Impulse nerve cells are surrounded. Nerve impulses - the alphabet of the brain - are of an electrochemical nature. Hormones work on the brain

Neurons communicate with each other using "neural messages". These "messages" are like electric current running through wires. Sometimes, when transmitted from one neuron to another, these impulses are converted into chemical messages.

nerve impulses

Information is transferred between neurons like an electric current in wires. These messages are encoded: they are a sequence of exactly the same impulses. The code itself lies in their frequency, that is, in the number of pulses per second. The impulses are transmitted from cell to cell, from the dendrite in which they originate to the axon through which they pass. But there is also a difference from electrical networks - impulses are transmitted not with the help of electrons *, but with the help of more complex particles - ions.

Medications that affect the speed of impulses

There are many chemicals that can change the characteristics of the transmission of nerve impulses. As a rule, they act at the synaptic level. Anesthetics and tranquilizers slow down, and sometimes even suppress, the transmission of impulses. And antidepressants and stimulants like caffeine, on the contrary, contribute to their better transmission.

With great speed

Nerve impulses must travel quickly through the body. Glial cells surrounding them help speed up their passage to neurons. They form the sheath of the nerve fiber, called myelin. As a result, the impulses go at breathtaking speed - more than 400 km / h.

chemical bonds

Messages transmitted from neuron to neuron must be converted from electrical to chemical form. This is due to the fact that, despite their large number, neurons never touch each other. But electrical impulses cannot be transmitted unless there is physical contact. Therefore, neurons use a special system called synapses to communicate with each other. In these places, the neurons are separated by a narrow space of the synaptic cleft. When an electrical impulse arrives at the first neuron, it releases chemical molecules called neurotransmitters from the synapse. These substances, produced by neurons, move through the synaptic cleft and enter the receptors of another neuron specially designed for them. The result is another electrical impulse.

An impulse travels between neurons in less than a thousandth of a second.

Distinguishing neurotransmitters

The brain produces about fifty neurotransmitters, which can be divided into two groups. The first consists of those that initiate the occurrence of a nerve impulse - they are called excitatory. Others, on the contrary, slow down its occurrence - these are inhibitory neurotransmitters. It should be noted that in most cases, the neuron releases only one type of neurotransmitters. And depending on whether it is excitatory or inhibitory, the neuron has a different effect on neighboring nerve cells.

artificial stimulation

An individual neuron or a group of neurons can be stimulated artificially with the help of electrodes inserted into them, which direct electrical impulses to precisely marked areas of the brain. This method is sometimes used in medicine, in particular for the treatment of patients suffering from Parkinson's disease. This disease, which manifests itself in old age, is accompanied by trembling of the limbs. This shaking can be stopped by constant stimulation of a specific area of ​​the brain.

Neuron - microcomputer

Each of the neurons is capable of receiving hundreds of messages per second. And in order not to be overloaded with information, he must be able to judge the degree of its significance and make a preliminary analysis of it. This computational activity takes place inside the cell. Excitatory impulses are added and inhibitory impulses are subtracted. And, in order for the neuron to generate its own impulse, it is necessary that the sum of the previous ones be greater than a certain value. If the addition of excitatory and inhibitory impulses does not exceed this limit, the neuron will be "silent".

information roads

In all this intricacies of neurons, there are beautifully marked paths. Similar ideas, similar memories pass, always firing the same neurons and synapses. It is still unknown how these circuit-like electronic communication circuits are created and maintained, but it is obvious that they exist and that the stronger they are, the more effective they are. Frequently used synapses work faster. This explains why we quickly recall things that we have seen or repeated several times. However, these bonds do not last forever. Some of them may disappear if they are not used enough, and new ones will appear in their place. If necessary, neurons are always able to create new connections.

The small green dots in the photo are hormones inside the blood vessels.

Chemical doping

When they say that an athlete used hormonal doping, this means that he took hormones either in the form of tablets or by injecting them directly into the blood. Hormones are either natural or artificial. The most common are growth hormones and steroids, due to which muscles become bigger and stronger, as well as erythropoietin, a hormone that speeds up delivery nutrients to the muscles.

The brain is capable of performing millions of operations in a fraction of a second.

Hormones work on the brain

For the exchange of information by the brain, another tool is also used - hormones. These chemicals are partly produced by the brain itself in a group of neurons located in the hypothalamus. These hormones control the production of others produced elsewhere in the body in the endocrine glands. They act differently from neurotransmitters, which are fixed directly on neurons and transported in the blood to distant organs of the body from the brain, such as breasts, ovaries, male testes, and kidneys. Fixing on their receptors, hormones cause various physiological reactions. They, for example, promote the growth of bones and muscles, control the feeling of hunger and thirst and, of course, affect sexual activity.

Synaptic transmission is the interaction of brain cells.

Neurons produce electrochemical perturbations that travel along their fibers. These disturbances, called nerve impulses or action potentials, are generated by small electrical currents along the nerve cell membrane. Neurons are capable of producing up to a thousand action potentials per second, in the sequence and duration of which information is encoded.

nerve impulses- electrochemical disturbances transmitted along nerve fibers; through them neurons interact with each other and with the rest of the body. The electrical nature of nerve impulses is determined by the structure of the cell membrane, which consists of two layers separated by a small gap. The membrane acts both as a capacitor - it accumulates an electric charge, collecting ions on itself, and as a resistance, blocking the current. In a neuron at rest, a cloud of negatively charged ions forms along the inner surface of the membrane, and positive ions along the outer surface.

A neuron, when activated, emits (also called "generates") a nerve impulse. It occurs in response to signals received from other cells, and is a brief reverse change in the potential difference of the membrane: inside it becomes positively charged for a moment, after which it quickly returns to a state of rest. During a nerve impulse, the membrane of a nerve cell lets in certain types of ions. Since the ions are electrically charged, their movement is electricity through the membrane.

neurons at rest. There are ions inside the neurons, but the neurons themselves are surrounded by ions in other concentrations. It is natural for particles to move from an area of ​​high concentration to an area of ​​low concentration, but the nerve cell membrane prevents this movement because it is basically impermeable.

It turns out that some ions are concentrated outside the membrane, while others are inside. As a result, the outer surface of the membrane is positively charged, while the inner surface is negatively charged. The membrane is thus polarized.

It all started with a squid. The mechanism of the action potential - waves of excitation on the cell membrane - was discovered in the early 1950s, in a classic experiment with microelectrodes inserted into the axons of a giant squid. These experiments proved that the action potential is generated by successive movements of ions across the membrane.

In the first phase of the action potential, the membrane briefly becomes permeable to sodium ions, and they fill the cell. This causes depolarization of the cell - the potential difference across the membrane is reversed, and inner surface membrane is positively charged. Following this, potassium ions rapidly leave the cell and the potential difference of the membrane returns to its original state. The penetration of potassium ions inside makes the charge on the membrane more negative than at rest, and the cell is thus hyperpolarized. During the so-called refractory period, the neuron cannot produce the next action potential, but quickly returns to a resting state.

Action potentials are generated at a structure called the axon hillock, which is where the axon grows out of the cell body. Action potentials move along the axon because depolarization of one segment of the fiber causes depolarization of the adjacent one. This wave of depolarization rolls away from the cell body and, upon reaching the terminal of the nerve cell, causes the release of neurotransmitters.

A single pulse lasts one thousandth of a second; Neurons encode information with a precisely timed sequence of impulses (spike discharges), but it is still unclear exactly how information is encoded. Neurons often fire action potentials in response to signals from other cells, but they also fire without any external signals. The frequency of basal pulsations, or spontaneous action potentials, varies with different types neurons and can change depending on the signals of other cells.

Few will pass. Ions cross the nerve cell membrane through barrel-shaped proteins called ion channels. They penetrate the membrane and form through pores. Ion channels have sensors that recognize changes in the potential difference of the membrane, and they open and close in response to these changes.

Human neurons contain more than a dozen different types such channels, and each of them passes only one type of ion. The activity of all these ion channels during the action potential is strictly regulated. They open and close in a certain order - so that neurons, in response to signals received from other cells, can generate sequences of nerve impulses.

Ohm's law.
Ohm's law explains how the electrical properties of the brain change with input. It describes the relationship between the potential difference (voltage) of the nerve cell membrane, its resistance, and the current flowing through it. According to this relationship, the current is directly proportional to the membrane voltage and is described by the equation I = U/R, where I is the electric current, U is the potential difference, and R is the resistance.

Faster than Usain Bolt.
The axons of the spinal cord and brain are isolated by thick myelin tissue produced by brain cells called oligodendrocytes. The oligodendrocyte has few branches, and each consists of a large, flat sheet of myelin repeatedly wrapped around a small segment of an axon belonging to another neuron. The myelin sheath along the length of the entire axon is uneven: it is interrupted at regular intervals, and the points of these interruptions are called nodes of Ranvier. Ion channels thicken just at these points, thereby ensuring the jumping of action potentials from one intercept to another. This accelerates the entire process of movement of action potentials along the axon - it occurs at a speed of up to 100 m / s.

A person acts as a kind of coordinator in our body. It transmits commands from the brain to muscles, organs, tissues and processes the signals coming from them. A nerve impulse is used as a kind of data carrier. What does he represent? At what speed does it work? These and a number of other questions can be answered in this article.

What is a nerve impulse?

This is the name of the wave of excitation that propagates through the fibers as a response to irritation of neurons. Thanks to this mechanism, information is transmitted from various receptors to the central nervous system. And from it, in turn, to different organs (muscles and glands). But what is this process at the physiological level? The mechanism of transmission of a nerve impulse is that the membranes of neurons can change their electrochemical potential. And the process of interest to us takes place in the area of ​​synapses. The speed of a nerve impulse can vary from 3 to 12 meters per second. In more detail about it, as well as about the factors that influence it, we will talk later.

Study of the structure and work

For the first time, the passage of a nerve impulse was demonstrated by the German scientists E. Goering and G. Helmholtz using a frog as an example. At the same time, it was found that the bioelectric signal propagates at the previously indicated speed. In general, this is possible due to the special construction. In some ways, they resemble an electrical cable. So, if we draw parallels with it, then the conductors are the axons, and the insulators are their myelin sheaths (they are the membrane of the Schwann cell, which is wound in several layers). Moreover, the speed of the nerve impulse depends primarily on the diameter of the fibers. The second most important is the quality of electrical insulation. By the way, the body uses myelin lipoprotein, which has the properties of a dielectric, as a material. Ceteris paribus, the larger its layer, the faster the nerve impulses will pass. Even at the moment it cannot be said that this system has been fully investigated. Much that relates to nerves and impulses still remains a mystery and a subject of research.

Features of the structure and functioning

If we talk about the path of a nerve impulse, then it should be noted that the fiber is not covered along its entire length. The construction features are such that the current situation would be best compared with the creation of insulating ceramic sleeves that are tightly strung on a rod electric cable(although in this case on the axon). As a result, there are small non-isolated electrical sections from which the ion current can safely flow out of the axon into the environment (or vice versa). This irritates the membrane. As a result, generation is caused in areas that are not isolated. This process is called the intercept of Ranvier. The presence of such a mechanism makes it possible to make the nerve impulse propagate much faster. Let's talk about this with examples. Thus, the speed of nerve impulse conduction in a thick myelinated fiber, the diameter of which fluctuates within 10-20 microns, is 70-120 meters per second. Whereas for those who have a suboptimal structure, this figure is 60 times less!

Where are they created?

Nerve impulses originate in neurons. The ability to create such "messages" is one of their main properties. The nerve impulse ensures the rapid propagation of the same type of signals along the axons over a long distance. Therefore, it is the most important means of the body for the exchange of information in it. Data on irritation are transmitted by changing the frequency of their repetition. A complex system of periodicals works here, which can count hundreds of nerve impulses in one second. According to a somewhat similar principle, although much more complicated, computer electronics work. So, when nerve impulses arise in neurons, they are encoded in a certain way, and only then are they transmitted. In this case, the information is grouped into special "packs", which have a different number and nature of the sequence. All this, put together, is the basis for the rhythmic electrical activity of our brain, which can be registered thanks to the electroencephalogram.

Cell types

Speaking about the sequence of passage of a nerve impulse, one cannot ignore (neurons), through which the transmission of electrical signals occurs. So, thanks to them, different parts of our body exchange information. Depending on their structure and functionality, three types are distinguished:

1. Receptor (sensitive). They encode and turn into nerve impulses all temperature, chemical, sound, mechanical and light stimuli.
2. Plug-in (also called conductor or closing). They serve to process and switch impulses. Most of them are found in the human brain and spinal cord.
3. Effector (motor). They receive commands from the central nervous system to perform certain actions (in the bright sun, close your eyes with your hand, and so on).

Each neuron has a cell body and a process. The path of a nerve impulse through the body begins precisely with the latter. Branches are of two types:

1. Dendrites. They are entrusted with the function of perceiving irritation of the receptors located on them.
2. Axons. Thanks to them, nerve impulses are transmitted from cells to the working organ.

Speaking about the conduction of a nerve impulse by cells, it is difficult not to talk about one interesting point. So when they are at rest, let's say the sodium-potassium pump is busy moving the ions in such a way as to achieve the effect of fresh water on the inside and salty on the outside. Due to the resulting imbalance of the potential difference across the membrane, up to 70 millivolts can be observed. For comparison, this is 5% of the usual ones. But as soon as the state of the cell changes, the resulting balance is disturbed, and the ions begin to change places. This happens when the path of a nerve impulse passes through it. Due to the active action of ions, this action is also called the action potential. When it reaches a certain value, then reverse processes begin, and the cell reaches a state of rest.

About the action potential

Speaking about the transformation of a nerve impulse and its propagation, it should be noted that it could be miserable millimeters per second. Then the signals from the hand to the brain would reach in minutes, which is clearly not good. This is where the previously discussed myelin sheath plays its role in strengthening the action potential. And all its "passes" are placed in such a way that they only have a positive effect on the speed of signal transmission. So, when an impulse reaches the end of the main part of one axon body, it is transmitted either to the next cell, or (if we talk about the brain) to numerous branches of neurons. In the latter cases, a slightly different principle works.

How does everything work in the brain?

Let's talk about which nerve impulse transmission sequence works in the most important parts of our central nervous system. Here, neurons are separated from their neighbors by small gaps, which are called synapses. The action potential cannot cross them, so it looks for another way to get to the next nerve cell. At the end of each process are small sacs called presynaptic vesicles. Each of them has special compounds - neurotransmitters. When an action potential arrives at them, molecules are released from the sacs. They cross the synapse and attach to special molecular receptors that are located on the membrane. In this case, the balance is disturbed and, probably, a new action potential appears. This is not yet known for certain, neurophysiologists are studying the issue to this day.

The work of neurotransmitters

When they transmit nerve impulses, there are several options for what will happen to them:

1. They will diffuse.
2. subjected to chemical degradation.
3. Return back to their bubbles (this is called recapture).

At the end of the 20th century, a startling discovery was made. Scientists have learned that drugs that affect neurotransmitters (as well as their release and reuptake) can change a person's mental state in a fundamental way. So, for example, a number of antidepressants like Prozac block the reuptake of serotonin. There are some reasons to believe that a deficiency in the brain neurotransmitter dopamine is to blame for Parkinson's disease.

Now researchers who study the borderline states of the human psyche are trying to figure out how all this affects the human mind. In the meantime, we do not have an answer to such a fundamental question: what causes a neuron to create an action potential? So far, the mechanism of "launching" this cell is a secret for us. Particularly interesting from the point of view of this riddle is the work of neurons in the main brain.

In short, they can work with thousands of neurotransmitters that are sent by their neighbors. Details regarding the processing and integration of this type of impulses are almost unknown to us. Although many research groups are working on this. At the moment, it turned out to find out that all received impulses are integrated, and the neuron makes a decision - whether it is necessary to maintain the action potential and transmit them further. The functioning of the human brain is based on this fundamental process. Well, then it is not surprising that we do not know the answer to this riddle.

Some theoretical features

In the article, "nerve impulse" and "action potential" were used as synonyms. Theoretically, this is true, although in some cases it is necessary to take into account some features. So, if you go into details, then the action potential is only part of the nerve impulse. With a detailed examination of scientific books, you can find out that this is only the change in the charge of the membrane from positive to negative, and vice versa. Whereas a nerve impulse is understood as a complex structural and electrochemical process. It spreads across the neuron membrane like a traveling wave of changes. An action potential is just an electrical component in a nerve impulse. It characterizes the changes that occur with the charge of a local section of the membrane.

Where are nerve impulses created?

Where do they start their journey? The answer to this question can be given by any student who diligently studied the physiology of arousal. There are four options:

1. Receptor ending of a dendrite. If it exists (which is not a fact), then the presence of an adequate stimulus is possible, which will first create a generator potential, and then a nerve impulse. Pain receptors work in a similar way.
2. The membrane of the excitatory synapse. As a rule, this is possible only in the presence of strong irritation or their summation.
3. Trigger zone of the dentrid. In this case, local excitatory postsynaptic potentials are formed as a response to a stimulus. If the first node of Ranvier is myelinated, then they are summed up on it. Due to the presence of a section of the membrane there, which has increased sensitivity, a nerve impulse occurs here.
4. Axon hillock. This is the name of the place where the axon begins. The mound is the most frequent create impulses on a neuron. In all other places that were considered earlier, their occurrence is much less likely. This is due to the fact that here the membrane has an increased sensitivity, as well as a reduced one. Therefore, when the summation of numerous excitatory postsynaptic potentials begins, the hillock reacts to them first of all.

An example of a spreading excitation

Story medical terms can lead to misunderstanding of some points. To eliminate this, it is worth briefly going through the stated knowledge. Let's take a fire as an example.

Think back to last summer's news bulletins (you might hear it again soon too). The fire is spreading! At the same time, trees and shrubs that burn remain in their places. But the front of the fire goes further and further from the place where the fire was. The nervous system works the same way.

It is often necessary to calm the excitation of the nervous system that has begun. But this is not so easy to do, as in the case of fire. To do this, artificial interference is made in the work of the neuron (in medicinal purposes) or use various physiological means. This can be compared to pouring water on a fire.

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Neurons

In higher animals, nerve cells form the organs of the central nervous system (CNS) - the head and spinal cord- and the peripheral nervous system (PNS), which includes nerves and their processes that connect the central nervous system with muscles, glands and receptors.

Structure

Nerve cells do not reproduce by mitosis (cell division). Neurons are called amitotic cells - if they are destroyed, they will not be restored. Ganglia are bundles of nerve cells outside the CNS. All neurons are made up of the following elements.

cell body. These are the nucleus and the cytoplasm.

Axon. It is a long, thin process that transmits information from the cell body to other cells through connections called synapses. Some axons are less than a centimeter long, while others are more than 90 cm long. Most axons are in a protective substance called myelin sheath, which helps speed up the transmission of nerve impulses. Narrowings on the axon after a certain interval are called nodes of Ranvier.

Dendrites. This is a network of short fibers that extend from the axon or cell body and connect the ends of axons from other neurons. Dendrites receive information for the cell by receiving and conducting signals. Each neuron can have hundreds of dendrites.

Structure of a neuron

Functions

Neurons contact each other in an electrochemical way, transmitting impulses throughout the body.

myelin sheath

. Schwann cells coil around one or more axons (A) to form the myelin sheath.
. It consists of several layers (perhaps 50-100) of plasma membranes. (b), between which the liquid cytosol circulates (cytoplasm devoid of hypochondria and other elements of the endoplasmic reticulum), with the exception of the uppermost layer (V).
. The myelin sheath around the long axon is divided into segments, each of which is formed by a separate Schwann cell.
. Adjacent segments are separated by narrowings called nodes of Ranvier (G) where the axon does not have a myelin sheath.

nerve impulses

In higher animals, signals are sent throughout the body and from the brain in the form of electrical impulses transmitted through nerves. Nerves create impulses when there is a physical, chemical, or electrical change in the cell membrane.

1 resting neuron

A resting neuron has a negative charge inside the cell membrane (a) and a positive charge outside this membrane (b). This phenomenon is called the residual potential of the membrane.

It is supported by two factors:

Different permeability of the cell membrane for sodium and potassium ions, which have the same positive charge. Sodium diffuses (passes) into the cell more slowly than potassium leaves it.

Sodium-potassium exchange, in which more positive ions leave the cell than enter it. As a result, more positive ions accumulate outside the cell membrane than inside it.

2 Stimulated neuron

When a neuron is stimulated, the permeability of some area (c) of the cell membrane changes. Positive sodium ions (g) begin to enter the cell faster than in the resting position, which leads to an increase in the positive potential inside the cell. This phenomenon is called depolarization.

3 Nerve impulse

Depolarization gradually spreads to the entire cell membrane (e). Gradually, the charges on the sides of the cell membrane change (not for a while). This phenomenon is called reverse polarization. This is, in fact, a nerve impulse transmitted along the cell membrane of a nerve cell.

4 Repolarization

The permeability of the cell membrane changes again. Positive sodium ions (Na+) begin to leave the cell (e). Finally, a positive charge is again formed outside the cell, and a positive one inside it. This process is called repolarization.

NERVE IMPULSE

NERVE IMPULSE

A wave of excitation, which spreads along the nerve fiber and serves to transmit information from the periphery. receptor (sensitive) endings to the nerve centers, inside the center. nervous system and from it to the executive apparatus - the muscles and glands. N.'s passage and. accompanied by transient electric. processes, to-rye it is possible to register both extracellular, and intracellular electrodes.

Generation, transfer and processing N. and. carried out by the nervous system. Main structural element nervous system of higher organisms is a nerve cell, or neuron, consisting of a cell body and numerous. processes - dendrites (Fig. 1). One of the processes in non-ripheric. neurons has 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 amyelinated. The pulpy fibers have myelin, formed by special. a membrane, edges like isolation is wound on an axon. The length of sections of a continuous myelin sheath is from 200 microns to 1 mm, they are interrupted by the so-called. interceptions of Ranvier with a width of 1 μm. The myelin sheath plays the role of insulation; the nerve fiber in these areas is passive, electrically active only in the nodes of Ranvier. Meleless fibers do not have insulated. plots; their structure is homogeneous along the entire length, and the membrane has an electric. activity over the entire surface.

Nerve fibers end on the bodies or dendrites of other nerve cells, but are separated from them by an intermediate

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 dendritic or muscle membrane is post-synaptic (see Fig. Cell structures).

Under normal conditions, a series of N. and. constantly run along the nerve fiber, arising on the dendrites or the cell body and spreading along the axon in the direction from the cell body (the axon can conduct N. and. in both directions). The frequency of these periodic discharges carries information about the strength of the irritation that caused them; eg, with moderate activity, the frequency is ~ 50-100 impulses / s. There are cells, to-rye are discharged with a frequency of ~ 1500 impulses/s.

Speed ​​of distribution 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 . ~ 100-120 m/s.

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

After excitation, a refractory period occurs, during which the excitability of the nerve fiber is reduced. Distinguish abs. the refractory period, when the fiber cannot be excited by any stimuli, and refers. refractory period, when possible, but its threshold is above normal. Abs. the refractory period limits the transmission frequency of N. from above and. The nerve fiber has the property of accommodation, that is, it gets used to constantly acting irritation, which is expressed in a gradual increase in the threshold of excitability. This leads to a decrease in N.'s frequency and. and even to their complete disappearance. If irritation builds up slowly, then excitation may not occur even after reaching the threshold.

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

Along N.'s nerve fiber and. distributed in the form of electricity. potential. In the synapse, there is a change in the propagation mechanism. When N. and. reaches the presynaptic endings, in synaptic. the gap is allocated active chem. - m e d i a t o r. The mediator diffuses through the synaptic. gap and changes the permeability of postsynaptic. membrane, as a result of which it appears, again generating a propagating . This is how chemo works. synapse. There is also an electric synapse when . the neuron is electrically excited.

N.'s excitation and. Phys. ideas about the appearance of electric. potentials in cells are based on the so-called. membrane theory. Cell membranes separate electrolytes of different concentrations and possess is-Byrate. permeability for certain ions. Thus, the axon membrane is a thin layer of lipids and proteins with a thickness of ~7 nm. Her electric resistance at rest ~ 0.1 ohm. m 2, and the capacity is ~ 10 mf / m 2. Inside the axon, the concentration of K + ions is high and the concentration of Na + and Cl - ions is low, while in the environment it is vice versa.

At rest, the axon membrane is permeable to K + ions. Due to the difference in concentrations C 0 K . in ext. and C in ext. solutions, a potassium membrane potential is established on the membrane

Where T - abs. pace-pa, e - charge of an electron. On the axon membrane, a resting potential of ~ -60 mV is indeed observed, corresponding to the indicated f-le.

Ions Na + and Cl - penetrate the membrane. To maintain the necessary non-equilibrium distribution of ions, the cell uses an active transport system, which uses cellular energy to work. Therefore, the state of rest of the nerve fiber is not thermodynamically equilibrium. It is stationary due to the action of ion pumps, and the membrane potential in 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 a weak current pulse is passed through the axon, leading to depolarization of the membrane, then after removing the external. exposure potential monotonously returns to the initial level. Under these conditions, the axon behaves like a passive electrical circuit. circuit consisting of a capacitor and a DC. resistance.

Rice. 2. Development of the action potential in the nervous systemlokne: A- subthreshold ( 1 ) and suprathreshold (2) irritation; b-membrane response; with supra-threshold irritation, full sweat appearsaction cycle; V is the 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 level of rest, and at first it even skips a little (the region of hyperpolarization, Fig. 2). The response of the membrane does not depend on the perturbation; this impulse is called action potential. At the same time, an ion 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. Hodg-kin and A. F. Huxley in 1952. The total ion current is made up of three components: potassium, sodium, and leakage current. When the membrane potential is shifted by the threshold value j* (~ 20mV), 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 begins to develop potassium (and at the same time decrease sodium). As a result, the sodium current is replaced by a potassium current directed outward. This current corresponds to a decrease in the action potential.

The empirical ur-tion for the description of sodium and potassium currents. The behavior of the membrane potential during spatially homogeneous 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:

the value 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 the linear equations

Coefficient dependence. a . and b on the membrane potential j (Fig. 3) are selected from the condition of the best match

Rice. 3. Dependence of coefficientsa. Andbfrom membranespotential.

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

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

N.'s distribution and. N. and. can propagate along the fiber without attenuation and with post. speed. This is due to the fact that the energy necessary for signal transmission does not come from a single center, but is drawn in place, at each point of the fiber. In accordance with the two types of fibers, there are two ways of N.'s transmission and

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

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

Type of functionality I specific to a biologically excitable environment. However, equation (5), apart from the form I, has a more general character and describes many physical. phenomena, eg. combustion process. Therefore N.'s transfer and. likened to the burning of a powder cord. If in a running flame the process of ignition 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 providing distributionnerve impulse.

Ur-tion of Hodgkin - Huxley for N.'s distribution and. solved numerically. The solutions obtained, together with the accumulated experiments. data showed that N.'s distribution and. does not depend on the details of the excitation process. Qualities. a picture of N.'s distribution and. can be obtained using simple models, reflecting only general properties arousal. Such approach allowed to count also the N.'s form and. in a homogeneous fiber, their change in the presence of inhomogeneities, and even complex modes of propagation of excitation in active media, for example. in the heart muscle. There are several math. models of this kind. The simplest of them is this. The ion current flowing through the membrane during the passage of N. and. is sign-alternating: at 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 is shifted by the threshold value j*. At this moment, a current appears, directed inside the fiber and equal in absolute value j". After t "the current changes to the opposite, equal to j". This continues for time ~t". The self-similar solution of equation (5) can be found as a function of the variable t = x/ u , where u - speed of distribution of N. and. (Fig. 2, b).

In real fibers, the time t" is large enough, so only it determines the speed u , for which the f-la is valid: . Given that j" ~ ~d, R~d 2 and WITH~ d, Where d- fiber diameter, we find, in agreement with experiment, that u ~d 1/2 . Using a piecewise constant approximation, the shape of the action potential is found.

Ur-tion (5) for the spreading N. and. actually admits two solutions. The second solution turns out to be unstable; it gives N. and. with a much lower speed and potential amplitude. The presence of the second, unstable solution has an analogy in the theory of combustion. When a flame propagates with a lateral heat sink, an unstable regime may also occur. A simple analytic N.'s model and. can be improved, taking into account the additions. details.

At change of section and at branching of nervous fibers N.'s passage and. may be difficult or even completely blocked. In an expanding fiber (Fig. 6), the pulse velocity decreases as it approaches expansion, and after expansion, it begins to increase until it reaches a new stationary value. N.'s delay 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 a fiber, a cut detains N. and.

At the return movement of N. and. (from wide fiber to narrow) there is no blocking, but the change in speed is the opposite. At the approach to narrowing N.'s speed and. increases and then begins to fall to a new stationary value. On the speed graph (Fig., 6 A) results in a kind of hysteresis loop.

Rie. 6. Passage of nerve impulses by expandingrunning 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. In the branch node, various options for passing and blocking impulses. At the nonsynchronous N.'s approach and. the blocking condition depends on the time offset. If the time between pulses is small, then they help each other to 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 came up first, but failed to excite the third fiber, partially transfers the node into a refractory state. Besides, there is a synchronization effect: in process of N.'s approach and. to the node, their delay relative to each other decreases.

N.'s interaction and. Nerve fibers in the body are combined into bundles or nerve trunks, forming a kind of stranded cable. All fibers in a 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. , a cut influences membrane potential of the next fibers. Usually such an influence is negligible and the communication lines work without mutual interference, but it manifests itself in the pathological. and arts. conditions. Processing nerve trunks special. chem. substances, it is possible to observe not only mutual interference, but also the transfer of excitation to neighboring fibers.

Known experiments on the interaction of two nerve fibers placed in a limited volume of external. solution. If N. runs along one of the fibers and., then the excitability of the second fiber changes at the same time. Change goes through three stages. At first, the excitability of the second fiber falls (the excitation threshold rises). This decrease in excitability precedes the action potential traveling along the first fiber and lasts approximately until the potential in the first fiber reaches its maximum. Then the excitability grows, this stage coincides in time with the process of reducing 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 N.'s passage and. on two fibers it was sometimes possible to achieve their synchronization. Despite the fact that own N.'s speeds and. in different fibers are different, at the same time. excitation 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.

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

Where R 1 and R 2 - longitudinal resistances of the first and second fibers, R 3 - longitudinal resistance 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 analytic model solution leads to the following. picture. When one fiber is excited, an alternating membrane potential is induced in the adjacent one: first, the fiber hyperpolarizes, then depolarizes, and finally, hyperpolarizes again. These three phases correspond to a decrease, an increase, and a new decrease in the excitability of the fiber. At normal values ​​of the parameters, the shift of the membrane potential in the second phase towards depolarization does not reach the threshold, so there is no transfer of excitation to the adjacent fiber. At the same time excitation of two fibers, system (6) allows a joint self-similar solution, which corresponds to two N. and. moving at the same speed per post. distance from each other. If there is a slow N. and. ahead, then it slows down the fast impulse, not releasing it forward; both are moving at a relatively slow speed. If there is a fast II ahead. and., then it pulls up a slow impulse. The collective velocity turns out to be close to the intrinsic velocity. fast impulse speed. In complex neural structures, the appearance of auto will.

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

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

The excitable medium can be three-dimensional, although it is more often considered as two-dimensional. The excitement which arose in to. point on the surface, propagates in all directions in the form of an annular wave. The excitation wave can go around obstacles, but cannot be reflected from them, nor is it reflected from the boundary of the medium. When waves collide with each other, their mutual annihilation occurs; 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 cardiac neuromuscular syncytium - the union of nerve and muscle fibers into a single conducting system capable of transmitting excitation in any direction. Neuromuscular syncytia contract synchronously, obeying a wave of excitation, which is sent by a single control center - the pacemaker. A single rhythm is sometimes disturbed, 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 the occurrence of such a regime, the perimeter of the obstacle must exceed the wavelength of excitation, which is ~ 5 cm in the human atrium. atrial contraction with a frequency of 3-5 Hz. A more complex mode of excitation is ventricular fibrillation of the heart, when otd. elements of the heart muscle begin to contract without external. commands and without communication with neighboring elements with a frequency of ~ 10 Hz. Fibrillation leads to the cessation of blood circulation.

The emergence and maintenance of spontaneous activity of an excitable medium are 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 pacemaker of the heart works.

Sources of excitation can also arise due to complex spaces. organization of the excitation mode, for example. reverberator of the type of a rotating spiral wave, appearing in the simplest excitable medium. Another kind of reverb occurs in an environment consisting of two types of elements with different excitation thresholds; the reverb 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 an environment that is inhomogeneous in terms of 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 regimes, which are 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., The problem of excitability, L., 1969; Tasaki I., Nervous excitement, trans. from English, M., 1971; V. S. Markin, V. F. Pastushenko, Yu. A. Chizmadzhev, Theory of Excitable Media, Moscow, 1981. V. S. Markin.

NERNSTA THEOREM- the same as Third law of thermodynamics.

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

NERNSTA-ETTINGSHAUSEN EFFECT- the appearance of electricity. fields E ne in the conductor, in which there is a temperature gradient T , in a direction perpendicular to the magnetic field H . Distinguish between 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 H And T . In the absence of a magnet. fields of thermoelectric the field compensates for the flow of charge carriers created by the temperature gradient, and compensation takes place 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 one, 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 magn. 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 the energy. In this case, the currents of cold and hot carriers in the transverse direction ( | T And | H ) cannot cancel each other out. This gives rise to a field E | ne , the value of which is determined from the condition of equality 0 of the total current j = 0.

Field value E | does not depend on T, H and properties of the substance, characterized by the coefficient. Nernst-Ettingsha-Usen N | :

IN semiconductors Under the influence T charge carriers of different signs move in the same direction, and in the magnetic. the field is deflected in opposite directions. As a result, the direction of the Nernst-Ettingshausen field created by the charges different sign, does not depend on the sign of the supports. 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.

Since the coefficient N | is determined by the dependence of the relaxation time t of carriers on their energy, then N.-E. e. sensitive to the mechanism scattering of charge carriers. Scattering of charge carriers reduces the influence of the magnetic. 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 magn. field of hot carriers. At r < 0 направление E | ne is opposite and is determined by cold carriers.

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

Measurements N.-E. e. in semiconductors allow you to determine r, i.e. restore the function t(). Usually at high temp-pax in the area of ​​own. semiconductor conductivity N | < 0 due to the scattering of carriers on the optical. phonons. When the temperature drops, an area appears with N | > 0, corresponding to the impurity conductivity and scattering of carriers Chap. arr. on phonons ( r< < 0). При ещё более низких T ionization scattering dominates. impurities with N | < 0 (r > 0).

In weak magnetic 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, the coefficient. N | - tensor. By the amount N | affect the drag of electrons by photons (increases N | ), anisotropy of the Fermi surface, etc.

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

Magn. field, bending the trajectories of electrons (see above), reduces their mean free path l in the direction T . Since the mean free path (relaxation time t) depends on the energy of the electrons, the decrease l is not the same for hot and cold carriers: it is smaller for the group for which m is smaller. T. o., magn. field changes the role of fast and slow carriers in energy transfer, and thermoelectric. the field that ensures 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 (during scattering of carriers by acoustic phonons), or decreases if m increases with increasing (during scattering 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 the transfer processes is small (~ kT), N || small: In a semiconductor with two types of carriers N ||~ ~ g/kT. At low temp-pax N|| can also increase due to the influence of electron drag by phonons. In strong magnetic fields total thermoelectric field in magn. the field "saturates" and is independent of the carrier scattering mechanism. In ferromagnet. metals N.-E. e. has features associated with the presence of spontaneous magnetization.

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