Last updated: 28/10/2013
The first article in the series “Fundamentals of Human and Animal Physiology.” Basic concepts are discussed, and the most important property of living cells - the presence of a resting potential - is discussed in detail.
Have you ever wondered how perfect the system is? human body? Everything in it is ordered down to the smallest detail, and if you try to delve into the processes occurring in our body in one second, you can experience overwhelming amazement. The more you learn about how precisely and thoughtfully each individual cell operates, the more convinced you become of the greatness of Mother Nature.
Before delving into the details of the structure and operation human body, it is worth clarifying what level of organization we are talking about. Our body is a complex system that can be divided into sections - organ systems, which, in turn, consist of cells. And every single cell is no less complex organized by the system. Since the basics of anatomy are taught in school, it will be much more interesting to talk about the smaller components of our body - the cells. The mechanisms that ensure their vital activity are chemical and physical interactions various substances among themselves. Today, thanks to the development of molecular methods, quite a lot is known about this, but some mysteries have still not been solved.
Communication into one whole huge amount structures different sizes- from a cell to a whole organ - is provided primarily by such a property of living things as excitability, that is, the ability to enter a state of physiological activity under the influence of some external stimuli. All cells of the human and animal body are excitable to one degree or another. The response to any stimulus is ultimately always some kind of movement.
Excitable cells have three important properties - physical rather than purely biological. This is the presence of two potentials, rest and action, and conductivity - the ability to transmit a signal. The electrical potential of a cell is provided by different concentrations of ions on both sides of the plasma membrane. The fact that the membrane of a living cell is semi-permeable (that is, it allows certain ions to pass through, but not others) was known back in the late 19th century. Later, the mechanisms of transport of molecules and ions into and out of the cell became known.
The cell membrane is a double layer of phospholipids. These are polar organic compounds that have two ends - a hydrophilic (interacts well with water) head and two hydrophobic (repel water molecules) tails. As part of the membrane, the heads of some phospholipids face the environment external to the cell, while others face the cytoplasm. The tails thus end up in the middle. In addition to phospholipids, the membrane contains glycolipids and cholesterol, which are compounds close to phospholipids. Proteins that perform transport, protective and receptor functions are embedded in the lipid layer.
This structure of the membrane precisely ensures its selective permeability for different molecules.
The electric potential is formed by a set of so-called potential-forming ions. These are chemical particles that carry an electrical charge. The most important of them are simple ions: potassium (K+), sodium (Na+), chlorine (Cl-) and calcium (Ca+).
The main ion that provides the resting potential is potassium, since the membrane permeability for it is much higher than for other ions. Thanks to diffusion (so-called passive transport), potassium freely passes through the membrane. It follows a concentration gradient - that is, from where the concentration is greater to where the concentration is less. Since its concentration in the cell is about forty times greater, it comes out. Since potassium moves freely, sooner or later equilibrium must be established on opposite sides of the membrane. This does not happen due to the operation of a special active transport system. This system pumps excess sodium ions out of the cell.
The fact is that sodium freely penetrates the cell membrane only in small quantities - for it the permeability of the membrane is low. In addition, there is more of it in the external environment, so transport must go against the concentration gradient - and therefore with energy consumption.
This is necessary to maintain constant electronegativity, because the presence of a resting potential makes the cell ready for excitation and physiological activity. And if sodium is not pumped out, potassium cannot get back in, and the charge on the membrane will decrease.
Active transport is carried out by a special protein in the membrane of an excitable cell. It is called potassium-sodium-dependent ATPase. Due to its structure, the protein is able to rotate in the membrane and exchange sodium for potassium. Sodium will remain outside, potassium will gradually come out.
This system is called the sodium-potassium pump. Up to 20% of the energy of a living cell is spent on its functioning. This is very important: when this pump is blocked, which can be caused by certain toxins, the cells lose their ability to excite, and the consequences can be very serious for the entire body.
IN embryonic development true resting potential appears in cells only when the potassium-sodium pump is fully formed. Some physiologists believe that it is from this moment - and not at all from the first heartbeat - that the embryo should be considered alive.
Both potassium and sodium are positively charged, so the cell ends up with a positive charge on the outside and a negative charge on the inside. The difference in charges creates a resting potential on the membrane; in different cells it has different meaning. The average (for warm-blooded animals) is minus 60 millivolts, and in the most excitable cells - nerve cells - up to minus 90.
Thus, the electrochemical potential created by potassium and sodium ions is one of the main properties of excitable living cells. Chlorine and calcium ions play a big role in the formation of another potential - .
1. For a cell, the external environment is, naturally, not that of the entire organism, but an intercellular substance or some kind of cavity inside the body.
2. Further - worse! If you are interested in studying physiology, be prepared for the fact that it is still possible to understand the essence of the processes, but to understand how it happened and who arranged it all so accurately is already difficult. But amazingly interesting.
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All living cells have the ability, under the influence of stimuli, to move from a state of physiological rest to a state of activity or excitation.
Excitation- is a complex of active electrical, chemical and functional changes in excitable tissues (nervous, muscular or glandular), with which the tissue responds to external influence. Play an important role in arousal electrical processes, ensuring the conduction of excitation along nerve fibers and bringing tissue into an active (working) state.
Living cells have an important property: the inner surface of the cell is always negatively charged relative to its outer side. Between the outer surface of the cell, charged electropositively with respect to the protoplasm, and inside cell membrane there is a potential difference that fluctuates between 60-70 mV. According to P. G. Kostyuk (2001), in a nerve cell this difference ranges from 30 to 70 mV. The potential difference between the outer and inner sides of the cell membrane is called membrane potential, or resting potential(Fig. 2.1).
The resting membrane potential is present on the membrane as long as the cell is alive and disappears when the cell dies. L. Galvani showed back in 1794 that if you damage a nerve or muscle by making a cross section and applying electrodes connected to a galvanometer to the damaged part and to the site of damage, the galvanometer will show a current that always flows from the undamaged part of the tissue to the site of the cut . He called this flow a quiescent current. In their physiological essence, resting current and resting membrane potential are one and the same. The potential difference measured in this experiment is 30-50 mV, since when tissue is damaged, part of the current is shunted into the intercellular space and the fluid surrounding the structure under study. The potential difference can be calculated using the Nernst formula:
where R is the gas constant, T is the absolute temperature, F is the Faraday number, [K] int. and [K] adv. - potassium concentration inside and outside the cell.
Rice. 2.1.
The cause of the resting potential is common to all cells. Between the protoplasm of the cell and the extracellular environment there is an uneven distribution of ions (ion asymmetry). Composition of human blood according to salt balance resembles the composition of ocean water. Extracellular environment in the central nervous system also contains a lot of sodium chloride. The ionic composition of the cell cytoplasm 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 environment, and is approximately equal to the extracellular concentration of Na. The main counterions for K + in the cytoplasm are organic anions, in particular the 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 must rush into it. is happening.
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, covered on the inside with a layer of protein molecules, and on the outside with a layer of carbohydrates (mucopolysaccharides). Some cellular proteins are embedded directly in the lipid bilayer. 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. Protein channels, or pores, provide selective permeability (diffusion) through the membrane of ions and molecules corresponding to their 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 flow chemical reactions at the surface of the membrane. Structural proteins ensure the connection of cells into organs and the maintenance of subcellular structure.
All these proteins are specific, but not strictly. Under certain conditions, a particular protein can simultaneously be a pump, an enzyme, and a receptor. Through membrane channels, water molecules, as well as ions corresponding to the size of the pores, enter and exit the cell. The permeability of the membrane for different cations is not the same and changes with different functional states fabrics. At rest, the membrane is 25 times more permeable to potassium ions than to sodium ions, and when excited, sodium permeability is approximately 20 times higher than potassium. At rest, equal concentrations of potassium in the cytoplasm and sodium in the extracellular environment should provide an equal number of positive charges on both sides of the membrane. But since the permeability for potassium ions is 25 times higher, potassium, leaving the cell, makes its surface more and more positively charged in relation to the inner side of the membrane, near which negatively charged molecules of aspartic, histamine and others, too large for the pores of the membrane, increasingly accumulate amino acids that “release” potassium outside the cell, but “prevent” it from going far due to its negative charge. Negative charges accumulate on the inside of the membrane, and positive charges on the outside. A potential difference arises. 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 resting potential, allowing it to remain at a certain level. Thus, the value of 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 per unit time from the cell outward, and positively charged sodium ions diffusing through the membrane in the opposite direction. The higher this 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, since it actively removes (pumps out) sodium ions penetrating into it from the protoplasm and introduces (pumps) potassium ions into it. The source of energy for the operation of the ion pump is the breakdown of ATP (adenosine triphosphate), which occurs under the influence of the enzyme adenosine triphosphatase, localized in the cell membrane and activated by the same ions, i.e. potassium and sodium (sodium-potassium-dependent ATPase).
Rice. 2.2.
This is a large protein, exceeding the thickness of the cell membrane. The molecule of this protein, penetrating the membrane, binds predominantly sodium and ATP on the inside, and potassium and various inhibitors such as glycosides on the outside. In this case, a membrane current occurs. Thanks to this current, the appropriate direction of ion transport is ensured. Ion transfer occurs in three stages. First, the ion combines with a carrier molecule to form an ion-transporter complex. This complex then passes through the membrane or transfers charge across it. Finally, the ion is released from the carrier on the opposite side of the membrane. At the same time, a similar process occurs, transporting ions in the opposite direction. If the pump transfers 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 membrane potential. To make this contribution, the ion pump must transport sodium and potassium in a ratio of 3:2, i.e., for every 2 potassium ions entering the cell, it must remove 3 sodium ions from the cell. Working with maximum load, each pump is capable of pumping about 130 potassium ions and 200 sodium ions per second through the membrane. This is the maximum speed. IN real conditions The operation of each pump is adjusted according to the needs of the cell. Most neurons have between 100 and 200 ion pumps per square micron of membrane surface. Consequently, the membrane of any nerve cell contains 1 million ion pumps capable of moving 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 certain mechanisms in different cells is not the same, which means 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 thinner the fiber, the higher the ratio of the surface size to the volume of the cytoplasm; accordingly, the activity of the pumps required 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 nerve tissue, and therefore from her functional purpose. Electrical polarization of the membrane is the main condition for cell excitability. This is her constant readiness for action. This is the cell's reserve of potential energy, which it can use in case the nervous system needs its immediate response.
Resting potential
Membranes, including plasma membranes, are in principle impenetrable to charged particles. True, the membrane contains Na + /K + -ATPase (Na + /K + -ATPase), which actively transports Na + ions from the cell in exchange for K + ions. This transport is energy-dependent and is associated with the hydrolysis of ATP (ATP). Due to the work of the “Na + ,K + -pump”, the unbalanced distribution of Na + and K + ions between the cell and the environment is maintained. Since the cleavage of one ATP molecule ensures the transfer of three Na + ions (out of the cell) and two K + ions (into the cell), this transport is electrogenic, i.e., the cell cytoplasm is negatively charged with respect to the extracellular space.
Electrochemical potential. The contents of the cell are negatively charged relative to the extracellular space. The main reason for the appearance of an electric potential on the membrane (membrane potential Δψ) is the existence of specific ion channels. Transport of ions through the channels occurs along a concentration gradient or under the influence of membrane potential. In an unexcited cell, some of the K + channels are in an open state and K + ions constantly diffuse from to environment(along the concentration gradient). Leaving the cell, K+ ions carry away a positive charge, which creates resting potential equal to approximately -60 mV. From the permeability coefficients of various ions, it is clear that the channels permeable to Na + and Cl – are predominantly closed. Phosphate ions and organic anions, such as proteins, are practically unable to pass through membranes. Using the Nernst equation, it can be shown that the membrane potential is primarily determined by K + ions, which make the main contribution to the conductivity of the membrane.
Ion channels. The membranes have channels that are permeable to Na + , K + , Ca 2+ and Cl – ions. These channels are most often in a closed state and open only when short time. Channels are divided into voltage-gated (or electrically excitable), such as fast Na + channels, and ligand-gated (or chemoexcitable), such as nicotinic cholinergic channels. Channels are integral membrane proteins consisting of many subunits. Depending on changes in membrane potential or interaction with the corresponding ligands, neurotransmitters and neuromodulators, receptor proteins can be in one of two conformational states, which determines the permeability of the channel (“open” - “closed”, etc.).
A nerve cell under the influence of a chemical signal (less often an electrical impulse) leads to the appearance action potential. This means that the resting potential of -60 mV jumps to +30 mV and after 1 ms it returns to its original value. The process begins with the opening of the Na + channel. Na + ions rush into the cell (along the concentration gradient), which causes a local reversal of the sign of the membrane potential. In this case, the Na + channels immediately close, i.e., the flow of Na + ions into the cell lasts a very short time. Due to a change in membrane potential, voltage-gated K + channels open (for several ms) and K + ions rush in the opposite direction, out of the cell. As a result, the membrane potential takes on its original value and even exceeds it for a short time resting potential. After this she becomes excitable again.
During one pulse, a small portion of Na + and K + ions passes through the membrane, and the concentration gradients of both ions are maintained (the level of K + is higher in the cell, and the level of Na + is higher outside the cell). Therefore, as the cell receives new impulses, the process of local reversal of the sign of the membrane potential can be repeated many times. The propagation of an action potential across the surface of a nerve cell is based on the fact that local reversal of the membrane potential stimulates the opening of neighboring voltage-gated ion channels, resulting in excitation propagating in the form of a depolarizing wave throughout the cell.
Lecture 2. General physiology of excitable tissues. Resting potential. Action potential.
۩ The essence of the excitation process. The essence of the excitation process can be formulated as follows. All cells of the body have an electrical charge, which is created by the unequal concentration of anions and cations inside and outside the cell. The different concentrations of anions and cations inside and outside the cell are a consequence of the unequal permeability of the cell membrane to various ions and the operation of ion pumps. The process of excitation begins with the action of a stimulus on the excitable cell. First, the permeability of its membrane for sodium ions increases very quickly and quickly returns to normal, then for potassium ions and also quickly, but with some lag, returns to normal. As a result, ions move into and out of the cell according to an electrochemical gradient - this is the process of excitation. Excitation is possible only if the cell constantly maintains a resting potential (membrane potential) and when it is irritated, the permeability of the cell membrane quickly changes.
۩ Resting potential. Resting potential (RP) - this is the difference in electrical potential between the internal and external environments of the cell in its state of rest. In this case, a negative charge is registered inside the cell. The magnitude of PP in different cells is different. Thus, in skeletal muscle fibers a PP of 60-90 mV is recorded, in neurons - 50-80 mV, in smooth muscles - 30-70 mV, in cardiac muscle - 80-90 mV. Cell organelles have their own variable membrane potentials.
The immediate reason for the existence of the resting potential is the unequal concentration of anions and cations inside and outside the cell (see Table 1!).
Table 1. Intra- and extracellular ion concentrations in muscle cells.
|
Intracellular concentration, mM |
Extracellular concentration, mM |
|||
|
A- (large molecular intracellular anions) |
A-(large molecular intracellular anions) |
|||
|
Small quantity |
Small quantity |
|||
|
Very few |
Basic quantity |
|||
The uneven distribution of ions inside and outside the cell is a consequence of the unequal permeability of the cell membrane to various ions and the operation of ion pumps that transport ions into and out of the cell against the electrochemical gradient. Permeability - this is its ability to pass water, uncharged and charged particles according to the laws of diffusion and filtration. It is defined:
Channel sizes and particle sizes;
The solubility of particles in the membrane (the cell membrane is permeable to lipids soluble in it and impermeable to peptides).
Conductivity – is the ability of charged particles to pass through a cell membrane according to an electrochemical gradient.
The different permeability of different ions plays an important role in the formation of PP:
Potassium is the main ion ensuring the formation of PP, since its permeability is 100 times higher than the permeability for sodium. When the potassium concentration in the cell decreases, the PP decreases, and when it increases, it increases. He can move in and out of the cell. At rest, the number of incoming potassium ions and outgoing potassium ions is balanced and the so-called potassium equilibrium potential is established, which is calculated using the Nernst equation. Its mechanism is as follows: since the electrical and concentration gradients oppose each other, potassium tends to go out along the concentration gradient, and the negative charge inside the cell and the positive charge outside the cell prevents this. Then the number of incoming ions becomes equal to the number of outgoing ions.
Sodium enters the cell. Its permeability is small compared to the permeability of potassium, so its contribution to the formation of PP is small.
Chlorine enters the cell in small quantities, since the permeability of the membrane for it is small, and it is balanced by the amount of sodium ions (opposite charges attract). Consequently, its contribution to the formation of PP is small.
Organic anions (glutamate, aspartate, organic phosphates, sulfates) cannot leave the cell at all, since they are large. Therefore, due to them, a negative charge is formed inside the cell.
The role of calcium ions in the formation of the PP is that they interact with the external negative charges of the cell membrane and negative carboxyl groups of the interstitium, neutralizing them, which leads to the stabilization of the PP.
In addition to the above ions, the surface charges of the membrane (mostly negative) also play an important role in the formation of PP. They are formed by glycoproteins, glycolipids and phospholipids: fixed external negative charges, neutralizing the positive charges of the outer surface of the membrane, reduce the PP, and fixed internal negative charges of the membrane, on the contrary, increase the PP, summing up with anions inside the cell. Thus, resting potential is algebraic sum all positive and negative charges of ions outside and inside the cell and surface charges of the cell membrane.
The role of ion pumps in the formation of PP. Ion pump is a protein molecule that ensures the transfer of an ion with a direct expenditure of energy, contrary to electrical and concentration gradients. As a result of the coupled transport of sodium and potassium, a constant difference in the concentrations of these ions is maintained inside and outside the cell. One ATP molecule provides one cycle of the Na/K pump - the transfer of three sodium ions outside the cell and two potassium ions inside the cell. Thus, the PP increases. The normal value of the resting potential is a necessary condition for the formation of an action potential, that is, for the formation of the excitation process.
۩Action potential. Action potential is an electrophysiological process that is expressed in rapid fluctuations in membrane potential due to changes in membrane permeability and diffusion of ions into and out of the cell. Role of PD is to ensure the transmission of signals between nerve cells, nerve centers and working organs; in the muscles, the PD ensures the process of electromechanical coupling. PD is subject to the “all or nothing” law. If the strength of stimulation is small, then a local potential arises that does not spread.
The action potential consists of three phases: depolarization, that is, the disappearance of the PP; inversion - changing the sign of the cell charge to the opposite; repolarization – restoration of the original MP.
Mechanism of action potential occurrence.
Depolarization phase . When a stimulus acts on a cell, the initial partial depolarization of the cell membrane occurs without changing its permeability to ions. When depolarization reaches approximately 50% of the threshold value, the permeability of the membrane to Na + increases, and at first relatively slowly. During this period, the driving force ensuring the movement of Na + into the cell is the concentration and electrical gradients. Let us remember that the inside of the cell is negatively charged (opposite charges attract), and the concentration of Na + outside the cell is 12 times greater than inside the cell. The condition that ensures further entry of Na + into the cell is an increase in the permeability of the cell membrane, which is determined by the state of the gate mechanism of sodium channels. The gating mechanism of sodium channels is located on the outer and inner sides of the cell membrane, the gating mechanism of potassium channels is located only on the inner side of the membrane. Sodium channels have an activation m-gate, which is located on the outside of the cell membrane, and an inactivation h-gate, located on the inside of the membrane. Under resting conditions, the activation m-gate is closed, and the inactivation h-gate is open. The potassium activation gate is closed, but the potassium inactivation gate is not. When cell depolarization reaches a critical value, which is usually 50 mV, the permeability of the membrane to Na + increases sharply, since a large number of voltage-dependent m-gates of sodium channels open and sodium ions rush into the cell in an avalanche. The developing depolarization of the cell membrane causes an additional increase in its permeability and, accordingly, sodium conductivity: more and more new activation m-gates open. As a result, the PP disappears, that is, it becomes equal to zero. The depolarization phase ends here. Its duration is approximately 0.2-0.5 ms.
Inversion phase . The process of membrane recharging represents the second phase of the AP – the inversion phase. The inversion phase is divided into ascending and descending components. Rising part . After the disappearance of the PP, the entry of sodium ions into the cell continues, since the sodium activation m-gate is still open. As a result, the charge inside the cell becomes positive and the charge outside becomes negative. Within a fraction of a millisecond, sodium ions continue to enter the cell. Thus, the entire ascending part of the AP peak is provided mainly by the entry of Na + into the cell. Descending component of the inversion phase . Approximately 0.2-0.5 ms after the onset of depolarization, the increase in AP stops as a result of the closing of the sodium inactivation h-gate and the opening of the potassium activation gate. Since potassium is located predominantly inside the cell, it, according to the concentration gradient, begins to quickly leave it, as a result of which the number of positively charged ions in the cell decreases. The cell's charge begins to decrease again. During the downward component of the inversion phase, the exit of potassium ions from the cell is also facilitated by the electrical gradient. K+ is pushed out of the cell by the positive charge and attracted by the negative charge from outside the cell. This continues until the positive charge inside the cell completely disappears. Potassium leaves the cell not only through controlled channels, but also through uncontrolled channels - leak channels. The AP amplitude consists of the AP value and the inversion phase value, which is 10-50 mV in different cells.
Repolarization phase . While the activation potassium channels are open, K+ still continues to leave the cell, according to the chemical gradient. The charge inside the cell becomes negative, and outside - positive, therefore, the electrical gradient sharply inhibits the release of potassium ions from the cell. But since the strength of the chemical gradient is greater than the strength of the electrical gradient, potassium ions continue to leave the cell very slowly. Then the activation potassium gate closes, leaving only the exit of potassium ions through leak channels, that is, along the concentration gradient through uncontrolled channels.
Thus, PD is caused by a cyclic process of sodium ions entering the cell and the subsequent release of potassium from it. The role of Ca 2+ in the occurrence of AP in nerve cells is insignificant. However, Ca 2+ plays a very important role in the occurrence of cardiac muscle action potential, in the transmission of impulses from one neuron to another, from nerve fiber to muscle fiber, and in ensuring muscle contraction.
Following the AP, trace phenomena (characteristic of neurons) arise - first a trace hyperpolarization, and then a trace depolarization. Trace hyperpolarization cell membrane is usually a consequence of the still remaining increased permeability of the membrane to potassium ions. Trace depolarization is associated with a short-term increase in membrane permeability for Na + and its entry into the cell according to chemical and electrical gradients.
In addition, there are: a) the so-called phase absolute refractoriness, or complete inexcitability of the cell. It occurs at the peak of the AP and lasts 1-2 ms; and b) relative refractory phase– a period of partial restoration of the cell, when strong irritation can cause new excitation. Relative refractoriness corresponds to the final part of the repolarization phase and the subsequent hyperpolarization of the cell membrane. In neurons, following hyperpolarization, partial depolarization of the cell membrane is possible. During this period, the next action potential can be caused by weaker stimulation, since the MP is somewhat less than usual. This period is called exaltation phase(period of increased excitability).
The rate of phase changes in cell excitability determines its lability. Lability, or functional mobility, is the speed of one cycle of excitation. A measure of the lability of an excitable formation is the maximum number of APs that it can reproduce in 1 second. Typically the excitation lasts less than 1 ms and is explosion-like. Such an “explosion” proceeds powerfully, but ends quickly.
Potential Document
... . Excitability fabrics and its measure. Laws of irritation excitable fabrics: strength, time actions irritant... potential peace(MPP); 2) membrane potential actions(MPD); 3) potential basal metabolic gradient (metabolic potential). Potential ...
Between the outer surface of the cell and its cytoplasm at rest there is a potential difference of about 0.06-0.09 V, and the cell surface is charged electropositively with respect to the cytoplasm. This potential difference is called resting potential or membrane potential. Accurate measurement of the resting potential is only possible with the help of microelectrodes designed for intracellular current drainage, very powerful amplifiers and sensitive recording instruments - oscilloscopes.
The microelectrode (Fig. 67, 69) is a thin glass capillary, the tip of which has a diameter of about 1 micron. This capillary is filled with saline solution, a metal electrode is immersed in it and connected to an amplifier and an oscilloscope (Fig. 68). As soon as the microelectrode pierces the membrane covering the cell, the oscilloscope beam is deflected down from its original position and established at a new level. This indicates the presence of a potential difference between the outer and inner surfaces of the cell membrane.
The origin of the resting potential is most fully explained by the so-called membrane-ion theory. According to this theory, all cells are covered with a membrane that is unequally permeable to different ions. In this regard, inside the cell in the cytoplasm there are 30-50 times more potassium ions, 8-10 times less sodium ions and 50 times less chlorine ions than on the surface. At rest, the cell membrane is more permeable to potassium ions than to sodium ions. Diffusion of positively charged potassium ions from the cytoplasm to the cell surface imparts outer surface membranes have a positive charge.
Thus, the cell surface at rest carries a positive charge, whereas inner side The membrane turns out to be negatively charged due to chlorine ions, amino acids and other large organic anions, which practically do not penetrate through the membrane (Fig. 70).
If a section of a nerve or muscle fiber is exposed to a sufficiently strong stimulus, then excitation occurs in this section, manifested in a rapid oscillation of the membrane potential and called action potential.
The action potential can be recorded either using electrodes applied to the outer surface of the fiber (extracellular lead) or a microelectrode inserted into the cytoplasm (intracellular lead).
With extracellular abduction, you can find that the surface of the excited area is very short period, measured in thousandths of a second, becomes charged electronegatively with respect to the resting area.
The cause of the action potential is a change in the ionic permeability of the membrane. When irritated, the permeability of the cell membrane to sodium ions increases. Sodium ions tend to enter the cell because, firstly, they are positively charged and are attracted inward electrostatic forces, secondly, their concentration inside the cell is low. At rest, the cell membrane was poorly permeable to sodium ions. Irritation changed the permeability of the membrane, and the flow of positively charged sodium ions from external environment cells into the cytoplasm significantly exceeds the flow of potassium ions from the cell to the outside. As a result, the inner surface of the membrane becomes positively charged, and the outer surface becomes negatively charged due to the loss of positively charged sodium ions. At this moment the peak of the action potential is recorded.
The increase in membrane permeability to sodium ions lasts for a very short time. Following this, reduction processes occur in the cell, leading to the fact that the permeability of the membrane for sodium ions again decreases, and for potassium ions increases. Since potassium ions are also positively charged, when they leave the cell, they restore the original relationship between the outside and inside the cell.
Accumulation of sodium ions inside the cell during repeated excitation does not occur because sodium ions are constantly evacuated from it due to the action of a special biochemical mechanism called the “sodium pump”. There is also evidence of active transport of potassium ions using the “sodium-potassium pump”.
Thus, according to the membrane-ion theory, the selective permeability of the cell membrane is of decisive importance in the origin of bioelectric phenomena, which determines the different ionic composition on the surface and inside the cell, and, consequently, the different charge of these surfaces. It should be noted that many provisions of the membrane-ion theory are still debatable and require further development.
