but due to the propagation of the wave through all the cells of the heart, it contracts, which helps to push the blood through the vascular bed, enriching all tissues and organs with it.

Also, it could not normally perform its function without an action potential. Organs could not receive signals to perform a particular function, as a result of which they would simply be useless. In addition, the improvement in the transmission of a nerve impulse in nerve fibers (the appearance of myelin and interceptions of Ranvier) made it possible to transmit a signal in a matter of fractions of a second, which led to the development of reflexes and conscious movements.

In addition to these organ systems, the action potential is also formed in many other cells, but in them it plays a role only in the performance of the cell's specific functions.

Occurrence of an action potential in the heart

The main organ whose work is based on the principle of action potential formation is the heart. Due to the existence of nodes for the formation of impulses, the work of this organ is carried out, the function of which is to deliver blood to tissues and organs.

The action potential is generated in the heart at the sinus node. It is located at the confluence of the vena cava in the right atrium. From there, the impulse propagates along the fibers of the conduction system of the heart - from the node to the atrioventricular junction. Passing more precisely, along its legs, the impulse passes to the right and left ventricles. In their thickness are smaller pathways - Purkinje fibers, through which excitation reaches every cell of the heart.

The action potential of cardiomyocytes is composite, i.e. depends on the contraction of all cells of the heart tissue. In the presence of a block (a scar after a heart attack), the formation of an action potential is disturbed, which is recorded on the electrocardiogram.

Nervous system

How is PD formed in neurons - cells nervous system. Here everything is carried out a little easier.

An external impulse is perceived by processes of nerve cells - dendrites associated with receptors located both in the skin and in all other tissues (resting potential and action potential also replace each other). Irritation provokes the formation of an action potential in them, after which the impulse goes through the body of the nerve cell to its long process - the axon, and from it through the synapses to other cells. Thus, the formed wave of excitation reaches the brain.

A feature of the nervous system is the presence of two types of fibers - covered with myelin and without it. The occurrence of an action potential and its transmission in those fibers where there is myelin is carried out much faster than in demyelinated ones.

This phenomenon is observed due to the fact that the propagation of AP along myelinated fibers occurs due to “jumps” - the impulse jumps over the myelin sections, which, as a result, reduces its path and, accordingly, accelerates its propagation.

resting potential

Without the development of the resting potential, there would be no action potential. The resting potential is understood as the normal, unexcited state of the cell, in which the charges inside and outside its membrane are significantly different (that is, the membrane is positively charged outside and negatively charged inside). The resting potential shows the difference between the charges inside and outside the cell. Normally, it ranges from -50 to -110 meV. in nerve fibers given value usually equal to -70 meV.

It is due to the migration of chloride ions into the cell and the creation of a negative charge on the inside of the membrane.

When the concentration of intracellular ions changes (as mentioned above), PP replaces AP.

Normally, all cells of the body are in an unexcited state, therefore, a change in potentials can be considered a physiologically necessary process, since without them the cardiovascular and nervous systems could not carry out their activities.

Significance of research on resting and action potentials

The resting potential and the action potential allow you to determine the state of the body, as well as individual organs.

Fixing the action potential from the heart (electrocardiography) allows you to determine its condition, as well as the functional ability of all its departments. If you study a normal ECG, you can see that all the teeth on it are a manifestation of the action potential and the subsequent resting potential (respectively, the occurrence of these potentials in the atria displays the P wave, and the spread of excitation in the ventricles - the R wave).

As for the electroencephalogram, the occurrence of various waves and rhythms on it (in particular, alpha and beta waves in a healthy person) is also due to the occurrence of action potentials in brain neurons.

These studies allow timely detection of the development of a particular pathological process and determine almost 50 percent of the successful treatment of the underlying disease.

Action potential (AP)- this is an electrophysiological process, expressed in a rapid fluctuation of the membrane potential due to the movement of ions into and out of the cell and capable of spread without decrement(no fade). PD provides signal transmission between nerve cells, nerve centers and working organs; in muscles, AP provides the process of electromechanical coupling.

BUT. Characteristics of the action potential (AP). Schematically, PD is shown in fig. 1.3. The AP value fluctuates in the range of 80-130 mV, the duration of the peak AP of the nerve fiber is 0.5-1 ms, the skeletal muscle fiber is up to 10 ms, taking into account the slowing down of depolarization at its end. The duration of AP of the heart muscle, 300-400 ms. The amplitude of AP does not depend on the strength of stimulation - it is always maximum for a given cell under specific conditions: AP obeys the "all or nothing" law, but does not obey the law of power relations - the law of force. AP either does not occur at all when the cell is stimulated, if it is small, or it occurs and reaches a maximum value if the irritation is threshold or suprathreshold.

It should be noted that weak (subthreshold) irritation can cause local potential. It obeys the law of force - with an increase in the strength of the stimulus, its magnitude increases.

There are four phases in PD:

1 - depolarization, i.e., the disappearance of the cell charge - a decrease in the membrane potential to zero;

2 - inversion, i.e., a change in the charge of the cell to the opposite, when the inner side of the cell membrane is charged positively, and the outer side is negatively charged (lat. shuegzyu - turning over);

3 - repolarization, i.e., restoration of the initial charge of the cell, when the inner surface of the cell membrane is again charged negatively, and the outer - positively;

4 - trace hyperpolarization.

B. Mechanism of occurrence of PD. If the action of the stimulus on the cell membrane leads to the onset of AP development, then the process of AP development itself causes phase changes in the permeability of the cell membrane, which ensures the rapid movement of Na + into the cell, and K + - out of the cell. This is the most common variant of the occurrence of PD. The value of the membrane potential at the same time first decreases, and then recovers again to its original level.

On the oscilloscope screen, the marked changes in the membrane potential appear as a peak potential - PD. It arises as a result of ion concentration gradients accumulated and maintained by ion pumps inside and outside the cell, i.e. due to potential energy in the form of electrochemical gradients of ions. If you block the process of energy production, action potentials will occur for a certain period of time. But after the disappearance of ion concentration gradients (elimination of potential energy), the cell will not generate AP. Consider the phases of PD.

1. Depolarization phase(see fig. 1.3 - 1). Under the action of a depolarizing stimulus on the cell (mediator, electric current), the initial partial depolarization of the cell membrane occurs without changing its permeability to ions. When depolarization reaches approximately 50% of the threshold value (50% of the threshold potential), the permeability of the cell membrane for Na + begins to increase, and at the first moment, relatively slowly.

Naturally, the rate of Na+ entry into the cell is low in this case. During this period, as well as during the entire first phase (depolarization), the driving force that ensures the entry of Hg!a + into the cell, are the concentration and electrical gradients. Recall that the cell inside is negatively charged (opposite charges are attracted to each other), and the concentration of Na + outside the cell is 10-12 times greater than inside the cell.

Condition ensuring the entry of No + into the cell is an increase in the permeability of the cell membrane, which is determined by the state of the gate mechanism of the Na channels (in some cells, for example, in cardiomyocytes, in smooth muscle fibers, important role in the emergence of PD, controlled channels for Ca 2+ also play).

When the cell depolarization reaches a critical value (E, the critical level of depolarization - CUD), which is usually 50 mV (other values ​​are possible), the permeability of the membrane for Na * increases sharply - it opens big number voltage-dependent gates of the Na-channels - and Na + rushes into the cell like an avalanche.

As a result of the intense flow of Na + into the cell, the process of depolarization proceeds very quickly. Developing depolarization of the cell membrane causes additional an increase in its permeability and, of course, the conductivity of Na + - more and more gates of Na channels open, which gives the current Na + into the cell a character regenerative process. As a result, the PP disappears and becomes equal to zero. The depolarization phase ends here.

2. phase of inversion. After the disappearance of the PP, the entry of Na + into the cell continues, therefore the number of positive ions in the cell exceeds the number of negative ions, the charge inside the cell becomes positive, outside - negative. The process of recharging the membrane is the second phase of the action potential - the phase of inversion (Fig. 1.3 - 2).

Now the electrical gradient prevents the entry of Na + into the cell (positive charges repel each other), Na-conductivity decreases. Nevertheless, for a certain period of time (fractions of a millisecond) N+ continues to enter the cell, which is evidenced by the continuing increase in AP. This means that the concentration gradient, which ensures the movement of Na + into the cell, is stronger than the electric one, which prevents the entry of Na + into the cell.

During membrane depolarization, its permeability for Ca 2+ also increases, it also goes into the cell, but in nerve fibers, neurons and skeletal muscle cells, the role of Ca 2+ in the development of PD is small.a. In smooth muscle cells and myocardium, its role is essential. Thus, the entire ascending part of the AP peak in most cases is provided mainly by the entry of N + into the cell.

Approximately 0.5-1 ms or more after the onset of depolarization (this time depends on the type of cell), the growth of AP stops due to the closing of the gates of sodium channels and the opening of the gates of K-channels, i.e., an increase in the permeability for K + and a sharp increase its exit from the cell (see Fig. 1.3 - 2). The growth of the AP peak is also prevented by the electrical gradient of Na + (the cell inside is positively charged at this moment), as well as the release of K + from the cell through leakage channels.

Since K + is located predominantly inside the cell, it, according to the concentration gradient, quickly leaves the cell after the gates of K + channels open, as a result of which the number of positively charged ions in the cell decreases. The charge of the cell starts to decrease again. In the inversion phase, the release of K + from the cell is also facilitated by the electrical gradient. K+ is pushed out of the cell by the positive charge and is attracted by the negative charge outside the cell.

This continues until the complete disappearance of the positive charge inside the cell (until the end of the inversion phase - Fig. 1.3-2, dotted line), when the next phase of AP begins - the repolarization phase. Potassium leaves the cell not only through controlled channels, the gates of which are open, but also through uncontrolled channels - leakage channels, which somewhat slows down the ascending part of the AP and accelerates the course of the descending component of the AP.

Thus, a change in the resting membrane potential leads to the sequential opening and closing of the electrically controlled gates of ion channels and the movement of ions according to the electrochemical gradient - the emergence of AP. All phases are regenerative - it is only necessary to reach a critical level of depolarization, then AP develops due to the potential energy of the cell in the form of electrochemical gradients, i.e., it is secondary active.

The AP amplitude is the sum of the PP value (membrane potential of the resting cell) and the inversion phase value, which is 10–50 mV for different cells. If the membrane potential of a resting cell is small, the AP amplitude of this cell is not large.

3. Repolarization phase(Fig. 1.3-3) is due to the fact that the permeability of the cell membrane for K + is still high (the gates of potassium channels are open), K + continues to quickly leave the cell, according to the concentration gradient. Since the cell now again has a negative charge inside, and a positive charge outside (see Fig. 1.3 - 3), the electrical gradient prevents K + from leaving the cell, which reduces its conductivity, although it continues to exit.

This is explained by the fact that the action of the concentration gradient is much more pronounced than the electric gradient. The entire descending part of the AP peak is due to the release of K + from the cell. Often, at the end of AP, there is a slowdown in repolarization, which is explained by a decrease in the permeability of the cell membrane for K + and a slowdown in its release from the cell due to partial closure of the K-channel gates. The second reason for the slowing down of the K + current from the cell is associated with an increase in the positive potential of the outer surface of the cell and the formation of an oppositely directed electrical gradient.

Thus, leading role in the occurrence of PD plays Ya + , entering the cell with an increase in the permeability of the cell membrane and providing the entire ascending part of the AP peak. When Ma + is replaced in the medium by another ion, for example, choline, PD does not occur in the nerve and muscle cells of the skeletal muscles. However, the permeability of the membrane for K + also plays an important role. If the increase in permeability for K + is prevented by tetraethylammonium, the membrane, after its depolarization, repolarizes much more slowly, only due to slow uncontrolled channels (ion leakage channels) through which K + will leave the cell.

Role of Ca 2+ in the occurrence of PD in the nerve and muscle cells of the skeletal muscles is insignificant. However, Ca 2+ plays an important role in the occurrence of AP in cardiac and smooth muscles, in the transmission of impulses from one neuron to another, from a nerve fiber to a muscle fiber, and in the provision of muscle contraction.

4. trace hyperpolarization cell membrane (Fig. 1.3-4) is usually a consequence of the still remaining increased permeability of the cell membrane for K +, it is characteristic of neurons. The gates of K-channels are not yet completely closed, so K + continues to leave the cell according to the concentration gradient, which leads to hyperpolarization of the cell membrane.

Gradually, the permeability of the cell membrane returns to its original state (sodium and potassium gates return to their original state), and the membrane potential becomes the same as it was before cell excitation. The Na/K-pump is not directly responsible for the phases of the action potential, although it continues to work during the development of PD.

trace depolarization is also characteristic of neurons; it can also be registered in skeletal muscle cells. Its mechanism has not been studied enough. Perhaps this is due to a short-term increase in the permeability of the cell membrane for Na + and its entry into the cell according to the concentration and electrical gradients.

AT. stock of ions in the cell providing the occurrence of excitation (AP), is huge. The concentration gradients of ions practically do not change as a result of one excitation cycle. The cell can be excited up to 510 times without recharging, that is, without the operation of the Na / K-pump.

The number of impulses that a nerve fiber generates and conducts depends on its thickness, which determines the supply of ions. The thicker the nerve fiber, the greater the supply of ions and the more impulses it can generate (from several hundred to several hundred thousand) without the participation of the N / K-pump. However, in thin C-fibers, about 1% of the Na + and K + concentration gradients are spent on the occurrence of one PD.

Thus, if you block the production of energy, then the cell will be repeatedly excited even in this case. In reality, the Na / K-pump constantly transfers Na + from the cell, and K + returns it to the cell, as a result, the concentration gradient Na + and K + is constantly maintained, which is carried out due to the direct consumption of energy by the source which is ATP.