Nerve electrical impulses. Motoneuron. Nerve impulse. Synapse. What is a nerve impulse

Conduction of nerve impulses along nerve fibers and through synapses. The high-voltage potential that occurs when a receptor is excited in a nerve fiber is 5-10 times greater than the receptor's irritation threshold. Conducting an excitation wave along the nerve fiber is ensured by the fact that each subsequent section of it is irritated by the high-voltage potential of the previous section. In the fleshy nerve fibers, this potential does not spread continuously, but abruptly; he jumps over one or even several interceptions of Ranvier, in which he strengthens. The duration of the excitation between two adjacent interceptions of Ranvier is equal to 5-10% of the duration of the high-voltage potential.

Conduction of a nerve impulse along a nerve fiber occurs only under the condition of its anatomical continuity and its normal physiological state. Violation of the physiological properties of the nerve fiber by severe cooling or poisoning with poisons and drugs stops the conduction of the nerve impulse even with its anatomical continuity.

Nerve impulses are conducted in isolation along individual motor and sensory nerve fibers that are part of the mixed nerve, which depends on the insulating properties of the myelin sheaths covering them. In non-fleshy nerve fibers, the biocurrent propagates continuously along the fiber and, thanks to the connective tissue sheath, does not pass from one fiber to another. nerve impulse can spread along the nerve fiber in two directions: centripetal and centrifugal. Therefore, there are three rules for conducting a nerve impulse in nerve fibers: 1) anatomical continuity and physiological integrity, 2) isolated conduction, and 3) bilateral conduction.

2-3 days after separation nerve fibers from the body of the neuron, they begin to regenerate, or degenerate, and the conduction of nerve impulses stops. Nerve fibers and myelin are destroyed and only the connective tissue sheath is preserved. If the cut ends of the nerve fibers, or nerve, are connected, then after the degeneration of those areas that are separated from the nerve cells, restoration, or regeneration, of the nerve fibers begins from the bodies of the neurons, from which they grow into the preserved connective tissue membranes. Regeneration of nerve fibers leads to the restoration of impulse conduction.

Unlike nerve fibers through neurons nervous system nerve impulses are conducted in only one direction - from the receptor to the working organ. It depends on the nature of the conduction of the nerve impulse through the synapses. In the nerve fiber above the presynaptic membrane there are many tiny vesicles of acetylcholine. When the biocurrent reaches the presynaptic membrane, some of these vesicles burst, and acetylcholine passes through the smallest holes in the presynaptic membrane into the synaptic cleft.
There are sites in the postsynaptic membrane that have a special affinity for acetylcholine, which causes the temporary appearance of pores in the postsynaptic membrane, which makes it temporarily permeable to ions. As a result, excitation and a high-voltage potential arise in the postsynaptic membrane, which propagates along the next neuron or innervated organ. Therefore, the transmission of excitation through the synapses occurs chemically through the mediator, or mediator, acetylcholine, and the conduction of excitation along the next neuron is again carried out electrically.

The action of acetylcholine on the conduction of a nerve impulse through the synapse is short-lived; it is quickly destroyed, hydrolyzed by the enzyme cholinesterase.

Since the chemical transmission of a nerve impulse in a synapse occurs within a fraction of a millisecond, in each synapse the nerve impulse is delayed for this time.

Unlike nerve fibers, in which information is transmitted according to the “all or nothing” principle, that is, discretely, in synapses, information is transmitted according to the “more or less” principle, that is, gradually. The more the mediator acetylcholine is formed up to a certain limit, the higher the frequency of high-voltage potentials in the subsequent neuron. After this limit, excitation turns into inhibition. Thus, the digital information transmitted along the nerve fibers passes in synapses into measuring information. measuring electronic machines,

in which there are certain relationships between actually measured quantities and the quantities that they represent, are called analog, working on the principle of "more or less"; we can assume that a similar process takes place in synapses and its transition to digital occurs. Consequently, the nervous system functions according to a mixed type: both digital and analog processes are performed in it.

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 an electric current 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.

Information is transferred between neurons like current in wires. Electrical 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 through electrons, but through ions.

Synapse

Despite their multiplicity, neurons never touch each other. But electrical impulses cannot be transmitted unless there is physical contact. Therefore, messages transmitted from neuron to neuron must be converted from electrical to another form. The nervous system uses chemicals to transfer information between neurons.

A synapse is a point of contact between two neurons or between a neuron and a receiving cell.

The synaptic space is slit-shaped. When an electrical impulse arrives at a neuron, it releases chemical molecules called neurotransmitters from the synapse. Through diffusion, they move through the synaptic cleft and fall on the receptors of another neuron specially designed for them. The result is another electrical impulse.

Two types of neurotransmitters

The brain produces about fifty types of neurotransmitters, which can be divided into two types. Excitatory mediators contribute to the emergence of a nerve impulse. Inhibitory neurotransmitters, on the contrary, slow down its occurrence. In most cases, a neuron releases only one type of neurotransmitter.

excitation limit

Each of the neurons is capable of receiving hundreds of messages per second. He judges the degree of its significance and makes its preliminary analysis. In a neuron, excitatory impulses are added and inhibitory impulses are subtracted. In order for the neuron to generate its own impulse, the resulting amount must be greater than a certain value.

The role of repetition

Similar ideas, similar memories fire the same neurons and synapses. Frequently used synapses work faster. Therefore, we quickly recall what we have seen or repeated several times. However, these connections can disappear if they are not used enough, and new ones appear in their place.

Glial cells

Another type of nerve cells is glial cells. There are 10 times more of them than the neurons themselves. They are called "nourishers of neurons" because they contribute to their nutrition, removal of their waste products and protection from external enemies. But the latest research suggests that they are needed not only for the care of neurons. Apparently, they are also involved in information processing, in addition, they are necessary for memory to work!

Nerve fibers

The processes of neurons are surrounded by membranes and combined into bundles, which are called nerve fibers. The number of nerve fibers in different nerves ranges from 10 2 to 10 5 .

The sheath of the nerve fiber consists of glial cells and facilitates the passage of nerve impulses through the body. It is called the myelin sheath.

The role of hormones in the brain

To exchange information, the brain uses special chemical compounds - hormones. Some of them are produced by the brain itself, and some by the endocrine glands. Hormones cause various physiological responses.

3. THE HUMAN BRAIN

The outer layer of the brain consists of two large hemispheres, which hide deeper formations underneath. The surface of the hemispheres is covered with grooves and convolutions, which increase their surface.

Main parts of the brain

The human brain can be divided into three main parts:

forebrain

brain stem

cerebellum

gray and white matter

The substance of the brain consists of gray and white areas. Gray areas are clusters of neurons. There are more than 100 billion of them, and they are the ones who process information. The white matter of the brain is the axons. Through them, information is transmitted that is processed by neurons. Gray matter is also concentrated in the inner part of the spinal cord.

Brain nutrition

The brain needs food to function properly. Unlike other cells in the body, brain cells can only process glucose. The brain also needs oxygen. Without it, mitochondria will not be able to produce enough energy. But since blood supplies glucose and oxygen to the brain, nothing should interfere with normal blood flow to maintain brain health. If the blood stops flowing to the brain, after ten seconds the person loses consciousness. Although the brain weighs only 2.5% of the body weight, it receives 20% of the blood circulating in the body and the corresponding amount of oxygen constantly, day and night.

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 flows) and electrical (changes in the electrical potential of the membrane: depolarization, positive polarization and repolarization). © 2012-2019 Sazonov V.F..

It can be said 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, the term "action potential" is also used as a synonym for a nerve impulse. Although the action potential is only electrical component nerve impulse.

action potential - this is a sharp abrupt change in the membrane potential from negative to positive and vice versa.

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

A nerve impulse is a complex structural-electro-chemical process that propagates along the neuron membrane in the form of a traveling wave of changes.

action potential - this is only the electrical component of a nerve impulse, characterizing changes in the electric charge (potential) in a local section of the membrane during the passage of a nerve impulse through it (from -70 to +30 mV and vice versa). (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 generated?

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

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

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

2) receptor end of the dendrite,

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

4) postsynaptic membrane of the excitatory synapse.

Locations of nerve impulses:

1. The axon hillock is the main originator 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 parent (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 lowered the critical level of depolarization (CDL) compared to the rest of the membrane. Therefore, when numerous excitatory postsynaptic potentials (EPSPs) begin to sum up on the membrane of a neuron, which arise in various places on the postsynaptic membranes of all its synaptic contacts, then the FEC is reached first of all on the axon hillock. It is there that this suprathreshold depolarization for the colliculus opens voltage-sensitive sodium channels into which the 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 the local potentials (excitatory and inhibitory) arising on the neuron - and the first one works 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 scatters along the entire 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 the nerve impulse running through the entire membrane.

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

If the 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 KUD, voltage-dependent sodium ion channels open at this end and an action potential and a nerve impulse are born. The nerve impulse runs along the dendrite to the body of the neuron, and then along its axon to the presynaptic endings 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 are picked up precisely at the receptor endings of the dendrites.

3. First interception 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, sum 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 (lower threshold), so it is in this area that the critical level of depolarization (CDL) is most easily overcome, after which voltage-controlled ion channels for sodium open - and an action potential (nerve impulse) arises.

4. The postsynaptic membrane of the 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 the material taken from the educational and methodological manual of the author of this site, which you can refer to in your bibliography:

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.

All processes of membrane changes occurring in the course of propagating excitation are well studied and described in the scientific and educational literature. But this description is not always easy to understand, because there are too many components involved in this process (from the point of view of an ordinary student, 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 - the development of the action potential.

Chemical phenomena - the movement of ionic flows.

Structural phenomena - the behavior of ion channels.

Three sides of the process spreading excitement

1. Action potential (AP)

action potential - this is an abrupt change in the constant membrane potential from negative to positive polarization and vice versa.

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

At the electrical level changes begin as a change in 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 (about -50 mV), this is a relatively simple linear decrease in electronegativity proportional to the strength of the stimulus. But then the cooler beginsself-reinforcing depolarization, it does not develop at a constant rate, butwith acceleration . Figuratively speaking, depolarization accelerates so much that it jumps over the zero mark without noticing it, and even goes into positive polarization. After reaching the peak (usually +30 mV), the reverse process begins -repolarization , i.e. restoration of the negative polarization of the membrane.

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

Ascending branch of the chart:

resting potential - the initial ordinary 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 self-opening of sodium channels), a spike begins from this point - a high-amplitude part of the action potential;

self-reinforcing steeply increasing depolarization;

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

"overshoot" - positive polarization (inversion, or reversion, of the membrane charge);

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

Descending branch of the chart:

repolarization - restoration of the former electronegativity of the membrane;

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

transition of the critical level of depolarization (-50 mV) - the termination of the phase of relative refractoriness (non-excitability) and the return of excitability;

trace processes (trace depolarization or trace hyperpolarization);

restoration of the resting potential - the norm (-70 mV).

So, first - depolarization, then - repolarization. First, the loss of electronegativity, then the 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, it sounds strange to say that water does not conduct electricity. But in fact it is. Water itself is an insulator, 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» perform cations, as they are more mobile. Electric currents in cells are flows of ions.

So, it is important to realize that all electric currents that pass through the membrane areion streams . There is simply no current familiar to us from physics in the form of a flow of electrons in cells, as in water systems. References to electron flows would be a mistake.

At the chemical level we, describing the spreading excitation, 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 increases sharply, and then it suddenly stops at the spike of the action potential. The incoming flow of sodium just causes depolarization, since sodium ions bring positive charges into the cell with them (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, takes positive charges out of the cell with it. Negative charges remain inside the cell in the majority, and due to this, electronegativity increases. This is the restoration of polarization due to the outgoing flow of potassium ions. Note that the outflow of potassium ions occurs almost simultaneously with the appearance of the sodium flow, but increases slowly and lasts 10 times longer. Despite the duration of the potassium flow of the ions themselves, little is consumed - only one millionth of the potassium reserve 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 is 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 for understanding its essence. But still, it all starts with the work of ion channels. It is the state of ion channels that predetermines the behavior of ions, and the behavior of ions, in turn, is accompanied by electrical phenomena. Start the process of arousalsodium 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 “unstoppable” and massive. The opening of the channels allows sodium to enter the cell and causes depolarization. Then, after about 2-5 milliseconds, theyautomatic closing . This closure of the channels abruptly cuts off the movement of sodium ions into the cell, and therefore cuts off the rise in electrical potential. 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 an activation gate and close with an inactivation gate, but this should be discussed earlier, in the topic “Excitation”. We won't stop there.

In parallel with the opening of sodium channels with a slight delay in time, there is an increasing opening of potassium channels. They are slow compared to sodium. The opening of additional potassium channels enhances the release of positive potassium ions from the cell. Potassium release counteracts the "sodium" depolarization and causes polarity restoration (electronegativity restoration). But sodium channels are ahead of potassium channels, they fire about 10 times faster. Therefore, the incoming flow of positive sodium ions into the cell is ahead of the compensating outflow of potassium ions. And therefore, depolarization develops at a faster rate than the polarization that opposes it, caused by the leakage of potassium ions. That is why, until the sodium channels close, the restoration of polarization will not begin.

Fire as a metaphor for spreading excitement

In order to understand the meaningdynamic excitation process, i.e. To understand its propagation along the membrane, one must imagine that the processes described above capture first the nearest, and then all new, more and more distant sections of the membrane, until they run through the entire membrane completely. If you have seen the “live wave” that fans at the stadium arrange 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 successive flow of transmembrane ion currents in neighboring areas.

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

How will the phenomenon of inhibition look like in this metaphor?

The answer is obvious - braking will look like extinguishing a fire, like reducing combustion and extinguishing the fire. But if the fire spreads on its own, then extinguishing 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) either you have to wait until everything burns down and the fire depletes all combustible reserves, (2) either you need to pour water on burning areas so that they go out, (3) or you need to water the nearest areas untouched by fire in advance, so they don't catch fire.

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

Hardly nerve cell is able to "extinguish" this "fire" of excitation that has begun. Therefore, the first method is suitable only for artificial intervention in the work of neurons (for example, in medicinal 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.

AUTOWAVE IN ACTIVELY EXCITABLE MEDIA (ABC)

When a wave propagates in actively excitable media, there is no energy transfer. Energy is not transferred, but released when excitation reaches the ABC section. 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, land reclamation), when an 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 medium. The flame spreads over an area with distributed energy reserves - trees, deadwood, dry moss.

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

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