The concentration of sodium ions inside the cell. Membrane potential of the cell, or resting potential

Effects of Na/K-pump (sodium-potassium pump) on membrane potential and cell volume. Constant cell volume.

On fig. 1.9 shows various components membrane current and are given intracellular ion concentrations that ensure their existence. An outward current of potassium ions is observed through potassium channels, since the membrane potential is somewhat more electropositive than the equilibrium potential for potassium ions. Total conductance of sodium channels much lower than potassium, i.e. sodium channels are open much less frequently than potassium channels at resting potential; however, about the same number of sodium ions enter the cell as potassium ions leave it, because large concentration and potential gradients are needed for the diffusion of sodium ions into the cell. The Na/K pump provides ideal compensation for passive diffusion currents as it transports sodium ions out of the cell and potassium ions into it. Thus, the pump is electrogenic due to the difference in the number of charges transferred into and out of the cell, which, at normal speed of its operation, creates a membrane potential that is about 10 mV more electronegative than if it was formed only due to passive ion flows. As a result, the membrane potential approaches the potassium equilibrium potential, which reduces the leakage of potassium ions. Na/K pump activity regulated intracellular concentration of sodium ions. The speed of the pump slows down as the concentration of sodium ions to be removed from the cell decreases (Fig. 1.8), so that the operation of the pump and the flow of sodium ions into the cell balance each other, maintaining the intracellular concentration of sodium ions at a level of about 10 mmol / l.

To maintain a balance between pumping and passive membrane currents, many more Na/K-pump molecules are needed than channel proteins for potassium and sodium ions. When the channel is open, tens of thousands of ions pass through it in a few milliseconds, and since the channel is usually opened several times per second, more than 105 ions pass through it in total during this time. A single pump protein moves several hundred sodium ions per second, so the plasma membrane must contain about 1000 times more pump molecules than channel molecules. Measurements of channel currents at rest showed an average of one potassium and one sodium open channel per 1 µm2 of the membrane; it follows that about 1000 Na/K pump molecules should be present in the same space; the distance between them averages 34 nm; the diameter of the pump protein, as well as the channel protein, is 8–10 nm. Thus, the membrane is sufficiently densely saturated with pumping molecules.

The fact that flow of sodium ions into the cell, but potassium ions - out of the cell compensated by the operation of the pump, has another consequence, which is to maintain a stable osmotic pressure and a constant volume. Inside the cell there is a high concentration of large anions, mainly proteins (A in Table 1.1), which are not able to penetrate the membrane (or penetrate it very slowly) and therefore are a fixed component inside the cell. To balance the charge of these anions, an equal number of cations is needed. Thanks to action of Na/K-pump these cations are mainly potassium ions. Significant increase intracellular concentration of ions could only occur with an increase in the concentration of anions due to the flow of Cl along the concentration gradient into the cell (Table 1.1), but the membrane potential counteracts this. The incoming Cl current is observed only until the equilibrium potential for chloride ions is reached; this is observed when the chloride ion gradient is almost opposite to the potassium ion gradient, since the chloride ions are negatively charged. Thus, a low intracellular concentration of chloride ions is established, corresponding to a low extracellular concentration of potassium ions. The result is a limitation of the total number of ions in the cell. If the membrane potential drops during blockade of the Na/K pump, for example, during anoxia, then the equilibrium potential for chloride ions decreases, and the intracellular concentration of chloride ions increases accordingly. Restoring the balance of charges, potassium ions also enter the cell; the total concentration of ions in the cell increases, which increases the osmotic pressure; this forces water to enter the cell. The cell swells. Such swelling is observed in vivo under energy deficient conditions.

Any living cell is covered with a semi-permeable membrane through which passive movement and active selective transport of positively and negatively charged ions is carried out. Due to this transfer between the outer and inner surface of the membrane there is a difference in electric charges (potentials) - the membrane potential. There are three different manifestations of the membrane potential - resting membrane potential, local potential, or local response, And action potential.

If external stimuli do not act on the cell, then the membrane potential remains constant for a long time. The membrane potential of such a resting cell is called the resting membrane potential. For the outer surface of the cell membrane, the resting potential is always positive, and for the inner surface of the cell membrane, it is always negative. It is customary to measure the resting potential on the inner surface of the membrane, because the ionic composition of the cytoplasm of the cell is more stable than that of the interstitial fluid. The magnitude of the resting potential is relatively constant for each type of cell. For striated muscle cells, it ranges from -50 to -90 mV, and for nerve cells from -50 to -80 mV.

The resting potential is caused by different concentration of cations and anions outside and inside the cell, as well as selective permeability for them the cell membrane. The cytoplasm of a resting nerve and muscle cell contains approximately 30–50 times more potassium cations, 5–15 times less sodium cations, and 10–50 times less chloride anions than the extracellular fluid.

At rest, almost all sodium channels of the cell membrane are closed, and most potassium channels are open. Whenever potassium ions encounter an open channel, they pass through the membrane. Since there are much more potassium ions inside the cell, the osmotic force pushes them out of the cell. Released potassium cations increase the positive charge on the outer surface of the cell membrane. As a result of the release of potassium ions from the cell, their concentration inside and outside the cell should soon equalize. However, this is prevented by the electrical repulsive force of positive potassium ions from the positively charged outer surface of the membrane.

The greater the value of the positive charge on the outer surface of the membrane, the more difficult it is for potassium ions to pass from the cytoplasm through the membrane. Potassium ions will leave the cell until the electrical repulsion force becomes equal to the osmotic pressure K+. At this level of potential on the membrane, the entry and exit of potassium ions from the cell are in equilibrium, so the electric charge on the membrane at this moment is called potassium equilibrium potential. For neurons, it is from -80 to -90 mV.

Since almost all sodium channels of the membrane are closed in a resting cell, Na + ions enter the cell along the concentration gradient in an insignificant amount. They only to a very small extent compensate for the loss of the positive charge by the internal environment of the cell, caused by the release of potassium ions, but cannot significantly compensate for this loss. Therefore, the penetration into the cell (leakage) of sodium ions leads only to a slight decrease in the membrane potential, as a result of which the resting membrane potential has a slightly lower value compared to the potassium equilibrium potential.

Thus, potassium cations leaving the cell, together with an excess of sodium cations in the extracellular fluid, create a positive potential on the outer surface of the membrane of the resting cell.

At rest, the plasma membrane of the cell is well permeable to chloride anions. Chlorine anions, which are more abundant in the extracellular fluid, diffuse into the cell and carry a negative charge with them. Complete equalization of the concentrations of chlorine ions outside and inside the cell does not occur, because. this is prevented by the electrical mutual repulsion of like charges. Created chlorine equilibrium potential, at which the entry of chloride ions into the cell and their exit from it are in equilibrium.

The cell membrane is practically impermeable to large anions of organic acids. Therefore, they remain in the cytoplasm and, together with the incoming chloride anions, provide a negative potential on the inner surface of the membrane of the resting nerve cell.

The most important significance of the resting membrane potential is that it creates an electric field that acts on the macromolecules of the membrane and gives their charged groups a certain position in space. It is especially important that this electric field determines the closed state of the activation gates of sodium channels and the open state of their inactivation gates (Fig. 61, A). This ensures the state of rest of the cell and its readiness for excitation. Even a relatively small decrease in the resting membrane potential opens the activation "gates" of sodium channels, which brings the cell out of its resting state and gives rise to excitation.

Both of these elements are in the first group of the Mendeleev system - they are neighbors and in many respects similar to each other. Active, typical metals, the atoms of which easily part with their single outer electron, passing into the ionic state, these elements form numerous salts that are widespread in nature. However, closer examination reveals that the biological functions of sodium and potassium are not the same. Potassium salts are better absorbed by the soil complex, so plant tissues contain relatively more potassium, while sodium salts predominate in sea water. In biological machines, both of these ions sometimes act together, sometimes in exactly the opposite way.

Both ions take part in the propagation of electrical impulses along the nerve. In the resting nerve, in its inner part, a negative charge is concentrated (Fig. 20, a), and on the outer side it is positive; the concentration of potassium ions is greater than the concentration of sodium ions inside the nerve. When irritated, the permeability of the nerve fiber membrane changes, and sodium ions rush into the nerve faster than potassium ions have time to leave from there (Fig. 20, b). As a result, a negative charge appears on the outer side of the nerve fiber (there is a lack of cations), and a positive charge appears inside the nerve (where there is now an excess of cations) (Fig. 20c). On the outer side of the fiber, diffusion of sodium ions begins to occur from neighboring sections to the one that is depleted in sodium ions. Energetic diffusion leads to the appearance of a negative charge already in neighboring regions (Fig. 20, d), while the initial state is restored in the initial one. Thus, the state of polarization (plus - inside, minus - outside) moved along the nerve fiber. Further, all processes are repeated, and the nerve impulse spreads quite quickly throughout the nerve. Therefore, the propagation mechanism electrical impulse along the nerve due to the different permeability of the nerve fiber membrane in relation to sodium and potassium ions.

The question of the permeability of cell membranes for certain substances is extremely important. The passage of a substance through a biological membrane does not always resemble simple diffusion through a porous partition. So, for example, glucose and other carbohydrates pass through the erythrocyte membrane with the help of a special carrier that carries molecules through the membrane. In this case, special conditions must be met - the carbohydrate molecule must have a certain shape, it must be bent so that its contour acquires the shape of a chair, otherwise the transfer may not take place. The concentration of carbohydrates in the external environment is greater than inside the erythrocyte, so this transfer is called passive.

There are cases when the membrane is tightly closed for certain ions: in particular, in mitochondria, the inner membrane does not allow potassium ions to pass at all. However, these ions enter the mitochondria if the environment contains the antibiotics valinomycin or gramicidin. Valinomycin specializes mainly in potassium ions (it can also carry rubidium and cesium ions), and gramicidin carries, in addition to potassium, also sodium, lithium, rubidium and cesium ions.

It was found that the molecules of such conductors have the shape of a donut, the radius of the hole of which is such that a potassium, sodium or other alkali metal ion is placed inside the donut. These antibiotics were called ionophores ("ion carriers"). On fig. 21 shows diagrams of the transport of ions through the membrane by molecules of valinomycin and gramicidin. It is very likely that the toxic effect that antibiotics have on various microorganisms is precisely due to the fact that in their presence, the membranes begin to let in those ions that are not supposed to be there; this disrupts the functioning of the chemical systems of the microorganism cell and leads to its death or to serious disorders that stop its reproduction.

An essential role in biological machines is played by active transfers across membranes (see Chap. 8). The question arises: where does the energy necessary for active transfer come from, and is it possible to carry it out without a special carrier?

As for energy, it is ultimately supplied by the same universal ATP molecules or creatine phosphate, the hydrolysis of which is accompanied by the release of large amounts of energy. But regarding carriers, the question is less clear, although there is no doubt that potassium and sodium metal ions cannot be dispensed with here.

The concentration of various substances in the cell (protein and mineral) is higher than in the environment; for this reason, most often the cell is under the threat of excessive penetration of water into it (as a result of osmosis). In order to get rid of this, the cell pumps sodium ions into the environment and thereby equalizes the osmotic pressure. For this reason, the concentration of sodium ions in the cell is less than in the environment. Here again the difference between sodium and potassium is revealed. Sodium is removed, and the concentration of potassium ions is relatively greater inside the cell. So, a red blood cell contains about five times more potassium than sodium.

And the content of potassium is high in the muscles: per 100 g of raw muscle tissue, potassium contains 366 mg, and sodium 65 mg. Potassium in muscles facilitates the transition from the globular form of actin to the fibrillar form, which is connected to myosin (see above).

There are some cases when an enzyme activated by potassium ion is inhibited by sodium ions, and vice versa. Therefore, the discovery of an enzyme that requires both ions for its action attracted the attention of biochemists. This enzyme accelerates the hydrolysis of ATP and is called (K + Na) ATPase. To understand its role and mechanism of action, we must again turn to the transfer processes.

As we have already pointed out, the concentration of potassium ions is increased inside the cells, and there is relatively more sodium in the surrounding cellular environment. The pumping out of sodium ions from the cell leads to increased entry of potassium ions into the cell, as well as other substances (glucose, amino acids). Sodium and potassium ions can be exchanged according to the "ion for ion" principle, and then there is no potential difference on both sides of the cell membrane. But if there are more potassium ions inside the cell than sodium ions left from there, a potential jump (about 100 mV) may occur; the sodium pumping system is called the "sodium pump". If a potential difference appears in this case, then the term "electrogenic sodium pump" is used.

The introduction of large amounts of potassium ions into the cell is necessary, since potassium ions promote protein synthesis (in ribosomes), and also accelerate the process of glycolysis.

It is in the cell membrane that (K + Na) ATP-ase is located - a protein with a molecular weight of 670,000, which has not yet been separated from the membranes. This enzyme hydrolyzes ATP, and the energy of hydrolysis is used to transport it in the direction of increasing concentration.

A remarkable property of (K + Na) ATP-ase is that in the process of ATP hydrolysis it is activated from the inside of the cell by sodium ions (and thus ensures the excretion of sodium), and from the outside of the cell (from the side of the environment) by potassium ions (facilitating their introduction into the cell) ; as a result, the distribution of ions of these metals necessary for the cell occurs. It is interesting to note that sodium ions in the cell cannot be replaced by any other ions. ATPase is activated from the inside only by sodium ions, but the potassium ions acting from the outside can be replaced by rubidium or ammonium ions.

For the functions of individual organs, in particular the heart, not only the concentration of potassium, sodium, calcium and magnesium ions is important, but also their ratio, which should lie within certain limits. The ratio of the concentrations of these ions in human blood does not differ too much from the corresponding ratio characteristic of sea ​​water. It is possible that biological evolution, from the first forms of life that arose in the waters of the primary ocean or on its shallows, to its higher forms, has preserved some chemical "imprints" of the distant past ...

Returning to the beginning of this chapter, we again recall the multifunctionality of ions, their ability to perform a wide variety of duties in organisms. Calcium, sodium, potassium, and cobalt exhibit this ability in different ways. Cobalt forms a strong complex of the corrine type, and this complex already catalyzes various reactions. Calcium, sodium, potassium act as activators. But the magnesium ion can act both as an activator and as an integral part of a strong complex compound - chlorophyll, one of the most important compounds created by nature.

The outstanding scientist K. A. Timiryazev devoted a work to chlorophyll, which he called "The Sun, Life and Chlorophyll", indicating in it that it is chlorophyll that is the link that connects the processes of energy release in the Sun with life on Earth.

In the next chapter, we will consider the properties of this interesting compound.

Na + /K + pump or Na + /K + ATP-ase is also, like ion channels, a complex of integral membrane proteins that can not only open the way for the ion to move along the gradient, but actively move ions against the concentration gradient. The mechanism of the pump is shown in Figure 8.

The protein complex is in the E1 state, in this state the pump is sensitive to sodium ions and 3 sodium ions bind to the enzyme from the cytoplasmic side

After the binding of sodium ions, ATP is hydrolyzed and released energy, necessary for the transport of ions against the concentration gradient, ADP inorganic phosphate is released (which is why the pump is called Na + / K + ATPase).

The pump changes conformation and enters the E2 state. In this case, the binding sites of sodium ions turn outward. In this state, the pump has a low affinity for sodium and ions are released into the extracellular environment.

In the E2 conformation, the enzyme has a high affinity for potassium and binds 2 ions.

There is a transfer of potassium, its release into the intracellular environment and the attachment of an ATP molecule - the pump returned to the E1 conformation, again acquired an affinity for sodium ions and is included in a new cycle.

Figure 8 Mechanism of Na + /K + ATP-ase

Note that the Na + /K + pump carries 3 sodium ion from the cell in exchange for 2 potassium ion. Therefore the pump is electrogenic: in total, one positive charge is removed from the cell in one cycle. The transport protein performs 150 to 600 cycles per second. Because pump operation is a multi-stage chemical reaction, like all chemical reactions, it is highly dependent on temperature. Another characteristic of the pump is the presence of a saturation level, which means that the speed of the pump cannot increase indefinitely as the concentration of transported ions increases. In contrast, the flow of a passively diffusing substance grows in proportion to the difference in concentrations.

In addition to the Na + /K + pump, the membrane also contains a calcium pump; this pump pumps out calcium ions from the cell. The calcium pump is present in very high density in the sarcoplasmic reticulum of muscle cells. The reticulum cisterns accumulate calcium ions as a result of the splitting of the ATP molecule.

So, the result of the Na + /K + pump is the transmembrane difference in the concentrations of sodium and potassium. Learn the concentrations of sodium, potassium and chlorine (mmol/l) outside and inside the cell!

The concentration of ions inside and outside the cell

So, there are two facts that need to be taken into account in order to understand the mechanisms that maintain the resting membrane potential.

1 . The concentration of potassium ions in the cell is much higher than in the extracellular environment. 2 . The membrane at rest is selectively permeable to K + , and for Na + the permeability of the membrane at rest is negligible. If we take the permeability for potassium as 1, then the permeability for sodium at rest will be only 0.04. Consequently, there is a constant flow of ions K + from the cytoplasm along a concentration gradient. The potassium current from the cytoplasm creates a relative deficit of positive charges on the inner surface; for anions, the cell membrane is impermeable; as a result, the cytoplasm of the cell becomes negatively charged with respect to the environment surrounding the cell. This potential difference between the cell and the extracellular space, the polarization of the cell, is called the resting membrane potential (RMP).

The question arises: why does the current of potassium ions not continue until the ion concentrations outside and inside the cell are balanced? It should be remembered that this is a charged particle, therefore, its movement also depends on the charge of the membrane. The intracellular negative charge, which is created due to the current of potassium ions from the cell, prevents new potassium ions from leaving the cell. The flow of potassium ions stops when the action of the electric field compensates for the movement of the ion along the concentration gradient. Therefore, for a given difference in ion concentrations on the membrane, the so-called EQUILIBRIUM POTENTIAL for potassium is formed. This potential (Ek) is equal to RT/nF *ln Koutside/Kinside, (n is the valency of the ion.) or

Ek=61,5 log Koutside /  Kinside

Membrane potential (MP) to a large extent depends on the equilibrium potential of potassium, however, part of the sodium ions still penetrate into the resting cell, as well as chloride ions. Thus, the negative charge that the cell membrane has depends on the equilibrium potentials of sodium, potassium and chlorine and is described by the Nernst equation. The presence of this resting membrane potential is extremely important, because it determines the cell's ability to excite - a specific response to a stimulus.

The fulfillment by a neuron of its main functions - the generation, conduction and transmission of a nerve impulse becomes possible primarily because the concentration of a number of ions inside and outside the cell differs significantly. The ions K+, Na+, Ca2+, Cl- are of the greatest importance here. There is 30-40 times more potassium in the cell than outside, and about 10 times less sodium. In addition, there are much less chloride and free calcium ions in the cell than in the intercellular medium.

The difference between the concentrations of sodium and potassium is created by a special biochemical mechanism called sodium-potassium pump. It is a protein molecule embedded in the neuron membrane (Fig. 6) and actively transporting ions. Using the energy of ATP (adenosine triphosphoric acid), such a pump exchanges sodium for potassium in a ratio of 3: 2. To transfer three sodium ions from the cell to the environment and two potassium ions in the opposite direction (i.e. against the concentration gradient), the energy of one molecule is required ATP.

When neurons mature, sodium-potassium pumps are embedded in their membrane (up to 200 such molecules can be located per 1 μm2), after which potassium ions are pumped into the nerve cell and sodium ions are removed from it. As a result, the concentration of potassium ions in the cell increases, and sodium decreases. The speed of this process can be very high: up to 600 Na+ ions per second. In real neurons, it is determined, first of all, by the availability of intracellular Na + and increases sharply when it penetrates from outside. In the absence of either of the two types of ions, the operation of the pump stops, since it can only proceed as a process of exchanging intracellular Na+ for extracellular K+.

Similar transport systems also exist for Cl- and Ca2+ ions. In this case, chloride ions are removed from the cytoplasm into the intercellular environment, and calcium ions are usually transferred into the cellular organelles - mitochondria and channels of the endoplasmic reticulum.

To understand the processes occurring in a neuron, it is necessary to know that there are ion channels in the cell membrane, the number of which is set genetically. ion channel is a hole in a special protein molecule embedded in the membrane. A protein can change its conformation (spatial configuration), as a result of which the channel is in an open or closed state. There are three main types of such channels:

- permanently open;

- potential-dependent (voltage-dependent, electrosensitive) - the channel opens and closes depending on the transmembrane potential difference, i.e. potential difference between the outer and inner surfaces of the cytoplasmic membrane;

- chemo-dependent (ligand-dependent, chemosensitive) - the channel opens depending on the impact on it of one or another substance specific to each channel.

Microelectrode technique is used to study electrical processes in the nerve cell. Microelectrodes make it possible to record electrical processes in one single neuron or nerve fiber. Usually these are glass capillaries with a very thin tip less than 1 µm in diameter, filled with an electrically conductive solution (for example, potassium chloride).

If two electrodes are placed on the cell surface, no potential difference is recorded between them. But if one of the electrodes pierces the cytoplasmic membrane of the neuron (i.e., the tip of the electrode is in the internal environment), the voltmeter will register a potential jump up to approximately -70 mV (Fig. 7). This potential is called the membrane potential. It can be registered not only in neurons, but also in a less pronounced form in other cells of the body. But only in nerve, muscle and glandular cells, the membrane potential can change in response to the action of an irritant. In this case, the membrane potential of the cell, which is not affected by any stimulus, is called resting potential(PP). In different nerve cells, the value of PP is different. It ranges from -50 to -100 mV. What causes this PP?

The initial (before the development of PP) state of the neuron can be characterized as devoid of internal charge, i.e. the number of cations and anions in the cytoplasm of the cell is equal due to the presence of large organic anions, for which the neuron membrane is impermeable. In reality, this pattern is observed in the early stages. embryonic development nervous tissue. Then, as it matures, the genes that trigger the synthesis are turned on. permanently open K+ channels. After their incorporation into the membrane, K+ ions are able to freely exit the cell (where there are many of them) into the intercellular environment (where there are much fewer of them) due to diffusion.

But this does not lead to an equilibrium of potassium concentrations inside and outside the cell, because. the release of cations leads to the fact that more and more uncompensated negative charges remain in the cell. This causes the formation of an electrical potential that prevents the release of new positively charged ions. As a result, the release of potassium continues until the force of the concentration pressure of potassium, due to which it leaves the cell, and the action of the electric field that prevents this, are balanced. As a result, a potential difference arises between the external and internal environment of the cell, or an equilibrium potassium potential, which is described Nernst equation:

EK = (RT / F) (ln [K+]o / [K+ ]i),

where R is the gas constant, T is the absolute temperature, F is the Faraday number, [K+]o is the concentration of potassium ions in the external solution, [K+ ]i is the concentration of potassium ions in the cell.

The equation confirms the dependence, which can be derived even by logical reasoning - the greater the difference in the concentrations of potassium ions in the external and internal environment, the greater (in absolute value) PP.

Classical studies of PP were carried out on giant squid axons. Their diameter is about 0.5 mm, so the entire contents of the axon (axoplasm) can be removed without any problems and the axon can be filled with a potassium solution, the concentration of which corresponds to its intracellular concentration. The axon itself was placed in a potassium solution with a concentration corresponding to the intercellular medium. After that, the RI was recorded, which turned out to be -75 mV. The equilibrium potassium potential calculated by the Nernst equation for this case turned out to be very close to that obtained in the experiment.

But the RI in a squid axon filled with true axoplasm is approximately -60 mV . Where does the 15 mV difference come from? It turned out that not only potassium ions, but also sodium ions are involved in the creation of PP. The fact is that in addition to potassium channels, neuron membranes also contain permanently open sodium channels. There are much fewer of them than potassium ones, however, the membrane still allows a small amount of Na + ions to enter the cell, and therefore, in most neurons, the RP is -60-(-65) mV. The current of sodium is also proportional to the difference between its concentrations inside and outside the cell - therefore, the smaller this difference, the greater the absolute value of the PP. The sodium current also depends on the PP itself. In addition, a very small amount of Cl- ions diffuse through the membrane. Therefore, when calculating the real PP, the Nernst equation is supplemented with data on the concentrations of sodium and chlorine ions inside and outside the cell. In this case, the calculated indicators turn out to be very close to the experimental ones, which confirms the correctness of the explanation of the origin of PP by diffusion of ions through the neuron membrane.

Thus, the final level of the resting potential is determined by the interaction of a large number of factors, the main of which are the currents K +, Na + and the activity of the sodium-potassium pump. The final value of PP is the result of the dynamic balance of these processes. By acting on any of them, it is possible to shift the level of PP and, accordingly, the level of excitability of the nerve cell.

As a result of the events described above, the membrane is constantly in a state of polarization - its inner side is charged negatively with respect to the outer one. The process of reducing the potential difference (i.e., reducing the PP in absolute value) is called depolarization, and increasing it (increasing PP in absolute value) is called hyperpolarization.

Publication date: 2015-10-09; Read: 361 | Page copyright infringement

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2–1. The resting membrane potential is:

1) potential difference between the outer and inner surfaces of the cell membrane in a state of functional rest *

2) feature only excitable tissue cells

3) rapid fluctuation of the cell membrane charge with an amplitude of 90-120 mV

4) potential difference between the excited and unexcited sections of the membrane

5) potential difference between the damaged and undamaged sections of the membrane

2–2. In a state of physiological rest, the inner surface of the membrane of an excitable cell in relation to the outer one is charged:

1) positive

2) as well as the outer surface of the membrane

3) negative*

4) has no charge

5) there is no correct answer

2–3. A positive shift (decrease) in the resting membrane potential under the action of a stimulus is called:

1) hyperpolarization

2) repolarization

3) exaltation

4) depolarization*

5) static polarization

2–4. A negative shift (increase) in the resting membrane potential is called:

1) depolarization

2) repolarization

3) hyperpolarization*

4) exaltation

5) reversion

2–5. The descending phase of the action potential (repolarization) is associated with an increase in membrane permeability to ions:

2) calcium

2–6. Inside the cell, compared with the intercellular fluid, the concentration of ions is higher:

3) calcium

2–7. An increase in potassium current during the development of an action potential causes:

1) rapid repolarization of the membrane*

2) membrane depolarization

3) membrane potential reversal

4) trace depolarization

5) local depolarization

2–8. With complete blockade of fast sodium channels of the cell membrane, the following is observed:

1) reduced excitability

2) decrease in the amplitude of the action potential

3) absolute refractoriness*

4) exaltation

5) trace depolarization

2–9. The negative charge on the inner side of the cell membrane is formed as a result of diffusion:

1) K+ from the cell and the electrogenic function of the K-Na pump *

2) Na+ into the cell

3) C1 - from the cell

4) Ca2+ into the cell

5) there is no correct answer

2–10. The value of the rest potential is close to the value of the equilibrium potential for the ion:

3) calcium

2–11. The rising phase of the action potential is associated with an increase in ion permeability:

2) there is no correct answer

3) sodium*

2–12. Specify the functional role of the resting membrane potential:

1) its electric field affects the state of protein channels and membrane enzymes*

2) characterizes the increase in cell excitability

3) is the main unit of encoding information in the nervous system

4) ensures the operation of diaphragm pumps

5) characterizes a decrease in cell excitability

2–13. The ability of cells to respond to the action of stimuli with a specific reaction, characterized by rapid, reversible membrane depolarization and a change in metabolism, is called:

1) irritability

2) excitability*

3) lability

4) conductivity

5) automation

2–14. Biological membranes, participating in the change in intracellular content and intracellular reactions due to the reception of extracellular biologically active substances, perform the function:

1) barrier

2) receptor-regulatory *

3) transport

4) cell differentiation

2–15. The minimum stimulus force necessary and sufficient for a response to occur is called:

1) threshold*

2) superthreshold

3) submaximal

4) subthreshold

5) maximum

2–16. With an increase in the threshold of irritation, the excitability of the cell:

1) increased

2) decreased*

3) has not changed

4) everything is correct

5) there is no correct answer

2–17. Biological membranes, participating in the conversion of external stimuli of non-electrical and electrical nature into bioelectrical signals, perform mainly the function of:

1) barrier

2) regulatory

3) cell differentiation

4) transport

5) action potential generation*

2–18. The action potential is:

1) a stable potential that is established on the membrane when two forces are in balance: diffusion and electrostatic

2) the potential between the outer and inner surfaces of the cell in a state of functional rest

3) fast, actively propagating, phase fluctuation of the membrane potential, accompanied, as a rule, by recharging the membrane *

4) a slight change in the membrane potential under the action of a subthreshold stimulus

5) prolonged, congestive depolarization of the membrane

2–19. Membrane permeability for Na+ in the depolarization phase of the action potential:

1) sharply increases and a powerful sodium current enters the cell *

2) sharply decreases and a powerful sodium current leaving the cell appears

3) does not change significantly

4) everything is correct

5) there is no correct answer

2–20. Biological membranes, participating in the release of neurotransmitters in synaptic endings, perform mainly the function of:

1) barrier

2) regulatory

3) intercellular interaction*

4) receptor

5) action potential generation

2–21. The molecular mechanism that ensures the removal of sodium ions from the cytoplasm and the introduction of potassium ions into the cytoplasm is called:

1) voltage-gated sodium channel

2) non-specific sodium-potassium channel

3) chemodependent sodium channel

4) sodium-potassium pump*

5) leakage channel

2–22. The system of movement of ions through the membrane along the concentration gradient, not requiring a direct expenditure of energy is called:

1) pinocytosis

2) passive transport*

3) active transport

4) persorption

5) exocytosis

2–23. The level of membrane potential at which an action potential occurs is called:

1) resting membrane potential

2) critical level of depolarization*

3) trace hyperpolarization

4) zero level

5) trace depolarization

2–24. With an increase in the concentration of K + in the extracellular environment with a resting membrane potential in an excitable cell, the following will occur:

1) depolarization*

2) hyperpolarization

3) transmembrane potential difference will not change

4) stabilization of the transmembrane potential difference

5) there is no correct answer

2–25. The most significant change when exposed to a fast sodium channel blocker will be:

1) depolarization (decrease in resting potential)

2) hyperpolarization (increased resting potential)

3) decrease in the steepness of the depolarization phase of the action potential *

4) slowing down the repolarization phase of the action potential

5) there is no correct answer

3. MAIN PATTERNS OF IRRITATION

EXCITABLE TISSUES

3–1. The law according to which, with an increase in the strength of the stimulus, the response gradually increases until it reaches a maximum, is called:

1) "all or nothing"

2) strength-duration

3) accommodation

4) forces (power relations) *

5) polar

3–2. The law according to which an excitable structure responds to threshold and suprathreshold stimuli with the maximum possible response is called:

2) "all or nothing" *

3) strength-duration

4) accommodation

5) polar

3–3. The minimum time during which a current equal to twice the rheobase (twice the threshold force) causes excitation is called:

1) good time

2) accommodation

3) adaptation

4) chronaxia*

5) lability

3–4. The structure obeys the law of force:

1) cardiac muscle

2) single nerve fiber

3) single muscle fiber

4) whole skeletal muscle*

5) single nerve cell

The law "All or nothing" obeys the structure:

1) whole skeletal muscle

2) nerve trunk

3) cardiac muscle*

4) smooth muscle

5) nerve center

3–6. The adaptation of a tissue to a slowly increasing stimulus is called:

1) lability

2) functional mobility

3) hyperpolarization

4) accommodation*

5) braking

3–7. The paradoxical phase of parabiosis is characterized by:

1) a decrease in response with an increase in the strength of the stimulus *

2) a decrease in the response with a decrease in the strength of the stimulus

3) an increase in response with an increase in the strength of the stimulus

4) the same response with an increase in the strength of the stimulus

5) lack of response to stimuli of any strength

3–8. The irritation threshold is an indicator of:

1) excitability*

2) contractility

3) lability

4) conductivity

5) automation

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ROLE OF ACTIVE ION TRANSPORT IN THE FORMATION OF MEMBRANE POTENTIAL

One of the advantages of an “ideal” membrane that allows any one ion to pass through is the maintenance of the membrane potential for an arbitrarily long time without energy expenditure, provided that the penetrating ion is initially distributed unevenly on both sides of the membrane. At the same time, the membrane of living cells is permeable to one degree or another for all inorganic ions present in the solution surrounding the cell. Therefore, the cells must

we somehow maintain the intracellular concentration of ions at a certain level. Sufficiently indicative in this respect are sodium ions, on the example of the permeability of which, in the previous section, the deviation of the muscle membrane potential from the equilibrium potassium potential was analyzed. According to the measured concentrations of sodium ions outside and inside the muscle cell, the equilibrium potential calculated by the Nernst equation for these ions will be about 60 mV, and with a plus sign inside the cell. The membrane potential, calculated according to the Goldman equation and measured using microelectrodes, is 90 mV with a minus sign inside the cell. Thus, its deviation from the equilibrium potential for sodium ions will be 150 mV. Under the action of such a high potential, even at low permeability, sodium ions will enter through the membrane and accumulate inside the cell, which, accordingly, will be accompanied by the release of potassium ions from it. As a result of this process, the intra- and extracellular concentrations of ions will equalize after some time.

In fact, this does not happen in a living cell, since sodium ions are constantly being removed from the cell with the help of the so-called ion pump. The assumption about the existence of an ion pump was put forward by R. Dean in the 40s of the XX century. and was an extremely important addition to the membrane theory of the formation of the resting potential in living cells. It has been experimentally shown that the active "pumping out" of Na + from the cell comes with the obligatory "pumping" of potassium ions into the cell (Fig. 2.8). Since the permeability of the membrane for sodium ions is small, their entry from the external environment into the cell will occur slowly, therefore

Low concentration of K+ High concentration of Na++

the pump will effectively maintain a low concentration of sodium ions in the cell. The permeability of the membrane for potassium ions at rest is quite high, and they easily diffuse through the membrane.

It is not necessary to spend energy to maintain a high concentration of potassium ions, it is maintained due to the emerging transmembrane potential difference, the mechanisms of which are detailed in the previous sections. The transfer of ions by the pump requires the expenditure of metabolic energy of the cell. The energy source of this process is the energy stored in macroergic bonds of ATP molecules. Energy is released due to the hydrolysis of ATP with the help of the enzyme adenosine triphosphatase. It is believed that the same enzyme directly carries out the transfer of ions. In accordance with the structure of the cell membrane, ATPase is one of the integral proteins built into the lipid bilayer. A feature of the carrier enzyme is its high affinity on the outer surface for potassium ions, and on the inner surface for sodium ions. The action of inhibitors of oxidative processes (cyanides or azides) on the cell, cooling of the cell blocks the hydrolysis of ATP, as well as the active transfer of sodium and potassium ions. Sodium ions gradually enter the cell, and potassium ions leave it, and as the ratio [K +] o / [K +], - decreases, the resting potential will slowly decrease to zero. We discussed the situation when the ion pump removes one positively charged sodium ion from the intracellular environment and, accordingly, transfers one positively charged potassium ion from the extracellular space (ratio 1: 1). In this case, the ion pump is said to be electrically neutral.

At the same time, it was experimentally found that in some nerve cells the ion pump removes more sodium ions in the same period of time than it pumps in potassium ions (the ratio can be 3:2). In such cases, the ion pump is electrogenic, T.

Physiologia_Answer

That is, he himself creates a small but constant total current of positive charges from the cell and additionally contributes to the creation of a negative potential inside it. Note that the additional potential created by the electrogenic pump in a resting cell does not exceed a few millivolts.

Let us summarize the information about the mechanisms of formation of the membrane potential - the resting potential in the cell. The main process, due to which most of the potential with a negative sign is created on the inner surface of the cell membrane, is the occurrence of an electric potential that delays the passive exit of potassium ions from the cell along its concentration gradient through potassium channels - in-

tegral proteins. Other ions (for example, sodium ions) participate in the creation of the potential only to a small extent, since the membrane permeability for them is much lower than for potassium ions, i.e. the number of open channels for these ions at rest is small . An extremely important condition for maintaining the resting potential is the presence in the cell (in the cell membrane) of an ion pump (integral protein), which ensures the concentration of sodium ions inside the cell at a low level and thereby creates the prerequisites for the main potential-forming intracellular ions become potassium ions. A small contribution to the resting potential can be made directly by the ion pump itself, but on condition that its work in the cell is electrogenic.

The concentration of ions inside and outside the cell

So, there are two facts that need to be taken into account in order to understand the mechanisms that maintain the resting membrane potential.

1 . The concentration of potassium ions in the cell is much higher than in the extracellular environment. 2 . The membrane at rest is selectively permeable to K+, and for Na+, the permeability of the membrane at rest is negligible. If we take the permeability for potassium as 1, then the permeability for sodium at rest will be only 0.04. Consequently, there is a constant flow of K+ ions from the cytoplasm along the concentration gradient. The potassium current from the cytoplasm creates a relative deficit of positive charges on the inner surface; for anions, the cell membrane is impermeable; as a result, the cytoplasm of the cell turns out to be negatively charged with respect to the environment surrounding the cell. This potential difference between the cell and the extracellular space, the polarization of the cell, is called the resting membrane potential (RMP).

The question arises: why does the current of potassium ions not continue until the ion concentrations outside and inside the cell are balanced? It should be remembered that this is a charged particle, therefore, its movement also depends on the charge of the membrane. The intracellular negative charge, which is created due to the current of potassium ions from the cell, prevents new potassium ions from leaving the cell. The flow of potassium ions stops when the action of the electric field compensates for the movement of the ion along the concentration gradient. Therefore, for a given difference in ion concentrations on the membrane, the so-called EQUILIBRIUM POTENTIAL for potassium is formed. This potential (Ek) is equal to RT/nF *ln /, (n is the valency of the ion.) or

Ek=61.5 log/

Membrane potential (MP) to a large extent depends on the equilibrium potential of potassium, however, part of the sodium ions still penetrate into the resting cell, as well as chloride ions. Thus, the negative charge that the cell membrane has depends on the equilibrium potentials of sodium, potassium and chlorine and is described by the Nernst equation. The presence of this resting membrane potential is extremely important, because it determines the cell's ability to excite - a specific response to a stimulus.

cell excitation

IN excitement cells (transition from rest to an active state) occurs with an increase in the permeability of ion channels for sodium, and sometimes for calcium. The reason for the change in permeability can also be a change in the potential of the membrane - electrically excitable channels are activated, and the interaction of membrane receptors with biologically active substance- receptor - controlled channels, and mechanical action. In any case, for the development of arousal, it is necessary initial depolarization - a slight decrease in the negative charge of the membrane, caused by the action of the stimulus. An irritant can be any change in the parameters of the external or internal environment of the body: light, temperature, chemicals (impact on taste and olfactory receptors), stretching, pressure. Sodium rushes into the cell, an ion current occurs and the membrane potential decreases - depolarization membranes.

Table 4

Change in membrane potential during cell excitation.

Pay attention to the fact that sodium enters the cell along the concentration gradient and along the electrical gradient: the sodium concentration in the cell is 10 times lower than in the extracellular environment and the charge in relation to the extracellular one is negative. At the same time, potassium channels are also activated, but sodium (fast) ones are activated and inactivated within 1–1.5 milliseconds, and potassium channels take longer.

Changes in the membrane potential are usually depicted graphically. The upper figure shows the initial depolarization of the membrane - a change in potential in response to the action of a stimulus. For each excitable cell, there is a special level of membrane potential, upon reaching which the properties of sodium channels change dramatically. This potential is called critical level of depolarization (KUD). When the membrane potential changes to the KUD, fast, potential-dependent sodium channels open, the flow of sodium ions rushes into the cell. With the transition of positively charged ions into the cell, in the cytoplasm, the positive charge increases. As a result, the transmembrane potential difference decreases, the MP value decreases to 0, and then, as sodium further enters the cell, the membrane is recharged and the charge is reversed (overshoot) - now the surface becomes electronegative with respect to the cytoplasm - the membrane is completely DEPOLARIZED - the middle figure. There is no further charge change because sodium channels are inactivated- more sodium cannot enter the cell, although the concentration gradient changes very slightly. If the stimulus has such a force that it depolarizes the membrane to the FCD, this stimulus is called a threshold stimulus, it causes excitation of the cell. The potential reversal point is a sign that the entire range of stimuli of any modality has been translated into the language nervous system- impulses of excitation. Impulses or excitation potentials are called action potentials. Action potential (AP) - a rapid change in the membrane potential in response to the action of a threshold stimulus. AP has standard amplitude and time parameters that do not depend on the strength of the stimulus - the "ALL OR NOTHING" rule. The next stage is the restoration of the resting membrane potential - repolarization(bottom figure) is mainly due to active ion transport. The most important process of active transport is the work of Na / K - a pump that pumps sodium ions out of the cell, while simultaneously pumping potassium ions into the cell. Restoration of the membrane potential occurs due to the current of potassium ions from the cell - potassium channels are activated and allow potassium ions to pass until the equilibrium potassium potential is reached. This process is important because until the MPP is restored, the cell is not able to perceive a new excitation impulse.

HYPERPOLARIZATION - a short-term increase in MP after its restoration, which is due to an increase in the permeability of the membrane for potassium and chlorine ions. Hyperpolarization occurs only after PD and is not characteristic of all cells. Let us try once again to graphically represent the phases of the action potential and the ionic processes underlying the changes in the membrane potential (Fig.

Resting potential of a neuron

nine). Let us plot the values ​​of the membrane potential in millivolts on the abscissa axis, and the time in milliseconds on the ordinate axis.

1. Membrane depolarization to KUD - any sodium channels can open, sometimes calcium, both fast and slow, and voltage-dependent, and receptor-controlled. It depends on the type of stimulus and cell type.

2. Rapid entry of sodium into the cell - fast, voltage-dependent sodium channels open, and depolarization reaches the potential reversal point - the membrane is recharged, the sign of the charge changes to positive.

3. Restoration of the potassium concentration gradient - pump operation. Potassium channels are activated, potassium passes from the cell to the extracellular environment - repolarization, restoration of the MPP begins

4. Trace depolarization, or negative trace potential - the membrane is still depolarized relative to the MPP.

5. Trace hyperpolarization. Potassium channels remain open and additional potassium current hyperpolarizes the membrane. After that, the cell returns to the initial level of MPP. The duration of PD is for different cells from 1 to 3-4 ms.

Figure 9 Action potential phases

Notice the three potential values ​​that are important and constant for each cell of its electrical characteristics.

1. MPP - electronegativity of the cell membrane at rest, providing the ability to excite - excitability. In the figure, MPP \u003d -90 mV.

2. KUD - the critical level of depolarization (or the threshold for generating a membrane action potential) - this is the value of the membrane potential, upon reaching which they open fast, potential dependent sodium channels and the membrane is recharged due to the entry of positive sodium ions into the cell. The higher the electronegativity of the membrane, the more difficult it is to depolarize it to the FCD, the less excitable such a cell is.

3. Potential reversal point (overshoot) - such a value positive membrane potential, at which positively charged ions no longer penetrate the cell - a short-term equilibrium sodium potential. In the figure + 30 mV. The total change in the membrane potential from –90 to +30 will be 120 mV for a given cell, this value is the action potential. If this potential arose in a neuron, it will spread along the nerve fiber, if in muscle cells it will spread along the membrane of the muscle fiber and lead to contraction, in glandular to secretion - to the action of the cell. This is the specific response of the cell to the action of the stimulus, excitation.

When exposed to a stimulus subthreshold strength there is an incomplete depolarization - LOCAL RESPONSE (LO).

Incomplete or partial depolarization is a change in the charge of the membrane that does not reach the critical level of depolarization (CDL).

Figure 10. Change in membrane potential in response to the action of a stimulus of subthreshold strength - local response

The local response has basically the same mechanism as AP, its ascending phase is determined by the entry of sodium ions, and the descending phase is determined by the exit of potassium ions.

However, the LO amplitude is proportional to the strength of subthreshold stimulation, and not standard, as in PD.

Table 5

It is easy to see that there are conditions in cells under which a potential difference should arise between the cell and the intercellular medium:

1) cell membranes are well permeable to cations (primarily potassium), while the permeability of membranes to anions is much less;

2) the concentrations of most substances in cells and in the intercellular fluid are very different (compare with what was said on p.

). Therefore, a double electric layer will appear on the cell membranes ("minus" on the inside of the membrane, "plus" on the outside), and a constant potential difference must exist on the membrane, which is called the resting potential. The membrane is said to be polarized at rest.

For the first time, the hypothesis about the similar nature of the PP of cells and the diffusion potential of Nernst was expressed in 1896.

Knowledge base

student of the Military Medical Academy Yu.V. Chagovets. Now this point of view is confirmed by numerous experimental data. True, there are some discrepancies between the measured PP values ​​and those calculated using formula (1), but they are explained by two obvious reasons. Firstly, there is not one cation in the cells, but many (K, Na, Ca, Mg, etc.). This can be taken into account by replacing the Nernst formula (1) with a more complex formula, eaten away by Goldman:

Where pK is the permeability of the membrane for potassium, pNa is the same for sodium, pCl is the same for chlorine; [K + ] e is the concentration of potassium ions outside the cell, [K + ] i is the same inside the cell (similarly for sodium and chlorine); the ellipsis denotes the corresponding terms for other ions. Chlorine ions (and other anions) go in the opposite direction to potassium and sodium ions, so the signs "e" and "i" for them are in reverse order.

The calculation using the Goldman formula gives a much better agreement with the experiment, but some discrepancies still remain. This is explained by the fact that when deriving formula (2), the work of active transport was not considered. Accounting for the latter makes it possible to achieve almost complete agreement with experiment.

19. Sodium and potassium channels in the membrane and their role in bioelectrogenesis. Gate mechanism. Features of potential-dependent channels. The mechanism of the action potential. The state of the channels and the nature of ion flows in different phases of PD. The role of active transport in bioelectrogenesis. Critical membrane potential. The all-or-nothing law for excitable membranes. Refractory.

It turned out that the selective filter has a "rigid" structure, that is, it does not change its clearance under different conditions. Transitions of the channel from open to closed and vice versa are related to the operation of a non-selective filter, a gate mechanism. Under the gate processes occurring in one or another part of the ion channel, which is called the gate, we understand any changes in the conformation of the protein molecules that form the channel, as a result of which its pair can open or close. Therefore, it is customary to call the gate functional groups of protein molecules that provide gate processes. It is important that the gates are set in motion by physiological stimuli, that is, those that are present in natural conditions. Among physiological stimuli, shifts in the membrane potential play a special role.

There are channels that are controlled by the potential difference across the membrane, being open at some values ​​of the membrane potential and closed at others. Such channels are called potential-dependent. It is with them that the generation of PD is connected. Due to their special significance, all ion channels of biomembranes are divided into 2 types: voltage-dependent and voltage-independent. The natural stimuli that control the movement of the gate in the channels of the second type are not shifts in the membrane potential, but other factors. For example, in chemosensitive channels, the role of the control stimulus belongs to chemicals.

An essential component of a voltage-gated ion channel is a voltage sensor. This is the name of a group of protein molecules that can respond to changes in the electric field. So far, there is no specific information about what they are and how they are located, but it is clear that the electric field can interact in the physical environment only with charges (either free or bound). It was assumed that Ca2+ (free charges) serves as a voltage sensor, since changes in its content in the intercellular fluid lead to the same consequences as shifts in the membrane potential. For example, a ten-fold decrease in the concentration of calcium ions in the interstitium is equivalent to a depolarization of the plasma membrane by approximately 15 mV. However, later it turned out that Ca2+ is necessary for the operation of the voltage sensor, but is not itself. PD is generated even when the concentration of free calcium in the intercellular medium falls below 10~8 mol. In addition, the Ca2+ content in the cytoplasm generally has little effect on the ionic conductivity of the plasma membrane. Obviously, the voltage sensor is bound charges - groups of protein molecules with a large dipole moment. They are embedded in a lipid bilayer, which is characterized by a rather low viscosity (30 - 100 cP) and low dielectric constant. This conclusion was drawn from the study of the kinetic characteristics of the motion of the voltage sensor with shifts in the membrane potential. This movement is a typical displacement current.

The modern functional model of the sodium voltage-dependent channel provides for the existence of two types of gates in it, operating in antiphase. They differ in inertial properties. More mobile (light) are called m-gates, more inertial (heavy) - h - gates. At rest, the h-gates are open, m-gates are closed, the movement of Na+ through the channel is impossible. When the plasmolemma is depolarized, the gates of both types begin to move, but due to the unequal inertia, the m-gates have time to

open before the h-gate closes. At this moment, the sodium channel is open and Na + rushes through it into the cell. The delay in the movement of the h-gate relative to the m-gate corresponds to the duration of the depolarization phase of AP. When the h-gate closes, the flow of Na + through the membrane will stop and repolarization will begin. Then the h - and m - gates return to their original state. Potential-dependent sodium channels are activated (turned on) during rapid (jump-like) depolarization of the plasma membrane. ,

PD is created due to faster diffusion of sodium ions through the plasma membrane compared to anions that form salts with it in the intercellular medium. Therefore, depolarization is associated with the entry of sodium cations into the cytoplasm. With the development of PD, sodium does not accumulate in the cell. When excited, there is an incoming and outgoing flow of sodium. The occurrence of AP is not caused by a violation of ionic concentrations in the cytoplasm, but by a drop in the electrical resistance of the plasma membrane due to an increase in its permeability to sodium.

As already mentioned, under the action of threshold and suprathreshold stimuli, the excitable membrane generates AP. This process is characterized law "all or nothing. It is the antithet of gradualism. The meaning of the law is that the AP parameters do not depend on the intensity of the stimulus. Once the IMF is reached, changes in the potential difference across the excitable membrane are determined only by the properties of its voltage-gated ion channels, which provide the incoming current. Among them, the external stimulus opens only the most sensitive ones. Others open at the expense of the previous ones, already regardless of the stimulus. They talk about the spantane nature of the process of involving ever new potential-dependent ion channels in the transmembrane transport of ions. So the amplitude. The duration and steepness of the leading and trailing fronts of AP depend only on the ionic gradients on the cell membrane and the kinetic characteristics of its channels. The all-or-nothing law is the most characteristic property of single cells and fibers that have an excitable membrane. It is not characteristic of most multicellular formations. The exception is the structures organized according to the type of syncytium.

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