Nervous electrical impulses. Motor neuron. Nervous impulse. Synapse. What is a nerve impulse
Conducting nerve impulses along nerve fibers and through synapses. The high-voltage potential arising from excitation of a receptor in a nerve fiber is 5-10 times greater than the threshold of receptor stimulation. Conduction of an excitation wave along the nerve fiber is ensured by the fact that each subsequent section is irritated by the high-voltage potential of the previous section. In the pulpy nerve fibers, this potential does not spread continuously, but in leaps and bounds; he jumps over one or even several of Ranvier's interceptions, 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 strong 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 separate motor and sensory nerve fibers, which are part of the mixed nerve, which depends on the insulating properties of the myelin sheaths covering them. In the non-fleshy nerve fibers, the biocurrent spreads continuously along the fiber and, thanks to the connective tissue sheath, does not pass from one fiber to another. Nerve impulse can propagate along the nerve fiber in two directions: centripetal and centrifugal. Consequently, 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 the separation of 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 is preserved. If you connect the cut ends of the nerve fibers, or nerve, then after the degeneration of those areas that are separated from the nerve cells, restoration, or regeneration, begins of nerve fibers from the side of the neuronal bodies, from which they grow into the preserved connective tissue sheaths. 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. The nerve fiber above the presynaptic membrane contains many tiny bubbles 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.
The postsynaptic membrane contains areas with 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 along an innervated organ. Therefore, the transmission of excitation through synapses occurs chemically by means of an intermediary, or transmitter, acetylcholine, and the conduction of excitation through the next neuron is again carried out electrically.
The effect of acetylcholine on the conduction of a nerve impulse through the synapse is short-lived; it is rapidly 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, up to a certain limit, the acetylcholine mediator is formed, the higher the frequency of high-voltage potentials in the subsequent neuron. After this limit, the excitation turns into inhibition. Thus, digital information transmitted along nerve fibers is transferred at synapses into measurement information. Measuring electronic machines,
in which there are certain ratios between the actually measured quantities and the quantities 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 takes place. Consequently, the nervous system functions in a mixed type: both digital and analog processes take place in it.
Synaptic transmission is the interaction of brain cells.
Neurons produce electrochemical disturbances that travel along their fibers. These perturbations, 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 arises 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 nerve cell membrane allows certain types of ions into the interior. Since the ions are electrically charged, their movement is an electric current through the membrane.
Resting neurons. There are ions inside the neurons, but the neurons themselves are surrounded by ions in different concentrations. Particles tend to move from an area with a high concentration to an area with a low one, however, the membrane of the nerve cell prevents this movement, since it is basically impenetrable.
It turns out that some ions are concentrated outside the membrane, while others - inside. As a result, the outer surface of the membrane is positively charged and the inner one is negatively charged. The membrane is thus polarized.
It all started with squid. The mechanism of the action potential - excitation waves on the cell membrane - was elucidated 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 the cell to depolarize - the potential difference across the membrane is reversed, and the inner surface of the membrane is charged positively. 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 into the interior makes the charge on the membrane more negative than at rest, and the cell thus becomes hyperpolarized. During the so-called refractory period, the neuron cannot produce the next action potential, but quickly returns to a state of rest.
Action potentials are generated in 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 fiber segment causes depolarization of an adjacent one. This wave of depolarization rolls in the direction away from the cell body and, upon reaching the terminal of the nerve cell, causes the release of neurotransmitters.
A single impulse lasts one thousandth of a second; neurons encode information with a precisely timed sequence of impulses (spike discharges), but it is still unclear how the information is encoded. Neurons often produce action potentials in response to signals from other cells, but they also generate impulses without any external signals. The frequency of basal pulsations, or spontaneous action potentials, varies in different types neurons and can change depending on the signals of other cells.
Few will pass. Ions travel across the nerve cell membrane through barrel-shaped proteins called ion channels. They penetrate the membrane and form through pores. The ion channels have sensors that detect changes in the membrane potential difference, they open and close in response to these changes.
Human neurons contain more than a dozen different types of such channels, and each of them passes only one type of ions. The activity of all these ion channels during the action potential is strictly regulated. They open and close in a specific 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 in response to incoming signals. It describes the relationship between the potential difference (voltage) of a nerve cell membrane, its resistance and the current flowing through it. According to this relationship, the current is directly proportional to the voltage on the membrane and is described by the equation I \u003d 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 myelinated tissue produced by brain cells by oligodendrocytes. An oligodendrocyte has few branches, and each consists of a large flat sheet of myelin wrapped many times around a small segment of an axon belonging to another neuron. The myelin sheath is uneven along the length of the entire axon: it is interrupted at regular intervals, and the points of these interruptions are called Ranvier interceptions. The ionic channels thicken just at these points, thereby providing a jump of action potentials from one interception 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 transmitted between neurons like current in wires. Electrical impulses are transmitted from cell to cell, from the dendrite in which they arise, to the axon through which they travel. But there is also a difference from electrical networks - impulses are transmitted not by electrons, but by ions.
Synapse
Despite their multiplicity, neurons never touch each other. But electrical impulses cannot be transmitted if there is no physical contact. Therefore, messages transmitted from neuron to neuron must turn from electrical to another form. The nervous system uses chemicals to transfer information between neurons.
A synapse is a place of contact between two neurons or between a neuron and a receiving cell.
The synaptic space is shaped like a gap. When an electrical impulse arrives at a neuron, it releases chemical molecules called neurotransmitters from the synapse. By means of diffusion, they move through the synaptic cleft and enter 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 neurotransmitters contribute to the emergence of a nerve impulse. In contrast, inhibitory neurotransmitters 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 a preliminary analysis of it. In the neuron, excitatory impulses are added and inhibitory impulses are subtracted. For the neuron to generate its own impulse, the resulting sum must be greater than a certain value.
The role of repetitions
Similar ideas, similar memories, drive the same neurons and synapses. Frequently used synapses are faster. Therefore, we quickly recall what we saw or repeated several times. However, these connections can disappear if they are not used enough, and new ones arise in their place.
Glial cells
Another type of nerve cell is glial cells. There are 10 times more of them than the neurons themselves. They are called "nurses of neurons" because they contribute to their nutrition, the removal of their waste products and protection from external enemies. But recent research suggests that they are needed for more than just caring for neurons. Apparently, they also participate in information processing, in addition, they are necessary for the memory to work!
Nerve fibers
The processes of neurons are surrounded by membranes and are combined into bundles called nerve fibers. The number of nerve fibers in various 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's 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 trigger various physiological responses.
3. HUMAN BRAIN
The outer layer of the brain consists of two large hemispheres, which hide deeper formations under them. The surface of the hemispheres is covered with grooves and convolutions, which increase their surface.
The main parts of the brain
The human brain can be roughly divided into three main parts:
forebrain
brain stem
cerebellum
Gray and white matter
The brain matter is composed of gray and white areas. The 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, which is processed by neurons. Gray matter is also concentrated in the inner part of the spinal cord.
Brain nutrition
The brain needs nutrition 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 the blood supplies glucose and oxygen to the brain, nothing must interfere with normal blood flow to maintain brain health. If blood stops flowing to the brain, within ten seconds the person will lose consciousness. Although the brain weighs only 2.5% of body weight, it constantly receives 20% of the blood circulating in the body and the corresponding amount of oxygen, day and night.
Nervous 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 membrane's electrical potential: depolarization, positive polarization, and repolarization). © 2012-2019 Sazonov V.F ..
In short:
"Nerve impulse is a wave of changes moving across the neuron membrane. " © 2012-2019 Sazonov V.F ..
But in 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 Is a sharp jump-like change in the membrane potential from negative to positive and vice versa.
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 the nerve impulse, which characterizes the changes in the electrical charge (potential) at the local section of the membrane during the passage of the 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 born?
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) axonal mound (this is the transition of the neuron body to the axon),
2) the receptor end of the dendrite,
3) the first interception of Ranvier on the dendrite (dendrite trigger zone),
4) postsynaptic membrane of the excitatory synapse.
Places of origin of nerve impulses:
1. The axonal mound is the main originator of nerve impulses.
The axonal mound is the very beginning of the axon, where it begins on the body of the neuron. It is the axonal mound that is the main originator (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 axonal hillock has an increased sensitivity to excitation and a lower critical level of depolarization (KUD) in comparison with the rest of the membrane. Therefore, when numerous excitatory postsynaptic potentials (EPSPs), which arise in various places on the postsynaptic membranes of all its synaptic contacts, begin to accumulate on the neuron membrane, it is on the axonal hillock that KUD is achieved first. It is there that depolarization, which is above the threshold for the hillock, opens up potential-sensitive sodium channels, into which the flow of sodium ions enters, which generates an action potential and a nerve impulse.
So, the axonal 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 KUD, generating a nerve impulse.
It is also important to consider the following fact. From the axonal hillock, the nerve impulse spreads across the entire membrane of its neuron: both along the axon to the presynaptic endings, and along the dendrites to postsynaptic "beginnings". In this case, all local potentials are removed from the neuron membrane and from all its synapses. 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 a neuron has a receptor end, then an adequate stimulus can act on it and generate at this end first a generator potential, and then a nerve impulse. When the generator potential reaches the KUD, then 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 dendritic endings of pain neurons, work. Nerve impulses in painful neurons are raised precisely at the receptor endings of the dendrites.
3. First interception of Ranvier on dendrite (dendrite trigger zone).
Local excitatory postsynaptic potentials (EPSPs) at the ends of the dendrite, which are formed in response to excitations arriving at the dendrite through synapses, are summed up at the first Ranvier interception of this dendrite, if, of course, it is myelinated. There is a section of the membrane with increased sensitivity to excitation (lowered threshold), therefore, it is in this area that the critical level of depolarization (CCD) is most easily overcome, after which voltage-gated ion channels for sodium open up and an action potential (nerve impulse) arises.
4. Postsynaptic membrane of the excitatory synapse.
In rare cases, EPSP at the excitatory synapse can be so strong that it reaches the KUD 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 have triggered simultaneously (spatial summation), or due to the fact that several impulses in a row have come to this synapse (time summation).
Video:Conducting a nerve impulse along a nerve fiber
Action potential as a nerve impulse
Below is the material taken from the training manual of the author of this site, which may well be referred to in your list of references:
Sazonov V.F. Concept and types of inhibition in the physiology of the central nervous system: Teaching aid. Part 1. Ryazan: RSPU, 2004. 80 p.
All processes of membrane changes occurring in the course of propagating excitement are 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, not a 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 an action potential.
Chemical phenomena - the movement of ionic streams.
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 polarization to positive and vice versa.
Usually, the membrane potential in neurons of the central nervous system changes from –70 mV to +30 mV, and then returns to its original state, ie. to –70 mV. As you can see, the concept of an 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 exciting potential. Up to the critical level of depolarization (approximately –50 mV), this is a relatively simple linear decrease in electronegativity, proportional to the strength of the stimulus. And then the cooler one beginsself-reinforcing depolarization, it does not develop at a constant rate, butwith acceleration ... Figuratively speaking, depolarization accelerates so 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 flow of the action potential:
Ascending branch of the graph:
resting potential - the initial normal polarized electronegative state of the membrane (–70 mV);
growing 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), from this point a spike begins - a high-amplitude part of the action potential;
self-reinforcing steeply increasing depolarization;
transition of the zero mark (0 mV) - change of the membrane polarity;
"Overshoot" - positive polarization (inversion, or reversion, of the membrane charge);
peak (+30 mV) - the peak of the membrane polarity change process, the peak of the action potential.
Descending branch of the chart:
repolarization - restoration of the previous electronegativity of the membrane;
transition of the zero mark (0 mV) - reverse change of the membrane polarity 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);
resting potential restoration is 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. It sounds strange to many people to say that water does not conduct electricity. 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»Perform cations, as they are more mobile. Electric currents in cells are streams of ions.
So, it is important to realize that everything electric currentsthat go through the membrane areion flows ... The current familiar to us from physics in the form of a flow of electrons in cells, as in water systems, simply does not exist. References to electron streams would be a mistake.
At the chemical level when describing propagating excitation, we must consider how the characteristics of the ionic fluxes 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 with them positive charges into the cell (which reduce electronegativity). Then, after the adhesion, the outward flow of potassium ions significantly increases, which causes repolarization. After all, potassium, as we have repeatedly said, carries with it positive charges from the cell. Negative charges remain inside the cell in the majority, and due to this, electronegativity is enhanced. This is the restoration of polarization due to the outgoing flow of potassium ions. Note that the outgoing flow of potassium ions occurs almost simultaneously with the appearance of the sodium flow, but it grows 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 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 one due to the release of potassium ions from the cell.
3. Ionic channels
All three aspects of the excitation process - electrical, chemical and structural - are necessary to understand its essence. Still, it all starts with the work of ion channels. It is the state of the ion channels that predetermines the behavior of ions, and the behavior of ions, in turn, is accompanied by electrical phenomena. Start the arousal processsodium channels .
At the molecular structural level the membrane sodium channels are opened. At first, this process proceeds in proportion to the strength of the external influence, and then it becomes simply “irrepressible” 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 growth of the electrical potential. The potential growth stops and we see a spike on the graph. This is the top of the curve on the chart, 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 are opened by activation gates and closed by 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 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. The release of potassium counteracts the "sodium" depolarization and causes the 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 outstrips the compensating release of potassium ions. And therefore, depolarization develops at an outstripping rate compared to the opposing polarization caused by the leakage of potassium ions. This is why, until the sodium channels are closed, polarization recovery will not begin.
Fire as a metaphor for spreading excitement
To move on to understanding 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 a "live wave" that fans in 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 sequential flow of transmembrane ionic currents in adjacent areas.
When we were looking for a figurative example, analogy or metaphor that can clearly convey the meaning of the spreading excitement, we settled on the image of a fire. Indeed, the spreading excitement is like a wildfire, when the burning trees remain in place, and the fire front spreads and goes further and further in all directions from the fire.
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 need to wait until everything burns out and the fire depletes all combustible reserves, (2) or you need to water the burning areas to extinguish them, (3) or you need to water the nearest areas untouched by fire in advance, so they don't catch fire.
Is it possible to "extinguish" the wave of propagating excitement?
Hardly nerve cell is able to "extinguish" this "fire" of excitement 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 “pour water” on some areas and put a block on the propagation of excitement.
© Sazonov V.F. Concept and types of inhibition in the physiology of the central nervous system: Teaching aid. Part 1. Ryazan: RSPU, 2004. 80 p.
AUTWAVES IN ACTIVE-EXCITABLE ENVIRONMENTS (ABC)
When a wave propagates in actively excitable media, no energy transfer occurs. Energy is not transferred, but released when excitement 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 (ABC)
The excitation wave propagates in the ABC without attenuation; the passage of a wave of excitement is associated with refractoriness - the non-excitability of the environment for a certain period of time (period of refractoriness).
