There is more sodium or potassium in the cell. Changes in extracellular potassium concentration (K)

The mineral composition of cells differs sharply from the mineral composition of the external environment. In the cell, as a rule, the concentration of potassium, magnesium and phosphorus ions predominates, and in the environment - sodium and chlorine. This is clearly seen from the data in Table 7.

Inside the cell, mineral substances are also distributed unevenly between the cytoplasm, its organelles and the nucleus. Thus, the concentration of sodium in the nucleus of frog eggs is three times higher than in the cytoplasm, and potassium is two times higher (Table 8).

Mitochondria are also able to accumulate potassium and especially calcium. Its concentration in isolated mitochondria can exceed the concentration of calcium in the surrounding saline solution by 3500 times. This uneven distribution is explained by the fact that these substances in the nucleus and mitochondria are partially connected.

Salt asymmetry depends on the functional state of the cell, and with the death of the latter, it is lost; the content of salts in the cell and the environment is leveled. The isolation of cells and tissues from the body is usually accompanied by a slight loss of potassium and an increase in the amount of sodium.

Rice. 25. Dependence of the concentration of sodium and chlorine ions in muscle fibers on their concentration in the environment, meq% (Fenn, Cobb and Marsh, 1934–1935)

When the concentration of sodium and chlorine ions in the medium changes, their content in cells changes in direct proportion (Fig. 25). For many other ions (K+, Ca2+, Mg2+, etc.), proportionality is not observed. The dependence of potassium concentration in frog muscles on its concentration in the medium is shown in Figure 26.

Rice. Fig. 26. Dependence of the concentration of potassium ions in frog muscles (C cl, meq per 100 g of muscle) on their concentration in the medium (C av, meq %)

Almost all mineral ions penetrate cells, though at very different rates. Using the isotope technique, it was shown that there is a constant exchange of cell ions for environmental ions even with a stationary (unchanging) distribution. In this case, the inward flow of the ion is equal to its flow in the opposite direction. Ion fluxes are usually expressed in pmol (1 pmol equals 10-12 M). Table 9 shows the fluxes of potassium and sodium ions into the cell for different objects. Mineral ions penetrate faster into those cells that have a higher level of metabolism. In some cells, the presence of fractions of ions with different exchange rates (fast and slow fractions) was found, which is associated with their different states inside the cell. Ions can be in the cell in a free ionized form and in a non-ionized state associated with proteins, nucleic acids, phospholipids. Almost all calcium and magnesium are found in the protoplasm in bound form. The mineral anions of the cell, apparently, are entirely in a free state.

In terms of the rate of penetration into the cell, cations can differ by tens and hundreds of times (Table 10).

As for anions, monovalent ones penetrate several times faster than divalent ones. Exceptionally high anion permeability is observed for erythrocytes. According to the rate of penetration into human erythrocytes, anions can be arranged in the following row: I (1.24) > CNS - (1.09), NO 3 - (l.09) > Cl - (1.00) > SO 4 2- ( 0.21) > HPO 4 2- (0.15).

Rice. 27. Dependence of the magnitude of the flow of potassium ions into erythrocytes on their concentration in the medium. The abscissa shows the concentration of potassium ions in the medium, mM; along the y-axis - the flow of potassium ions into erythrocytes, μM/g h

The values ​​of ion fluxes into the cell do not depend directly on their concentration. With an increase in the ion concentration in the external medium, the flux first increases rapidly, and then its increase decreases. This can be seen in curve (1), in Figure 27, which shows the dependence of the flow of potassium ions into human erythrocytes on its concentration in the medium. This curve has two components. One of them (2) reflects a linear dependence - it is a passive component and reflects diffusion. The other component (3) indicates the saturation process and is associated with ion transport and energy consumption, therefore it is called active and it can be expressed by the Michaelis-Menten formula.

When cells are excited and damaged, mineral ions are redistributed between the cell and the environment: cells lose potassium ions and are enriched with sodium and chlorine ions. Physiological activity is accompanied by an increase in the rate of exchange of cellular ions for the corresponding ions of the environment and an increase in the permeability for ions.

With each impulse running through the nerve fiber, the fiber loses a certain amount of potassium ions and approximately the same amount of sodium ions enter the fiber (Table 11). When cells are excited, the permeability for lithium, rubidium, cesium, choline, and calcium ions also increases. So, with one contraction of the skeletal muscle, the entry of calcium into the cell increases by 0.2 pmol / cm 2.

It has now been proven that the ionic asymmetry inherent in all living cells is provided by the activity of membranes that have the function of active transport. With its help, sodium ions are pumped out of the cell, and potassium ions are introduced into the cell. This transport function is carried out by enzyme systems with ATPase activity dependent on potassium and sodium.

The scheme of transport of potassium and sodium ions is shown in Figure 28. It is believed that when the form of the carrier x changes into y, when ATP energy is required, phosphorylation occurs: x + ATP → xATP → xP + ADP, where xP corresponds to y.

Rice. 28. Scheme of transport of sodium and potassium ions through the surface membrane (according to Glynn)

The membranes of the sarcoplasmic reticulum of muscle fibers have a powerful active transport system that carries potassium ions in a certain direction. What is the specific mechanism of the transport system is unknown. There are ideas about mobile single carriers, and about collective transport, and about relay race transmission.

Positively charged potassium ions into the environment from the cytoplasm cells in the process of establishing osmotic equilibrium. Anions of organic acids that neutralize the charge of potassium ions in the cytoplasm cannot leave the cell, however, potassium ions, whose concentration in the cytoplasm is high compared to the environment, diffuse from the cytoplasm until the electric charge they create begins to balance their concentration gradient on the cell membrane.

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✪ Membrane potentials - Part 1

✪ Resting potential: - 70 mV. Depolarization, repolarization

✪ Resting potential

Subtitles

I'll draw a small cage. It will be a typical cell, and it is filled with potassium. We know that cells love to accumulate it inside themselves. Lots of potassium. Let its concentration be somewhere around 150 millimoles per liter. Huge amount of potassium. Let's put it in parentheses, because parentheses denote concentration. There is also some potassium on the outside. Here the concentration will be approximately 5 millimoles per liter. I will show you how the concentration gradient will be set. It doesn't happen on its own. This requires a lot of energy. Two potassium ions are pumped in, and at the same time three sodium ions leave the cell. So potassium ions get inside initially. Now that they're inside, will they be held here on their own? Of course not. They find anions, small molecules or atoms with a negative charge, and position themselves near them. Thus, the total charge becomes neutral. Each cation has its own anion. And usually these anions are proteins, some structures that have a negative side chain. It can be chloride, or, for example, phosphate. Anything. Any of these anions will do. I'll draw a few more anions. So here are two potassium ions that just got inside the cell, this is what it looks like now. If everything is good and static, then this is how they look. And in fact, to be completely fair, there are also small anions here, which are here on a par with potassium ions. There are small holes in the cell through which potassium can flow out. Let's see what it will look like and how it will affect what happens here. So we have these little channels. Only potassium can pass through them. That is, these channels are very specific for potassium. Nothing else can pass through them. No anions, no proteins. Potassium ions, as it were, are looking for these channels and reasoning: “Wow, how interesting! So much potassium here! We should go outside." And all these potassium ions just leave the cell. They go outside. And as a result, an interesting thing happens. Most of them have moved outside. But there are already a few potassium ions outside. I said that there was this little ion here, and it could theoretically get inside. He can get into this cage if he wants to. But the fact is that in total, in total, you have more movements outward than inward. Now I'm erasing this path because I want you to remember that we have more potassium ions that tend to escape due to the presence of a concentration gradient. This is the first stage. Let me write it down. The concentration gradient causes potassium to move outward. Potassium begins to move out. Comes out of the cell. And then what? Let me draw it in the process of going outside. This potassium ion is here now, and this one is here. Only anions remain. They remained after the departure of potassium. And these anions begin to produce a negative charge. Very large negative charge. Only a few anions moving back and forth create a negative charge. And the potassium ions on the outside think it's all very interesting. There is a negative charge here. And since it is there, they are attracted to it, since they themselves have a positive charge. They are drawn to a negative charge. They want to return. Now think. You have a concentration gradient that pushes potassium out. But, on the other hand, there is a membrane potential - in this case negative - which arises from the fact that potassium left behind an anion. This potential stimulates potassium to come back. One force, concentration, pushes the potassium ion out, another force, the membrane potential, which is created by potassium, forces it back in. I'll free up some space. Now I will show you something interesting. Let's build two curves. I'll try not to miss anything on this slide. I'll draw everything here and then a small fragment of it will be visible. We build two curves. One of them will be for the concentration gradient, and the other for the membrane potential. It will be potassium ions outside. If you follow them for time - this time - you get something like this. Potassium ions tend to go outside and reach equilibrium at a certain point. Let's do the same with time on this axis. This is our membrane potential. We start at the zero time point and get a negative result. The negative charge will get bigger and bigger. We start at the zero point of the membrane potential, and it is at the point where potassium ions begin to come out that the following happens. In general terms, everything is very similar, but this happens, as it were, in parallel with changes in the concentration gradient. And when these two values ​​equalize with each other, when the number of potassium ions going out is equal to the number of potassium ions that come back in, you get such a plateau. And it turns out that the charge in this case is minus 92 millivolts. At this point, where there is practically no difference in terms of the total movement of potassium ions, equilibrium is observed. It even has its own name - "equilibrium potential for potassium." Upon reaching the value of minus 92 - and it differs depending on the type of ions - upon reaching minus 92 for potassium, an equilibrium of potentials is created. I will write down that the charge for potassium is minus 92. This happens only when the cell is permeable to only one element, for example, to potassium ions. And still the question may arise. You might be thinking, “Wait a second! If the potassium ions move outward - which they do - then don't we have a lower concentration at a certain point, because the potassium has already gone out of here, and a higher concentration here is provided by moving the potassium outward? Technically it is. Here, outside, contains more potassium ions. And I did not mention that the volume also changes. This results in a higher concentration. And the same is true for the cell. Technically, there is a lower concentration. But actually I didn't change the value. And the reason is the following. Look at these values, these are moths. And that's a huge number, right? 6.02 times 10 to the minus 23 power is not a small number at all. And if you multiply it by 5, it will come out approximately - let me quickly calculate what we got. 6 multiplied by 5 is 30. And here are millimoles. 10 to 20 moles. It's just a huge amount of potassium ions. And to create a negative charge, they need very little. That is, the changes caused by the movements of the ions will be insignificant compared to 10 to the 20th power. This is why concentration changes are not taken into account.

Discovery history

The resting potential for most neurons is about -60 mV - -70 mV. The cells of non-excitable tissues also have a potential difference on the membrane, which is different for cells of different tissues and organisms.

Resting potential formation

The PP is formed in two stages.

First stage: the creation of negligible (-10 mV) negativity inside the cell due to an unequal asymmetric exchange of Na + for K + in a ratio of 3: 2. As a result, more positive charges leave the cell with sodium than return into it with potassium. This feature of the sodium-potassium pump, which exchanges these ions through the membrane with the expenditure of ATP energy, ensures its electrogenicity.

The results of the operation of membrane ion exchanger pumps at the first stage of the formation of PP are as follows:

1. Deficiency of sodium ions (Na +) in the cell.

2. An excess of potassium ions (K +) in the cell.

3. The appearance of a weak electric potential on the membrane (-10 mV).

Second phase: the creation of a significant (-60 mV) negativity inside the cell due to the leakage of K + ions from it through the membrane. Potassium ions K + leave the cell and take positive charges out of it, bringing the negative to -70 mV.

So, the resting membrane potential is a deficit of positive electric charges inside the cell, which occurs due to the leakage of positive potassium ions from it and the electrogenic action of the sodium-potassium pump.

I expressed the idea of ​​two forms of convertible energy in 1975. Two years later, this view was supported by Mitchell. Meanwhile, in the group of A. Glagolev, experiments began to test one of the predictions of this new concept.

I reasoned as follows. If the proton potential is a bargaining chip, then the cell must have a sufficient amount of such "banknotes".

This requirement was met when it was about ATP. The cell always contains rather large amounts of ATP, and measures have been taken to stabilize this amount under conditions of changing conjuncture - continuously varying rates of ATP formation and use. There is a special substance - creatine phosphate, which is involved in only one reaction - ADP phosphorylation:

ADP + creatine phosphate ⇔ ATP + creatine.

When ATP is in excess and ADP is in short supply, the reaction goes from right to left and creatine phosphate accumulates, which under these conditions becomes much larger than ATP. But as soon as the level of ADP rises and ATP decreases, the reaction changes direction, and creatine phosphate becomes a supplier of ATP. Thus, creatine phosphate performs its function as a stabilizer, a buffer of the ATP level.

And what about the proton potential?

A simple calculation allows you to convert one energy "currency" into another. This calculation shows that the amount of energy accumulated, for example, by a bacterial cell in the form of a proton potential, turns out to be almost a thousand times less than the amount of ATP if the proton potential is in electric form. This quantity is of the same order as the number of potential generators and consumers in the bacterial membrane.

This situation creates a special need for a buffer system that stabilizes the level of the proton potential. Otherwise, even a short-term excess of the total rate of processes consuming the potential over the rate of its generation will lead to the disappearance of the potential and the shutdown of all systems fed by the potential.

So, there must be a buffer for the proton potential, like creatine phosphate for ATP. But what kind of component did nature choose for such a role?

Thinking about this problem, I tried to find some potential-related biological system, the function of which would be unknown.

One of the old mysteries of biology: why does a cell absorb potassium ions and throw out sodium ions, creating a costly asymmetry in the distribution of these similar ions between the cytoplasm and the environment? Practically in any living cell there are much more potassium ions than sodium ions, while in the environment sodium is in huge excess over potassium. Maybe Na + is a poison for the cell?

No, it's not. While some enzyme systems do work better in KCl than in NaCl, this appears to be a secondary adaptation to the "high potassium" and "low sodium" internal environment of the cell. Over a huge period of biological evolution, the cell could adapt to the natural ratio of alkali metal ions in the external environment. Halophilic bacteria live in a saturated solution of NaCl, and the concentration of Na + in their cytoplasm sometimes reaches a mole per liter, which is almost a thousand times higher than the concentration of Na + in ordinary cells. So Na+ is not a poison.

Note that the same halophilic bacteria maintain an intracellular concentration of K + of about 4 moles per liter, spending enormous amounts of energy resources on the creation of a sodium-potassium gradient.

Excitable animal cells, such as neurons, are known to use the sodium-potassium gradient to conduct nerve impulses. But what about other types of cells, such as bacteria?

Let's turn to the mechanism of transport of K + and Na + through the bacterial membrane. It is known that between the bacterial cytoplasm and the external environment there is a difference in electrical potentials, supported by the work of generator proteins in the bacterial membrane. By pumping out protons from inside the cell to the outside, generator proteins thereby charge the inside of the bacterium negatively. Under these conditions, the accumulation of K + ions inside the cell could occur simply due to electrophoresis - the movement of a positively charged potassium ion into the negatively charged cytoplasm of the bacterium.

In this case, the potassium flow should discharge the membrane preliminarily charged by proton generators.

In turn, the discharge of the membrane should immediately activate the operation of the generators.

This means that the energy resources spent on generating the electrical potential difference between the cell and the environment will be used to concentrate K+ ions inside the cell. The final balance of such a process will be the exchange of intracellular H + ions for extracellular K + ions (H + ions are pumped out by generator proteins, K + ions enter inside, moving in the electric field created by the movement of H + ions).

Therefore, not only an excess of K + ions will be created inside the cell, but also a deficiency of H + ions.

This deficit can be used to pump out Na + ions. This can be done in the following way. It is known that bacteria have a special carrier of sodium ions, which exchanges Na + for H + (this carrier is called Na + /H + -antiporter). Under conditions of H+ deficiency in the cytoplasm, the antiport can compensate for the proton deficiency by transferring H+ from the external environment into the cell. The transporter can produce such an antiport in only one way: by exchanging the external for the internal Na + . This means that the movement of H + ions into the cell can be used to pump out Na + ions from the same cell.

So we created a potassium-sodium gradient: we accumulated K + inside the cell and pumped out Na + from there. The driving force behind these processes was the proton potential created by generator proteins. (The direction of the potential was such that the inside of the cell was charged negatively and there was a shortage of hydrogen ions.)

Let us now assume that the proton generators have been turned off for some reason. What will happen to the potassium-sodium gradient under these new conditions?

Of course, it will dissipate: K + ions will flow out of the cell into the environment, where there are few of them, Na + ions will enter inside, where these ions are in short supply.

But here's what's interesting. Scattering, the potassium-sodium gradient itself will turn out to be a generator of the proton potential of the same direction that was formed during the operation of protein generators.

Indeed, the release of the K + ion as a positively charged particle creates a diffusion potential difference on the cell membrane with a minus sign inside the cell. The entry of Na + with the participation of Na + /H + - antiporter will be accompanied by the release of H + , that is, the creation of a deficiency of H + inside the cell.

So what happens? When protein generators work, the proton potential created by them is spent on the formation of a potassium-sodium gradient. But when they are turned off (or their power is not enough to satisfy the numerous consumers of the potential), the potassium-sodium gradient, dissipating, itself begins to generate a proton potential.

After all, this is the proton potential buffer, the very buffer that is so necessary for the operation of membrane energy systems!

Schematically, this concept can be depicted as follows:

Potassium-sodium gradient ↓ external energy resources → proton potential → work.

But if such a scheme is correct, then the potassium-sodium gradient should prolong the cell's performance under conditions when energy resources have been exhausted.

A. Glagolev and I. Brown checked the validity of this conclusion. A mutant of Escherichia coli lacking proton ATP synthetase was taken. For such a mutant, the oxidation of substrates with oxygen is the only energy resource suitable to form a proton potential. As was shown at the time by J. Adler and his collaborators, the mutant is mobile as long as there is oxygen in the medium.

Glagolev and Brown repeated Adler's experiment and became convinced that the depletion of the oxygen supply in the solution actually stops bacteria if they are in a medium with KCl. Under these conditions, there is no potassium-sodium gradient: there is a lot of potassium both in the cells and in the environment, and there is no sodium either there or here.

Now let's take the medium with NaCl. Under such conditions, there should be both gradients of interest to us: potassium (a lot of potassium inside and little outside) and sodium (a lot of sodium outside and little inside). The hypothesis predicted that in such a situation, mobility would persist for some time even in anoxic conditions, since energy conversion is possible:

potassium-sodium gradient → proton potential → flagellum rotation.

Indeed, the bacteria moved for another 15-20 minutes after the measuring device registered zero level of CO in the medium.

But especially illustrative, as one would expect, was the experiment with salt-loving bacteria, which transport very large amounts of K + and Na + ions to create a potassium-sodium gradient. Such bacteria quickly stopped in the dark under anoxic conditions if there was KCl in the medium, and still moved after nine (!) hours if KCl was replaced by NaCl.

This value - nine hours - is interesting primarily as an illustration of the volume of the energy reservoir, which is a potassium-sodium gradient in salt-loving bacteria. In addition, it acquires a special meaning if we remember that salt-loving bacteria have bacteriorhodopsin and, therefore, are capable of converting light energy into a proton potential. It is clear that such a transformation is possible only during the daylight hours. And what about at night? So it turns out that the energy stored during the day in the form of a potassium-sodium gradient is enough for the whole night.

The statement that the potassium-sodium gradient plays the role of a proton potential buffer allows us to understand not only the biological function of this gradient, but also the reason that for many years prevented the elucidation of its significance for the life of the cell. The idea of ​​the buffer role of the potassium-sodium gradient could not be born before the proton potential was discovered and it was proved that it serves as a convertible form of energy. All these years, the problem of potassium and sodium was just waiting in the wings.

Article for the competition "bio/mol/text": The resting potential is an important phenomenon in the life of all body cells, and it is important to know how it is formed. However, this is a complex dynamic process, difficult to understand as a whole, especially for undergraduate students (biological, medical and psychological specialties) and unprepared readers. However, when considering the points, it is quite possible to understand its main details and stages. The paper introduces the concept of the rest potential and identifies the main stages of its formation using figurative metaphors that help to understand and remember the molecular mechanisms of the formation of the rest potential.

Membrane transport structures - sodium-potassium pumps - create the prerequisites for the emergence of a resting potential. These prerequisites are the difference in the concentration of ions on the inner and outer sides of the cell membrane. Separately, the difference in concentration for sodium and the difference in concentration for potassium manifest themselves. An attempt of potassium ions (K +) to equalize their concentration on both sides of the membrane leads to its leakage from the cell and the loss of positive electric charges along with them, due to which the overall negative charge of the inner surface of the cell is significantly increased. This "potassium" negativity makes up most of the resting potential (−60 mV on average), and the smaller part (−10 mV) is the "exchange" negativity caused by the electrogenicity of the ion exchange pump itself.

Let's understand in more detail.

Why do we need to know what the resting potential is and how it arises?

Do you know what "animal electricity" is? Where do biocurrents come from in the body? How can a living cell in an aquatic environment turn into an "electric battery" and why does it not instantly discharge?

These questions can only be answered if we find out how the cell creates for itself a difference in electrical potentials (resting potential) across the membrane.

It is quite obvious that in order to understand how the nervous system works, it is first necessary to understand how its separate nerve cell, the neuron, works. The main thing that underlies the work of a neuron is the movement of electrical charges through its membrane and, as a result, the appearance of electrical potentials on the membrane. We can say that a neuron, preparing for its nervous work, first stores energy in electrical form, and then uses it in the process of conducting and transmitting nervous excitation.

Thus, our very first step in studying the workings of the nervous system is to understand how the electrical potential appears on the membrane of nerve cells. This is what we will do, and we will call this process resting potential formation.

Definition of the concept of "resting potential"

Normally, when a nerve cell is at physiological rest and ready to work, it has already had a redistribution of electrical charges between the inner and outer sides of the membrane. Due to this, an electric field arose, and an electric potential appeared on the membrane - resting membrane potential.

Thus, the membrane is polarized. This means that it has a different electrical potential of the outer and inner surfaces. It is quite possible to register the difference between these potentials.

This can be verified by inserting a microelectrode connected to a recording device into the cell. As soon as the electrode enters the cell, it instantly acquires a certain constant electronegative potential with respect to the electrode located in the fluid surrounding the cell. The value of the intracellular electrical potential in nerve cells and fibers, for example, giant squid nerve fibers, at rest is about −70 mV. This value is called the resting membrane potential (RMP). At all points of the axoplasm, this potential is practically the same.

Nozdrachev A.D. etc. Beginnings of Physiology.

A little more physics. Macroscopic physical bodies are, as a rule, electrically neutral, i.e. they contain equal amounts of both positive and negative charges. You can charge a body by creating in it an excess of charged particles of one type, for example, by friction against another body, in which an excess of charges of the opposite type is formed in this case. Taking into account the presence of an elementary charge ( e), the total electric charge of any body can be represented as q= ±N× e, where N is an integer.

resting potential- this is the difference in electrical potentials available on the inner and outer sides of the membrane when the cell is in a state of physiological rest. Its value is measured from inside the cell, it is negative and averages -70 mV (millivolts), although it can vary in different cells: from -35 mV to -90 mV.

It is important to consider that in the nervous system, electric charges are not represented by electrons, as in ordinary metal wires, but by ions - chemical particles that have an electric charge. And in general, in aqueous solutions, not electrons, but ions move in the form of an electric current. Therefore, all electric currents in cells and their environment are ion currents.

So, inside the cell at rest is negatively charged, and outside - positively. This is characteristic of all living cells, with the exception, perhaps, of erythrocytes, which, on the contrary, are negatively charged from the outside. More specifically, it turns out that positive ions (Na + and K + cations) will prevail outside around the cell, and negative ions (anions of organic acids that are not able to freely move through the membrane, like Na + and K +) will prevail inside.

Now we just need to explain how everything turned out that way. Although, of course, it is unpleasant to realize that all our cells except erythrocytes only look positive on the outside, but inside they are negative.

The term "negativity", which we will use to characterize the electrical potential inside the cell, will be useful to us for the simplicity of explaining changes in the level of the resting potential. What is valuable in this term is that the following is intuitively clear: the greater the negativity inside the cell, the lower the potential is shifted to the negative side from zero, and the smaller the negativity, the closer the negative potential is to zero. This is much easier to understand than each time to figure out what exactly the expression “potential increases” means - an increase in absolute value (or “modulo”) will mean a shift in the rest potential down from zero, but simply “increase” means a shift in potential up to zero. The term "negativity" does not create similar ambiguity problems.

The essence of resting potential formation

Let's try to figure out where the electric charge of nerve cells comes from, although no one rubs them, as physicists do in their experiments with electric charges.

Here, one of the logical traps awaits the researcher and student: the internal negativity of the cell does not arise from the appearance of extra negative particles(anions), but, conversely, due to loss of some positive particles(cations)!

So where do the positively charged particles go from the cell? Let me remind you that these are sodium ions that have left the cell and accumulated outside - Na + - and potassium ions - K +.

The main secret of the appearance of negativity inside the cell

Let's open this secret right away and say that the cell loses some of its positive particles and becomes negatively charged due to two processes:

1. at first, she exchanges her “own” sodium for “foreign” potassium (yes, some positive ions for others, just as positive);
2. then these “named” positive potassium ions leak out of it, along with which positive charges leak out of the cell.

These two processes we need to explain.

The first stage of creating internal negativity: the exchange of Na + for K +

Protein proteins are constantly working in the membrane of the nerve cell. exchanger pumps(adenosine triphosphatase, or Na + /K + -ATPase), embedded in the membrane. They change the "own" sodium of the cell to the external "foreign" potassium.

But after all, when exchanging one positive charge (Na +) for another of the same positive charge (K +), there can be no shortage of positive charges in the cell! Right. But, nevertheless, because of this exchange, very few sodium ions remain in the cell, because almost all of them have gone outside. And at the same time, the cell is overflowing with potassium ions, which were pumped into it by molecular pumps. If we could taste the cytoplasm of a cell, we would notice that as a result of the work of exchange pumps, it turned from salty to bitter-salty-sour, because the salty taste of sodium chloride was replaced by the complex taste of a rather concentrated solution of potassium chloride. In the cell, the concentration of potassium reaches 0.4 mol / l. Solutions of potassium chloride in the range of 0.009-0.02 mol / l have a sweet taste, 0.03-0.04 - bitter, 0.05-0.1 - bitter-salty, and starting from 0.2 and above - a complex taste , consisting of salty, bitter and sour.

What is important here is that exchange of sodium for potassium - unequal. For every cell given three sodium ions she gets everything two potassium ions. This results in the loss of one positive charge with each ion exchange event. So already at this stage, due to unequal exchange, the cell loses more “pluses” than it receives in return. In electrical terms, this amounts to approximately −10 mV of negativity inside the cell. (But remember that we still have to find an explanation for the remaining -60 mV!)

To make it easier to remember the operation of exchanger pumps, it can be figuratively expressed as follows: "The cell loves potassium!" Therefore, the cell drags potassium towards itself, despite the fact that it is already full of it. And therefore, she unprofitably exchanges it for sodium, giving 3 sodium ions for 2 potassium ions. And so it spends on this exchange the energy of ATP. And how to spend! Up to 70% of all neuron energy consumption can be spent on the work of sodium-potassium pumps. (That's what love does, even if it's not real!)

By the way, it is interesting that the cell is not born with a ready-made resting potential. She still needs to create it. For example, during differentiation and fusion of myoblasts, the potential of their membrane changes from –10 to –70 mV, i.e. their membrane becomes more negative - it becomes polarized in the process of differentiation. And in experiments on multipotent mesenchymal stromal cells of the human bone marrow, artificial depolarization, which counteracts the resting potential and reduces the negativity of cells, even inhibited (depressed) cell differentiation.

Figuratively speaking, it can be expressed as follows: By creating the potential for rest, the cell is "charged with love." It's love for two things:

1. the love of the cell for potassium (therefore, the cell forcibly drags him to itself);
2. the love of potassium for freedom (therefore, potassium leaves the cell that has captured it).

We have already explained the mechanism of cell saturation with potassium (this is the work of exchange pumps), and we will explain the mechanism of potassium leaving the cell below, when we proceed to the description of the second stage of creating intracellular negativity. So, the result of the activity of membrane ion exchanger pumps at the first stage of the formation of the resting potential is as follows:

1. Sodium deficiency (Na +) in the cell.
2. Excess potassium (K +) in the cell.
3. Appearance of a weak electric potential on the membrane (–10 mV).

We can say this: at the first stage, the ion pumps of the membrane create a difference in ion concentrations, or a concentration gradient (difference), between the intracellular and extracellular environment.

The second stage of creating negativity: the leakage of K + ions from the cell

So, what begins in a cell after its membrane sodium-potassium exchanger pumps work with ions?

Due to the resulting sodium deficiency inside the cell, this ion strives at every opportunity rush inward: solutes always tend to equalize their concentration in the entire volume of the solution. But this does not work well for sodium, since sodium ion channels are usually closed and open only under certain conditions: under the influence of special substances (transmitters) or with a decrease in negativity in the cell (membrane depolarization).

At the same time, there is an excess of potassium ions in the cell compared to the external environment - because the membrane pumps forcibly pumped it into the cell. And he, also striving to equalize his concentration inside and outside, strives, on the contrary, get out of the cell. And he succeeds!

Potassium ions K + leave the cell under the action of a chemical concentration gradient on opposite sides of the membrane (the membrane is much more permeable to K + than to Na +) and carry away positive charges with them. Because of this, negativity grows inside the cell.

Here it is also important to understand that sodium and potassium ions, as it were, "do not notice" each other, they react only "to themselves." Those. sodium reacts to the concentration of sodium, but "does not pay attention" to how much potassium is around. Conversely, potassium reacts only to the concentration of potassium and "does not notice" sodium. It turns out that in order to understand the behavior of ions, it is necessary to consider separately the concentrations of sodium and potassium ions. Those. it is necessary to separately compare the sodium concentration inside and outside the cell and separately the potassium concentration inside and outside the cell, but it makes no sense to compare sodium with potassium, as it happens in textbooks.

According to the law of equalization of chemical concentrations, which operates in solutions, sodium "wants" to enter the cell from the outside; the electric force also draws him there (as we remember, the cytoplasm is negatively charged). He wants to want something, but he cannot, since the membrane in its normal state does not pass it well. The sodium ion channels present in the membrane are normally closed. If, nevertheless, it enters a little, then the cell immediately exchanges it for external potassium with the help of its sodium-potassium exchange pumps. It turns out that sodium ions pass through the cell as if in transit and do not linger in it. Therefore, sodium in neurons is always in short supply.

But potassium just can easily go out of the cell! The cage is full of him, and she can't keep him. It exits through special channels in the membrane - "potassium leak channels", which are normally open and release potassium.

K + -leak channels are constantly open at normal values ​​of the resting membrane potential and show bursts of activity during membrane potential shifts that last several minutes and are observed at all potential values. An increase in K + leakage currents leads to membrane hyperpolarization, while their suppression leads to depolarization. ...However, the existence of a channel mechanism responsible for leakage currents remained in question for a long time. Only now it has become clear that potassium leakage is a current through special potassium channels.

Zefirov A.L. and Sitdikova G.F. Ion channels of an excitable cell (structure, function, pathology).

From chemical to electrical

And now - once again the most important thing. We must consciously move from movement chemical particles to the movement electric charges.

Potassium (K +) is positively charged, and therefore, when it leaves the cell, it takes out of it not only itself, but also a positive charge. Behind him from the inside of the cell to the membrane stretch "minuses" - negative charges. But they cannot seep through the membrane - unlike potassium ions - because. there are no suitable ion channels for them, and the membrane does not let them through. Remember the -60 mV negativity that we didn't explain? This is the very part of the resting membrane potential, which is created by the leakage of potassium ions from the cell! And that's a big part of the resting potential.

There is even a special name for this component of the resting potential - concentration potential. concentration potential - this is part of the resting potential, created by a deficit of positive charges inside the cell, formed due to the leakage of positive potassium ions from it.

Well, now a little physics, chemistry and mathematics for lovers of accuracy.

Electrical forces are related to chemical forces by the Goldman equation. Its particular case is the simpler Nernst equation, which can be used to calculate the transmembrane diffusion potential difference based on different concentrations of ions of the same species on opposite sides of the membrane. So, knowing the concentration of potassium ions outside and inside the cell, we can calculate the potassium equilibrium potential E K:

Where E k - equilibrium potential, R is the gas constant, T is the absolute temperature, F- Faraday's constant, K + ext and K + ext - concentrations of ions K + outside and inside the cell, respectively. The formula shows that to calculate the potential, the concentrations of ions of the same type - K + are compared with each other.

More precisely, the final value of the total diffusion potential, which is created by the leakage of several types of ions, is calculated using the Goldman-Hodgkin-Katz formula. It takes into account that the resting potential depends on three factors: (1) the polarity of the electric charge of each ion; (2) membrane permeability R for each ion; (3) [concentrations of the corresponding ions] inside (int) and outside the membrane (ex). For the squid axon membrane at rest, the conductance ratio is R K: PNa :P Cl = 1:0.04:0.45.

Conclusion

So, the rest potential consists of two parts:

1. −10 mV, which are obtained from the "asymmetric" operation of the membrane exchanger pump (after all, it pumps out more positive charges (Na +) from the cell than it pumps back with potassium).
2. The second part is potassium leaking out of the cell all the time, carrying away positive charges. His contribution is the main one: −60 mV. In sum, this gives the desired −70 mV.

Interestingly, potassium will stop leaving the cell (more precisely, its input and output are equalized) only at a cell negativity level of −90 mV. In this case, the chemical and electrical forces will equalize, pushing potassium through the membrane, but directing it in opposite directions. But this is hindered by sodium constantly leaking into the cell, which carries with it positive charges and reduces the negativity for which potassium “fights”. And as a result, the equilibrium state at the level of −70 mV is maintained in the cell.

Now the resting membrane potential is finally formed.

Scheme of Na + /K + -ATPase clearly illustrates the "asymmetric" exchange of Na + for K +: pumping out excess "plus" in each cycle of the enzyme leads to a negative charge of the inner surface of the membrane. What this video does not say is that ATPase is responsible for less than 20% of the resting potential (-10 mV): the remaining "negativity" (-60 mV) comes from leaving the cell through the "potassium leak channels" of K ions + , striving to equalize their concentration inside and outside the cell.

Literature

1. Jacqueline Fischer-Lougheed, Jian-Hui Liu, Estelle Espinos, David Mordasini, Charles R. Bader, et. al. (2001). Human Myoblast Fusion Requires Expression of Functional Inward Rectifier Kir2.1 Channels . J Cell Biol. 153 , 677-686;
2. Liu J.H., Bijlenga P., Fischer-Lougheed J. et al. (1998). Role of an inward rectifier K + current and of hyperpolarization in human myoblast fusion . J Physiol. 510 , 467–476;
3. Sarah Sundelacruz, Michael Levin, David L. Kaplan. (2008). Membrane Potential Controls Adipogenic and Osteogenic Differentiation of Mesenchymal Stem Cells. PLOS ONE. 3 , e3737;
4. Pavlovskaya M.V. and Mamykin A.I. Electrostatics. Dielectrics and conductors in an electric field. Direct current / Electronic manual for the general course of physics. St. Petersburg: St. Petersburg State Electrotechnical University;
5. Nozdrachev A.D., Bazhenov Yu.I., Barannikova I.A., Batuev A.S. and others. Beginnings of Physiology: A Textbook for High Schools / Ed. acad. HELL. Nozdrachev. St. Petersburg: Lan, 2001. - 1088 p.;
6. Makarov A.M. and Luneva L.A. Fundamentals of electromagnetism / Physics at the Technical University. T. 3;
7. Zefirov A.L. and Sitdikova G.F. Ion channels of an excitable cell (structure, function, pathology). Kazan: Art-cafe, 2010. - 271 p.;
8. Rodina T.G. Sensory analysis of food products. Textbook for university students. M.: Academy, 2004. - 208 p.;
9. Kolman J. and Rem K.-G. Visual biochemistry. M.: Mir, 2004. - 469 p.;
10. Shulgovsky V.V. Fundamentals of neurophysiology: Textbook for university students. Moscow: Aspect Press, 2000. - 277 p.

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. Hence, 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 be a change in the potential of the membrane - electrically excitable channels are activated, and the interaction of membrane receptors with a biologically active substance - receptor - controlled channels, and a mechanical effect. 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 of the nervous system - excitation impulses. 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 operation of the Na/K pump, which 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's try once again to graphically represent the phases of the action potential and the ionic processes underlying the changes in the membrane potential (Fig. 9). 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 AP 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 the 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 subthreshold stimulus - local response

The local response has basically the same mechanism as PD, 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.