Membrane resting potential (MPS) is potential difference between the outer and inner sides of the membrane under conditions when the cell is not excited. The cell cytoplasm is negatively charged to the extracellular fluid by the uneven distribution of anions and cations on both sides of the membrane. The potential difference (voltage) for different cells has a value from -50 to -200 mV (minus means that inside the cell is more negatively charged than outside). The resting resting membrane potential occurs on the membranes of all cells - excitatory (nerves, muscles, secretory cells) and non-forgetful.
MPS is necessary to maintain the excitability of cells such as muscle and nerve. It also affects the transport of all charged particles in any type of cell: it promotes the passive transport of anions from the cell and cations to the cell.
The formation and maintenance of the membrane potential is provided by various types of ion pumps (in particular, sodium-potassium pump or sodium-potassium ATPase) and ion channels (potassium, sodium, chlorine ion channels).
Recording resting potential
A special microelectrode technique is used to register the resting potential. A microelectrode is a thin glass tube, with an elongated end, with a diameter of less than 1 μm, filled with an electrolyte solution (usually potassium chloride). The reference electrode is a silver chlorinated plate located in the extracellular space, both electrodes are connected to an oscilloscope. First, both electrodes are embedded in the extracellular space and there is no potential difference between them, if you enter a recording microelectrode through the membrane into the cell, the oscilloscope will show an abrupt potential shift of up to about -80 mV. This potential shift is called the resting membrane potential.
Formation of resting potential
Two factors lead to the emergence of the resting resting membrane potential: firstly, the concentration of various ions differs externally and from the middle of the cell, secondly, the membrane is semipermeable: some ions can penetrate through it, others not. Both of these phenomena depend on the presence of special proteins in the membrane: ion concentration pumps create concentration gradients, and ion channels provide membrane permeability to ions. The most important role in the formation of membrane potential is played by potassium, sodium and chlorine ions. The concentrations of these ions are visible on both sides of the membrane. For a mammalian neuron, the K + concentration is 140 mmol inside the cell and only 5 mM from the outside, the Na + concentration gradient is almost the opposite - 150 mmol outside and 15 mM inside. This ion distribution is supported by a sodium-potassium pump in the plasma membrane - a protein that uses ATP energy to pump K + into the cell and download Na + from it. There is also a concentration gradient for other ions, for example, chloride anion Cl -.
The concentration gradients of potassium and sodium cations are a chemical form of potential energy. Ion channels are involved in the conversion of energy into electrical energy - pores are formed by clusters of special transmembrane proteins. When ions diffuse through the channel, they carry a unit of electric charge. Any total movement of positive or negative ions through the membrane will create a voltage, or potential difference, on either side of the membrane.
The ion channels participating in the MPS have selective permeability, that is, they allow only a certain type of ion to penetrate. In the membrane of a neuron, at rest, open potassium channels (those that mainly pass only potassium), most of the sodium channels are closed. The diffusion of K + ions through potassium channels is crucial for creating membrane potential. Since the concentration of K + is much higher inside the cell, the chemical gradient favors the outflow of these cations from the cell; therefore, anions that cannot pass through the potassium channels begin to dominate in the cytoplasm.
The outflow of potassium ions from the cell is limited by the membrane potential itself, since at a certain level, the accumulation of negative charges in the cytoplasm will limit the movement of cations outside the cell. Thus, the main factor in the occurrence of MPS is the distribution of potassium ions under the influence of electric and chemical potentials.
Equilibrium potential
In order to determine the effect of the motion of a specific ion through a semipermeable membrane on the formation of the membrane potential, model systems are built. Such a model system consists of a vessel divided into two cells by an artificial semipermeable membrane into which ion channels are integrated. An electrode can be immersed in each cell and the potential difference measured.
Consider the case where an artificial membrane is permeable only to potassium. A concentration gradient similar to that of a neuron is created on two sides of the membrane of the model system: a 140 mM potassium chloride (KCl) solution is placed in the cell corresponding to the cytoplasm (inner cell), and a 5 mmol solution is placed in the cell corresponding to the extracellular fluid (outer cell) KCl. Potassium ions will diffuse through the membrane into the outer cell along a concentration gradient. But since Cl - anions cannot penetrate through the membrane, an excess of negative charge cannot arise in the inner cell, which will impede the cation outflow. When such model neurons reach equilibrium, the effect of the chemical and electrical potential will be balanced, and no total K + diffusion will be observed. The value of the membrane potential, viinkai under such conditions, is called the equilibrium potential for a specific ion (E ion). The equilibrium potential for potassium is approximately -90 mV.
A similar experiment can be carried out for sodium, by installing between the cells a membrane that penetrates only for this cation, and placing a solution of sodium chloride with a concentration of 150 mM in the outer cell, and 15 mM in the inner one. Sodium will move into the inner cell, the rivone important potential for it will be approximately 62 mV.
The number of ions must diffuse to generate an electric potential is very small (approximately 10 -12 mol K + per 1 cm 2 of membrane), this fact has two important consequences. In general, this means that the concentration of ions that can penetrate through the membrane remain stable outside and inside the cell, even after their movement has provided the electrical potential of the room. Secondly, scanty ion fluxes through the membrane, strikingly to establish the potential, do not violate the electroneutrality of the cytoplasm and extracellular fluid as a whole, the charge distribution occurs only in the region immediately adjacent to the plasma membrane.
Nernst equation
The equilibrium potential for a specific ion, for example, for potassium, can be calculated using the Nernst equation, which looks like this:
,where R is the universal gas constant, T is the absolute temperature (according to the Kelvin scale), z is the ion charge, F is the Faraday number, o, i is the potassium concentration outside and inside the cell, respectively. Since the described processes occur at a body temperature of 310 ° K, and decimal logarithms in calculus are easier to use than natural ones, this equation is converted as follows:
Substituting the K + concentration in the Nernst equation, we obtain the equilibrium potential for potassium, -90 mV. Since the outer side of the membrane is taken at zero potential, the minus sign means that under the conditions of equilibrium potassium potential, the inner Storn of the membrane is relatively more electronegative. Similar calculations can be performed for the equilibrium Natium potential, it is 62 mV.
Goldman equations
Although the equilibrium potential for potassium ions is -90 mV, the MPS of the neuron is slightly less negative. This difference reflects an insignificant but constant sequence of Na + ions through the membrane at rest. Since the concentration gradient for sodium is the opposite of that for potassium, Na + moves inside the cell and shifts the total charge on the inside of the membrane to the positive side. In fact, the MPS of a neuron is from -60 to -80 mV. This value is much closer to E K than to E Na, because at rest in the neuron there are a lot of potassium channels and very little sodium channels open. Also, the movement of chlorine ions affects the installation of MPS. In 1943, David Goldaman proposed improving the Nernst equation so that it reflects the influence of various ions on the membrane potential, this equation takes into account the relative permeability of the membrane for each type of ion:
where R is the universal gas constant, T is the absolute temperature (according to the Kelvin scale), z is the ion charge, F is the Faraday number, [ion] o, [ion] i is the concentration of ions inside and inside the cells, P is the relative permeability of the membrane for corresponding ion. The value of the charge in this equation is not stored, but it is taken into account - for chlorine, the external and internal concentrations are interchanged, since its charge is 1.
The value of the resting resting membrane potential for various tissues
- Separated muscle -95 mV;
- Unrefined muscles -50 mV;
- Astroglia from -80 to -90 mV;
- Neurons -70 mV.
The role of the sodium-potassium pump in the formation of MPS
The membrane resting potential can exist only under the condition of an uneven distribution of ions, provided by the functioning of the sodium-potassium pump. In addition, this protein also makes electrogenic power - it transfers 3 sodium cations in exchange for 2 potassium ions moving inside the cell. Thus, Na + -K + -ATPase reduces MPS by 5-10 mV. Suppression of the activity of this protein leads to an insignificant (by 5-10 mV) instantaneous increase in the membrane potential, after which it will exist for some time at a fairly stable level, while the concentration gradients of Na + and K + remain. Subsequently, these gradients will begin to decrease, due to the penetration of the membrane to ions, and after a few tens of minutes electric potential on the membrane will disappear.
It has been established that the most important ions that determine the membrane potentials of cells are inorganic ions K +, Na +, SG, and also, in some cases, Ca 2 +. It is well known that the concentrations of these ions in the cytoplasm and in the intercellular fluid differ by a factor of ten.
From the table. 11.1 it is seen that the concentration of K + ions inside the cell is 40-60 times higher than in the intercellular fluid, while for Na + and SG the concentration distribution is the opposite. The uneven distribution of the concentrations of these ions on both sides of the membrane is ensured both by their different permeability and by the strong electric field of the membrane, which is determined by its resting potential.
Indeed, at rest, the total flux of ions through the membrane is zero, and then from the Nernst – Planck equation it follows that
Thus, at rest, concentration gradients - and
electrical potential - directed onto the membrane
opposite to each other, and therefore in a resting cell, a high and constant difference in the concentrations of the main ions ensures the maintenance of an electric voltage on the cell membrane, which is called equilibrium membrane potential.
In turn, the resting potential arising on the membrane prevents the ions from leaving the K + cell and the excessive entry of SG into it, thereby maintaining their concentration gradients on the membrane.
The full expression for the membrane potential, taking into account the diffusion fluxes of these three types of ions, was obtained by Goldman, Hodgkin and Katz:
where R to P Na, P C1 - membrane permeability for the corresponding ions.
Equation (11.3) with high accuracy determines the membrane resting potentials of various cells. It follows from this that for the resting resting membrane potential, it is not the absolute values \u200b\u200bof the membrane permeability for different ions that are important, but their relations, since, having divided both parts of the fraction under the logarithm sign, for example, by P k, we will move on to the relative ion permeabilities.
In cases where the permeability of one of these ions is much greater than the others, equation (11.3) goes over to the Nernst equation (11.1) for this ion.
From the table. 11.1 shows that the membrane resting potential of cells is close to the Nernst potential for K + and CB ions, but differs significantly from it in Na +. This is evidence
The fact that at rest the membrane is well permeable to K + and SG ions, while for Na + ions its permeability is very low.
Despite the fact that the Nernst equilibrium potential for hypertension is closest to the resting potential of the cell, the latter is predominantly potassium in nature. This is due to the fact that the high intracellular concentration of K + cannot significantly decrease, since K + ions must balance the negative volume charge of the anions inside the cell. Intracellular anions are mainly large organic molecules (proteins, residues of organic acids, etc.) that cannot pass through channels in the cell membrane. The concentration of these anions in the cell is almost constant and their total negative charge prevents a significant exit of potassium from the cell, maintaining its high intracellular concentration together with the Na-K pump. However, the main role in the initial establishment inside the cell of a high concentration of potassium ions and a low concentration of sodium ions belongs to the Na-K pump.
The distribution of C1 ions is set in accordance with the membrane potential, since there are no special mechanisms in the cell to maintain the concentration of SG. Therefore, due to the negative charge of chlorine, its distribution turns out to be inverse with respect to the distribution of potassium on the membrane (see table 11.1). Thus, the concentration diffusions of K + from the cell and C1 into the cell are almost balanced by the membrane resting potential of the cell.
As for Na +, at rest its diffusion is directed into the cell under the influence of both a concentration gradient and the electric field of the membrane, and the entry of Na + into the cell is limited at rest only by the low permeability of the membrane for sodium (sodium channels are closed). Indeed, Hodgkin and Katz experimentally established that at rest, the permeability of the squid axon membrane for K +, Na + and SG are treated as 1: 0.04: 0.45. Thus at rest cell membrane low permeability only for Na +, and for SG it is permeable almost as good as for K +. In nerve cells, permeability for hypertension is usually lower than for K +, but in muscle fibers, permeability for hypertension is even somewhat predominant.
Despite the low permeability of the cell membrane for Na + at rest, there is, albeit a very small, passive transfer of Na + into the cell. This current of Na + should lead to a decrease in the potential difference on the membrane and to the exit of K + from the cell, which would ultimately lead to an equalization of the concentrations of Na + and K + on both sides of the membrane. This does not happen due to the operation of the Na + - K + pump, which compensates for the leakage currents of Na + and K + and thus maintains the normal values \u200b\u200bof the intracellular concentrations of these ions and, therefore, the normal value of the resting potential of the cell.
For most cells, the resting resting membrane potential is (-bO) - (- 10) mV. At first glance, it might seem that this is a small value, but it must be taken into account that the membrane thickness is also small (8-10 nm), so the electric field strength in the cell membrane is huge and amounts to about 10 million volts per 1 m (or 100 kV per 1 cm):
Air, for example, does not withstand such an electric field intensity (electrical breakdown in air occurs at 30 kV / cm), and the membrane withstands. This is a normal condition for its activity, since it is such an electric field that is necessary to maintain the difference in the concentrations of sodium, potassium, and chlorine ions on the membrane.
The magnitude of the resting potential, which is different for cells, can change with changing conditions of their life. Thus, disruption of bioenergetic processes in the cell, accompanied by a drop in the intracellular level of macroergic compounds (in particular, ATP), first of all excludes the component of the resting potential associated with the work of Ma + -K + -ATPase.
Cell damage usually leads to an increase in the permeability of cell membranes, as a result of which the differences in membrane permeability for potassium and sodium ions are reduced; the resting potential decreases, which can cause a violation of a number of cell functions, for example, excitability.
- Since the intracellular potassium concentration is maintained almost constant, even relatively small changes in the extracellular concentration of K * can have a noticeable effect on resting potential and cell activity. Similar changes in the concentration of K "in the blood plasma occur with some pathologies (for example, renal failure).
Membrane potential
At rest, there is a potential difference between the outer and inner surfaces of the cell membrane, which is called the membrane potential [MP], or, if it is an excitable tissue cell, the rest potential. Since the inner side of the membrane is negatively charged with respect to the outer, taking the potential of the external solution as zero, the MP is written with a minus sign. Its value in different cells ranges from minus 30 to minus 100 mV.
The first theory of the emergence and maintenance of membrane potential was developed by J. Bernstein (1902). Based on the fact that the cell membrane has high permeability to potassium ions and low permeability to other ions, he showed that the membrane potential can be determined using the Nernst formula.
In 1949-1952 A. Hodgkin, E. Huxley, B. Katz created a modern membrane-ion theory, according to which the membrane potential is determined not only by the concentration of potassium ions, but also by sodium and chlorine, as well as by the uneven permeability of these cell membranes. The cytoplasm of nerve and muscle cells contains 30-50 times more potassium ions, 8-10 times less sodium ions and 50 times less chlorine ions than extracellular fluid. The permeability of the membrane for ions is due to ion channels, protein macromolecules, penetrating the lipid layer. Some channels are constantly open, others (voltage-dependent) open and close in response to changes in the MP. Potential-dependent channels are divided into sodium, potassium, calcium and chlorine. In a state of physiological rest, the membrane of nerve cells is 25 times more permeable to potassium ions than to sodium ions.
Thus, according to the updated membrane theory, the asymmetric distribution of ions on both sides of the membrane and the associated creation and maintenance of the membrane potential is due to both the selective permeability of the membrane for different ions and their concentration on both sides of the membrane, and more precisely, the value of the membrane potential can be calculated according to the formula.
The polarization of the membrane at rest is explained by the presence of open potassium channels and a transmembrane gradient of potassium concentrations, which leads to the release of part of the intracellular potassium into the surrounding cell environment, i.e., to the appearance of a positive charge on the outer surface of the membrane. Organic anions are large-molecular compounds for which the cell membrane is impermeable, create a negative charge on the inner surface of the membrane. Therefore, the greater the difference in potassium concentrations on both sides of the membrane, the more it comes out and the higher the MP value. The transition of potassium and sodium ions through the membrane according to their concentration gradient should ultimately lead to an equalization of the concentration of these ions inside the cell and in its environment. But this does not happen in living cells, since there are sodium-potassium pumps in the cell membrane, which ensure the removal of sodium ions from the cell and the introduction of potassium ions into it, working with the expenditure of energy. They also take a direct part in the creation of MP, since per unit time sodium ions are removed from the cell more than potassium is introduced (in a ratio of 3: 2), which ensures a constant current of positive ions from the cell. The fact that sodium excretion depends on the presence of metabolic energy is proved by the fact that under the action of dinitrophenol, which blocks metabolic processes, the sodium yield decreases by about 100 times. Thus, the emergence and maintenance of the membrane potential is due to the selective permeability of the cell membrane and the operation of the sodium-potassium pump.
All living cells have the ability, under the influence of stimuli, to switch from a state of physiological rest to a state of activity or arousal.
Excitation is a complex of active electrical, chemical and functional changes in excitable tissues (nervous, muscle or glandular) by which the tissue responds to external influences. An important role in excitation is played by electrical processes that ensure that excitation is carried out along nerve fibers and brings tissues into an active (working) state.
Membrane potential
Living cells have an important property: the inner surface of the cell is always negatively charged with respect to its outer side. Between the outer surface of the cell, charged electropositively with respect to the protoplasm, and the inner side of the cell membrane, there is a potential difference that ranges from 60-70 mV. According to P. G. Kostyuk (2001), this difference in a nerve cell ranges from 30 to 70 mV. The potential difference between the outer and inner sides of the cell membrane is called membrane potentialor resting potential (Fig. 2.1).
The membrane resting potential is present on the membrane as long as the cell is alive, and disappears with cell death. L. Galvani in 1794 showed that if you damage a nerve or muscle by making a cross-section and applying electrodes to the damaged part and the damage site connected to the galvanometer, the galvanometer will show the current that always flows from the undamaged part of the tissue to the incision site . He called this current a quiescent current. In its physiological essence, the quiescent current and the membrane quiescent potential are one and the same. The potential difference measured in this experiment is 30–50 mV, since when the tissue is damaged, part of the current is shunted in the intercellular space and the fluid surrounding the studied structure. The potential difference can be calculated by the Nernst formula:
where R is the gas constant, T is the absolute temperature, F is the Faraday number, [K] int. and [K] Nar. - the concentration of potassium inside and outside the cell.
Fig. 2.1.
The cause of the resting potential is common to all cells. Between the protoplasm of the cell and the extracellular medium there is an uneven distribution of ions (ion asymmetry). The composition of human blood in salt balance resembles the composition of ocean water. Extracellular medium in the central nervous system also contains a lot of sodium chloride. The ionic composition of the cytoplasm of the cells is poorer. Inside the cells there are 8-10 times less Na + ions and 50 times less C ions! ". The main cation of the cytoplasm is K +. Its concentration inside the cell is 30 times higher than in the extracellular medium, and is approximately equal to the extracellular concentration of Na. The main counterions for K + in the cytoplasm are organic anions, in particular anions of aspartic, histamine and other amino acids. Such asymmetry is a violation of thermodynamic equilibrium. In order to restore it, potassium ions must gradually leave the cell, and sodium ions should strive into it. However, this is not going on.
The first obstacle to equalizing the difference in ion concentrations is the plasma membrane of the cell. It consists of a double layer of phospholipid molecules coated internally with a layer of protein molecules, and externally with a layer of carbohydrates (mucopolysaccharides). Some of the cellular proteins are embedded directly in the double lipid layer. These are internal proteins.
Membrane proteins of all cells are divided into five classes: pumps, channels, receptors, enzymes and structural proteins. Pumps serve to move ions and molecules against concentration gradients, using metabolic energy for this. Protein channels, or pores provide selective permeability (diffusion) through the membrane of ions and molecules corresponding to them in size. Receptor proteins possessing high specificity, they recognize and bind, attaching to the membrane, many types of molecules necessary for the life of the cell at any given time. Enzymes accelerate the course of chemical reactions at the surface of the membrane. Structural proteins provide the connection of cells into organs and the maintenance of the subcellular structure.
All of these proteins are specific, but not strict. Under certain conditions, a particular protein can be both a pump, and an enzyme, and a receptor. Through the channels of the membrane, water molecules, as well as ions corresponding to pore sizes, enter and leave the cell. The permeability of the membrane for different cations is not the same and varies with different functional states of the tissue. At rest, the membrane is 25 times more permeable to potassium ions than to sodium ions, and when excited, the sodium permeability is about 20 times higher than potassium. At rest, equal concentrations of potassium in the cytoplasm and sodium in the extracellular medium should provide an equal amount of positive charges on both sides of the membrane. But since the permeability for potassium ions is 25 times higher, the potassium leaving the cell makes its surface more and more positively charged with respect to the inner side of the membrane, around which negatively charged aspartic, histamine, and other molecules that are too large for the pores of the membrane accumulate amino acids that "released" potassium outside the cell, but "did not allow" it to go far due to its negative charge. Negative charges accumulate on the inside of the membrane, and positive charges on the outside. There is a potential difference. The diffuse current of sodium ions into the protoplasm from the extracellular fluid keeps this difference at the level of 60-70 mV, preventing it from increasing. The diffuse current of sodium ions at rest is 25 times weaker than the counter current of potassium ions. Sodium ions, penetrating into the cell, reduce the value of the resting potential, allowing it to stay at a certain level. Thus, the resting potential of muscle and nerve cells, as well as nerve fibers, is determined by the ratio of the number of positively charged potassium ions diffusing outward from the cell per unit time and positively charged sodium ions diffusing in the opposite direction through the membrane. The higher the ratio, the greater the resting potential, and vice versa.
The second obstacle that keeps the potential difference at a certain level is the sodium-potassium pump (Fig. 2.2). It is called sodium-potassium or ionic, because it actively removes (pumps out) sodium ions that penetrate it from the protoplasm and introduces (injects) potassium ions into it. The source of energy for the operation of the ion pump is the decomposition of ATP (adenosine triphosphate), which occurs under the influence of the enzyme adenosine triphosphatase, which is localized in the cell membrane and activated by the same ions, i.e. potassium and sodium (sodium-potassium-dependent ATPase).
Fig. 2.2.
This is a large protein that is larger than the thickness of the cell membrane. The molecule of this protein, penetrating the membrane through, binds mainly sodium and ATP from the inside, and potassium and various glycoside inhibitors from the outside. In this case, a membrane current arises. Due to this current, an appropriate direction of ion transfer is ensured. Ion transfer occurs in three stages. First, the ion combines with the carrier molecule to form an ion-carrier complex. Then this complex passes through the membrane or transfers charge through it. Finally, the ion is released from the carrier on the opposite side of the membrane. At the same time, a similar process takes place, transferring ions in the opposite direction. If the pump carries out the transfer of one sodium ion to one potassium ion, then it simply maintains a concentration gradient on both sides of the membrane, but does not contribute to the creation of the membrane potential. To make this contribution, the ion pump must transfer sodium and potassium in a ratio of 3: 2, i.e., by 2 potassium ions entering the cell, it must remove 3 sodium ions from the cell. Working at maximum load, each pump is able to pump about 130 potassium ions and 200 sodium ions per second through the membrane. This is top speed. In real conditions, the operation of each pump is regulated in accordance with the needs of the cell. In most neurons, between 100 and 200 ion pumps per square micron of membrane surface. Therefore, the membrane of any nerve cell contains 1 million ion pumps capable of transporting up to 200 million sodium ions per second.
Thus, the membrane potential (resting potential) is created as a result of both passive and active mechanisms. The degree of participation of various mechanisms in different cells is not the same, which implies that the membrane potential may be different in different structures. The activity of the pumps may depend on the diameter of the nerve fibers: the finer the fiber, the higher the ratio of surface size to cytoplasm volume, respectively, and the activity of the pumps necessary to maintain the difference in ion concentrations on the surface and inside the fiber should be greater. In other words, the membrane potential may depend on the structure of the nervous tissue, and hence on its functional purpose. The electric polarization of the membrane is the main condition ensuring the excitability of the cell. This is her constant readiness for action. This is a reserve of potential energy of the cell, which it can use if the nervous system needs its immediate reaction.
Membrane resting potential (WFP) or resting potential (PP) is called the potential difference of a resting cell between the inner and outer sides of the membrane. The inner side of the cell membrane is negatively charged relative to the outer. Taking the potential of the external solution as zero, the MPP is written with a minus sign. Value WFPdepends on the type of fabric and varies from -9 to -100 mv. Therefore, at rest, the cell membrane polarized.The decrease in the value of MPP is called depolarizationincrease - hyperpolarizationrestoration of initial value WFP-repolarizationmembranes.
Fundamentals of Membrane Theory of Origin WFPboil down to the following. At rest, the cell membrane is well permeable to K + ions (in a number of cells and to SG), less permeable to Na + and practically impermeable to intracellular proteins and other organic ions. K + ions diffuse from the cell in a concentration gradient, while non-penetrating anions remain in the cytoplasm, providing the appearance of a potential difference across the membrane.
The resulting potential difference prevents the exit of K + from the cell and, at a certain value, the equilibrium sets in between the output of K + in the concentration gradient and the entrance of these cations in the electric gradient that has arisen. The membrane potential at which this equilibrium is achieved is called equilibrium potential.Its value can be calculated from the Nernst equation:
10 In nerve fibers, signals are transmitted using action potentials, which are rapid changes in the membrane potential, rapidly propagating along the membrane of the nerve fiber. Each action potential begins with a rapid shift of the resting potential from a normal negative value to a positive value, then it almost as quickly returns to a negative potential. When conducting a nerve signal, the action potential moves along the nerve fiber until it ends. The figure shows the changes that occur on the membrane during the action potential, with the transfer of positive charges into the fiber at the beginning and the return of positive charges outside at the end. In the lower part of the figure, successive changes in the membrane potential over several 1/10000 sec are graphically presented, illustrating the explosive onset of the action potential and almost equally fast recovery. Stage of rest. This stage is represented by the membrane resting potential, which precedes the action potential. The membrane during this stage is polarized due to the presence of a negative membrane potential of -90 mV. Depolarization phase. At this time, the membrane suddenly becomes highly permeable to sodium ions, allowing a huge number of positively charged sodium ions to diffuse into the axon. The normal polarized state of -90 mV is immediately neutralized by the positively charged sodium ions entering inside, as a result, the potential rapidly rises in the positive direction. This process is called depolarization. In large nerve fibers, a significant excess of positive sodium ions entering the interior usually leads to the membrane potential “slipping” beyond the zero level, becoming slightly positive. In some smaller fibers, as in most neurons of the central nervous system, the potential reaches zero level without “jumping” it. The phase of repolarization. Within a few fractions of a millisecond after a sharp increase in the permeability of the membrane for sodium ions, the sodium channels begin to close and the potassium channels open. As a result, the rapid diffusion of potassium ions to the outside restores the normal negative membrane resting potential. This process is called membrane repolarization. action potential For a more complete understanding of the factors that cause depolarization and repolarization, it is necessary to study the features of two other types of transport channels in the nerve fiber membrane: electrically controlled sodium and potassium channels. Electro-controlled sodium and potassium channels. A necessary participant in the processes of depolarization and repolarization during the development of the action potential in the membrane of the nerve fiber is an electrically controlled sodium channel. An electrically controlled potassium channel also plays an important role in increasing the rate of membrane repolarization. Both types of electrically controlled channels exist in addition to the Na + / K + pump and K * / Na + leak channels. Electrically controlled sodium channel. The top of the figure shows an electrically controlled sodium channel in three different states. This canal has two gates: some near the outer part of the canal, which are called activation gates, others - at the inner part of the canal, which are called inactivation gates. The upper left part of the figure shows the state of these gates at rest when the membrane resting potential is -90 mV. Under these conditions, the activation gate is closed and impedes the entry of sodium ions into the fiber. Activation of the sodium channel. When the membrane resting potential shifts in the direction of less negative values, rising from -90 mV to zero, a sudden conformational change in the activation gates occurs at a certain level (usually between -70 and -50 mV), as a result, they go into a completely open state . This condition is called the activated state of the channel, in which sodium ions can freely enter through it into the fiber; while the sodium permeability of the membrane increases in the range from 500 to 5000 times. Inactivation of the sodium channel. In the upper right part of the figure, the third state of the sodium channel is shown. The increase in potential opening the activation gate closes the inactivation gate. However, inactivation gates close within a few tenths of a millisecond after opening the activation gates. This means that the conformational change leading to the closure of the inactivation gates is a slower process than the conformational change that opens the activation gates. As a result, after a few tenths of a millisecond after opening the sodium channel, the inactivation gate closes, and sodium ions can no longer penetrate the fiber. From this moment, the membrane potential begins to return to the resting level, i.e. the process of repolarization begins. There is another important characteristic of the sodium channel inactivation process: inactivation gates do not reopen until the membrane potential returns to a value equal to or close to the level of the initial resting potential. In this regard, the reopening of sodium channels is usually impossible without preliminary repolarization of the nerve fiber.
13The mechanism for conducting excitation along nerve fibers depends on their type. There are two types of nerve fibers: myelin and non-myelin. The metabolism processes in myelin-free fibers do not provide quick compensation for energy consumption. The spread of excitation will go with a gradual attenuation - with decrement. Decremental behavior of excitation is characteristic of a low-organized nervous system. Excitation propagates due to small circular currents that occur inside the fiber or into the surrounding fluid. A potential difference arises between the excited and unexcited sections, which contributes to the occurrence of circular currents. Current will propagate from “+” charge to “-”. At the exit point of the circular current, the permeability of the plasma membrane for Na ions increases, resulting in depolarization of the membrane. A potential difference arises again between the newly excited section and the neighboring unexcited, which leads to the appearance of circular currents. Excitation gradually covers adjacent sections of the axial cylinder and so extends to the end of the axon. In myelin fibers, due to the perfection of metabolism, excitation passes without decay, without decrement. Due to the large radius of the nerve fiber due to the myelin sheath, an electric current can enter and exit the fiber only in the interception region. When irritation occurs, depolarization occurs in the area of \u200b\u200binterception A, the neighboring interception B is polarized at this time. Between the intercepts, a potential difference occurs, and circular currents appear. Due to the circular currents, other interceptions are excited, while the excitation propagates saltatory, spasmodically from one interception to another. There are three laws of nerve fiber stimulation. The law of anatomical and physiological integrity. Conducting impulses along the nerve fiber is possible only if its integrity is not broken. The law of isolated excitation. There are a number of features of the spread of excitation in the peripheral, pulp and cordless nerve fibers. In peripheral nerve fibers, excitation is transmitted only along the nerve fiber, but is not transmitted to neighboring ones located in the same nerve trunk. In the pulp nerve fibers, the myelin sheath plays the role of an insulator. Due to myelin, the resistivity increases and the electric capacity of the shell decreases. In serene nerve fibers, excitation is transmitted in isolation. The law of two-way excitation. Nerve fiber conducts nerve impulses in two directions - centripetal and centrifugal.
14 Synapses Is a specialized structure that ensures the transmission of a nerve impulse from a nerve fiber to an effector cell - muscle fiber, neuron or secretory cell.
Synapses - these are the junction of the nerve process (axon) of one neuron with the body or the process (dendrite, axon) of another nerve cell (intermittent contact between nerve cells).
All structures that provide signal transmission from one nervous structure to another - synapses .
Value - transmits nerve impulses from one neuron to another \u003d\u003e provides transmission of excitation along the nerve fiber (signal propagation).
A large number of synapses provides a large area for transmitting information.
Synapse structure:
1. Presynaptic membrane- belongs to a neuron from which a signal is transmitted.
2. Synaptic cleftfilled with a liquid with a high content of Ca ions.
3. Postsynaptic membrane- belongs to the cells to which the signal is transmitted.
There is always a gap between neurons filled with interstitial fluid.
Depending on the density of the membranes, there are:
- symmetrical(with the same membrane density)
- asymmetric(the density of one of the membranes is higher)
Presynaptic membrane covers the axon extension of the transmitting neuron.
Extension - synaptic button / synaptic plaque.
On the plaque - synaptic vesicles (vesicles).
On the inside of the presynaptic membrane - protein / hexogonal lattice(necessary for the release of the mediator) in which the protein is located - neurin . Filled with synaptic vesicles that contain mediator- a special substance involved in the transmission of signals.
The composition of the membrane of the vesicles includes - stenin (protein).
Postsynaptic membrane covers the effector cell. It contains protein molecules that are selectively sensitive to the mediator of this synapse, which ensures interaction.
These molecules are part of the postsynaptic membrane channels + enzymes (many) that can break the connection of the mediator with receptors.
Postsynaptic membrane receptors.
The postsynaptic membrane contains receptors that are related to the mediator of this synapse.
Between them is snap gap . It is filled with intercellular fluid having a large number of calcium. It has a number of structural features - it contains protein molecules that are sensitive to a mediator transmitting signals.
15 Synaptic Delay of Excitation
In order for the excitation to spread along the reflex arc, a certain time is spent. This time consists of the following periods:
1. the period temporarily necessary for the excitation of receptors (receptor) and for conducting excitation pulses along afferent fibers to the center;
2. the period of time necessary for the spread of excitation through the nerve centers;
3. the period of time necessary for the spread of excitation along the efferent fibers to the working body;
4. The latent period of the working body.
16 Braking plays an important role in processing the information received in the central nervous system. This role is especially pronounced in presynaptic inhibition. It more precisely regulates the excitation process, since individual nerve fibers can be blocked by this inhibition. Hundreds and thousands of impulses at different terminals can approach one exciting neuron. However, the number of pulses reaching the neuron is determined by presynaptic inhibition. Inhibition of the lateral paths provides the selection of significant signals from the background. Blockade of inhibition leads to a wide irradiation of excitation and convulsions, for example, when presynaptic inhibition is turned off by biculin.