Is a nerve impulse an electrical impulse or not?

There are different points of view: chemical and electrical. Googling results.

Dmitriy. Why are nerves not wires, and why is a nerve impulse not a current? (4.09.2013)

PHYSICAL ENCYCLOPEDIA:

NERVOUS IMPULSE - wave of excitement, the edges spread along the nerve fiber and serve to transmit information from the peripheral. receptor (sensitive) endings to the nerve centers, inside the center. nervous system and from it to the executive apparatus - muscles and glands. Passage of N. and. accompanied by transitional electrical processes that can be recorded with both extracellular and intracellular electrodes... Along the nerve fiber, the nerve impulse spreads in the form of an electrical wave. potential. At the synapse, the propagation mechanism changes. When N. and. reaches presynaptic. endings, in synaptic. the gap releases an active chemical. substance - me d i a t o r. The transmitter diffuses through the synaptic. gap and changes the permeability of postsynaptic. membrane, as a result of which a potential arises on it, again generating a propagating pulse. This is how chem works. synapse. There is also electric. synapse when trace . the neuron is excited electrically...The state of rest of the nerve fiber...stationary due to the action ion pumps , and the membrane potential under open-circuit conditions is determined from the equality to zero of the total electric current...
The process of nervous excitation develops as follows (see also Biophysics). If you pass a weak current pulse through the axon, leading to depolarization of the membrane, then after removing the external. impact, the potential monotonically returns to its original level. In these conditions the axon behaves like a passive electrical circuit consisting of a capacitor and DC. resistance.
If current pulse exceeds a certain threshold value, the potential continues to change even after the disturbance is turned off...

The nerve fiber membrane is a nonlinear ionic conductor , the properties of which significantly depend on the electrical fields.

ION PUMPS molecular structures built into biol. membranes and implementing ion transport towards higher electrochemical potential

SEMENOV S.N. ABOUT THE PHONON NATURE OF THE NERVE IMPULSE FROM THE POSITION OF THE DYNAMICS OF EVOLUTION. (29.05.2013)
Semenov S.N. Phonon is a quantum of a biological (cellular) membrane.

MOLECULAR-MECHANICAL MODEL OF THE STRUCTURE AND FUNCTIONING OF BIOLOGICAL MEMBRANES
INTRODUCTION TO QUANTUM PHONON BIOLOGY OF MEMBRANES.
S.N. Semyonov, Publication date: September 8, 2003
Contact the author: [email protected]

Nikolaev L.A. “Metals in living organisms” - Moscow: Education, 1986 - p.127
In a popular science form, the author talks about the role of metals in biochemical processes occurring in living organisms. The book will help broaden the horizons of students.
Both ions (sodium and potassium) take part in the propagation of electrical impulses along the nerve.

The electrical nature of nerve impulses and the excitability of a nerve cell.
Even on the eve of the 19th century, Galvani experimentally proved that there is a certain connection between electricity and the functioning of muscles and nerves.
The establishment of the electrical nature of skeletal muscle excitation led to the practical application of this property in medicine. The Dutch physiologist Willern Einthoven contributed greatly to this. In 1903, he created a particularly sensitive galvanometer, so sensitive that it could be used to record changes in the electrical potential of the contracting heart muscle. Over the next three years, Einthoven recorded changes in the potential of the heart during its contraction (this recording is called an electrocardiogram) and compared the features of peaks and valleys with various types of cardiac pathologies.
The electrical nature of the nerve impulse was more difficult to detect; at first it was believed that the emergence of an electric current and its spread along the nerve fiber was caused by chemical changes in the nerve cell. The reason for such a purely speculative judgment was the results of experiments by the 19th century German physiologist Emile Du Bois-Raymond, who, using a highly sensitive galvanometer, was able to register a weak electric current in a nerve when it was stimulated.
As technology developed, studies of the electrical nature of the nerve impulse became more and more elegant. By placing tiny electrodes (microelectrodes) on various parts of the nerve fiber, researchers using an oscilloscope learned to record not only the magnitude of the electrical potential that arises when the nerve is excited, but also its duration, propagation speed and other electrophysiological parameters. For their work in this area, American physiologists Joseph Erlanger and Herbert Spencer Hesser were awarded the Nobel Prize in Medicine and Physiology in 1944.
If electrical impulses of increasing strength are applied to a nerve cell, then initially, until the strength of the impulse reaches a certain value, the cell will not respond to these impulses. But as soon as the strength of the impulse reaches a certain value, the cell suddenly becomes excited and immediately the excitation begins to spread along the nerve fiber. A nerve cell has a certain threshold of excitation, and to any stimulus exceeding this threshold, it responds with excitation only of a certain intensity. Thus, the excitability of a nerve cell obeys the “all or nothing” law, and in all nerve cells of the body the nature of excitation is the same.

http://med-000.ru/kak-funkcioniruet-nerv/elektrich...

Ionic theory of nerve impulses, the role of potassium and sodium ions in nervous excitation.

The excitation of the nerve cell itself is due to movement of ions across the cell membrane. Typically, the inside of the cell contains an excess of potassium ions, while the outside of the cell contains an excess of sodium ions. At rest, the cell does not release potassium ions and does not allow sodium ions into itself, preventing the concentrations of these ions on both sides of the membrane from becoming equal. The cell maintains the ion gradient through the operation of a sodium pump, which pumps sodium ions out as they enter the cell through the membrane. The different concentrations of sodium ions on both sides of the cell membrane create a potential difference of about 1/10 of a volt across it. When the cell is stimulated, the potential difference drops, which means the cell is excited. The cell cannot respond to the next stimulus until the potential difference between the outer and inner sides of the membrane is restored again. This “rest” period lasts a few thousandths of a second, and is called the refractory period.
After the cell is excited, the impulse begins to spread along the nerve fiber. The propagation of an impulse is a series of sequential excitations of fragments of a nerve fiber, when the excitation of the previous fragment causes the excitation of the next, and so on until the very end of the fiber. The propagation of the impulse occurs only in one direction, since the previous fragment, which has just been excited, cannot be re-excited immediately, since it is in the “rest” stage.
The fact that the emergence and propagation of a nerve impulse is caused by a change in the ionic permeability of the nerve cell membrane was first proven by British neurophysiologists Alan Lloyd Hodgkin and Andrew Fielding Huxley, as well as Australian researcher John Carew Iccles.

Action potential or nerve impulse, a specific response that occurs in the form of an excitatory wave and flows along the entire nerve pathway. This reaction is a response to a stimulus. The main task is to transmit data from the receptor to the nervous system, and after that it directs this information to the desired muscles, glands and tissues. After the passage of the pulse, the surface part of the membrane becomes negatively charged, while its inner part remains positive. Thus, a nerve impulse is a sequentially transmitted electrical change.

The exciting effect and its distribution are subject to physico-chemical nature. The energy for this process is generated directly in the nerve itself. This happens due to the fact that the passage of an impulse leads to the formation of heat. Once it has passed, the attenuation or reference state begins. In which only a fraction of a second the nerve cannot conduct a stimulus. The speed at which the pulse can be delivered ranges from 3 m/s to 120 m/s.

The fibers through which excitation passes have a specific sheath. Roughly speaking, this system resembles an electrical cable. The composition of the membrane can be myelin or non-myelin. The most important component of the myelin sheath is myelin, which plays the role of a dielectric.

The speed of the pulse depends on several factors, for example, on the thickness of the fibers; the thicker it is, the faster the speed develops. Another factor in increasing conduction speed is the myelin itself. But at the same time, it is not located over the entire surface, but in sections, as if strung together. Accordingly, between these areas there are those that remain “bare”. They cause current leakage from the axon.

An axon is a process that is used to transmit data from one cell to the rest. This process is regulated by a synapse - a direct connection between neurons or a neuron and a cell. There is also a so-called synaptic space or cleft. When an irritating impulse arrives at a neuron, neurotransmitters (molecules of a chemical composition) are released during the reaction. They pass through the synaptic opening, eventually reaching the receptors of the neuron or cell to which the data needs to be conveyed. Calcium ions are necessary for the conduction of a nerve impulse, since without this the neurotransmitter cannot be released.

The autonomic system is provided mainly by non-myelinated tissues. Excitement spreads through them constantly and continuously.

The transmission principle is based on the appearance of an electric field, so a potential arises that irritates the membrane of the adjacent section and so on throughout the fiber.

In this case, the action potential does not move, but appears and disappears in one place. The transmission speed through such fibers is 1-2 m/s.

Laws of conduct

There are four basic laws in medicine:

• Anatomical and physiological value. Excitation is carried out only if there is no violation in the integrity of the fiber itself. If unity is not ensured, for example, due to infringement, drug use, then the conduction of a nerve impulse is impossible.
• Isolated conduction of irritation. Excitation can be transmitted along the nerve fiber, without spreading to neighboring ones.
• Bilateral conduction. The path of impulse conduction can be of only two types - centrifugal and centripetal. But in reality, the direction occurs in one of the options.
• Non-decremental implementation. The impulses do not subside, in other words, they are carried out without decrement.

Chemistry of impulse conduction

The irritation process is also controlled by ions, mainly potassium, sodium and some organic compounds. The concentration of these substances is different, the cell is negatively charged inside itself, and positively charged on the surface. This process will be called potential difference. When a negative charge oscillates, for example, when it decreases, a potential difference is provoked and this process is called depolarization.

Stimulation of a neuron entails the opening of sodium channels at the site of stimulation. This may facilitate the entry of positively charged particles into the cell. Accordingly, the negative charge is reduced and an action potential or nerve impulse occurs. After this, the sodium channels close again.

It is often found that it is the weakening of polarization that promotes the opening of potassium channels, which provokes the release of positively charged potassium ions. This action reduces the negative charge on the cell surface.

The resting potential or electrochemical state is restored when potassium-sodium pumps are activated, with the help of which sodium ions leave the cell and potassium ions enter it.

As a result, we can say that when electrochemical processes are resumed, impulses occur that travel along the fibers.

Candidate of Biological Sciences L. Chailakhyan, researcher at the Institute of Biophysics of the USSR Academy of Sciences

Magazine reader L. Gorbunova (village of Tsybino, Moscow region) writes to us: “I am interested in the mechanism of signal transmission through nerve cells.”

1963 Nobel Prize laureates (from left to right): A. Hodgkin, E. Huxley, D. Eccles.

Scientists' ideas about the mechanism of nerve impulse transmission have recently undergone significant changes. Until recently, Bernstein's views dominated science.

The human brain is, without a doubt, the highest achievement of nature. A kilogram of nervous tissue contains the quintessence of the whole person, starting from the regulation of vital functions - the work of the heart, lungs, digestive tract, liver - and ending with his spiritual world. Here are our thinking abilities, our entire perception of the world, memory, reason, our self-awareness, our “I”. Knowing the mechanisms of how the brain works is knowing yourself.

The goal is great and tempting, but the object of research is incredibly complex. Just kidding, this kilogram of tissue represents a complex system of communication between tens of billions of nerve cells.

However, the first significant step towards understanding how the brain works has already been taken. It may be one of the easiest, but it is extremely important for everything that follows.

I mean the study of the mechanism of transmission of nerve impulses - signals running along the nerves, as if through wires. It is these signals that are the alphabet of the brain, with the help of which the senses send information-dispatches about events in the outside world to the central nervous system. The brain encodes its orders to the muscles and various internal organs with nerve impulses. Finally, individual nerve cells and nerve centers speak the language of these signals.

Nerve cells - the main element of the brain - are varied in size and shape, but in principle they have a single structure. Each nerve cell consists of three parts: a body, a long nerve fiber - an axon (its length in humans ranges from several millimeters to a meter) and several short branched processes - dendrites. Nerve cells are isolated from each other by membranes. But the cells still interact with each other. This happens at the junction of cells; this junction is called a “synapse”. At a synapse, the axon of one nerve cell and the body or dendrite of another cell meet. Moreover, it is interesting that excitation can be transmitted only in one direction: from the axon to the body or dendrite, but in no case back. A synapse is like a kenotron: it transmits signals in only one direction.

In the problem of studying the mechanism of a nerve impulse and its propagation, two main questions can be distinguished: the nature of the conduction of a nerve impulse or excitation within one cell - along a fiber, and the mechanism of transmission of a nerve impulse from cell to cell - through synapses.

What is the nature of the signals transmitted from cell to cell along nerve fibers?

People have been interested in this problem for a long time; Descartes assumed that the propagation of the signal was associated with the transfusion of fluid through the nerves, as if through tubes. Newton thought it was a purely mechanical process. When the electromagnetic theory appeared, scientists decided that a nerve impulse is analogous to the movement of current through a conductor at a speed close to the speed of propagation of electromagnetic oscillations. Finally, with the development of biochemistry, a point of view emerged that the movement of a nerve impulse is the propagation along a nerve fiber of a special biochemical reaction.

Yet none of these ideas came to fruition.

Currently, the nature of the nerve impulse has been revealed: it is a surprisingly subtle electrochemical process, which is based on the movement of ions through the cell membrane.

The work of three scientists made a major contribution to the discovery of this nature: Alan Hodgkin, professor of biophysics at the University of Cambridge; Andrew Huxley, Professor of Physiology, University of London, and John Eccles, Professor of Physiology, University of Canberra, Australia. They were awarded the Nobel Prize in Medicine for 1963.

The famous German physiologist Bernstein was the first to suggest the electrochemical nature of the nerve impulse at the beginning of this century.

By the early twentieth century, quite a lot was known about nervous excitation. Scientists already knew that a nerve fiber can be excited by electric current, and the excitation always occurs under the cathode - under the minus. It was known that the excited area of ​​the nerve is charged negatively in relation to the non-excited area. It was found that the nerve impulse at each point lasts only 0.001-0.002 seconds, that the magnitude of excitation does not depend on the strength of the irritation, just as the volume of the bell in our apartment does not depend on how hard we press the button. Finally, scientists have established that the carriers of electric current in living tissues are ions; Moreover, inside the cell the main electrolyte is potassium salts, and in the tissue fluid - sodium salts. Inside most cells, the concentration of potassium ions is 30-50 times higher than in the blood and in the intercellular fluid that washes the cells.

And based on all this data, Bernstein suggested that the membrane of nerve and muscle cells is a special semi-permeable membrane. It is permeable only to K + ions; for all other ions, including negatively charged anions inside the cell, the path is closed. It is clear that potassium, according to the laws of diffusion, will tend to leave the cell, an excess of anions appears in the cell, and a potential difference will appear on both sides of the membrane: outside - plus (excess cations), inside - minus (excess of anions). This potential difference is called the resting potential. Thus, at rest, in an unexcited state, the inside of the cell is always negatively charged compared to the outer solution.

Bernstein suggested that at the moment of excitation of the nerve fiber, structural changes occur in the surface membrane, its pores seem to increase, and it becomes permeable to all ions. In this case, naturally, the potential difference disappears. This causes a nerve signal.

Bernstein's membrane theory quickly gained recognition and existed for over 40 years, until the middle of our century.

But already at the end of the 30s, Bernstein's theory encountered insurmountable contradictions. It was dealt a major blow in 1939 by the subtle experiments of Hodgkin and Huxley. These scientists were the first to measure the absolute values ​​of the membrane potential of a nerve fiber at rest and during excitation. It turned out that upon excitation, the membrane potential did not simply decrease to zero, but crossed zero by several tens of millivolts. That is, the inner part of the fiber changed from negative to positive.

But it is not enough to overthrow a theory, we must replace it with another: science does not tolerate a vacuum. And Hodgkin, Huxley, Katz in 1949-1953 propose a new theory. It is called sodium.

Here the reader has the right to be surprised: until now there has been no talk about sodium. That's the whole point. Scientists have established with the help of labeled atoms that not only potassium ions and anions are involved in the transmission of nerve impulses, but also sodium and chlorine ions.

There are enough sodium and chlorine ions in the body; everyone knows that blood tastes salty. Moreover, there is 5-10 times more sodium in the intercellular fluid than inside the nerve fiber.

What could this mean? Scientists have suggested that upon excitation, at the first moment, the permeability of the membrane only to sodium sharply increases. The permeability becomes tens of times greater than for potassium ions. And since there is 5-10 times more sodium outside than inside, it will tend to enter the nerve fiber. And then the inside of the fiber will become positive.

And after some time - after excitation - equilibrium is restored: the membrane begins to allow potassium ions to pass through. And they go outside. Thus, they compensate for the positive charge that was introduced into the fiber by sodium ions.

It was not at all easy to come to such ideas. And here's why: the diameter of the sodium ion in solution is one and a half times larger than the diameter of potassium and chlorine ions. And it is completely unclear how a larger ion passes where a smaller one cannot pass.

It was necessary to radically change the view on the mechanism of ion transition through membranes. It is clear that reasoning about pores in the membrane alone is not sufficient here. And then the idea was put forward that ions could cross the membrane in a completely different way, with the help of secret allies for the time being - special organic carrier molecules hidden in the membrane itself. With the help of such a molecule, ions can cross the membrane anywhere, not just through the pores. Moreover, these taxi molecules distinguish their passengers well; they do not confuse sodium ions with potassium ions.

Then the general picture of the propagation of a nerve impulse will look like this. At rest, carrier molecules, negatively charged, are pressed to the outer boundary of the membrane by the membrane potential. Therefore, the permeability for sodium is very small: 10-20 times less than for potassium ions. Potassium can cross the membrane through pores. As the excitation wave approaches, the pressure of the electric field on the carrier molecules decreases; they throw off their electrostatic “shackles” and begin to transfer sodium ions into the cell. This further reduces the membrane potential. There is a kind of chain process of recharging the membrane. And this process continuously spreads along the nerve fiber.

Interestingly, nerve fibers spend only about 15 minutes a day on their main job - conducting nerve impulses. However, the fibers are ready for this at any second: all elements of the nerve fiber work without interruption - 24 hours a day. Nerve fibers in this sense are similar to interceptor aircraft, whose engines are constantly running for instant departure, but the departure itself can only take place once every few months.

We have now become acquainted with the first half of the mysterious act of passing a nerve impulse along one fiber. How is excitation transmitted from cell to cell, through junctions - synapses? This question was explored in the brilliant experiments of the third Nobel laureate, John Eccles.

Excitation cannot directly transfer from the nerve endings of one cell to the body or dendrites of another cell. Almost all of the current flows through the synaptic cleft into the outer fluid, and a tiny fraction of it enters the neighboring cell through the synapse, unable to cause excitation. Thus, in the region of synapses, the electrical continuity in the propagation of the nerve impulse is disrupted. Here, at the junction of two cells, a completely different mechanism comes into force.

When excitation approaches the end of the cell, the site of the synapse, physiologically active substances - mediators, or intermediaries - are released into the intercellular fluid. They become a link in the transfer of information from cell to cell. The mediator chemically interacts with the second nerve cell, changes the ionic permeability of its membrane - as if punching a hole into which many ions rush, including sodium ions.

So, thanks to the work of Hodgkin, Huxley and Eccles, the most important states of a nerve cell - excitation and inhibition - can be described in terms of ionic processes, in terms of structural and chemical rearrangements of surface membranes. Based on these works, it is already possible to make assumptions about the possible mechanisms of short-term and long-term memory, and about the plastic properties of nervous tissue. However, this is a conversation about mechanisms within one or more cells. This is just the ABC of the brain. Apparently, the next stage, perhaps much more difficult, is the discovery of the laws by which the coordinating activity of thousands of nerve cells is built, the recognition of the language that the nerve centers speak among themselves.

In our knowledge of how the brain works, we are now at the level of a child who has learned the letters of the alphabet, but does not know how to connect them into words. However, the time is not far when scientists, using the code - elementary biochemical acts occurring in a nerve cell, will read the most fascinating dialogue between the nerve centers of the brain.

Detailed description of illustrations

Scientists' ideas about the mechanism of nerve impulse transmission have recently undergone significant changes. Until recently, Bernstein's views dominated science. In his opinion, in a state of rest (1) the nerve fiber is charged positively on the outside and negatively on the inside. This was explained by the fact that only positively charged potassium ions (K +) can pass through the pores in the fiber wall; Large negatively charged anions (A –) are forced to remain inside and create an excess of negative charges. Excitation (3) according to Bernstein is reduced to the disappearance of the potential difference, which is caused by the fact that the pore size increases, anions come out and equalize the ionic balance: the number of positive ions becomes equal to the number of negative ones. The work of 1963 Nobel Prize winners A. Hodgkin, E. Huxley and D. Eccles changed our previous ideas. It has been proven that positive sodium ions (Na +), negative chlorine ions (Cl –) and negatively charged carrier molecules are also involved in nervous excitation. The resting state (3) is formed in principle in the same way as was previously thought: an excess of positive ions is outside the nerve fiber, an excess of negative ones is inside. However, it has been established that during excitation (4) it is not the equalization of charges that occurs, but a recharging: an excess of negative ions is formed outside, and an excess of positive ions inside. This is explained by the fact that when excited, carrier molecules begin to transport positive sodium ions through the wall. Thus, the nerve impulse (5) is a recharge of the electrical double layer moving along the fiber. And from cell to cell, excitation is transmitted by a kind of chemical “battering ram” (6) - an acetylcholine molecule, which helps ions break through the wall of the neighboring nerve fiber.

RESEARCH WORK

Electrical nature of the nerve impulse

Introduction 3

Experiments by L. Galvani and A. Volta 3

Biocurrents in living organisms 4

The effect of irritability. 5

Nerve cell and nerve impulse transmission 6

Action of nerve impulse on various parts of the body 8

Exposure to electrical activity for medical purposes 9

Reaction speed 10

Conclusion 11

Literature 11

Application

Introduction

“No matter how wonderful the laws and phenomena

electricity,

appearing to us in the world

inorganic or

dead matter, interest,

which they

imagine, it can hardly

compare with that

which is inherent in the same force

in connection with the nervous

system and life"

M. Faraday

Purpose of the work: To determine the factors influencing the propagation of a nerve impulse.

This work had the following tasks:

1. Study the history of the development of the science of bioelectricity.

2. Consider electrical phenomena in living nature.

3. Investigate the transmission of nerve impulses.

4. Check in practice what affects the speed of nerve impulse transmission.

Experiments by L. Galvani and A. Volta

Back in the 18th century. Italian physician Luigi Galvani (1737-1787) discovered that if electrical voltage is applied to the headless body of a frog, contractions of its legs are observed. So he showed the effect of electric current on muscles, so he is rightly called the father of electrophysiology. In other experiments, he hung the leg of a dissected frog on a brass hook. At the moment when, swinging, the paw touched the iron grating of the balcony where the experiments were carried out, a contraction of the paw was again observed. Galvani suggested the existence of a potential difference between the nerve and the paw - “animal electricity”. He explained the contraction of the muscle by the action of an electric current arising in the tissues of the frog when the circuit is closed through the metal.

Galvani's compatriot, Alessandro Volta (1745-1827), carefully studied the electrical circuit used by Galvani and proved that it contains two dissimilar metals that are closed through a saline solution, i.e. it looks like a chemical current source. The neuromuscular preparation, he argued, in this experiment serves only as a sensitive galvanometer.

Galvani could not admit defeat. He threw a nerve onto the muscle under various conditions to prove that even without metal it was possible to obtain muscle contraction using electricity of “animal origin.” One of his followers finally succeeded. It turned out that an electric current occurs in cases where a nerve is thrown onto a damaged muscle. This is how electrical currents were discovered between healthy and damaged tissue. That's what they were called -damage currents. Later it was shown that any activity of nerves, muscles and other tissues is accompanied by the generation of electrical currents.

Thus, the presence of biocurrents in living organisms has been proven. Nowadays, they are recorded and examined with sensitive instruments - oscilloscopes.

Biocurrents in living organisms

The first information about the study of electrical phenomena in living nature is interesting. The object of observation was electric fish. Through experiments on the electric stingray, Faraday established that the electricity created by a special organ of this fish is completely identical to electricity received from a chemical or other source, although it is a product of the activity of a living cell. Subsequent observations showed that many fish have special electrical organs, a kind of “battery” that generates high voltages. Thus, the giant stingray creates a discharge voltage of 50-60 V, the Nile electric catfish 350 V, and the electrophorus eel - over 500 V. However, this high voltage does not have any effect on the body of the fish itself!

The electrical organs of these fish consist of muscles that have lost the ability to contract: muscle tissue serves as a conductor, and connective tissue as an insulator. Nerves from the spinal cord go to the organ, and in general it is a fine-plate structure of alternating elements. For example, the eel has from 6,000 to 10,000 elements connected in series to form a column, and about 70 columns in each organ located along the body. In adults, this organ accounts for about 40% of the total body weight. The role of electrical organs is great, they serve for defense and attack, and are also part of a very sensitive navigation and location system.

The effect of irritability.

One of the most important functions of the body, calledirritability, - ability to respond to environmental changes. The highest irritability is in animals and humans, which have specialized cells that form nervous tissue. Nerve cells - neurons - are adapted for a quick and specific response to various stimuli coming from the external environment and the tissues of the body itself. Reception and transmission of irritations occurs with the help of electrical impulses propagating along certain paths.

Nerve cell and nerve impulse transmission

A nerve cell, neuron, is a star-shaped body and consists of thin processes - axons and dendrites. The end of the axon passes into thin fibers that end in muscle or synapses. In an adult, the length of the axon can reach 1-1.5 m with a thickness of about 0.01 mm. The cell membrane plays a special role in the formation and transmission of nerve impulses.

The fact that a nerve impulse is an electric current pulse has only been provenby the middle of the 20th century, mainly by the work of A. Hodgkin’s group. In 1963, A. Hodgkin, E. Huxley and J. Eccles were awarded the Nobel Prize in Physiology or Medicine “for their discoveries concerning ionic mechanisms involved in excitation and inhibition in the peripheral and central regions of the nerve cell membrane.” The experiments were carried out on giant neurons (diameter 0.5 mm) - squid axons.

Certain parts of the membrane have semiconductor and ion-selective properties - they allow ions of the same sign or one element to pass through. The appearance of membrane potential, on which the work of the body’s information and energy-transforming systems depends, is based on this selective ability. In the external solution, more than 90% of the charged particles are sodium and chlorine ions. In the solution inside the cell, the bulk of the positive ions are potassium ions, and the negative ones are large organic ions. The concentration of sodium ions outside is 10 times higher than inside, and potassium ions inside is 30 times higher than outside. Due to this, an electrical double layer appears on the cell wall. Since the membrane at rest is highly permeable, a potential difference of 60-100 mV arises between the internal part and the external environment, and the internal part is negatively charged. This potential difference is calledresting potential.

When the cell is stimulated, the electrical double layer is partially discharged. When the resting potential decreases to 15-20 mV, the permeability of the membrane increases, and sodium ions rush into the cell. Once a positive potential difference between both membrane surfaces is reached, the flow of sodium ions dries up. At the same moment, channels for potassium ions open, and the potential shifts to the negative side. This in turn reduces the supply of sodium ions and the potential returns to its resting state.

The signal arising in the cell spreads along the axon due to the conductivity of the electrolyte located inside it. If the axon has special insulation - the myelin sheath - then the electrical impulse travels through these areas faster, and the overall speed is determined by the size and number of non-insulated areas. The impulse speed in the axon is 100 m/s.

How is the signal transmitted across the gap? It turned out that the synapse membrane is heterogeneous in structure - in the central regions it has “windows” with low resistance, and at the edge the resistance is high. The heterogeneity of the membrane is created in a special way: with the help of a special protein - coppectin. The molecules of this protein form a special structure - copnexon, which in turn consists of six molecules and has a channel inside. Thus, the synapse connects two cells with many small tubes passing inside protein molecules. The gap between the membranes is filled with an insulator. In birds, the protein myelin acts as an insulator.

When the change in potentials in the muscle fiber reaches the excitation threshold of the electrically excitable membrane, an action potential arises in it and the muscle fiber contracts.

The action of nerve impulses on various parts of the body

For more than one millennium, humanity has been puzzling over what happens in the brain of every person. It is now known that in the brain thoughtsare born under the influence of electric current, but the mechanism has not been studied. Reflecting on the interaction of chemical and physical phenomena, Faraday said: “Wonderful as are the laws and phenomena of electricity which we have observed in the world of inorganic matter and inanimate nature, the interest which they present can hardly be compared with that which is caused by the same force in combination with life."

In humans, an electromagnetic field has also been found, generated by bioelectric potentials on the surface of cells. The Soviet inventor S.D. Kirlian managed to make this phenomenon visual in the literal sense of the word. He proposed photographing the human body by placing it between two large metal walls to which an alternating electrical voltage was applied. In an environment with an increased electromagnetic field, microcharges appear on human skin, and the places where the nerve endings emerge are most active. In photographs taken using the Kirlian method, they are visible in the form of small, brightly glowing dots. These points, as it turned out, are located exactly in those places of the body in which it is recommended to immerse silver needles during acupuncture treatment.

Thus, using the recording of brain biocurrents as feedback, it is possible to assess the degree of prayer immersion of the patient.

It is now known that certain areas of the brain are responsible for emotions and creative activity. It is possible to determine whether a particular area of ​​the brain is in an excited state, but it is impossible to decipher these signals, so we can say with confidence that humanity will not soon learn to read minds.

A person’s thought is a product of the brain, associated with bioelectric phenomena in it and in other parts of the body. It is the biocurrents that arise in the muscles of a person who thinks about clenching his fingers into a fist, captured and amplified by appropriate equipment, that compress the fingers of a mechanical hand.

Academics psychiatristVladimir Mikhailovich Bekhterev and biophysicistPyotr Petrovich Lazarev recognized that under some special conditions, not yet precisely known to science, the electrical energy of one brain can influence the brain of another person at a distance. If this brain is “tuned” accordingly, they assumed, it is possible to evoke in it “resonant” bioelectric phenomena and, as a product of them, corresponding ideas.

The study of electrical phenomena in the body has brought significant benefits. Let's list the most famous ones.

Exposure to electrical activity for medical purposes

О Electrochemistry is widely used in medicine and physiology. The potential difference between two points of the cell is determined using microelectrodes. With their help, you can measure the oxygen content in the blood: a catheter is inserted into the blood, the basis of which is a platinum electrode, placed together with a reference electrode in an electrolyte solution, which is separated from the analyzed blood by a porous hydrophobic Teflon film; oxygen dissolved in the blood diffuses through the pores of the Teflon film to the platinum electrode and is reduced there.

O In the process of life, the state of the organ, and therefore its electrical activity, changes over time. A method for studying their operation, based on recording electric field potentials on the surface of a body, is called electrography. The name of the electrogram indicates the organs or tissues being studied: heart - electrocardiogram, brain - electroencephalogram, muscles - electromyogram, skin - galvanic skin response, etc.

O In medical practice, electrophoresis is widely used to separate proteins, amino acids, antibiotics, and enzymes in order to monitor the progress of the disease. Iontophoresis is just as common.

O The well-known “artificial kidney” device, to which a patient is connected in case of acute renal failure, is based on the phenomenon of electrodialysis. Blood flows in a narrow gap between two membranes, washed with physiological solution, while waste products - products of metabolism and tissue breakdown - are removed from it.

O Researchers from the USA have proposed treating epilepsy with electrical stimulation. To do this, a tiny device is sewn under the skin in the upper chest, programmed to stimulate the vagus nerve for 30 hours at intervals of 5-15 minutes. Its effect has been tested in the USA, Canada, and Germany. In patients who were not helped by medications, after 3 months the number of seizures decreased by 25%, after 1.5 years - by 50%.

Speed ​​reaction

One of the features that characterizes the brain is its reaction speed. It is determined by the time during which the first impulse moves from the receptors of the organ that received the irritation to the organ that produces the body’s response. From the survey I conducted, it follows that reaction speed and attentiveness are influenced by many factors. In particular, it may decrease for the following reasons: uninteresting and (or) educational material presented monotonously by the teacher; poor discipline in the classroom; unclear purpose and lesson plan; stale indoor air; classroom temperature is too hot or too cold; extraneous noise; the presence of new unnecessary benefits, fatigue at the end of the day.

There are also individual reasons for inattention: learning the material is too easy or too difficult; unpleasant family events; illness, overwork; watching a large number of films; late falling asleep.

Conclusion

Words have a huge influence on human nervous activity. The more listeners trust the speaker, the brighter the emotional coloring of the words they perceive and the stronger their effect. The patient trusts the doctor, the student trusts the teacher, so you should be especially careful when choosing words that stimulate the second signal system. So, a flight school cadet who was already a good flyer suddenly began to experience insurmountable fear. It turned out that the pilot instructor, who was authoritative for him, left him a note when leaving: “I hope to see you soon, but be careful with the spin.”

With a word you can both cause a disease and successfully cure it. Word treatment - logotherapy - is part of psychotherapy. My next experience is direct proof of this. I asked two people to do the following: at the same time, with one hand, stroke the stomach in a circular motion, and with the other, touch the head in a straight line. It turned out that this was quite difficult to do - the movements were either simultaneously circular or linear. However, I influenced the subjects in different ways: I told one that he was about to succeed, and the other that he would not succeed. After some time, everything worked out for the first one, but nothing worked out for the other.

Personal indicators should be used as a guide when choosing a profession. If the reaction speed is low, then it is better not to choose professions that require a lot of attention and quick analysis of the situation (pilot, driver, etc.).

Literature

Voronkov G.Ya.Electricity in the world of chemistry. - M.: Knowledge, 1987.

Tretyakova S.V.Human nervous system. - Physics (“PS”), No. 47.

Platonov K.Interesting psychology. - M.: Liter, 1997.

Berkinblit M.B., Glagoleva E.G.Electricity in living organisms. - M.: Nauka, 1988.

The effect of fatigue on nerve electrical impulses

Purpose: to check the effect of physical activity on reaction speed.

Progress of the study:The usual simple reaction time is 100-200 ms for light, 120-150 ms for sound, and 100-150 ms for an electrocutaneous stimulus. I conducted an experiment using the method of Academician Platonov.At the beginning of the physical education lesson, we recorded the reaction time when catching a ball, then we checked this reaction after physical activity.

Reaction time before physical activity

Reaction time after physical Loads

Kocharyan Karen

0.13s

0.15s

Nikolaev Valery

0.15s

0.16s

Kazakov Vadim

0.14s

0.16s

Kuzmin Nikita

0.8s

0.1s

Safiullin Timur

0.13s

0.15s

Tukhvatullin Rishat

0.9s

0.11s

Farafonov Arthur

0.9s

0.11s

Conclusion: We recorded reaction time before and after physical activity. We concluded that fatigue slows down reaction time.Based on this, we can advise teachers, when drawing up a schedule, to place subjects that require maximum attention in the middle of the school day, when students are not yet tired and are capable of full mental activity.

NERVOUS IMPULSE

NERVOUS IMPULSE

A wave of excitation, edges, spreads along the nerve fiber and serves to transmit information from the peripheral. receptor (sensitive) endings to the nerve centers, inside the center. nervous system and from it to the executive apparatus - muscles and glands. Passage of N. and. accompanied by transitional electrical processes that can be recorded with both extracellular and intracellular electrodes.

Generation, transmission and processing of N. and. carried out by the nervous system. Basic The structural element of the nervous system of higher organisms is the nerve cell, or neuron, consisting of a cell body and numerous. processes - dendrites (Fig. 1). One of the processes in non-riferiforms. neurons have a large length - this is a nerve fiber, or axon, the length of which is ~ 1 m, and the thickness is from 0.5 to 30 microns. There are two classes of nerve fibers: pulpy (myelinated) and non-pulphate. The pulp fibers have myelin, formed by special fibers. membrane, the edges, like insulation, are wound onto the axon. The length of the sections of the continuous myelin sheath ranges from 200 µm to 1 mm, they are interrupted by the so-called. nodes of Ranvier 1 µm wide. The myelin sheath plays an insulating role; the nerve fiber in these areas is passive, electrically active only in the nodes of Ranvier. Non-pulp fibers are not insulated. plots; their structure is uniform along the entire length, and the membrane is electrically activity over the entire surface.

Nerve fibers end on the bodies or dendrites of other nerve cells, but are separated from them intermediately.

an eerie width of ~10 nm. This area of ​​contact between two cells is called. synapse. The axon membrane entering the synapse is called presynaptic, and the corresponding membrane of dendrites or muscles is post-synaptic (see.

Cellular structures).

Under normal conditions, a series of nerve fibers constantly run along the nerve fiber, arising on dendrites or the cell body and spreading along the axon in the direction from the cell body (the axon can conduct nerve fibers in both directions). The frequency of these periodic discharges carry information about the strength of the irritation that caused them; for example, with moderate activity, the frequency is ~ 50-100 impulses/s. There are cells that discharge at a frequency of ~1500 pulses/s. . Speed ​​of spread of N. and. u depends on the type of nerve fiber and its diameter d, . ~ u d . 1/2. In the thin fibers of the human nervous system u . ~ 1 m/s, and in thick fibers u

Each N. and. occurs as a result of irritation of the nerve cell body or nerve fiber. N. and. always has the same characteristics (shape and speed) regardless of the strength of stimulation, i.e., with subthreshold stimulation of N. and. does not occur at all, but when above the threshold it has full amplitude.

After excitation, a refractory period begins, during which the excitability of the nerve fiber is reduced. There are abs. the refractory period, when the fiber cannot be excited by any stimuli, and refers. refractory period, when possible, but its threshold is higher than normal. Abs. the refractory period limits from above the frequency of transmission of N. and. The nerve fiber has the property of accommodation, that is, it gets used to constant stimulation, which is expressed in a gradual increase in the threshold of excitability. This leads to a decrease in the frequency of N. and. and even to their complete disappearance. If stimulation increases slowly, then arousal may not occur even after reaching the threshold.

Fig.1. Diagram of the structure of a nerve cell.

Along the nerve fiber N. and. spreads in the form of electricity. potential. At the synapse, the propagation mechanism changes. When N. and. reaches presynaptic. endings, in synaptic. the gap releases an active chemical. - M e d i a t o r. The transmitter diffuses through the synaptic. gap and changes the permeability of postsynaptic. membrane, as a result of which it appears on it, again generating spreading. This is how chem works. synapse. There is also electric. synapse when . the neuron is excited electrically.

Excitement N. and. Phys. ideas about the appearance of electricity. potentials in cells are based on the so-called. membrane theory. Cell membranes separate electrolyte of different concentrations and have a birate. permeability for certain ions. Thus, the axon membrane is a thin layer of lipids and proteins ~7 nm thick. Her electric Resistance at rest ~ 0.1 Ohm. m2, and the capacity is ~ 10 mf/m2. Inside the axon, the concentration of K + ions is high and the concentration of Na + and Cl - ions is low, and in the environment - vice versa.

In the resting state, the axon membrane is permeable to K + ions. Due to the difference in concentrations C 0 K . in ext. and C in internal solutions, the potassium membrane potential is established on the membrane

Where T - abs. temp-pa, e - electron charge. A resting potential of ~-60 mV is indeed observed on the axon membrane, corresponding to the indicated value.

Na + and Cl - ions penetrate the membrane. To maintain the necessary non-equilibrium distribution of ions, the cell uses an active transport system, which consumes cellular energy for work. Therefore, the resting state of the nerve fiber is not thermodynamically equilibrium. It is stationary due to the action of ion pumps, and the membrane potential under open-circuit conditions is determined from the equality to zero of the total electric current. current.

The process of nervous excitation develops as follows (see also Biophysics). If you pass a weak current pulse through the axon, leading to depolarization of the membrane, then after removing the external. impact, the potential monotonically returns to its original level. Under these conditions, the axon behaves as a passive electrical current. circuit consisting of a capacitor and DC. resistance.

Rice. 2. Development of action potential in the nervous systemlocke: A- subthreshold ( 1 ) and suprathreshold (2) irritation; b-membrane response; with above-threshold stimulation, full sweat occursaction cial; V- ion current flowing through membrane when excited; G - approximation ion current in a simple analytical model.

If the current pulse exceeds a certain threshold value, the potential continues to change even after the disturbance is turned off; the potential becomes positive and only then returns to the resting level, and at first it even jumps a little (hyperpolarization region, Fig. 2). The response of the membrane does not depend on the disturbance; this impulse is called action potential. At the same time, an ionic current flows through the membrane, directed first inward and then outward (Fig. 2, V).

Phenomenological interpretation of the mechanism of occurrence of N. and. was given by A. L. Hodgkin and A. F. Huxley in 1952. The total ion current is composed of three components: potassium, sodium and leakage current. When the membrane potential shifts by a threshold value j* (~ 20 mV), the membrane becomes permeable to Na + ions. Na + ions rush into the fiber, shifting the membrane potential until it reaches the equilibrium sodium potential:

component ~ 60 mV. Therefore, the full amplitude of the action potential reaches ~120 mV. By the time the max. potential in the membrane, potassium begins to develop (and at the same time sodium decreases). As a result, the sodium current is replaced by a potassium current directed outward. This current corresponds to a decrease in the action potential.

Established empirically. equation for describing sodium and potassium currents. The behavior of the membrane potential during spatially uniform excitation of the fiber is determined by the equation:

Where WITH - membrane capacity, I- ion current, consisting of potassium, sodium and leakage current. These currents are determined by the post. emf j K , j Na and j l and conductivities g K, g Na and gl:

Size g l considered constant, conductivity g Na and g K is described using parameters m, h And P:

g Na, g K - constants; options t, h And P satisfy linear equations

Dependence of coefficient a . and b from the membrane potential j (Fig. 3) are selected from the best fit condition

Rice. 3. Dependence of coefficientsa. Andbfrom membranesgreat potential.

calculated and measured curves I(t). The choice of parameters was driven by the same considerations. Dependence of stationary values t, h And P from the membrane potential is shown in Fig. 4. There are models with a large number of parameters. Thus, the nerve fiber membrane is a nonlinear ionic conductor, the properties of which significantly depend on the electrical power. fields. The mechanism of excitation generation is poorly understood. The Hodgkin-Huxley equation provides only successful empirical evidence. description of the phenomenon, for which there is no specific physical. models. Therefore, an important task is to study the mechanisms of electrical flow. current through membranes, in particular through controlled electric. field ion channels.

Rice. 4. Dependence of stationary values t, h And P from membrane potential.

Distribution of N. and. N. and. can propagate along the fiber without attenuation and with DC. speed. This is due to the fact that the energy necessary for signal transmission does not come from a single center, but is drawn locally, at each point of the fiber. In accordance with the two types of fibers, there are two ways of transmitting N. and.: continuous and saltatory (saccade-like), when the impulse moves from one node of Ranvier to another, jumping over areas of myelin insulation.

In the case of unmyelinated fiber membrane potential j( x, t) is determined by the equation:

Where WITH - membrane capacity per unit length of fiber, R- the sum of longitudinal (intracellular and extracellular) resistances per unit fiber length, I- ionic current flowing through the membrane of a fiber of unit length. Electric current I is a functional of potential j, which depends on time t and coordinates X. This dependence is determined by equations (2) - (4).

Type of functionality I specific for a biologically excitable environment. However, equation (5), if we ignore the type I, is more general in nature and describes many physical. phenomena, for example combustion process. Therefore, N.’s transmission and. likened to the burning of a gunpowder cord. If in a running flame the ignition process is carried out due to thermal conductivity, then in N. and. excitation occurs with the help of the so-called. local currents (Fig. 5).

Rice. 5. Local currents that ensure propagationloss of nerve impulse.

Hodgkin-Huxley equation for the dissemination of N. and. were solved numerically. The obtained solutions together with the accumulated experiments. data showed that the spread of N. and. does not depend on the details of the excitation process. Quality picture of the spread of N. and. can be obtained using simple models that reflect only the general properties of excitation. This approach made it possible to calculate the shape of N. and. in a homogeneous fiber, their change in the presence of inhomogeneities, and even complex regimes of excitation propagation in active media, for example. in the heart muscle. There are several math. models of this kind. The simplest of them is this. The ionic current flowing through the membrane during the passage of nitrogen is alternating in sign: first it flows into the fiber, and then out. Therefore, it can be approximated by a piecewise constant function (Fig. 2, G). Excitation occurs when the membrane potential shifts by a threshold value j*. At this moment, a current appears, directed into the fiber and equal in magnitude j". After t" the current changes to the opposite, equal to j". This continues for a time ~ t ". A self-similar solution to equation (5) can be found as a function of the variable t = x/ d, , where u - speed of spread of N. and. (Fig. 2, b).

In real fibers, the time t" is quite long, so only it determines the speed u , for this type the following formula is valid: . Considering that j" ~ ~d, R~d 2 and WITH~ depends on the type of nerve fiber and its diameter Where d- fiber diameter, we find, in agreement with experiment, that u ~d 1/2 . Using piecewise constant approximation, the shape of the action potential is found.

Equation (5) for spreading N. and. actually allows two solutions. The second solution turns out to be unstable; it gives N. and. with a significantly lower speed and potential amplitude. The presence of a second, unstable solution has an analogy in the theory of combustion. When a flame propagates with a lateral heat sink, an unstable mode may also occur. Simple analytical model N. and. can be improved by taking into account additional details.

When the cross-section changes and when nerve fibers branch, N.’s passage and. may be difficult or even completely blocked. In an expanding fiber (Fig. 6), the pulse speed decreases as it approaches expansion, and after expansion it begins to increase until it reaches a new stationary value. Slowing down N. and. the stronger the greater the difference in cross sections. With a sufficiently large expansion of N. and. stops. There is a critical expansion of the fiber, which delays N. and.

With the reverse movement of N. and. (from wide fiber to narrow) blocking does not occur, but the change in speed is of the opposite nature. When approaching the narrowing, the speed of N. and. increases and then begins to decrease to a new stationary value. On the speed graph (Fig. 6 A) a kind of hysteresis loop is obtained.

Rie. 6. The passage of nerve impulses expandsto the fiber: A - change in pulse speed in depending on its direction; b-schematic image of an expanding fiber.

Another type of heterogeneity is fiber branching. At the branch node, different types are possible. options for passing and blocking impulses. With a non-synchronous approach, N. and. the blocking condition depends on the time offset. If the time between pulses is small, then they help each other penetrate into the wide third fiber. If the shift is large enough, then N. and. interfere with each other. This is due to the fact that N. and., who approached first, but failed to excite the third fiber, partially transfers the node to a refractory state. In addition, a synchronization effect occurs: as N. approaches and. towards the node their lag relative to each other decreases.

Interaction N. and. Nerve fibers in the body are combined into bundles or nerve trunks, forming something like a multi-core cable. All fibers in the bundle are independent. communication lines, but have one common “wire” - intercellular. When N. and. runs along any of the fibers, it creates an electric current in the intercellular fluid. , which affects the membrane potential of neighboring fibers. Usually such an influence is negligible and communication lines operate without mutual interference, but it manifests itself pathologically. and arts. conditions. By treating nerve trunks with special chem. substances, it is possible to observe not only mutual interference, but also the transfer of excitation to neighboring fibers.

There are known experiments on the interaction of two nerve fibers placed in a limited external volume. solution. If N. and. runs along one of the fibers, then the excitability of the second fiber simultaneously changes. Change goes through three stages. Initially, the excitability of the second fiber decreases (the excitation threshold increases). This decrease in excitability precedes the action potential traveling along the first fiber and lasts approximately until the potential in the first fiber reaches a maximum. Then the excitability increases; this stage coincides in time with the process of decreasing the potential in the first fiber. Excitability decreases again when a slight hyperpolarization of the membrane occurs in the first fiber.

At the same time passing N. and. using two fibers it was sometimes possible to achieve their synchronization. Despite the fact that own speed N. and. in different fibers are different, when they are simultaneously. excitement could arise collective N. and. If own speeds were the same, then the collective impulse had a lower speed. With a noticeable difference in property. speeds, the collective speed had an intermediate value. Only N. and. could synchronize, the speeds of which did not differ too much.

Math. a description of this phenomenon is given by a system of equations for the membrane potentials of two parallel fibers j 1 and j 2:

Where R 1 and R 2 - longitudinal resistance of the first and second fibers, R 3 - longitudinal resistance of the external environment, g = R 1 R 2 + R 1 R 3 . + R 2 R 3 . Ionic currents I 1 and I 2 can be described by one or another model of nervous excitation.

When using a simple analytical model solution leads to the following. picture. When one fiber is excited, an alternating membrane potential is induced in the neighboring one: first the fiber is hyperpolarized, then depolarized, and finally hyperpolarized again. These three phases correspond to a decrease, an increase, and a new decrease in fiber excitability. At normal parameter values, the shift of the membrane potential in the second phase towards depolarization does not reach the threshold, so transfer of excitation to the neighboring fiber does not occur. At the same time excitation of two fibers, system (6) allows a joint self-similar solution, which corresponds to two N. and., moving with the same speed at the station. distance from each other. If there is a slow N.I. ahead, then it slows down the fast impulse without releasing it forward; both move at relatively low speeds. If there is a fast II ahead. and., then it pulls a slow impulse behind it. The collective speed turns out to be close to the intrinsic speed. fast impulse speed. In complex neural structures, the appearance of auto-will.

Excitable media. Nerve cells in the body are united into neural networks, which, depending on the frequency of branching of the fibers, are divided into sparse and dense. In a rare network dep. are excited independently of each other and interact only at branch nodes, as described above.

In a dense network, excitation covers many elements at once, so that their detailed structure and the way they are connected to each other turn out to be unimportant. The network behaves as a continuous excitable medium, the parameters of which determine the occurrence and propagation of excitation.

An excitable medium can be three-dimensional, although more often it is considered two-dimensional. The excitement that arose in the k.-l. point on the surface, propagates in all directions in the form of a ring wave. An excitation wave can bend around obstacles, but cannot be reflected from them, nor is it reflected from the boundary of the medium. When waves collide with each other, they destroy each other; These waves cannot pass through each other due to the presence of a refractory region behind the excitation front.

An example of an excitable environment is the cardiac neuromuscular syncytium - the union of nerve and muscle fibers into a single conductive system capable of transmitting excitation in any direction. Neuromuscular syncytia contract synchronously, obeying a wave of excitation sent by a single control center - the pacemaker. The uniform rhythm is sometimes disrupted and arrhythmias occur. One of these modes is called. atrial flutter: these are autonomous contractions caused by the circulation of excitation around an obstacle, for example. superior or inferior vein. For such a regime to occur, the perimeter of the obstacle must exceed the excitation wavelength, which is ~ 5 cm in the human atrium. With flutter, periodic movement occurs. atrial contraction with a frequency of 3-5 Hz. A more complex mode of excitation is fibrillation of the ventricles of the heart, when the department. elements of the heart muscle begin to contract without external influence. commands and without communication with neighboring elements with a frequency of ~ 10 Hz. Fibrillation leads to cessation of blood circulation.

The emergence and maintenance of spontaneous activity in an excitable environment is inextricably linked with the emergence of wave sources. The simplest source of waves (spontaneously excited cells) can provide periodic. pulsation of activity, this is how the heart pacemaker works.

Sources of excitation can also arise due to complex spaces. organizing the excitation mode, for example. reverberator of the type of rotating spiral wave, appearing in the simplest excitable medium. Another type of reverberator occurs in a medium consisting of two types of elements with different excitation thresholds; the reverberator periodically excites one or the other elements, while changing the direction of its movement and generating plane waves.

The third type of source is the leading center (echo source), which appears in a medium that is heterogeneous in refractoriness or excitation threshold. In this case, a reflected wave (echo) appears on the inhomogeneity. The presence of such wave sources leads to the appearance of complex excitation modes studied in the theory of autowaves.

Lit.: Hodgkin A., Nerve impulse, trans. from English, M., 1965; Katz B., Nerve, muscle and synapse, trans. from English, M., 1968; Khodorov B.I., Problem of excitability, L., 1969; Tasaki I., Nervous excitement, trans. from English, M., 1971; Markin V.S., Pastushenko V.F., Chizmadzhev Yu.A., Theory of excitable media, M., 1981. V. S. Markin.

NERNST'S THEOREM- the same as Third law of thermodynamics.

NERNST EFFECT(longitudinal galvanothermomagnetic effect) - appearance in a conductor through which current flows j , located in a magnetic field H | j , temperature gradient T , directed along the current j ; the temperature gradient does not change sign when the field direction changes N to the opposite (even effect). Discovered by V. G. Nernst (W. N. Nernst) in 1886. AD. arises as a result of the fact that current transfer (charge carrier flow) is accompanied by heat flow. In fact, N. e. represents Peltier effect in conditions where the temperature difference arising at the ends of the sample leads to compensation of the heat flow associated with the current j , heat flow due to thermal conductivity. N. e. observed also in the absence of magnetism. fields.

NERNST-ETTINGSHAUSEN EFFECT- the appearance of electricity fields E ne in a conductor in which there is a temperature gradient T , in a direction perpendicular to the magnet. field N . There are transverse and longitudinal effects.

Transverse H.-E. e. consists in the appearance of electricity. fields E ne | (potential difference V ne | ) in a direction perpendicular to N And T . In the absence of magnetic thermoelectric fields the field compensates for the flow of charge carriers created by the temperature gradient, and compensation occurs only for the total current: electrons with an energy greater than the average (hot) move from the hot end of the sample to the cold, electrons with an energy less than the average (cold) - in the opposite direction. The Lorentz force deflects these groups of carriers in a direction perpendicular to T and mag. field, in different directions; the deflection angle (Hall angle) is determined by the relaxation time t of a given group of carriers, i.e., it differs for hot and cold carriers if t depends on energy. In this case, the currents of cold and hot carriers in the transverse direction ( | T And | N ) cannot compensate each other. This results in a field E | ne , the value of which is determined from the condition that the total current is equal to 0 j = 0.

Field size E | ne depends on T, N and properties of the substance, characterized by coefficient. Nernsta-Ettingsha-uzena N | :

IN semiconductors Under the influence T Charge carriers of different signs move in one direction, and in the magnetic direction. the fields are deviated in opposite directions. As a result, the direction of the Nernst-Ettingshausen field created by charges of different signs does not depend on the sign of the carriers. This significantly distinguishes the transverse N.-E. e. from Hall effect, where the direction of the Hall field is different for charges of different signs.

Because coefficient N | is determined by the dependence of the carrier relaxation time t on their energy, then N.-E. e. sensitive to mechanism charge carrier scattering. The scattering of charge carriers reduces the influence of the magnetic field. fields. If t ~ , then at r> 0 hot carriers scatter less often than cold ones and the direction of the field E | ne is determined by the direction of deflection in mag. hot carrier field. At r < 0 направление E | ne is opposite and is determined by cold carriers.

IN metals, where the current is carried by electrons with energy in the range ~ kT close Fermi surface, magnitude N | is given by the derivative d t /d. on the Fermi surface = const (usually for metals N | > 0, but, for example, for copper N | < 0).

Measurements N.-E. e. in semiconductors make it possible to determine r, i.e. restore the function t(). Usually at high temps in the property area. semiconductor conductivity N | < 0 due to scattering of carriers by optical devices. phonons. When the temperature decreases, an area appears with N | > 0, corresponding to impurity conductivity and scattering of carriers ch. arr. on phonons ( r< < 0). При ещё более низких T ionization scattering dominates. impurities with N | < 0 (r > 0).

In weak mag. fields (w with t<< 1, где w с - cyclotron frequency carriers) N | does not depend on H. In strong fields (w c t >> 1) coefficient N | proportional 1/ H 2. In anisotropic conductors, coefficient. N | - tensor. By the amount N | affect the entrainment of electrons by photons (increases N | ), anisotropy of the Fermi surface, etc.

Longitudinal H.-E. e. consists in the occurrence of electrical fields E || ne (potential difference V || ne) along T in the presence of H | T . Because along T there is thermoelectric. field E a = a T , where a is the coefficient. thermoelectric-trich. fields, then the appearance will be complementary. fields along T is equivalent to changing the field E a . when applying magnetic fields:

Magn. the field, bending the trajectories of electrons (see above), reduces their mean free path l in the direction T . Since the free travel time (relaxation time t) depends on the electron energy, then the decrease l is not the same for hot and cold carriers: it is less for that group, for a certain type it is less. Thus, mag. the field changes the role of fast and slow carriers in energy transfer, and thermoelectric. the field ensuring the absence of charge during energy transfer must change. At the same time, the coefficient N || also depends on the carrier scattering mechanism. Thermoelectric the current increases if m decreases with increasing carrier energy (when carriers are scattered by acoustic phonons), or decreases if m increases with increasing (when scattered by impurities). If electrons with different energies have the same t, the effect disappears ( N|| = 0). Therefore, in metals, where the energy range of electrons involved in transfer processes is small (~ kT), N || small: In a semiconductor with two types of carriers N ||~ ~ g/kT. At low temp. N|| may also increase due to the influence of electron drag by phonons. In strong magnetic fields complete thermoelectric. field in magnetic the field is “saturated” and does not depend on the carrier scattering mechanism. In ferromagnetic metals N.-E. e. has features associated with the presence of spontaneous magnetization.

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