Why does the membrane potential disappear in a dead cell. Membrane action potential

Electric charge, like mass, is a fundamental property of matter. There are two types of charges conventionally designated as positive and negative.

Every substance has an electrical charge, which can be positive, negative, or zero. For example, electrons are negatively charged, while protons are positively charged. Since each atom contains one or more electrons and an equal number of protons, total number charges in a macroscopic object is extremely large, but in general such an object is not charged or has a small charge.

The charge of an electron is absolute value the smallest.

Electric field. Coulomb's Law

Each charged object forms an electric field in the surrounding space. Electric field is a kind of matter through which charged objects interact with each other. A test charge introduced into the electric field of another charge "feels" the presence of this field. It will be attracted to the charge that creates an electric field, or repelled from it.

Coulomb's Law determines the electric force F acting between two point charges q 1 And q2:

k- a constant determined by the selected conditions; r- distance between charges.

According to Coulomb's law, a force acts in the direction of a line connecting two charges. The magnitude of the force acting on the charges is proportional to the magnitude of each of the charges and inversely proportional to the square of the distance between them.

The electric field can be represented as lines of force showing direction electrical forces. These forces are directed away from the charge when it is positive and towards the charge when it is negative. If a positive charge is placed in an electric field, it is subjected to a force in the direction of the field. negative charge subjected to a force opposite to the direction of the field.

Characteristics of the electric field

1) Tension electric field. Every electric charge generates an electric field around itself. If another charge q entered into this field, then a force will act on it F, proportional q and electric field strength E:

The electric field strength E (or simply strength) at any point is defined as the electric force F that acts on a positive charge q placed at this point:

E is a vector quantity, that is, it has both magnitude and direction. The unit of tension is volt per meter [V/m].

The principle of superposition (superposition) indicates that if an electric field is created by many charges, the total strength is determined by adding the strengths created by each charge, according to the rules of vector addition.

2) Electric potential. To move a charge against an electric force acting on it, work must be done. This work does not depend on the path of movement of the charge in the electric field, but depends on the initial and final position of the charge.

If a charge moves from one point to another against an electrical force, its electrostatic potential energy increases. The electric potential at any point is equal to the electrostatic potential energy Wp, which has a positive charge q at this point: φ = W p /q (4).

It can also be said that the electric potential at a point equals work, which must be done against electric forces in order to move a positive charge from a given point to a large distance, where the potential of the electric field is zero. The electrical potential is scalar value and is measured in volts ( IN).

The electric field strength is a negative gradient electrical potential- indicator of potential change with distance x: E → = - dφ/dx. With the help of instruments, you can measure the potential difference, but not the field strength. The latter can be calculated using the relationship between E → and Δφ : where Δφ = E l is the distance between two electric field currents.

Resting membrane potential

Each cell converts some of its metabolic energy into electrostatic energy. The source of the electric field of the cell is the plasma membrane. There is a potential difference between the inner and outer surfaces plasma membrane. This potential difference is called membrane potential .

The potential difference between the internal and external environments of the cell can be measured directly and quite accurately. For this, a microelectrode is used, which is a glass micropipette with a tip diameter of up to 1 micron filled with concentrated KCl solution. The microelectrode is connected to the voltage amplifier of the recording device. You can measure the membrane potential of muscle, nerve cells or cells of other tissues. Another electrode (reference) is placed on the tissue surface.

When the microelectrode tip is outside the cell, its potential with respect to the reference electrode is zero. If the end of the electrode is immersed in the cell, piercing the plasma membrane, the potential difference becomes sharply negative. On the scale of the measuring device, the potential difference between the internal and external environments of the cell is recorded. This potential difference is called transmembrane or membrane potential.


If the cell is at rest, its membrane potential is negative meaning and stable value. Usually it is called resting membrane potential . The resting membrane potential of cells of various tissues ranges from - 55 millivolts (mV) before - 100mV.

Under certain physiological conditions, changes can occur membrane potential. Changing it in a positive direction is called depolarization plasma membrane. The shift of the membrane potential in the negative direction is called hyperpolarization .

Biophysical foundations of the resting membrane potential

Electrical phenomena in the plasma membrane are determined by the distribution of ions between the inner and external parties membranes. From chemical analysis it is known that the concentration of ions in the intracellular fluid is very different from the concentration of ions in the extracellular fluid. The term "extracellular fluid" refers to all fluids outside the cells (intercellular substance, blood, lymph, etc.). The table shows the concentrations of major ions in mammalian muscle cells and extracellular fluid (millimoles per liter).

Exist significant differences between the concentration of basic ions inside and outside the cell. The extracellular fluid has a high concentration of sodium and chloride ions. The intracellular fluid has a high concentration of potassium and various organic anions (A -) (charged groups of proteins).

The difference between the concentrations of sodium and potassium in extracellular and intracellular fluids is due to the activity sodium potassium pump, which pumps out 3 sodium ions from the cell in one cycle and pumps 2 potassium ions into the cell against the electrochemical gradient of these ions. The main function of the sodium-potassium pump is to maintain a difference in the concentrations of sodium and potassium ions on both sides of the plasma membrane.

At rest, the permeability of the plasma membrane for potassium ions significantly exceeds the permeability of the membrane for sodium ions. In nerve cells, the permeability ratio of the corresponding ions is 1:0.04.

This fact makes it possible to explain the existence of the resting membrane potential.

Potassium ions tend to leave the cell due to their high internal concentration. In this case, intracellular anions do not move through the membrane due to their large size. An insignificant intake of sodium ions into the cell also does not compensate for the exit of potassium ions to the outside, since the permeability of the membrane at rest for sodium ions is low.

Consequently, the outside of the cell acquires an additional positive charge and an excess of negative charge remains inside.

Diffusion of potassium across the membrane is a limited process. Potassium ions penetrating the membrane create an electric field that delays the diffusion of other potassium ions. As potassium leaves the cell, the electric field increases and, eventually, the tension reaches such a value when the flow of potassium through the membrane stops. The state in which the flow of ions along their concentration gradient is balanced by the membrane potential is called state of electrochemical equilibrium ions. The value of this membrane equilibrium potential is determined by Nernst equation ( at the same time, it is considered that the membrane is permeable to only one type of ions ) :

R is the universal gas constant, T- thermodynamic temperature, z is the electric charge of the ion, F- Faraday's constant, i and o - intracellular and extracellular concentrations of potassium ions, respectively.

Calculations based on the Nernst equation indicate that the internal and external chloride ion concentration also corresponds to a state of electrochemical equilibrium, but the sodium concentration is far from equilibrium with the membrane potential of the membrane.

The Nernst equation shows that the concentration gradient of potassium ions determines the magnitude of the resting membrane potential only in the first approximation. The calculated values ​​of the membrane potential coincide with those obtained experimentally only at high concentration potassium outside the cell.

A more accurate value of the resting membrane potential can be calculated from the Goldman-Hodgkin equation, which takes into account the concentration and permeability of the membrane for the three main ions of intra- and extracellular fluids:

Also, the sodium-potassium pump is directly involved in maintaining the resting membrane potential, pumping out three sodium ions from the cell and pumping only two potassium ions. As a result, the resting membrane potential becomes more negative than it would be if it were created only by the passive movement of ions across the membrane.

action potential

If a short-term electricity, then the membrane potential undergoes successive changes that are specific and unique to excitable cells. Excitable tissues can also be stimulated by mechanical or chemicals, but in experimental work, as a rule, electrical stimuli are used.

Rice. 1. action potential nerve cell.

action potential - a rapid fluctuation in the magnitude of the membrane potential caused by the action of an electrical or other stimulus on an excitable cell.

On fig. 1 shows the action potential of a nerve cell recorded using a microelectrode. If a brief electrical stimulus is applied to the cell, the membrane potential rapidly decreases to zero. This deviation is characterized as depolarization phase And. Within a short time, the internal environment of the cell becomes electropositive with respect to the external ( membrane potential reversal phase, or overshot ). The membrane potential then returns to the level of the resting membrane potential ( repolarization stage ) (Fig. 2.).

Rice. 2. Action potential phases

The duration of an action potential is 0.5 to 1 millisecond in large nerve cells and a few milliseconds in skeletal muscle cells. Total amplitude - almost 100 - 120 mV, deviation from the zero line - about 30-50 mV.

The action potential plays a leading role in information processing in the nervous system. It has a constant amplitude, which is not a probability quantity. It has great importance in information processing nervous system. The coding of the intensity of stimulation is carried out by the number of action potentials and the frequency with which the action potentials follow each other.

Biophysical foundations of the action potential

The action potential arises from specific changes in ion permeability in the plasma membrane. The English physiologist Hodgkin showed that the main mechanism of the action potential is a short-term and very specific change in the permeability of the membrane for sodium ions. At the same time, sodium ions enter the cell until the membrane potential reaches the potential of the electrochemical equilibrium of sodium ions.

Rice. 3. Change in membrane permeability to sodium and potassium ions during an action potential

The permeability of the membrane for sodium under the action of an electrical stimulus on the cell increases approximately 500 times and becomes much greater than the permeability of the membrane for potassium ions. The concentration of sodium ions sharply increases in the cell. As a result, the membrane potential takes positive value, and the flow of sodium ions into the cell slows down.

During the occurrence of the action potential, the plasma membrane depolarizes. Rapid depolarization of the membrane under the action of an electrical stimulus causes an increase in its permeability to sodium ions. The increased intake of sodium ions into the cell enhances the depolarization of the membrane, which, in turn, causes a further increase in the permeability of the membrane for sodium, etc.

But the value of the membrane potential during depolarization does not reach the level of the potential of the electrochemical equilibrium of sodium ions. The reason for this is a decrease in the permeability of the membrane for sodium ions due to inactivation of sodium transmembrane transport. This process dramatically reduces the permeability of the membrane to sodium ions and stops the influx of sodium into the cell.

At this point, there is an increase in the permeability of the membrane for potassium ions, which leads to rapid decline the magnitude of the membrane potential to the level of the resting potential. The permeability of the membrane for potassium ions also decreases to its normal value. Thus, inactivation of the incoming sodium current and an increase in membrane permeability to potassium ions (outgoing current) limit the duration of the action potential and lead to repolarization membranes.

Thus, during an action potential, some sodium ions enter the cell. But this number is quite small. The change in the concentration of ions in large nerve cells is only about 1/300,000 of the initial value.

The main mechanism for changes in membrane permeability is due to events in the sodium and potassium channels of the membrane. The state of their gates is controlled by the magnitude of the membrane potential. Sodium channels have two types of gates. One of them, called the activation gate, is closed at rest and opens when the membrane depolarizes. The entry of sodium ions into the cell causes the opening of everything more activation gate. The second type of sodium channel gates - membranes that are inactivated with increasing depolarization are gradually closed, which stops the influx of sodium into the cell. Membrane depolarization also causes an additional number of potassium channels to open, resulting in an increase in membrane permeability to potassium ions and membrane repolarization occurs.

Rice. 4. Changes in the state of sodium and potassium channels of the membrane depending on the magnitude of the membrane potential

Action potential propagation

The action potential propagates along the membrane of nerve and muscle cells without a decrease in amplitude with distance. This process is due cable properties plasma membrane, i.e. the ability to conduct electricity over short distances. Local electric current flows into the cell in the active region (where the action potential occurs) and out of the cell in the adjacent inactive region. These ionic currents cause some changes in the membrane potential in the zone adjacent to the site of the action potential.

The cyclic local current reduces the membrane charge in the inactive zone and depolarizes it. If depolarization reaches threshold level, then the permeability of the membrane for sodium ions increases and an action potential arises. Thus, the action potential propagates along the nerve and muscle fibers at a constant speed.

Rice. 5. Action potential propagation along the nerve fiber membrane

The speed of action potential propagation in nerve fibers depends on their diameter. It is maximum in the thickest fibers, reaching about 100 meters per second.

resting potential

The membranes, including plasma membranes, are in principle impenetrable to charged particles. True, the membrane contains Na+/K+-ATP-ase (Na+/K+-ATP-ase), which actively transfers Na+ ions from the cell in exchange for K+ ions. This transport is energy dependent and is associated with the hydrolysis of ATP (ATP). Due to the operation of the “Na +, K + -pump”, a non-equilibrium distribution of Na + and K + ions between the cell and the environment is maintained. Since the splitting of one ATP molecules provides the transfer of three Na + ions (out of the cell) and two K + ions (into the cell), this transport is electrogenic, i.e. . the cytoplasm of the cell is negatively charged with respect to the extracellular space.

Electrochemical potential. The contents of the cell are negatively charged in relation to the extracellular space. The main reason for the appearance of an electric potential on the membrane (membrane potential Δψ) is the existence specific ion channels. The transport of ions through the channels occurs along a concentration gradient or under the action of a membrane potential. In an unexcited cell, part of the K + channels are located in open state and K+ ions are constantly diffusing from the neuron to environment(along the concentration gradient). Leaving the cell, K + ions carry away a positive charge, which creates a resting potential equal to approximately -60 mV. It can be seen from the permeability coefficients of various ions that the channels permeable to Na+ and Cl- are predominantly closed. Phosphate ions and organic anions, such as proteins, practically cannot pass through membranes. Using the Nernst equation (RT / ZF, where R is the gas constant, T is the absolute temperature, Z is the valency of the ion, F is the Faraday number), it can be shown that the membrane potential of the nerve cell is primarily determined by K + ions, which make the main contribution to the conductivity of the membrane.

ion channels. The nerve cell membranes have channels permeable to Na+, K+, Ca2+ and Cl- ions. These channels are most often in a closed state and open only for a short time. The channels are subdivided into voltage-gated (or electrically excitable), for example, fast Na+ channels, and ligand-gated (or chemo-excitable), for example, nicotinic cholinergic receptors. Channels are integral membrane proteins composed of many subunits. Depending on the change in the membrane potential or interaction with the corresponding ligands, neurotransmitters and neuromodulators (see Fig. 343), receptor proteins can be in one of two conformational states, which determines the permeability of the channel ("open" - "closed" - and etc.).

Active transport:

The stability of the ion gradient is achieved through active transport: membrane proteins transport ions across the membrane against electrical and (or) concentration gradients, consuming metabolic energy for this. The most important active transport process is the operation of the Na/K pump, which exists in almost all cells; the pump pumps sodium ions out of the cell while simultaneously pumping potassium ions into the cell. This ensures a low intracellular concentration of sodium ions and a high potassium ion. The concentration gradient of sodium ions on the membrane has specific functions associated with the transmission of information in the form of electrical impulses, as well as with the maintenance of other active transport mechanisms and regulation of cell volume. Therefore, it is not surprising that more than 1/3 of the energy consumed by the cell is spent on the Na/K pump, and in some of the most active cells up to 70% of the energy is spent on its operation.

Passive transport:

Free diffusion and transport processes, provided by ion channels and carriers, are carried out along a concentration gradient or an electric charge gradient (called together an electrochemical gradient). Such transport mechanisms are classified as "passive transport". For example, according to this mechanism, glucose enters the cells from the blood, where its concentration is much higher.

Ion pump:

Ion pumps (pumps) are integral proteins that provide active transport of ions against a concentration gradient. The energy for transport is the energy of ATP hydrolysis. There are Na + / K + pump (pumps Na + out of the cell in exchange for K +), Ca ++ pump (pumps Ca ++ out of the cell), Cl– pump (pumps Cl - out of the cell).

As a result of the operation of ion pumps, transmembrane ion gradients are created and maintained:

The concentration of Na +, Ca ++, Cl - inside the cell is lower than outside (in the intercellular fluid);

The concentration of K+ inside the cell is higher than outside.

Sodium - potassium pump- this is a special protein that penetrates the entire thickness of the membrane, which constantly pumps potassium ions into the cell, while simultaneously pumping sodium ions out of it; in this case, the movement of both ions occurs against the gradients of their concentrations. These functions are possible due to two the most important properties this protein. First, the shape of the carrier molecule can change. These changes occur as a result of the addition of a phosphate group to the carrier molecule due to the energy released during the hydrolysis of ATP (i.e., the decomposition of ATP to ADP and the residue phosphoric acid). Secondly, this protein itself acts as an ATPase (i.e., an enzyme that hydrolyzes ATP). Since this protein transports sodium and potassium and, in addition, has ATPase activity, it is called “sodium-potassium ATPase”.

Simplistically, the action of the sodium-potassium pump can be represented as follows.

1. C inside ATP and sodium ions enter the membrane to the carrier protein molecule, and potassium ions from the outside.

2. The carrier molecule hydrolyzes one ATP molecule.

3. With the participation of three sodium ions, due to the energy of ATP, a phosphoric acid residue is attached to the carrier (phosphorylation of the carrier); these three sodium ions themselves also attach to the carrier.

4. As a result of the addition of a phosphoric acid residue, the shape of the carrier molecule (conformation) changes so that sodium ions are on the other side of the membrane, already outside the cell.

5. Three sodium ions are released into the external environment, and instead of them, two potassium ions are combined with a phosphorylated carrier.

6. The addition of two potassium ions causes dephosphorylation of the carrier - the return of a phosphoric acid residue to them.

7. Dephosphorylation, in turn, causes the conformation of the carrier so that potassium ions are on the other side of the membrane, inside the cell.

8. Potassium ions are released inside the cell and the whole process is repeated.

The significance of the sodium-potassium pump for the life of each cell and the organism as a whole is determined by the fact that the continuous pumping out of the sodium cell and the injection of potassium into it is necessary for the implementation of many vital functions. important processes: osmoregulation and preservation of cell volume, maintaining a potential difference on both sides of the membrane, maintaining electrical activity in nerve and muscle cells, for active transport of other substances (sugars, amino acids) through membranes. Large quantities potassium is also required for protein synthesis, glycolysis, photosynthesis and other processes. Approximately one third of all ATP consumed animal cell at rest, it is spent precisely on maintaining the work of the sodium-potassium pump. If any external influence suppress the respiration of the cell, i.e., stop the supply of oxygen and the production of ATP, then the ionic composition of the internal contents of the cell will begin to gradually change. In the end, he will come into equilibrium with the ionic composition of the environment surrounding the cell; in this case, death occurs.

action potential excitable cell and its phases:

P.D, - rapid fluctuation of the membrane potential that occurs during excitation of nerves, mice. And other cells. Can spread.

1. rise phase

2. reversion or overshoot (the charge is reversed)

3. polarity recovery or repolarization

4.positive trace potential

5. negative trace. Potential

Local response- This is the process of membrane response to a stimulus in a certain area of ​​the neuron. Does not spread along axons. The greater the stimulus, the more the local response changes. At the same time, the level of depolarization does not reach the critical level, it remains subthreshold. As a result, a local response can have electrotonic effects on neighboring sections of the membrane, but cannot propagate in the same way as an action potential. The excitability of the membrane in places of local depolarization and in places of the electrotonic depolarization caused by it is increased.

Activation and inactivation of the sodium system:

The depolarizing shock of the current leads to the activation of sodium channels and an increase in sodium current. This provides a local response. Membrane potential shift up to critical level leads to a rapid depolarization of the cell membrane and provides a rise front for the action potential. If the Na+ ion is removed from external environment, no action potential is generated. A similar effect was obtained by adding TTX (tetrodotoxin), a specific blocker of sodium channels, to the perfusion solution. When using the voltage-clamp method, it was shown that in response to the action of a depolarizing current, a short-term (1-2 ms) incoming current flows through the membrane, which is replaced after a while by an outgoing current (Fig. 2.11). When replacing sodium ions with other ions and substances, such as choline, it was possible to show that the incoming current is provided by sodium current, i.e., in response to a depolarizing stimulus, an increase in sodium conductivity (gNa +) occurs. Thus, the development of the depolarization phase of the action potential is due to an increase in sodium conductivity.

Let us consider the principle of operation of ion channels using the sodium channel as an example. The sodium channel is believed to be closed at rest. When the cell membrane depolarizes to a certain level, the m-activation gate opens (activation) and increases the flow of Na + ions into the cell. A few milliseconds after the opening of the m-gate, the p-gate located at the exit of the sodium channels closes (inactivation) (Fig. 2.4). Inactivation develops very rapidly in the cell membrane, and the degree of inactivation depends on the magnitude and duration of the depolarizing stimulus.

The work of sodium channels is determined by the magnitude of the membrane potential in accordance with certain laws of probability. It is calculated that the activated sodium channel passes only 6000 ions per 1 ms. In this case, a very significant sodium current that passes through the membranes during excitation is the sum of thousands of single currents.

When generating a single action potential in a thick nerve fiber, the change in the concentration of Na + ions during internal environment is only 1/100,000 of internal content Na ions of the giant squid axon. However, for thin nerve fibers, this change in concentration can be quite significant.

In addition to sodium, other types of channels are installed in cell membranes that are selectively permeable to individual ions: K +, Ca2 +, and there are varieties of channels for these ions (see Table 2.1).

Hodgkin and Huxley formulated the principle of "independence" of channels, according to which the flows of sodium and potassium through the membrane are independent of each other.

Change in excitability during arousal:

1. Absolute refractoriness - i.e. complete non-excitability, determined first by the full employment of the "sodium" mechanism, and then by the inactivation of sodium channels (this approximately corresponds to the peak of the action potential).

2. Relative refractoriness - i.e. reduced excitability associated with partial sodium inactivation and the development of potassium activation. In this case, the threshold is increased, and the response [PD] is reduced.

3. Exaltation - i.e. increased excitability - supernormality, appearing from trace depolarization.

4. Subnormality - i.e. reduced excitability arising from trace hyperpolarization. The amplitudes of the action potential in the phase of trace negativity are somewhat reduced, and against the background of trace positivity, they are slightly increased.

The presence of refractory phases determines the intermittent (discrete) nature of the nervous signaling, and the ionic mechanism of the action potential ensures the standardity of the action potential (nerve impulses). In this situation, changes in external signals are encoded only by a change in the frequency of the action potential (frequency code) or a change in the number of action potentials.

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resting membrane potential (MPP) or resting potential (PP) is called the potential difference of a resting cell between the inner and outer sides of the membrane. The inner side of the cell membrane is charged negatively with respect to the outer. Taking the potential of the external solution as zero, the MPP is recorded with a minus sign. Value WFP depends on the type of tissue and varies from -9 to -100 mV. Therefore, at rest cell membranepolarized. A decrease in the MPP value is called depolarization increase - hyperpolarization, restoring the original value WFP-repolarization membranes.

The main provisions of the membrane theory of origin WFP come down to the following. At rest, the cell membrane is well permeable to K + ions (in some cells and to SG), less permeable to Na + and practically impermeable to intracellular proteins and other organic ions. K + ions diffuse out of the cell along a concentration gradient, while non-penetrating anions remain in the cytoplasm, providing the appearance of a potential difference across the membrane.

The resulting potential difference prevents the exit of K + from the cell, and at a certain value, an equilibrium occurs between the exit of K + along the concentration gradient and the entry of these cations along the resulting electrical gradient. The membrane potential at which this equilibrium is reached is called equilibrium potential. Its value can be calculated from the Nernst equation:

10 In nerve fibers, signals are transmitted by action potentials, which are rapid changes in membrane potential that propagate rapidly along the membrane of the nerve fiber. Each action potential begins with a rapid shift of the resting potential from a normal negative value to a positive value, then it returns almost as quickly to a negative potential. When a nerve signal is conducted, the action potential moves along the nerve fiber until it ends. The figure shows the changes that occur on the membrane during an action potential, with the transfer of positive charges into the fiber at the beginning and the return of positive charges to the outside at the end. The lower part of the figure graphically shows successive changes in membrane potential over several 1/10000 sec, illustrating the explosive onset of the action potential and a nearly equally rapid recovery. rest stage. This stage is represented by the resting membrane potential, which precedes the action potential. The membrane during this stage is polarized due to the presence of a negative membrane potential of -90 mV. phase of depolarization. At this time, the membrane suddenly becomes highly permeable to sodium ions, allowing a huge number of positively charged sodium ions to diffuse into the axon. The normal polarized state of -90 mV is immediately neutralized by the incoming positively charged sodium ions, causing the potential to rise rapidly in the positive direction. This process is called depolarization. In large nerve fibers, a significant excess of inward positive sodium ions usually causes the membrane potential to “jump” beyond zero level, becoming slightly positive. In some smaller fibers, as in most neurons of the central nervous system, the potential reaches the zero level without "jumping" it. phase of repolarization. Within a few fractions of a millisecond after a sharp increase in the permeability of the membrane to sodium ions, sodium channels begin to close and potassium channels open. As a result, rapid outward diffusion of potassium ions restores the normal negative resting membrane potential. This process is called membrane repolarization. action potential For a more complete understanding of the factors that cause depolarization and repolarization, it is necessary to study the features of two other types of transport channels in the nerve fiber membrane: electrically controlled sodium and potassium channels. Electrically operated sodium and potassium channels. A necessary participant in the processes of depolarization and repolarization during the development of an action potential in the nerve fiber membrane is an electrically controlled sodium channel. The electrically controlled potassium channel also plays important role in increasing the rate of membrane repolarization. Both types of electrically operated channels exist in addition to the Na+/K+ pump and K*/Na+ leakage channels. Electrically operated sodium channel. At the top of the figure, the electrically controlled sodium channel is shown in three different states. This channel has two gates: one near the outer part of the channel, which is called the activation gate, the other - near the inside of the channel, which is called the inactivation gate. The upper left side of the figure shows the resting state of this gate when the resting membrane potential is -90 mV. Under these conditions, the activation gates are closed and prevent the entry of sodium ions into the fiber. sodium channel activation. When the resting membrane potential shifts in the direction of less negative values, rising from -90 mV towards zero, at a certain level (usually between -70 and -50 mV) there is a sudden conformational change in the activation gate, as a result, they go into a fully open state. . This state is called the activated state of the channel, in which sodium ions can freely enter through it into the fiber; while the sodium permeability of the membrane increases in the range from 500 to 5000 times. sodium channel inactivation. The top right side of the figure shows the third state of the sodium channel. An increase in potential that opens the activation gate closes the inactivation gate. However, the inactivation gate closes within a few tenths of a millisecond after the activation gate opens. This means that the conformational change that leads to the closing of the inactivation gate is a slower process than the conformational change that opens the activation gate. As a result, several tenths of a millisecond after the opening of the sodium channel, the inactivation gate closes, and sodium ions can no longer penetrate into the fiber. From this moment, the membrane potential begins to return to the resting level, i.e. the process of repolarization begins. There is another important characteristic of the sodium channel inactivation process: the inactivation gate does not reopen until the membrane potential returns to a value equal to or close to the level of the initial resting potential. In this regard, the re-opening of sodium channels is usually impossible without prior repolarization of the nerve fiber.

13 The mechanism of conduction of excitation along nerve fibers depends on their type. There are two types of nerve fibers: myelinated and unmyelinated. Metabolic processes in unmyelinated fibers do not provide a quick compensation for energy expenditure. The spread of excitation will go with a gradual attenuation - with a decrement. The decremental behavior of excitation is characteristic of a low-organized nervous system. The excitation is propagated by small circular currents that occur inside the fiber or in the liquid surrounding it. A potential difference arises between the excited and unexcited areas, which contributes to the occurrence of circular currents. The current will spread from the "+" charge to "-". At the exit circular current the permeability of the plasma membrane for Na ions increases, resulting in membrane depolarization. Between the newly excited area and the adjacent unexcited potential difference again arises, which leads to the occurrence of circular currents. The excitation gradually covers the neighboring sections of the axial cylinder and thus spreads to the end of the axon. In myelin fibers, thanks to the perfection of metabolism, excitation passes without fading, without decrement. Due to the large radius of the nerve fiber, due to the myelin sheath, the electric current can enter and leave the fiber only in the area of ​​interception. When irritation is applied, depolarization occurs in the area of ​​​​intercept A, the adjacent intercept B is polarized at this time. Between the interceptions, a potential difference arises, and circular currents appear. Due to the circular currents, other interceptions are excited, while the excitation spreads in a saltatory way, abruptly from one interception to another. There are three laws of conduction of irritation along the nerve fiber. The law of anatomical and physiological integrity. Conduction of impulses along the nerve fiber is possible only if its integrity is not violated. The law of isolated conduction of excitation. There are a number of features of the spread of excitation in the peripheral, pulpy and non-pulmonic nerve fibers. In peripheral nerve fibers, excitation is transmitted only along the nerve fiber, but is not transmitted to neighboring nerve fibers that are in the same nerve trunk. In the pulpy nerve fibers, the role of an insulator is performed by the myelin sheath. Due to myelin, the resistivity increases and the electrical capacitance of the shell decreases. In the non-fleshy nerve fibers, excitation is transmitted in isolation. The law of bilateral excitation. Nerve fiber conducts nerve impulses in two directions - centripetal and centrifugal.

14 synapses - This is a specialized structure that ensures the transmission of a nerve impulse from a nerve fiber to an effector cell - a muscle fiber, neuron or secretory cell.

synapses- these are the junctions of the nerve process (axon) of one neuron with the body or process (dendrite, axon) of another nerve cell (intermittent contact between nerve cells).

All structures that provide signal transmission from one nervous structure to another - synapses .

Meaning- transmits nerve impulses from one neuron to another => ensures the transmission of excitation along the nerve fiber (signal propagation).

A large number of synapses provides a large area for the transmission of information.

Synapse structure:

1. presynaptic membrane- belongs to the neuron from which the signal is transmitted.

2. synaptic cleft, filled with a liquid with a high content of Ca ions.

3. postsynaptic membrane- belongs to the cells to which the signal is transmitted.

Between neurons there is always a gap filled with interstitial fluid.

Depending on the density of the membranes, there are:

- symmetrical(with the same membrane density)

- asymmetrical(the density of one of the membranes is higher)

presynaptic membrane covers the extension of the axon of the transmitting neuron.

Extension - synaptic button/synaptic plaque.

On plaque - synaptic vesicles (vesicles).

On the inside of the presynaptic membrane protein/hexagonal lattice(required for the release of the mediator), in which the protein is located - neuron . Filled with synaptic vesicles that contain mediator- a special substance involved in signal transmission.

The vesicle membrane contains - stenin (protein).

postsynaptic membrane covers the effector cell. Contains protein molecules that are selectively sensitive to the mediator of this synapse, which ensures interaction.

These molecules are part of the channels of the postsynaptic membrane + enzymes (many) that can destroy the connection of the mediator with receptors.

Receptors on the postsynaptic membrane.

The postsynaptic membrane contains receptors that are related to the mediator of this synapse.

Between them is snap cleft . It is filled with interstitial fluid a large number of calcium. It has a number of structural features - it contains protein molecules that are sensitive to a mediator that transmits signals.

15 Synaptic delay in the conduction of excitation

In order for the excitation to spread along the reflex arc, a certain time is spent. This period consists of the following periods:

1. the period temporarily necessary for the excitation of the receptors (receptor) and for the conduction of excitation impulses along the afferent fibers to the center;

2. the period of time necessary for the spread of excitation through the nerve centers;

3. the period of time required for the propagation of excitation along the efferent fibers to the working body;

4. latent period of the working body.

16 Inhibition plays an important role in the processing of information entering the CNS. This role is especially pronounced in presynaptic inhibition. It more precisely regulates the process of excitation, since this inhibition can block individual nerve fibers. One excitatory neuron can be approached by hundreds and thousands of impulses through different terminals. At the same time, the number of impulses reaching the neuron is determined by presynaptic inhibition. Inhibition of the lateral pathways ensures the selection of essential signals from the background. Blockade of inhibition leads to a wide irradiation of excitation and convulsions, for example, when presynaptic inhibition is turned off by bicuculin.

"Membrane Potential"

Made by Chetverikova R

1st year student

Faculty of Biology and Soil

Introduction

A bit of history

Electricity in a cage

Membrane potential

action potential

Irritation threshold

Characteristic properties of the action potential

Conclusion

Introduction

Modern science is developing rapidly, and the more we move along the path of progress, the more we are convinced that in order to solve any scientific tasks it is necessary to unite the efforts and achievements of several branches of science at once.

Previously, the concept of vitalism dominated, according to which biological phenomena are fundamentally incomprehensible on the basis of physics and chemistry, since there is a certain “ life force”, or entelechy, not subject to physical interpretation. In 20th century great physicist Bohr considered the problem of the relationship between biology and physics based on the concept of complementarity, a particular case of which is the uncertainty principle of quantum mechanics.

Bohr believed that not a single result of biological research could be unambiguously described except on the basis of the concepts of physics and chemistry. Development molecular biology led to an atomistic interpretation of the basic phenomena of life - such as heredity and variability. In recent decades, there has been a successful development and physical theory integral biological systems, based on the ideas of synergetics. Erwin Schrödinger came to an optimistic, although not entirely reassuring conclusion: “Although modern physics and chemistry cannot explain the processes occurring in a living organism, there is no reason to doubt the possibility of their scientific explanation". Today there is every reason to believe that modern physics does not meet the limits of its applicability to consideration biological phenomena. It is difficult to think that such boundaries will be found in the future.

On the contrary, the development of biophysics as a part of modern physics testifies to its unlimited possibilities.

In this example, one can clearly see how the achievements of physics helped scientists understand such a complex phenomenon.

A bit of history

Man discovered electricity in living organisms in ancient times. Or rather, I felt it, unaware of its existence. This concept did not exist then. For example, the ancient Greeks were wary of meeting in the water with fish, which, as the great scientist Aristotle wrote, "makes animals numb." The fish that instilled fear in people was an electric stingray and bore the name "torpedo". And only two hundred years ago, scientists finally understood the nature of this phenomenon.

Scientists have long wanted to understand what is the nature of the signals flowing through the nerves. Among the many theories that arose in the middle of the 18th century, under the influence of the general enthusiasm for electricity, there appeared the theory that the "electric fluid" is transmitted through the nerves.

The idea was in the air. Luigi Galvani, studying lightning discharges, used a frog neuromuscular preparation. Hanging it on a copper hook on the balcony railing, Galvani noticed that when the frog's legs touched the iron railing, muscle contraction occurred. Based on this, Galvani concludes that an electrical signal exists in a biological object. However, Galvani's contemporary Alessandro Volta ruled out a biological object and showed that an electric current can be obtained by contacting a set of metals separated by an electrolyte (voltaic column). So it was opened chemical source current (named, however, later, in honor of his scientific opponent a galvanic cell).

This controversy was the beginning of electrobiology. And now, half a century later, the German physiologist E. Dubois-Reymond confirmed Galvani's discovery by demonstrating the presence of electric fields in the nerves with the help of improved electrical measuring equipment. The answer to the question of how electricity appears in a cell was found half a century later.

Electricity in a cage

In 1890, Wilhelm Ostwald, who was working on semi-permeable artificial films, suggested that semi-permeability could be the cause of not only osmosis, but also electrical phenomena. Osmosis occurs when the membrane is selectively permeable, i.e. passes some particles and does not pass others. Most often, the permeability of the membrane depends on the particle size. Ions can also be such particles. Then the membrane will pass ions of only one sign, for example, positive. Indeed, if we look at the Nernst formula for the diffusion potential Vd arising at the boundary of two solutions with electrolyte concentrations C1 and C2:

where u is the speed of the faster ion, v is the speed of the slower ion, R is the universal gas constant, F is the Faraday number, T is the temperature, and assuming that the membrane is impermeable to anions, i.e. v = 0, then one can see, what should appear big values for Vd

(2)

Potential across a membrane separating two solutions

Thus, Ostwald combined the Nernst formula and the knowledge of semipermeable membranes. He suggested that the properties of such a membrane explained the potentials of muscles and nerves and the action of the electrical organs of fish.

Membrane potential (resting potential)

Under the membrane potential understand the potential difference between the inner (cytoplasmic) and outer surfaces of the membrane

With the help of electrophysiological studies, it was proved that in a state of physiological rest, there is a positive charge on the outer surface of the membrane, and on inner surface- negative.

Julius Bernstein created a theory according to which the difference in charges is determined by the different concentrations of sodium, potassium, chlorine ions inside and outside the cell. Inside the cell, the concentration of potassium ions is 30-50 times higher, the concentration of sodium ions is 8-10 times lower, and the concentration of chloride ions is 50 times less. According to the laws of physics, if living system was not regulated, then the concentration of these ions would be equal on both sides of the membrane and the membrane potential would disappear. But this does not happen, because cell membrane is active transport system. The membrane has special channels for one or another ion, each channel is specific and the transport of ions inside and outside the cell is largely active. In a state of relative physiological rest, sodium channels are closed, while potassium and chloride channels are open. This leads to the fact that potassium leaves the cell, and chlorine enters the cell, as a result of which the number of positive charges on the cell surface increases and the number of charges inside the cell decreases. Thus, a positive charge remains on the surface of the cell, and a negative one inside. This distribution of electronic charges ensures the preservation of the membrane potential.

molecular biology membrane potential

action potential

This leads to the fact that on the inner surface of the membrane accumulate positive charges, and on the outside - negative charges. This redistribution of charges is called depolarization.

In this state, the cell membrane does not exist for long (0.1-5 m.s.). In order for a cell to become capable of excitation again, its membrane must repolarize, i.e. return to resting potential. To return the cell to the membrane potential, it is necessary to “pump out” the sodium and potassium cations against the concentration gradient. This work is performed by the sodium-potassium pump, which restores the initial state of the concentration of sodium and potassium cations, i.e. membrane potential is restored.

Irritation threshold

For the occurrence of depolarization and subsequent excitation, the stimulus must have a certain value. The minimum strength of the acting stimulus that can cause excitation is called the threshold of irritation. The value above the threshold is called superthreshold, and below the threshold - subthreshold. Excitable formations obey the “all or nothing” law, which means that when an irritation is applied with a force equal to the threshold, maximum excitation occurs. Irritation below the subthreshold strength does not cause irritation.

To characterize the strength of the acting stimulus from the time of its action, a curve is drawn that reflects how long the threshold or superthreshold stimulus must act to cause excitation. The action of a threshold strength stimulus will cause excitation only if this stimulus will act for a certain time. The minimum current or excitation that must act on excitable formations in order to cause irritation is called the rheobase. The minimum time during which the stimulus with the force of one reobase must act in order to cause excitation is called the minimum useful time.

The value of the stimulation threshold depends not only on the duration of the current stimulus, but also on the steepness of the increase. When the steepness of the growth of the stimulus decreases below a certain value, no excitation occurs, no matter how strong we bring the stimulus. This is because at the site of application of the stimulus, the threshold constantly rises, and no matter how much the stimulus is brought to, excitation does not occur. Such a phenomenon, the adaptation of an excitable formation to a slowly increasing strength of the stimulus, is called accommodation.

Different excitable formations have different rates of accommodation, so the higher the rate of accommodation, the steeper the rise of the stimulus.

The same law works not only for electrical stimulators, but also for others (chemical, mechanical stimuli / stimulants).

Characteristic properties of the action potential

Polar law of irritation.

This law was first discovered by P.F. weather vane. He established that D.C. has a polar effect on excitable tissue. This is expressed in the fact that at the moment of closing the circuit, excitation occurs only under the cathode, and at the moment of opening - under the anode. Moreover, under the anode, when the circuit is opened, the excitation is much higher than when the circuit is closed under the cathode. This is due to the fact that a positively charged electrode (anode) causes membrane hyperpolarization, when the surfaces touch the cathode (negatively charged), it causes depolarization.

The all-or-nothing law

According to this law, a subthreshold stimulus does not cause excitation (nothing); with threshold stimulation, excitation takes on a maximum value (all). A further increase in the strength of the stimulus does not increase excitation.

For a long time it was believed that this law is a general principle excitable tissue. At the same time, it was believed that “nothing” is a complete absence of excitation, and “everything” is a complete manifestation of an excitable formation, i.e. his ability to excite.

However, with the help of microelectronic studies, it was proved that even under the action of a subthreshold stimulus in an excitable formation, ions are redistributed between the outer and inner surfaces of the membrane. If, with the help of a pharmacological preparation, the membrane permeability to sodium ions is increased or the permeability to potassium ions is reduced, then the amplitude of the action potentials increases. Thus, we can conclude that this law should be considered only, as a rule, characterizing the features of excitable education.

Carrying out excitation. Excitability.

In demyelinated and myelinated fibers, excitation is transmitted differently, this is due to the anatomical features of these fibers. Myelinated nerve fibers have nodes of Ranvier. Transmission of signals through such fibers is carried out using intercepts of Ranvier. The signal skips through the myelinated areas, and thus, the conduction of excitation through them occurs faster than in non-myelinated areas, the return of the impulse is impossible, since the threshold of irritation increases in the previous intercept.

Excitability is the ability to irritate or excite and, consequently, the occurrence of an action potential. The higher the threshold of irritation, the higher the excitation, and vice versa.

The value of the threshold of stimulation is inversely related to the duration (t) of the stimulus and the steepness of the increase in its strength

Thus, we see that without the help of physics it would not have been possible to discover the secret of electricity in living organisms, the transmission of nerve impulses, membrane potential - one of critical aspects modern biology.