Nerve impulses. Nerve impulse and the principle of its transmission Nerve electrical impulses

Nerve impulse - it is a moving wave of changes in the state of the membrane. It includes structural changes (opening and closing of membrane ion channels), chemical (changing transmembrane ion fluxes) and electrical (changes in the electrical potential of the membrane: depolarization, positive polarization and repolarization). © 2012-2019 Sazonov V.F..

We can say in short:

"Nerve impulse"is a wave of change moving across the membrane of a neuron." © 2012-2019 Sazonov V.F..

But in the physiological literature, it is customary to use the term “action potential” as a synonym for a nerve impulse. Although the action potential is only electrical component nerve impulse.

Action potential is a sharp abrupt change in membrane potential from negative to positive and back.

The action potential is the electrical characteristic (electrical component) of a nerve impulse.

A nerve impulse is a complex structural-electrochemical process that spreads across the neuron membrane in the form of a traveling wave of changes.

Action potential - this is only the electrical component of the nerve impulse, characterizing changes electric charge(potential) on a local section of the membrane during the passage of a nerve impulse through it (from -70 to +30 mV and back). (Click on the image on the left to see the animation.)

Compare the two pictures above (click on them) and, as they say, feel the difference!

Where are nerve impulses born?

Oddly enough, not all students who have studied the physiology of arousal can answer this question. ((

Although the answer is not complicated. Nerve impulses are born on neurons in just a few places:

1) axon hillock (this is the transition of the neuron body into the axon),

2) receptor end of the dendrite,

3) the first node of Ranvier on the dendrite (trigger zone of the dendrite),

4) postsynaptic membrane of the excitatory synapse.

Places of origin of nerve impulses:

1. The axon hillock is the main generator of nerve impulses.

The axon hillock is the very beginning of the axon, where it begins on the body of the neuron. It is the axon hillock that is the main generator (generator) of nerve impulses on a neuron. In all other places, the probability of the birth of a nerve impulse is much less. The fact is that the membrane of the axon hillock has increased sensitivity to excitation and a reduced critical level of depolarization (CLD) compared to other parts of the membrane. Therefore, when numerous excitatory postsynaptic potentials (EPSPs), which arise in a variety of places on the postsynaptic membranes of all its synaptic contacts, begin to be summed up on the membrane of a neuron, then the CUD is achieved first of all on the axon hillock. It is there that this supra-threshold depolarization for the colliculus opens voltage-sensitive sodium channels, into which a flow of sodium ions enters, generating an action potential and a nerve impulse.

So, the axon hillock is an integrative zone on the membrane; it integrates all local potentials (excitatory and inhibitory) arising on the neuron - and the first one is triggered to achieve the CUD, generating a nerve impulse.

It is also important to take into account the following fact. From the axon hillock, the nerve impulse spreads throughout the membrane of its neuron: both along the axon to the presynaptic endings, and along the dendrites to the postsynaptic “beginnings”. All local potentials are removed from the membrane of the neuron and from all its synapses, because they are “interrupted” by the action potential from a nerve impulse running across the entire membrane.

2. Receptor ending of a sensory (afferent) neuron.

If a neuron has a receptor ending, then an adequate stimulus can act on it and generate at this ending first a generator potential and then a nerve impulse. When the generator potential reaches the CUD, voltage-gated sodium ion channels open at this end and an action potential and nerve impulse are born. The nerve impulse travels along the dendrite to the body of the neuron, and then along its axon to the presynaptic terminals to transmit excitation to the next neuron. This is how, for example, pain receptors (nociceptors), which are the dendritic endings of pain neurons, work. Nerve impulses in pain neurons originate precisely at the receptor endings of dendrites.

3. First node of Ranvier on the dendrite (trigger zone of the dendrite).

Local excitatory postsynaptic potentials (EPSPs) at the ends of the dendrite, which are formed in response to excitations coming to the dendrite through synapses, are summed up at the first node of Ranvier of this dendrite, if, of course, it is myelinated. There is a section of the membrane with increased sensitivity to excitation (lowered threshold), so it is in this section that the critical level of depolarization (CLD) is most easily overcome, after which voltage-gated ion channels for sodium open - and an action potential (nerve impulse) arises.

4. Postsynaptic membrane of an excitatory synapse.

In rare cases, an EPSP at an excitatory synapse can be so strong that it reaches the CUD right there and generates a nerve impulse. But more often this is possible only as a result of the summation of several EPSPs: either from several neighboring synapses that fired simultaneously (spatial summation), or due to the fact that several impulses in a row arrived at a given synapse (temporal summation).

Video:Conduction of a nerve impulse along a nerve fiber

Action potential as a nerve impulse

Below is material taken from the educational manual of the author of this site, which can be referred to in your bibliography:

Sazonov V.F. Concept and types of inhibition in the physiology of the central nervous system: Educational and methodological manual. Part 1. Ryazan: RGPU, 2004. 80 p.

All processes of membrane changes that occur during spreading excitation have been quite well studied and described in the scientific and educational literature. But this description is not always easy to understand, since there are too many components involved in this process (from the point of view of an ordinary student, and not a child prodigy, of course).

To facilitate understanding, we propose to consider a single electrochemical process of propagating dynamic excitation from three sides, at three levels:

Electrical phenomena - development of action potential.

Chemical phenomena - movement of ion flows.

Structural phenomena - behavior of ion channels.

Three sides of the process spreading excitation

1. Action potential (AP)

Action potential is an abrupt change in constant membrane potential from negative to positive polarization and back.

Typically, the membrane potential in CNS neurons changes from –70 mV to +30 mV, and then returns to its original state, i.e. to –70 mV. As we can see, the concept of action potential is characterized through electrical phenomena on the membrane.

On an electrical level changes begin as a change from the polarized state of the membrane to depolarization. First, depolarization occurs in the form of a local excitatory potential. Up to a critical level of depolarization (approximately –50 mV), there is a relatively simple linear decrease in electronegativity, proportional to the strength of the applied stimulus. But then a cooler one beginsself-reinforcing depolarization, it does not develop at a constant speed, butwith acceleration . Figuratively speaking, depolarization accelerates so much that it jumps over the zero mark without noticing it, and even turns into positive polarization. After reaching the peak (usually +30 mV), the reverse process begins -repolarization , i.e. restoration of negative polarization of the membrane.

Let us briefly describe the electrical phenomena during the course of an action potential:

Ascending branch of the graph:

resting potential – the initial normal polarized electronegative state of the membrane (–70 mV);

increasing local potential – depolarization proportional to the stimulus;

critical level of depolarization (–50 mV) – a sharp acceleration of depolarization (due to the self-opening of sodium channels), from this point the spike begins – the high-amplitude part of the action potential;

self-reinforcing steeply increasing depolarization;

zero mark transition (0 mV) – change of membrane polarity;

“overshoot” – positive polarization (inversion, or reversion, of the membrane charge);

peak (+30 mV) – the peak of the process of changing membrane polarity, the peak of the action potential.

Descending branch of the graph:

repolarization – restoration of the membrane’s previous electronegativity;

transition of the zero mark (0 mV) – reverse change of membrane polarity to the previous, negative one;

transition to a critical level of depolarization (–50 mV) – cessation of the phase of relative refractoriness (non-excitability) and return of excitability;

trace processes (trace depolarization or trace hyperpolarization);

restoration of the resting potential is normal (–70 mV).

So, first - depolarization, then - repolarization. First - loss of electronegativity, then - restoration of electronegativity.

2. Ionic flows

Figuratively, we can say that charged ions are the creators of electrical potentials in nerve cells. For many people, the statement that water does not conduct electricity sounds strange. But in reality it is so. Water itself is a dielectric, not a conductor. In water, electric current is provided not by electrons, as in metal wires, but by charged ions: positive cations and negative anions. In living cells, the main “electrical work” is performed by cations, since they are more mobile. Electric currents in cells are flows of ions.

So, it is important to realize that all electrical currents that pass through the membrane areion flows . The current we are accustomed to from physics in the form of a flow of electrons in cells, as in aqueous systems, simply does not exist. References to electron flows would be a mistake.

On a chemical level In describing the propagating excitation, we must consider how the characteristics of the ion flows passing through the membrane change. The main thing in this process is that during depolarization, the flow of sodium ions into the cell sharply increases, and then it suddenly stops at the action potential spike. The incoming flow of sodium causes depolarization, since sodium ions bring positive charges into the cell (which reduces electronegativity). Then, after the spike, the outward flow of potassium ions increases significantly, which causes repolarization. After all, potassium, as we have repeatedly said, carries positive charges with it from the cell. The majority of negative charges remain inside the cell, and due to this, electronegativity increases. This is the restoration of polarization due to the outgoing flow of potassium ions. Note that the outgoing flow of potassium ions appears almost simultaneously with the appearance of the sodium flow, but increases slowly and lasts 10 times longer. Despite the duration of the potassium flow, the ions themselves are consumed a little - only one millionth of the potassium supply in the cell (0.000001 part).

Let's summarize. The ascending branch of the action potential graph is formed due to the entry of sodium ions into the cell, and the descending branch – due to the exit of potassium ions from the cell.

3. Ion channels

All three aspects of the excitation process - electrical, chemical and structural - are necessary to understand its essence. But still, it all starts with the work of ion channels. It is the state of ion channels that determines the behavior of ions, and the behavior of ions, in turn, is accompanied electrical phenomena. The process of excitation beginssodium channels .

At the molecular structural level membrane sodium channels open. At first, this process proceeds in proportion to the strength of external influence, and then it becomes simply “uncontrollable” and massive. The opening of the channels allows sodium to enter the cell and causes depolarization. Then, after about 2-5 milliseconds, theirautomatic closing . This closure of the channels abruptly interrupts the movement of sodium ions into the cell, and, therefore, interrupts the increase in electrical potential. The potential growth stops, and we see a spike on the chart. This is the top of the curve on the graph, then the process will go in the opposite direction. Of course, it is very interesting to understand that sodium channels have two gates, and they open with activation gates and close with inactivation gates, but this should be discussed earlier, in the topic “Excitation.” We will not dwell on this.

In parallel with the opening of sodium channels, with a slight time lag, there is an increasing opening of potassium channels. They are slow compared to sodium ones. The opening of additional potassium channels enhances the release of positive potassium ions from the cell. The release of potassium counteracts the "sodium" depolarization and causes restoration of polarity (restoration of electronegativity). But sodium channels are ahead of potassium channels; they work about 10 times faster. Therefore, the incoming flow of positive sodium ions into the cell is ahead of the compensating output of potassium ions. And therefore, depolarization develops at a faster pace than the counterpolarization caused by the leakage of potassium ions. This is why until the sodium channels close, restoration of polarization will not begin.

Fire as a metaphor for spreading excitement

In order to move on to understanding the meaningdynamic excitation process, i.e. To understand its spread along the membrane, one must imagine that the processes we described above first capture the nearest, and then new, more and more distant sections of the membrane, until they completely run across the entire membrane. If you have seen the “live wave” that fans create at the stadium by standing up and squatting, then it will be easy for you to imagine a membrane wave of excitation, which is formed due to the sequential flow of transmembrane ionic currents in adjacent areas.

When we were looking for a figurative example, analogy or metaphor that could clearly convey the meaning of spreading excitement, we settled on the image of a fire. Indeed, spreading excitation is similar to a forest fire, when burning trees remain in place, and the fire front spreads and goes further and further in all directions from the source of fire.

How will the phenomenon of inhibition look in this metaphor?

The answer is obvious - braking will look like putting out a fire, like reducing combustion and extinguishing the fire. But if the fire spreads on its own, then extinguishing it requires effort. From the extinguished area, the extinguishing process by itself will not go in all directions.

There are three options for fighting a fire: (1) you either have to wait until everything burns and the fire has exhausted all flammable reserves, (2) or you have to pour water on the burning areas so that they go out, (3) or you have to water the nearest areas untouched by the fire in advance, so that they don't catch fire.

Is it possible to “extinguish” the wave of spreading excitation?

It is unlikely that a nerve cell is able to “extinguish” this “fire” of excitement that has begun. Therefore, the first method is only suitable for artificial intervention in the functioning of neurons (for example, for therapeutic purposes). But it turns out that it is quite possible to “fill some areas with water” and block the spread of excitation.

© Sazonov V.F. The concept and types of inhibition in the physiology of the central nervous system: Educational manual. Part 1. Ryazan: RGPU, 2004. 80 p.

AUTOWAVES IN ACTIVELY EXCITABLE MEDIA (AEC)

When a wave propagates in actively excitable media, no energy transfer occurs. Energy is not transferred, but is released when excitation reaches the ABC site. An analogy can be drawn with a series of explosions of charges placed at some distance from each other (for example, when extinguishing forest fires, construction, reclamation work), when the explosion of one charge causes an explosion of a nearby one, and so on. A forest fire is also an example of wave propagation in an actively excitable environment. The flame spreads over an area with distributed energy reserves - trees, dead wood, dry moss.

Basic properties of waves propagating in actively excitable media (AEM)

The excitation wave propagates in the ABC without attenuation; the passage of an excitation wave is associated with refractoriness - the non-excitability of the environment for a certain period of time (refractory period).

What is a nerve impulse

Nature is very simple.
Otherwise nothing would work.
There’s just a lot of this simplicity.
Hence all the difficulties.

Although today a lot is known about the brain and its structure, the main question is: “How does it work?” no answer yet. The brain appears to us as a black box, at the input of which “some” signals reflecting circumstances are received through receptors - sense organs outside world, and the brain, in turn, processes them, stores them and sends “some” control commands to the working (executive) organs.

Unanswered questions remain about how this information is displayed, recorded (captured) and retrieved.

But, be that as it may, Science does not stand still, and scientists have made significant progress in brain research.

There are ideas about how neurons function, there are attempts to build a logical model of how the brain works. True, it is worth touching on the issues of information transfer between neurons and we immediately come across modest evasive hints about certain methods of transmitting excitation, chemical and electrical methods of signal transmission. The electrical nature of nerve impulses is mentioned, as if in passing.

The lack of specifics gives scope for mystical and pseudo-scientific imagination. Therefore, to understand the biophysical effects in the brain, attempts are constantly being made to introduce new postulates, for example, about the presence in nature of certain vital forces or torsion fields.

So, a modern model of how the brain works.
Today it is known for certain that the brain consists of a large number of individual logical elements - neurons. Each neuron can be excited by signals arriving at its inputs ( axons) from the outputs ( dendrites) other neurons directly connected to it. Having been excited, this neuron is in an excited (!!! and not charged) state and transmits excitation through its outputs to the inputs of the following logical elements - neurons.

Neuron– a specialized nerve cell with its own membrane, a set of intracellular organelles and neurofibrils. A long axial process-axon and short branching dendrites extend from its body. Dendrites receiving nerve impulses from other neurons transfer them to the axon, along which the excitation spreads without attenuation to other neurons or effectors - various kinds of executive organs (glands, muscles, etc.). Dictionary - Entomologist's Handbook I would also highlight the synapse. Synapse- the site of contact between two neurons or between a neuron and the effector cell receiving the signal. Serves to transmit nerve impulses between two cells.

This is practically all that science knows about how a neuron works. All other knowledge comes down to the classification of neurons by type, size, number of tails and other very important properties. And of course, a huge number of conclusions were drawn on the basis of an essentially erroneous idea about the electrical nature of nerve impulses.

Now let's make two assumptions.
First– information (excitation) is transmitted from neuron to neuron in the form of acoustic (sound) wave.
Second– a neuron is a single oscillatory system ( oscillatory circuit) and is capable of tuning to one or more resonant frequencies and being in a self-oscillating state, thereby ensuring memorization (storage) of information.
Then a nerve impulse is nothing more than an acoustic wave transmitted along the dendrites and axons of a neuron. The body of the neuron itself represents an acoustic oscillatory circuit or resonator, which, in the case of information transmission, is capable of modulating the nerve impulse passing through it, and in the case of storing information, is in a self-oscillating state at a certain frequency. Or, suppose, to perform the recording function, the cell changes its resonance parameters and continues to remain calm, and responds only when addressed.

Let's look at how this all works using the example of FIGURE......

R1-Rn - receptors. Information from the receptors passes through the inputs - dendrites, through the body of the neuron to the output - axon. The task of the nervous system is to convey information from the receptor to the brain. In the simplest circuit shown in Figure 1, this is only possible if the signals are individually distinguishable. That is, the output signal carries information about the specific receptor from which the nerve impulse began. Let's assume that in our case, nerve impulses differ in frequency.

Now let's make the task much more difficult. Suppose that a nerve impulse is transmitted from a receptor through a sequence of neurons, for example two. see Fig.2.
In this example, the nerve impulse at the output of the circuit must contain information not only about the receptor from which it came, but also about all the neurons through which it was transmitted. It can be assumed that each neuron involved in the transmission of an impulse brings its own information component to it. For example, modulation of the frequency signal coming from the receptor.

All nerve impulses are unique, like barcodes on goods in a supermarket, like fingerprints. They are unique and carry information about the fact of receptor irritation and the path traveled.
Millions of nerve impulses rush through the human nervous system every second. The scheme proposed above allows us to explain how completely different impulses can be transmitted along the same nerve channels, and how the impulse distribution service can work.

What do such assumptions tell us?

• Firstly, the acoustic idea gives us a more or less plausible, from the point of view of physics, theory of information transfer within a living organism.
• Secondly, it explains how information is stored in the brain.
• Thirdly, it makes it possible to explain life phenomena that are incomprehensible at this moment in time, and provides a tool for self-knowledge.
• Fourthly, this is a new paradigm in medicine, especially in therapy.

Rhetorical question: what is the cause of the disease, the pathology of the organ or the pathology of the signal governing the organ? Theoretically, both are possible, and with equal probability. So what does modern therapy treat (surgery is clearer)? And maybe placebo and homeopathy, which “real” doctors politely laugh at, are not such stupidity based on the patient’s self-hypnosis, but are precisely treatment by adjusting the control system. Treatment is indirect, through external functions of the brain, but what if treatment is possible through. For example, let’s remember modern battery-powered heart stimulators. And if you stimulate the heart not with electrical impulses according to the “ ” principle, but with a naturally occurring control (acoustic wave) signal. Maybe then there is no need for surgery; it is enough to apply an acoustic generator to any part of the body or to any neuron and the signal will find its target itself.

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) are released during the reaction process. chemical composition). 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 resuming electrochemical processes and impulses occur, rushing along the fibers.

The content of the article

NERVOUS SYSTEM, a complex network of structures that permeates the entire body and ensures self-regulation of its vital functions due to the ability to respond to external and internal influences (stimuli). The main functions of the nervous system are receiving, storing and processing information from external and internal environment, regulation and coordination of the activities of all organs and organ systems. In humans, like in all mammals, the nervous system includes three main components: 1) nerve cells (neurons); 2) glial cells associated with them, in particular neuroglial cells, as well as cells forming neurilemma; 3) connective tissue. Neurons provide the conduction of nerve impulses; neuroglia performs supporting, protective and trophic functions both in the brain and in the spinal cord, and the neurilemma, consisting mainly of specialized, so-called. Schwann cells, participates in the formation of peripheral nerve fiber sheaths; Connective tissue supports and binds together the various parts of the nervous system.

The human nervous system is divided in different ways. Anatomically, it consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system includes the brain and spinal cord, and the PNS, which provides communication between the central nervous system and various parts body - cranial and spinal nerves, as well as nerve ganglia and nerve plexuses lying outside the spinal cord and brain.

Neuron.

The structural and functional unit of the nervous system is the nerve cell – neuron. It is estimated that there are more than 100 billion neurons in the human nervous system. A typical neuron consists of a body (i.e., the nuclear part) and processes, one usually non-branching process, an axon, and several branching ones - dendrites. The axon carries impulses from the cell body to muscles, glands or other neurons, while the dendrites carry them into the cell body.

A neuron, like other cells, has a nucleus and a number of tiny structures - organelles ( see also CELL). These include the endoplasmic reticulum, ribosomes, Nissl bodies (tigroid), mitochondria, Golgi complex, lysosomes, filaments (neurofilaments and microtubules).

Nerve impulse.

If the stimulation of a neuron exceeds a certain threshold value, then a series of chemical and electrical changes occur at the point of stimulation that spread throughout the neuron. The transmitted electrical changes are called nerve impulses. Unlike a simple electrical discharge, which, due to the resistance of the neuron, will gradually weaken and will be able to cover only a short distance, a much slower “running” nerve impulse is constantly restored (regenerated) in the process of propagation.

Concentrations of ions (electrically charged atoms) - mainly sodium and potassium, as well as organic matter– outside the neuron and inside it are not the same, therefore the nerve cell at rest is charged negatively from the inside, and positively charged from the outside; As a result, a potential difference appears on the cell membrane (the so-called “resting potential” is approximately –70 millivolts). Any change that reduces the negative charge within the cell and thereby the potential difference across the membrane is called depolarization.

The plasma membrane surrounding the neuron is complex education, consisting of lipids (fats), proteins and carbohydrates. It is practically impenetrable to ions. But some of the protein molecules in the membrane form channels through which certain ions can pass. However, these channels, called ion channels, are not constantly open, but, like gates, can open and close.

When a neuron is stimulated, some of the sodium (Na+) channels open at the point of stimulation, allowing sodium ions to enter the cell. The influx of these positively charged ions reduces the negative charge of the inner surface of the membrane in the channel area, which leads to depolarization, which is accompanied by a sharp change in voltage and discharge - the so-called. “action potential”, i.e. nerve impulse. The sodium channels then close.

In many neurons, depolarization also causes potassium (K+) channels to open, causing potassium ions to flow out of the cell. The loss of these positively charged ions again increases the negative charge on the inner surface of the membrane. The potassium channels then close. Other membrane proteins also begin to work - the so-called. potassium-sodium pumps that move Na + out of the cell and K + into the cell, which, along with the activity of potassium channels, restores the original electrochemical state (resting potential) at the point of stimulation.

Electrochemical changes at the point of stimulation cause depolarization at an adjacent point on the membrane, triggering the same cycle of changes in it. This process is constantly repeated, and in each new point, where depolarization occurs, an impulse of the same magnitude is born as at the previous point. Thus, along with the renewed electrochemical cycle, the nerve impulse spreads along the neuron from point to point.

Nerves, nerve fibers and ganglia.

A nerve is a bundle of fibers, each of which functions independently of the others. The fibers in a nerve are organized into groups surrounded by specialized connective tissue that contains vessels that supply the nerve fibers with nutrients and oxygen and remove carbon dioxide and waste products. The nerve fibers through which impulses travel from peripheral receptors to the central nervous system (afferent) are called sensitive or sensory. Fibers that transmit impulses from the central nervous system to muscles or glands (efferent) are called motor or motor. Most nerves are mixed and consist of both sensory and motor fibers. A ganglion (nerve ganglion) is a collection of neuron bodies in the peripheral nervous system.

The axonal fibers in the PNS are surrounded by the neurilemma, a sheath of Schwann cells that are located along the axon, like beads on a string. A significant number of these axons are covered with an additional sheath of myelin (a protein-lipid complex); they are called myelinated (pulpy). Fibers surrounded by neurilemma cells, but not covered with a myelin sheath, are called unmyelinated (unmyelinated). Myelinated fibers are found only in vertebrates. The myelin sheath is formed from plasma membrane Schwann cells, which winds onto the axon like a skein of tape, forming layer after layer. The section of the axon where two adjacent Schwann cells touch each other is called the node of Ranvier. In the central nervous system, the myelin sheath of nerve fibers is formed by a special type of glial cells - oligodendroglia. Each of these cells forms the myelin sheath of several axons at once. Unmyelinated fibers in the CNS lack a sheath of any special cells.

The myelin sheath speeds up the conduction of nerve impulses that “jump” from one node of Ranvier to another, using this sheath as a connecting electrical cable. The speed of impulse conduction increases with thickening of the myelin sheath and ranges from 2 m/s (for unmyelinated fibers) to 120 m/s (for fibers especially rich in myelin). For comparison: the speed of propagation of electric current through metal wires is from 300 to 3000 km/s.

Synapse.

Each neuron has specialized connections to muscles, glands, or other neurons. The area of ​​functional contact between two neurons is called a synapse. Interneuron synapses are formed between different parts of two nerve cells: between an axon and a dendrite, between an axon and a cell body, between a dendrite and a dendrite, between an axon and an axon. A neuron that sends an impulse to a synapse is called presynaptic; the neuron receiving the impulse is postsynaptic. The synaptic space has the shape of a cleft. A nerve impulse propagating along the membrane of a presynaptic neuron reaches the synapse and stimulates the release of a special substance - a neurotransmitter - into a narrow synaptic cleft. Neurotransmitter molecules diffuse across the gap and bind to receptors on the membrane of the postsynaptic neuron. If a neurotransmitter stimulates a postsynaptic neuron, its action is called excitatory; if it suppresses, it is called inhibitory. The result of the summation of hundreds and thousands of excitatory and inhibitory impulses simultaneously flowing to a neuron is the main factor determining whether this postsynaptic neuron will generate a nerve impulse in this moment.

In a number of animals (for example, the lobster), a special connection is established between the neurons of certain nerves. close connection with the formation of either an unusually narrow synapse, the so-called. gap junction, or, if the neurons are in direct contact with each other, tight junction. Nerve impulses pass through these connections not with the participation of a neurotransmitter, but directly, through electrical transmission. Mammals, including humans, also have a few tight junctions of neurons.

Regeneration.

By the time a person is born, all of his neurons and most of the interneuron connections have already been formed, and in the future only a few new neurons are formed. When a neuron dies, it is not replaced by a new one. However, the remaining ones can take over the functions of the lost cell, forming new processes that form synapses with those neurons, muscles or glands with which the lost neuron was connected.

Cut or damaged PNS neuron fibers surrounded by the neurilemma can regenerate if the cell body remains intact. Below the site of transection, the neurilemma is preserved as a tubular structure, and that part of the axon that remains connected to the cell body grows along this tube until it reaches the nerve ending. In this way, the function of the damaged neuron is restored. Axons in the central nervous system that are not surrounded by a neurilemma are apparently unable to re-grow to the site of their previous termination. However, many neurons in the central nervous system can produce new short processes - branches of axons and dendrites that form new synapses. see also REGENERATION.

CENTRAL NERVOUS SYSTEM

The central nervous system consists of the brain and spinal cord and their protective membranes. The outermost is the dura mater, under it is the arachnoid (arachnoid), and then the pia mater, fused with the surface of the brain. Between the pia mater and the arachnoid membrane is the subarachnoid space, which contains cerebrospinal fluid, in which both the brain and spinal cord literally float. The action of the buoyant force of the fluid leads to the fact that, for example, the adult brain, which has an average mass of 1500 g, actually weighs 50–100 g inside the skull. The meninges and cerebrospinal fluid also play the role of shock absorbers, softening all kinds of shocks and shocks that tests the body and which could lead to damage to the nervous system.

The central nervous system is made up of gray and white matter. Gray matter is composed of cell bodies, dendrites, and unmyelinated axons, organized into complexes that include countless synapses and serve as information processing centers for many functions of the nervous system. White matter consists of myelinated and unmyelinated axons that act as conductors transmitting impulses from one center to another. The gray and white matter also contains glial cells.

CNS neurons form many circuits that perform two main functions: they provide reflex activity, as well as complex information processing in higher brain centers. These higher centers, such as the visual cortex (visual cortex), receive incoming information, process it, and transmit a response signal along the axons.

The result of the activity of the nervous system is one or another activity, which is based on the contraction or relaxation of muscles or the secretion or cessation of secretion of glands. It is with the work of muscles and glands that any way of our self-expression is connected.

Incoming sensory information is processed through a sequence of centers connected by long axons that form specific pathways, for example pain, visual, auditory. Sensory (ascending) pathways go in an ascending direction to the centers of the brain. Motor (descending) tracts connect the brain with motor neurons of the cranial and spinal nerves.

The pathways are usually organized in such a way that information (for example, pain or tactile) from the right side of the body enters the left side of the brain and vice versa. This rule also applies to the descending motor pathways: the right half of the brain controls the movements of the left half of the body, and the left half controls the right. From this general rule however, there are a few exceptions.

Brain

consists of three main structures: cerebral hemispheres, cerebellum and brainstem.

The cerebral hemispheres - the largest part of the brain - contain higher nerve centers that form the basis of consciousness, intelligence, personality, speech, and understanding. In each of the cerebral hemispheres, the following formations are distinguished: underlying isolated accumulations (nuclei) of gray matter, which contain many important centers; a large mass of white matter located above them; covering the outside of the hemispheres is a thick layer of gray matter with numerous convolutions that makes up the cerebral cortex.

The cerebellum also consists of an underlying gray matter, an intermediate mass of white matter, and an outer thick layer of gray matter that forms many convolutions. The cerebellum primarily provides coordination of movements.

Spinal cord.

Located inside the spinal column and protected by its bone tissue, the spinal cord has a cylindrical shape and is covered with three membranes. In a cross section, the gray matter is shaped like the letter H or a butterfly. Gray matter is surrounded by white matter. Sensitive fibers of the spinal nerves end in the dorsal (posterior) parts of the gray matter - the dorsal horns (at the ends of the H, facing the back). The bodies of motor neurons of the spinal nerves are located in the ventral (anterior) parts of the gray matter - the anterior horns (at the ends of the H, distant from the back). In the white matter there are ascending sensory pathways ending in the gray matter of the spinal cord, and descending motor pathways coming from the gray matter. In addition, many fibers in the white matter connect different parts of the gray matter of the spinal cord.

PERIPHERAL NERVOUS SYSTEM

The PNS provides two-way communication between the central parts of the nervous system and the organs and systems of the body. Anatomically, the PNS is represented by the cranial (cranial) and spinal nerves, as well as the relatively autonomous enteric nervous system, located in the intestinal wall.

All cranial nerves (12 pairs) are divided into motor, sensory or mixed. Motor nerves begin in the motor nuclei of the trunk, formed by the bodies of the motor neurons themselves, and sensory nerves are formed from the fibers of those neurons whose bodies lie in ganglia outside the brain.

31 pairs of spinal nerves depart from the spinal cord: 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. They are designated according to the position of the vertebrae adjacent to the intervertebral foramina from which these nerves emerge. Each spinal nerve has an anterior and a posterior root, which fuse to form the nerve itself. The posterior root contains sensory fibers; it is closely connected with the spinal ganglion (dorsal root ganglion), consisting of the cell bodies of neurons, the axons of which form these fibers. The anterior root consists of motor fibers formed by neurons whose cell bodies lie in the spinal cord.

AUTONOMIC NERVOUS SYSTEM

The autonomic, or autonomic, nervous system regulates the activity of involuntary muscles, the heart muscle, and various glands. Its structures are located both in the central nervous system and in the peripheral nervous system. The activity of the autonomic nervous system is aimed at maintaining homeostasis, i.e. a relatively stable state of the body's internal environment, such as a constant body temperature or blood pressure that meets the body's needs.

Signals from the central nervous system enter the working (effector) organs through pairs of sequentially connected neurons. The bodies of neurons of the first level are located in the CNS, and their axons end in the autonomic ganglia, which lie outside the CNS, and here they form synapses with the bodies of neurons of the second level, the axons of which are in direct contact with the effector organs. The first neurons are called preganglionic, the second - postganglionic.

In the part of the autonomic nervous system called the sympathetic nervous system, the cell bodies of preganglionic neurons are located in the gray matter of the thoracic (thoracic) and lumbar (lumbar) spinal cord. Therefore, the sympathetic system is also called the thoracolumbar system. The axons of its preganglionic neurons terminate and form synapses with postganglionic neurons in ganglia located in a chain along the spine. Axons of postganglionic neurons contact effector organs. The endings of postganglionic fibers secrete norepinephrine (a substance close to adrenaline) as a neurotransmitter, and therefore the sympathetic system is also defined as adrenergic.

The sympathetic system is complemented by the parasympathetic nervous system. The bodies of its preganglinar neurons are located in the brainstem (intracranial, i.e. inside the skull) and the sacral (sacral) part of the spinal cord. Therefore, the parasympathetic system is also called the craniosacral system. The axons of preganglionic parasympathetic neurons terminate and form synapses with postganglionic neurons in ganglia located near the working organs. The endings of postganglionic parasympathetic fibers release the neurotransmitter acetylcholine, on the basis of which the parasympathetic system is also called cholinergic.

As a rule, the sympathetic system stimulates those processes that are aimed at mobilizing the body’s forces in extreme situations or under stress. The parasympathetic system contributes to the accumulation or restoration of the body's energy resources.

The reactions of the sympathetic system are accompanied by the consumption of energy resources, an increase in the frequency and strength of heart contractions, an increase in blood pressure and blood sugar, as well as an increase in blood flow to skeletal muscles due to a decrease in its flow to internal organs and skin. All of these changes are characteristic of the “fear, flight or fight” response. The parasympathetic system, on the contrary, reduces the frequency and strength of heart contractions, lowers blood pressure, and stimulates the digestive system.

REFLEXES

When an adequate stimulus acts on the receptor of a sensory neuron, a volley of impulses appears in it, triggering a response action called a reflex act (reflex). Reflexes underlie most of the vital functions of our body. The reflex act is carried out by the so-called. reflex arc; This term refers to the path of transmission of nerve impulses from the point of initial stimulation on the body to the organ that performs the response action.

The reflex arc that causes contraction of a skeletal muscle consists of at least two neurons: a sensory neuron, whose body is located in the ganglion, and the axon forms a synapse with neurons of the spinal cord or brain stem, and a motor (lower, or peripheral, motor neuron), whose body is located in the gray matter, and the axon ends at the motor end plate on skeletal muscle fibers.

The reflex arc between the sensory and motor neurons may also include a third, intermediate, neuron located in the gray matter. The arcs of many reflexes contain two or more interneurons.

Reflex actions are carried out involuntarily, many of them are not realized. The knee jerk reflex, for example, is triggered by tapping the quadriceps tendon at the knee. This is a two-neuron reflex, its reflex arc consists of muscle spindles (muscle receptors), a sensory neuron, a peripheral motor neuron and a muscle. Another example is the reflexive withdrawal of the hand from a hot object: the arc of this reflex includes a sensory neuron, one or more interneurons in the gray matter of the spinal cord, a peripheral motor neuron, and a muscle.

Literature:

Bloom F., Leiserson A., Hofstadter L. Brain, Mind and Behavior. M., 1988
Human physiology, ed. R. Schmidt, G. Tevs, vol. 1. M., 1996

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

What is a nerve impulse?

This is the name for the excitation wave that spreads along 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 does this process represent at the physiological level? The mechanism of nerve impulse transmission is that neuron membranes can change their electrochemical potential. And the process that interests us occurs in the area of ​​synapses. The speed of the nerve impulse can vary from 3 to 12 meters per second. We will talk about it in more detail, as well as about the factors that influence it.

Study of structure and work

The passage of a nerve impulse was first demonstrated by German scientists E. Hering and G. Helmholtz using the example of a frog. It was then established that the bioelectric signal propagates at the previously indicated speed. In general, this is possible thanks to a special construction. In some ways, they resemble an electric cable. So, if we draw parallels with it, then the conductors are the axons, and the insulators are their myelin sheaths (they are a Schwann cell membrane, 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 factor is the quality of electrical insulation. By the way, the body uses lipoprotein myelin as a material, which has dielectric properties. All other things being equal, the larger its layer, the faster nerve impulses will travel. Even at the moment it cannot be said that this system has been fully explored. Much that relates to nerves and impulses still remains a mystery and a subject of research.

Features of structure and functioning

If we talk about the path of the nerve impulse, it should be noted that the fiber is not covered along its entire length. The design features are such that the current situation can best be compared with the creation of insulating ceramic couplings that are tightly strung on the rod of an electrical cable (albeit in this case on an axon). As a result, there are small non-insulated electrical areas from which ionic current can easily flow from the axon into 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 interception of Ranvier. The presence of such a mechanism allows the nerve impulse to spread 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 varies between 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. A nerve impulse ensures rapid propagation of similar signals along axons over a long distance. Therefore this is the most important tool organism for the exchange of information within it. Data on irritation are transmitted by changing their frequency. A complex system of periodicals operates here, which can count hundreds of nerve impulses in one second. Computer electronics works on a somewhat similar principle, although much more complicated. So, when nerve impulses arise in neurons, they are encoded in a certain way, and only then are transmitted. In this case, information is grouped into special “packs”, which have different numbers and patterns. All this, put together, forms the basis for the rhythmic electrical activity of our brain, which can be recorded using an electroencephalogram.

Cell types

Speaking about the sequence of passage of a nerve impulse, we cannot ignore the neurons through which electrical signals are transmitted. 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 transform into nerve impulses all temperature, chemical, sound, mechanical and light stimuli.
2. Insert (also called conductor or closure). They serve to process and switch impulses. The largest number 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 bright sunshine, 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 with the last one. There are two types of shoots:

1. Dendrites. They are entrusted with the function of perceiving irritation from 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 nerve impulses by cells, it is difficult not to talk about one interesting point. So, when they are at rest, then, let's say, the sodium-potassium pump is engaged in moving ions in such a way as to achieve the effect fresh water inside and salty outside. Due to the resulting imbalance, potential differences across the membrane can be observed up to 70 millivolts. For comparison, this is 5% of the usual ones. But as soon as the state of the cell changes, the resulting equilibrium is disrupted, 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 an action potential. When it reaches a certain point, they begin reverse processes, 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 amount to measly millimeters per second. Then signals from the hand to the brain would take minutes, which is clearly not good. This is where the previously discussed myelin sheath plays its role in enhancing 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 what transmission sequence of nerve impulses works in the most important parts of our central nervous system. Here, neurons are separated from their neighbors by small gaps called synapses. The action potential cannot pass through them, so it looks for another way to get to the next nerve cell. At the end of each process there are small sacs called presynaptic vesicles. Each of them contains 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 equilibrium is disturbed and, probably, a new action potential appears. This is not yet known for certain; neurophysiologists are still 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. Will undergo chemical breakdown.
3. They will return back to their bubbles (this is called recapture).

At the end of the 20th century, an amazing discovery was made. Scientists have learned that drugs that affect neurotransmitters (as well as their release and reuptake) can radically change a person's mental state. 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. Well, for now we don’t have an answer to this fundamental question: What causes a neuron to create an action potential? For now, the mechanism for “launching” this cell is a secret to 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 sent by their neighbors. Details regarding processing and integration of this type impulses are almost unknown to us. Although many research groups are working on this. At the moment, we have learned 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. In theory this is true, although in some cases it is necessary to take into account some features. So, if you go into detail, 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 name for a change in the charge of the membrane from positive to negative, and vice versa. Whereas a nerve impulse is understood as a complex structural-electrochemical process. It spreads across the neuron membrane as a traveling wave of change. The action potential is just the electrical component of a nerve impulse. It characterizes the changes that occur with the charge of a local area 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 has diligently studied the physiology of arousal. There are four options:

1. Receptor end of the dendrite. If it exists (which is not a fact), then it is possible that there is an adequate stimulus, which will first create a generator potential, and then a nerve impulse. Pain receptors work in a similar way.
2. Membrane of the excitatory synapse. As a rule, this is only possible in the presence of severe irritation or their summation.
3. Dendritic trigger zone. In this case, local excitatory postsynaptic potentials are formed as a response to the stimulus. If the first node of Ranvier is myelinated, then they are summed up on it. Due to the presence of a section of membrane there that has increased sensitivity, a nerve impulse arises here.
4. Axon hillock. This is the name given to the place where the axon begins. The mound is the most frequent one to 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 increased sensitivity, as well as decreased sensitivity. Therefore, when the summation of numerous excitatory postsynaptic potentials begins, the hillock reacts to them first.

Example of propagating excitation

Talking in medical terms may cause misunderstanding of certain points. To eliminate this, it is worth briefly going through the knowledge presented. Let's take a fire as an example.

Remember the news reports from last summer (you can also hear this again soon). The fire is spreading! At the same time, trees and bushes that burn remain in their places. But the fire front is moving further and further from the place where the fire was located. The nervous system works in a similar 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 functioning of the neuron (for therapeutic purposes) or various physiological means are used. This can be compared to pouring water on a fire.