As a result of the evolution of the nervous system of humans and other animals, complex information networks arose, the processes in which are based on chemical reactions. The most important element of the nervous system are specialized cells neurons. Neurons consist of a compact cell body containing a nucleus and other organelles. Several branched processes depart from this body. Most of these branches, called dendrites, serve as contact points for receiving signals from other neurons. One process, usually the longest, is called axon and sends signals to other neurons. The end of an axon can branch many times, and each of these smaller branches is able to connect with the next neuron.
In the outer layer of the axon there is a complex structure formed by many molecules that act as channels through which ions can enter - both inside and outside the cell. One end of these molecules, deviating, joins the target atom. After that, the energy of other parts of the cell is used to push this atom out of the cell, while the process, acting in the opposite direction, introduces another molecule into the cell. The most important is the molecular pump, which removes sodium ions from the cell and introduces potassium ions into it (sodium-potassium pump).
When the cell is at rest and does not conduct nerve impulses, the sodium-potassium pump moves potassium ions into the cell and pushes sodium ions out (think of a cell containing fresh water and surrounded by salt water). Due to this imbalance, the potential difference across the axon membrane reaches 70 millivolts (approximately 5% of the voltage of a conventional AA battery).
However, when the state of the cell changes and the axon is stimulated by an electrical impulse, the equilibrium on the membrane is disturbed, and the sodium-potassium pump begins to work in the opposite direction for a short time. Positively charged sodium ions enter the axon, and potassium ions are pumped out. For a moment, the internal environment of the axon acquires a positive charge. At the same time, the channels of the sodium-potassium pump are deformed, blocking the further influx of sodium, and potassium ions continue to go outside, and the original potential difference is restored. Meanwhile, sodium ions spread inside the axon, changing the membrane at the bottom of the axon. In this case, the state of the pumps located below changes, contributing to the further propagation of the pulse. A sharp change in voltage caused by the rapid movement of sodium and potassium ions is called action potential. When an action potential passes through a certain point on the axon, the pumps turn on and restore the resting state.
The action potential propagates rather slowly - no more than a fraction of an inch per second. In order to increase the speed of impulse transmission (because, after all, it is not good for a signal sent by the brain to reach the hand only after a minute), the axons are surrounded by a sheath of myelin, which prevents the inflow and outflow of potassium and sodium. The myelin sheath is not continuous - at certain intervals there are gaps in it, and the nerve impulse jumps from one "window" to another, due to this, the speed of impulse transmission increases.
When the impulse reaches the end of the main part of the body of the axon, it must be transmitted either to the next lower neuron, or, in the case of neurons in the brain, along numerous branches of many other neurons. For such transmission, a completely different process is used than for the transmission of an impulse along the axon. Each neuron is separated from its neighbor by a small gap called synapse. The action potential cannot jump through this gap, so some other way must be found to transmit an impulse to the next neuron. At the end of each process are tiny sacs called ( presynaptic) bubbles, each of which contains special compounds - neurotransmitters. Upon receipt of an action potential, neurotransmitter molecules are released from these vesicles, crossing the synapse and attaching to specific molecular receptors on the membrane of underlying neurons. When a neurotransmitter is attached, the equilibrium on the neuron membrane is disturbed. Now we will consider whether a new action potential arises with such an imbalance (neuroscientists continue to search for an answer to this important question until now).
After the neurotransmitters have transmitted a nerve impulse from one neuron to the next, they can simply diffuse, or undergo chemical breakdown, or return back to their vesicles (this process is awkwardly called reverse capture). At the end of the 20th century, an amazing scientific discovery was made - it turns out that drugs that affect the release and reuptake of neurotransmitters can radically change the mental state of a person. Prozac (Prozac *) and similar antidepressants block the reuptake of the neurotransmitter serotonin. It seems that Parkinson's disease is associated with a deficiency of the neurotransmitter dopamine in the brain. Borderline psychiatric researchers are trying to understand how these compounds affect the human mind.
There is still no answer to the fundamental question of what causes a neuron to initiate an action potential - in the professional language of neurophysiologists, the mechanism of “launching” a neuron is unclear. Of particular interest in this respect are the neurons of the brain, which can receive neurotransmitters sent by a thousand neighbors. Almost nothing is known about the processing and integration of these impulses, although many research groups are working on this problem. We only know that the process of integration of incoming impulses is carried out in the neuron and a decision is made whether or not to initiate an action potential and transmit the impulse further. This fundamental process governs the functioning of the entire brain. It is not surprising that this greatest mystery of nature remains, at least today, a mystery to science too!
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.
An action potential is an 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), therefore 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 graph:
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" 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 go 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 distribution 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 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?
It is unlikely that a 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, for 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. One can draw an analogy with a series of explosions of charges placed at some distance from each other (for example, when extinguishing forest fires, construction, land reclamation work), 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).
NERVE IMPULSE
NERVE IMPULSE
A wave of excitation, which spreads along the nerve fiber and serves to transmit information from the periphery. receptor (sensitive) endings to the nerve centers, inside the center. nervous system and from it to the executive apparatus - the muscles and glands. N.'s passage and. accompanied by transient electric. processes, to-rye it is possible to register both extracellular, and intracellular electrodes.
Generation, transfer and processing N. and. carried out by the nervous system. Main a structural element of the nervous system of higher organisms is a nerve cell, or a neuron, consisting of a cell body and numerous. processes - dendrites (Fig. 1). One of the processes in non-ripheric. neurons has a large length - this is a nerve fiber, or axon, the length of which is ~ 1 m, and the thickness is from 0.5 to 30 microns. There are two classes of nerve fibers: pulpy (myelinated) and amyelinated. The pulpy fibers have myelin, formed by special. a membrane, edges like isolation is wound on an axon. The length of sections of a continuous myelin sheath is from 200 microns to 1 mm, they are interrupted by the so-called. interceptions of Ranvier with a width of 1 μm. The myelin sheath plays the role of insulation; the nerve fiber in these areas is passive, electrically active only in the nodes of Ranvier. Meleless fibers do not have insulated. plots; their structure is homogeneous along the entire length, and the membrane has an electric. activity over the entire surface.
Nerve fibers end on the bodies or dendrites of other nerve cells, but are separated from them by an intermediate
an eerie width of ~10 nm. This area of contact between two cells is called. synapse. The axon membrane entering the synapse is called. presynaptic, and the corresponding dendritic or muscle membrane is post-synaptic (see Fig. Cell structures).
Under normal conditions, a series of N. and. constantly run along the nerve fiber, arising on the dendrites or the cell body and spreading along the axon in the direction from the cell body (the axon can conduct N. and. in both directions). The frequency of these periodic discharges carries information about the strength of the irritation that caused them; eg, with moderate activity, the frequency is ~ 50-100 impulses / s. There are cells, to-rye are discharged with a frequency of ~ 1500 impulses/s.
Speed of distribution of N. and. u .
depends on the type of nerve fiber and its diameter d, u .
~ d 1/2. In the thin fibers of the human nervous system u .
~ 1 m/s, and in thick fibers u .
~ 100-120 m/s.
Each N. and. occurs as a result of irritation of the body of a nerve cell or nerve fiber. N. and. always has the same characteristics (shape and speed) regardless of the strength of irritation, i.e., with subthreshold irritation of N. and. does not occur at all, but with suprathreshold - has a full amplitude.
After excitation, a refractory period occurs, during which the excitability of the nerve fiber is reduced. Distinguish abs. the refractory period, when the fiber cannot be excited by any stimuli, and refers. refractory period, when possible, but its threshold is above normal. Abs. the refractory period limits the transmission frequency of N. from above and. The nerve fiber has the property of accommodation, that is, it gets used to constantly acting irritation, which is expressed in a gradual increase in the threshold of excitability. This leads to a decrease in N.'s frequency and. and even to their complete disappearance. If irritation builds up slowly, then excitation may not occur even after reaching the threshold.
Fig.1. Diagram of the structure of a nerve cell.
Along N.'s nerve fiber and. distributed in the form of electricity. potential. In the synapse, there is a change in the propagation mechanism. When N. and. reaches the presynaptic endings, in synaptic. the gap is allocated active chem. - m e d i a t o r. The mediator diffuses through the synaptic. gap and changes the permeability of postsynaptic. membrane, as a result of which it appears, again generating a propagating . This is how chemo works. synapse. There is also an electric synapse when . the neuron is electrically excited.
N.'s excitation and. Phys. ideas about the appearance of electric. potentials in cells are based on the so-called. membrane theory. Cell membranes separate electrolytes of different concentrations and possess is-Byrate. permeability for certain ions. Thus, the axon membrane is a thin layer of lipids and proteins with a thickness of ~7 nm. Her electric resistance at rest ~ 0.1 ohm. m 2, and the capacity is ~ 10 mf / m 2. Inside the axon, the concentration of K + ions is high and the concentration of Na + and Cl - ions is low, while in the environment it is vice versa.
At rest, the axon membrane is permeable to K + ions. Due to the difference in concentrations C 0 K .
in ext. and C in ext. solutions, a potassium membrane potential is established on the membrane
where T - abs. pace-pa, e - charge of an electron. On the axon membrane, a resting potential of ~ -60 mV is indeed observed, corresponding to the indicated f-le.
Ions Na + and Cl - penetrate the membrane. To maintain the necessary non-equilibrium distribution of ions, the cell uses an active transport system, which uses cellular energy to work. Therefore, the state of rest of the nerve fiber is not thermodynamically equilibrium. It is stationary due to the action of ion pumps, and the membrane potential in open circuit conditions is determined from the equality to zero of the total electric. current.
The process of nervous excitation develops as follows (see also Biophysics). If a weak current pulse is passed through the axon, leading to depolarization of the membrane, then after removing the external. exposure potential monotonously returns to the initial level. Under these conditions, the axon behaves like a passive electrical circuit. circuit consisting of a capacitor and a DC. resistance.
Rice. 2. Development of the action potential in the nervous systemlokne: a- subthreshold ( 1
) and suprathreshold (2) irritation; b-membrane response; with supra-threshold irritation, full sweat appearsaction cycle; in is the ion current flowing through membrane when excited; G - approximation ion current in a simple analytical model.
If the current pulse exceeds a certain threshold value, the potential continues to change even after the disturbance is turned off; the potential becomes positive and only then returns to the level of rest, and at first it even skips a little (the region of hyperpolarization, Fig. 2). The response of the membrane does not depend on the perturbation; this impulse is called action potential. At the same time, an ion current flows through the membrane, directed first inward and then outward (Fig. 2, in).
Phenomenological interpretation of the mechanism of occurrence of N. and. was given by A. L. Hodg-kin and A. F. Huxley in 1952. The total ion current is made up of three components: potassium, sodium, and leakage current. When the membrane potential is shifted by the threshold value j* (~ 20mV), the membrane becomes permeable to Na + ions. Na + ions rush into the fiber, shifting the membrane potential until it reaches the equilibrium sodium potential:
component ~ 60 mV. Therefore, the full amplitude of the action potential reaches ~ 120 mV. By the time the max. potential in the membrane begins to develop potassium (and at the same time decrease sodium). As a result, the sodium current is replaced by a potassium current directed outward. This current corresponds to a decrease in the action potential.
The empirical ur-tion for the description of sodium and potassium currents. The behavior of the membrane potential during spatially homogeneous excitation of the fiber is determined by the equation:
where FROM - membrane capacity, I- ion current, consisting of potassium, sodium and leakage current. These currents are determined by the post. emf j K , j Na and j l and conductivities g K , g Na and gl:
the value g l considered constant, conductivity g Na and g K is described using parameters m, h and P:
g Na, g K - constants; options t, h and P satisfy the linear equations
Coefficient dependence. a .
and b on the membrane potential j (Fig. 3) are selected from the condition of the best match
Rice. 3. Dependence of coefficientsa.
andbfrom membranespotential.
calculated and measured curves I(t). The choice of parameters is caused by the same considerations. Dependence of stationary values t, h and P on the membrane potential is shown in fig. 4. There are models with a large number of parameters. Thus, the nerve fiber membrane is a non-linear ionic conductor, the properties of which significantly depend on the electric. fields. The mechanism of excitation generation is poorly understood. The Hodgkin-Huxley Urn gives only a successful empirical. description of the phenomenon, for which there is no specific physical. models. Therefore, an important task is to study the mechanisms of the flow of electric. current through membranes, in particular through controlled electric. field ion channels.
Rice. 4. Dependence of stationary values t, h and P
from the membrane potential.
N.'s distribution and. N. and. can propagate along the fiber without attenuation and with post. speed. This is due to the fact that the energy necessary for signal transmission does not come from a single center, but is drawn in place, at each point of the fiber. In accordance with the two types of fibers, there are two ways of N.'s transmission and
In the case of nonmyelination membrane potential fibers j( x, t) is determined by the equation:
where FROM - membrane capacitance per unit fiber length, R- the sum of longitudinal (intracellular and extracellular) resistances per unit fiber length, I- ion current flowing through the membrane of a fiber of unit length. Electric current I is a functional of the potential j, which depends on time t and coordinates X. This dependence is determined by equations (2) - (4).
Type of functionality I specific to a biologically excitable environment. However, equation (5), apart from the form I, has a more general character and describes many physical. phenomena, eg. combustion process. Therefore N.'s transfer and. likened to the burning of a powder cord. If in a running flame the process of ignition is carried out due to thermal conductivity, then in N. and. excitation occurs with the help of the so-called. local currents (Fig. 5).
Rice. 5. Local currents providing distributionnerve impulse.
Ur-tion of Hodgkin - Huxley for N.'s distribution and. solved numerically. The solutions obtained, together with the accumulated experiments. data showed that N.'s distribution and. does not depend on the details of the excitation process. Qualities. a picture of N.'s distribution and. can be obtained using simple models that reflect only the general properties of excitation. Such approach allowed to count also the N.'s form and. in a homogeneous fiber, their change in the presence of inhomogeneities, and even complex modes of propagation of excitation in active media, for example. in the heart muscle. There are several math. models of this kind. The simplest of them is this. The ion current flowing through the membrane during the passage of N. and. is sign-alternating: at first it flows into the fiber, and then out. Therefore, it can be approximated by a piecewise constant function (Fig. 2, G). Excitation occurs when the membrane potential is shifted by the threshold value j*. At this moment, a current appears, directed inside the fiber and equal in absolute value j". After t "the current changes to the opposite, equal to j". This continues for time ~t". The self-similar solution of equation (5) can be found as a function of the variable t = x/ u ,
where u -
speed of distribution of N. and. (Fig. 2, b).
In real fibers, the time t" is large enough, so only it determines the speed u ,
for which the f-la is valid: 
. Given that j" ~ ~d, R~d 2 and FROM~ d, where d- fiber diameter, we find, in agreement with experiment, that u ~d 1/2 .
Using a piecewise constant approximation, the shape of the action potential is found.
Ur-tion (5) for the spreading N. and. actually admits two solutions. The second solution turns out to be unstable; it gives N. and. with a much lower speed and potential amplitude. The presence of the second, unstable solution has an analogy in the theory of combustion. When a flame propagates with a lateral heat sink, an unstable regime may also occur. A simple analytic N.'s model and. can be improved, taking into account the additions. details.
At change of section and at branching of nervous fibers N.'s passage and. may be difficult or even completely blocked. In an expanding fiber (Fig. 6), the pulse velocity decreases as it approaches expansion, and after expansion, it begins to increase until it reaches a new stationary value. N.'s delay and. the stronger, the greater the difference in cross sections. With a sufficiently large expansion of N. and. stops. There is a critical expansion of a fiber, a cut detains N. and.
At the return movement of N. and. (from wide fiber to narrow) there is no blocking, but the change in speed is the opposite. At the approach to narrowing N.'s speed and. increases and then begins to fall to a new stationary value. On the speed graph (Fig., 6 a) results in a kind of hysteresis loop.
Rie. 6. Passage of nerve impulses by expandingrunning fiber: a - change in pulse speed in depending on its direction; b- schematic image of an expanding fiber.
Another type of heterogeneity is fiber branching. In the branch node, various options for passing and blocking impulses. At the nonsynchronous N.'s approach and. the blocking condition depends on the time offset. If the time between pulses is small, then they help each other to penetrate into the wide third fiber. If the shift is large enough, then N. and. interfere with each other. This is due to the fact that N. and., who came up first, but failed to excite the third fiber, partially transfers the node into a refractory state. Besides, there is a synchronization effect: in process of N.'s approach and. to the node, their delay relative to each other decreases.
N.'s interaction and. Nerve fibers in the body are combined into bundles or nerve trunks, forming a kind of stranded cable. All fibers in a bundle are independent. communication lines, but have one common "wire" - intercellular. When N. and runs along any of the fibers, it creates an electric current in the intercellular fluid. , a cut influences membrane potential of the next fibers. Usually such an influence is negligible and the communication lines work without mutual interference, but it manifests itself in the pathological. and arts. conditions. Processing nerve trunks special. chem. substances, it is possible to observe not only mutual interference, but also the transfer of excitation to neighboring fibers.
Known experiments on the interaction of two nerve fibers placed in a limited volume of external. solution. If N. runs along one of the fibers and., then the excitability of the second fiber changes at the same time. Change goes through three stages. At first, the excitability of the second fiber falls (the excitation threshold rises). This decrease in excitability precedes the action potential traveling along the first fiber and lasts approximately until the potential in the first fiber reaches its maximum. Then the excitability grows, this stage coincides in time with the process of reducing the potential in the first fiber. Excitability decreases again when a slight hyperpolarization of the membrane occurs in the first fiber.
At the same time N.'s passage and. on two fibers it was sometimes possible to achieve their synchronization. Despite the fact that own N.'s speeds and. in different fibers are different, at the same time. excitation could arise collective N. and. If own. speeds were the same, then the collective impulse had a lower speed. With a noticeable difference in property. speeds, the collective speed had an intermediate value. Only N. and. could synchronize, the speeds of which did not differ too much.
Matem. the description of this phenomenon is given by the system of equations for the membrane potentials of two parallel fibers j 1 and j 2:
where R 1 and R 2 - longitudinal resistances of the first and second fibers, R 3 -
longitudinal resistance of the environment, g = R 1 R 2 + R 1 R 3 .
+ R 2 R 3 .
Ionic currents I 1 and I 2 can be described by one or another model of nervous excitation.
When using a simple analytic model solution leads to the following. picture. When one fiber is excited, an alternating membrane potential is induced in the adjacent one: first, the fiber hyperpolarizes, then depolarizes, and finally, hyperpolarizes again. These three phases correspond to a decrease, an increase, and a new decrease in the excitability of the fiber. At normal values of the parameters, the shift of the membrane potential in the second phase towards depolarization does not reach the threshold, so there is no transfer of excitation to the adjacent fiber. At the same time excitation of two fibers, system (6) allows a joint self-similar solution, which corresponds to two N. and. moving at the same speed per post. distance from each other. If a slow N. and. is ahead, then it slows down the fast impulse, not releasing it forward; both are moving at a relatively slow speed. If there is a fast II ahead. and., then it pulls up a slow impulse. The collective velocity turns out to be close to the intrinsic velocity. fast impulse speed. In complex neural structures, the appearance of auto will.
excitable environments. Nerve cells in the body are combined into neural networks, which, depending on the frequency of branching of the fibers, are divided into rare and dense. In a rare network are excited independently of each other and interact only at branch nodes, as described above.
In a dense network, the excitation covers many elements at once, so that their detailed structure and the way they are interconnected turn out to be insignificant. The network behaves like a continuous excitable medium, the parameters of which determine the occurrence and propagation of excitation.
The excitable medium can be three-dimensional, although it is more often considered as two-dimensional. The excitement which arose in to. point on the surface, propagates in all directions in the form of an annular wave. The excitation wave can go around obstacles, but cannot be reflected from them, nor is it reflected from the boundary of the medium. When waves collide with each other, their mutual annihilation occurs; these waves cannot pass through each other due to the presence of a refractory region behind the excitation front.
An example of an excitable environment is cardiac neuromuscular syncytium - the union of nerve and muscle fibers into a single conducting system capable of transmitting excitation in any direction. Neuromuscular syncytia contract synchronously, obeying a wave of excitation, which is sent by a single control center - the pacemaker. A single rhythm is sometimes disturbed, arrhythmias occur. One of these modes is called atrial flutter: these are autonomous contractions caused by the circulation of excitation around an obstacle, for example. superior or inferior vein. For the occurrence of such a regime, the perimeter of the obstacle must exceed the wavelength of excitation, which is ~ 5 cm in the human atrium. atrial contraction with a frequency of 3-5 Hz. A more complex mode of excitation is ventricular fibrillation of the heart, when otd. elements of the heart muscle begin to contract without external. commands and without communication with neighboring elements with a frequency of ~ 10 Hz. Fibrillation leads to the cessation of blood circulation.
The emergence and maintenance of spontaneous activity of an excitable medium are inextricably linked with the emergence of wave sources. The simplest source of waves (spontaneously excited cells) can provide periodic. pulsation of activity, this is how the pacemaker of the heart works.
Sources of excitation can also arise due to complex spaces. organization of the excitation mode, for example. reverberator of the type of a rotating spiral wave, appearing in the simplest excitable medium. Another kind of reverb occurs in an environment consisting of two types of elements with different excitation thresholds; the reverb periodically excites one or the other elements, while changing the direction of its movement and generating plane waves.
The third type of source is the leading center (echo source), which appears in an environment that is inhomogeneous in terms of refractoriness or excitation threshold. In this case, a reflected wave (echo) appears on the inhomogeneity. The presence of such wave sources leads to the appearance of complex excitation regimes, which are studied in the theory of autowaves.
Lit.: Hodgkin A., Nerve impulse, trans. from English, M., 1965; Katz B., Nerve, muscle and synapse, trans. from English, M., 1968; Khodorov B. I., The problem of excitability, L., 1969; Tasaki I., Nervous excitement, trans. from English, M., 1971; V. S. Markin, V. F. Pastushenko, Yu. A. Chizmadzhev, Theory of Excitable Media, Moscow, 1981. V. S. Markin.
NERNSTA THEOREM- the same as Third law of thermodynamics.
NERNSTA EFFECT(longitudinal galvanothermomagnetic effect) - the appearance in the conductor, through which current flows j , located in the magnet. field H | j , temperature gradient T , directed along the current j ; temperature gradient does not change sign when field direction changes H to the opposite (even effect). Opened by W. G. Nernst (W. H. Nernst) in 1886. N. e. occurs as a result of the fact that current transfer (the flow of charge carriers) is accompanied by a heat flow. Actually N. e. represents Peltier effect under conditions when the temperature difference arising at the ends of the sample leads to compensation for the heat flux associated with the current j , the flow of heat due to thermal conductivity. N. e. observed also in the absence of a magnet. fields.
NERNSTA-ETTINGSHAUSEN EFFECT- the appearance of electricity. fields E ne in the conductor, in which there is a temperature gradient T
,
in a direction perpendicular to the magnetic field H
.
Distinguish between transverse and longitudinal effects.
Transverse H.-E. e. consists in the appearance of electricity. fields E ne |
(potential difference V ne |
) in a direction perpendicular to H
and T
. In the absence of a magnet. fields of thermoelectric the field compensates for the flow of charge carriers created by the temperature gradient, and compensation takes place only for the total current: electrons with an energy greater than the average (hot) move from the hot end of the sample to the cold one, electrons with an energy less than the average (cold) - in the opposite direction. The Lorentz force deflects these groups of carriers in a direction perpendicular to T
and magn. field, in different directions; the deflection angle (Hall angle) is determined by the relaxation time t of a given group of carriers, i.e., it differs for hot and cold carriers if t depends on the energy. In this case, the currents of cold and hot carriers in the transverse direction ( |
T
and |
H
) cannot cancel each other out. This gives rise to a field E |
ne ,
the value of which is determined from the condition of equality 0 of the total current j = 0.
Field value E |
does not depend on T, H and properties of the substance, characterized by the coefficient. Nernst-Ettingsha-Usen N |
:
AT semiconductors Under the influence T charge carriers of different signs move in the same direction, and in the magnetic. the field is deflected in opposite directions. As a result, the direction of the Nernst-Ettingshausen field created by charges of different signs does not depend on the sign of the carriers. This significantly distinguishes the transverse N.-E. e. from hall effect, where the direction of the Hall field is different for charges of different signs.
Since the coefficient N |
is determined by the dependence of the relaxation time t of carriers on their energy, then N.-E. e. sensitive to the mechanism scattering of charge carriers. Scattering of charge carriers reduces the influence of the magnetic. fields. If t ~ , then at r> 0 hot carriers scatter less often than cold ones and the direction of the field E |
ne is determined by the direction of deflection in magn. field of hot carriers. At r < 0 направление E |
ne is opposite and is determined by cold carriers.
AT metals, where the current is carried by electrons with energies in the interval ~ kT near Fermi surfaces, magnitude N |
is given by the derivative d t /d.
on the Fermi surface = const (usually for metals N |
> 0, but, for example, copper N |
< 0).
Measurements N.-E. e. in semiconductors allow you to determine r, i.e. restore the function t(). Usually at high temp-pax in the area of own. semiconductor conductivity N |
<
0 due to the scattering of carriers on the optical. phonons. When the temperature drops, an area appears with N |
> 0, corresponding to the impurity conductivity and scattering of carriers Chap. arr. on phonons ( r< < 0). При ещё более низких T ionization scattering dominates. impurities with N |
< 0 (r > 0).
In weak magnetic fields (w with t<< 1, где w с - cyclotron frequency carriers) N |
does not depend on H. In strong fields (w c t >> 1) coefficient. N |
proportional one/ H 2. In anisotropic conductors, the coefficient. N |
-
tensor. By the amount N |
affect the drag of electrons by photons (increases N |
),
anisotropy of the Fermi surface, etc.
Longitudinal H.-E. e. consists in the occurrence of electric-rich. fields E || ne (potential difference V || ne) along T
in the presence of H
|
T
. Because along T
there is a thermo-electric. field E a =
a T
,
where a is the coefficient. thermoelectric fields, then the appearance will complement. fields along T
is equivalent to changing the field E a .
when applying a magnet. fields:
Magn. field, bending the trajectories of electrons (see above), reduces their mean free path l in the direction T . Since the mean free path (relaxation time t) depends on the energy of the electrons, the decrease l is not the same for hot and cold carriers: it is smaller for the group for which m is smaller. T. o., magn. field changes the role of fast and slow carriers in energy transfer, and thermoelectric. the field that ensures the absence of charge during energy transfer must change. At the same time, the coefficient N || also depends on the carrier scattering mechanism. Thermoelectric the current increases if m decreases with increasing carrier energy (during scattering of carriers by acoustic phonons), or decreases if m increases with increasing (during scattering by impurities). If electrons with different energies have the same t, the effect disappears ( N|| = 0). Therefore, in metals, where the energy range of electrons involved in the transfer processes is small (~ kT), N || small: In a semiconductor with two types of carriers N ||~ ~ g/kT. At low temp-pax N|| can also increase due to the influence of electron drag by phonons. In strong magnetic fields total thermoelectric field in magn. the field "saturates" and is independent of the carrier scattering mechanism. In ferromagnet. metals N.-E. e. has features associated with the presence of spontaneous magnetization.
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The content of the article
NERVOUS SYSTEM, a complex network of structures that permeates the entire body and ensures self-regulation of its vital activity due to the ability to respond to external and internal influences (stimuli). The main functions of the nervous system are the receipt, storage and processing of information from the external and internal environment, the regulation and coordination of the activities of all organs and organ systems. In humans, as 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 that form 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 neurilemma, consisting mainly of specialized, so-called. Schwann cells, participates in the formation of sheaths of peripheral nerve fibers; connective tissue supports and links 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 of the body, includes cranial and spinal nerves, as well as nerve nodes (ganglia) and nerve plexuses that lie outside the spinal cord and brain.
Neuron.
The structural and functional unit of the nervous system is a nerve cell - a 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., a 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 the muscles, glands, or other neurons, while the dendrites carry them to the cell body.
In a neuron, as in other cells, there is a nucleus and a number of tiny structures - organelles ( see also CELL). These include the endoplasmic reticulum, ribosomes, Nissl bodies (tigroid), mitochondria, the 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, which spread throughout the neuron. Transmitted electrical changes are called nerve impulses. Unlike a simple electric discharge, which, due to the resistance of the neuron, will gradually weaken and be able to overcome only a short distance, a much slower “running” nerve impulse is constantly restored (regenerates) in the process of propagation.
The concentrations of ions (electrically charged atoms) - mainly sodium and potassium, as well as organic substances - outside the neuron and inside it are not the same, so the nerve cell at rest is negatively charged from the inside, and positively from the outside; as a result, a potential difference arises on the cell membrane (the so-called "resting potential" is approximately -70 millivolts). Any change that reduces the negative charge inside the cell and thereby the potential difference across the membrane is called depolarization.
The plasma membrane surrounding a neuron is a complex formation consisting of lipids (fats), proteins and carbohydrates. It is practically impermeable to ions. But some of the protein molecules in the membrane form channels through which certain ions can pass. However, these channels, called ionic channels, are not always open, but, like gates, they can open and close.
When a neuron is stimulated, some of the sodium (Na +) channels open at the point of stimulation, due to which sodium ions enter the cell. The influx of these positively charged ions reduces the negative charge of the inner surface of the membrane in the region of the channel, which leads to depolarization, which is accompanied by a sharp change in voltage and a discharge - a 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 leave 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 ensure the movement of Na + from the cell, and K + into the cell, which, along with the activity of potassium channels, restores the initial electrochemical state (resting potential) at the point of stimulation.
Electrochemical changes at the point of stimulation cause depolarization at the adjacent point of the membrane, triggering the same cycle of changes in it. This process is constantly repeated, and at each new point where depolarization occurs, an impulse of the same magnitude is born as at the previous point. Thus, together with the renewed electrochemical cycle, the nerve impulse propagates 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 clusters surrounded by specialized connective tissue, which contains vessels that supply the nerve fibers with nutrients and oxygen and remove carbon dioxide and waste products. Nerve fibers along which impulses propagate 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. Ganglion (ganglion) is a cluster of neuron bodies in the peripheral nervous system.
Axon fibers in the PNS are surrounded by a neurilemma - a sheath of Schwann cells that are located along the axon, like beads on a thread. A significant number of these axons are covered with an additional sheath of myelin (a protein-lipid complex); they are called myelinated (meaty). Fibers that are surrounded by neurilemma cells, but not covered with a myelin sheath, are called unmyelinated (non-myelinated). Myelinated fibers are found only in vertebrates. The myelin sheath is formed from the plasma membrane of the Schwann cells, which winds around the axon like a roll of ribbon, forming layer upon layer. The area of the axon where two adjacent Schwann cells touch each other is called the node of Ranvier. In the CNS, 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 sheaths of any special cells.
The myelin sheath accelerates 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 the thickening of the myelin sheath and ranges from 2 m / s (along unmyelinated fibers) to 120 m / s (along 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 a specialized connection to muscles, glands, or other neurons. The zone of functional contact between two neurons is called a synapse. Interneuronal 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 is slit-shaped. 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 through the cleft and bind to receptors on the membrane of the postsynaptic neuron. If the neurotransmitter stimulates the 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 at a given moment.
In a number of animals (for example, in the spiny lobster), a particularly close connection is established between the neurons of certain nerves with the formation of either an unusually narrow synapse, the so-called. gap junction, or, if 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, by electrical transmission. A few dense junctions of neurons are also found in mammals, including humans.
Regeneration.
By the time a person is born, all of his neurons and most of the interneuronal connections have already been formed, and later only single 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 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 with the cell body grows along this tube until it reaches the nerve ending. Thus, the function of the damaged neuron is restored. Axons in the CNS that are not surrounded by a neurilemma are apparently unable to grow back to the site of their former termination. However, many CNS neurons can give rise to new short processes - branches of axons and dendrites that form new synapses. see also REGENERATION.
CENTRAL NERVOUS SYSTEM
The CNS 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 soft and arachnoid membranes is the subarachnoid (subarachnoid) space containing the cerebrospinal (cerebrospinal) fluid, in which both the brain and the spinal cord literally float. The action of the buoyancy force of the fluid leads to the fact that, for example, the adult brain, which has an average weight 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 experiences the body and which could cause damage to the nervous system.
The CNS is made up of gray and white matter. Gray matter is made up of cell bodies, dendrites, and unmyelinated axons, organized into complexes that include countless synapses and serve as information processing centers for many of the functions of the nervous system. White matter consists of myelinated and unmyelinated axons, which act as conductors that transmit impulses from one center to another. The composition of gray and white matter also includes 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 by passing through a sequence of centers connected by long axons, which form specific pathways, such as pain, visual, auditory. Sensitive (ascending) pathways go in an ascending direction to the centers of the brain. Motor (descending) pathways connect the brain with the motor neurons of the cranial and spinal nerves.
Pathways are usually organized in such a way that information (for example, pain or tactile) from the right side of the body goes to the left side of the brain and vice versa. This rule also applies to 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. There are a few exceptions to this general rule, however.
Brain
consists of three main structures: the cerebral hemispheres, the cerebellum and the trunk.
The cerebral hemispheres - the largest part of the brain - contain higher nerve centers that form the basis of consciousness, intellect, personality, speech, and understanding. In each of the large hemispheres, the following formations are distinguished: isolated accumulations (nuclei) of gray matter lying in the depths, which contain many important centers; a large array of white matter located above them; covering the hemispheres from the outside, a thick layer of gray matter with numerous convolutions, constituting the cerebral cortex.
The cerebellum also consists of a deep gray matter, an intermediate array of white matter, and an outer thick layer of gray matter that forms many convolutions. The cerebellum provides mainly 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. On a transverse section, the gray matter has the shape of the letter H or a butterfly. Gray matter is surrounded by white matter. The sensory fibers of the spinal nerves end in the dorsal (posterior) sections of the gray matter - the posterior horns (at the ends of H facing the back). The bodies of the motor neurons of the spinal nerves are located in the ventral (anterior) sections of the gray matter - the anterior horns (at the ends of H, remote 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 the different parts of the gray matter of the spinal cord.
PERIPHERAL NERVOUS SYSTEM
The PNS provides a two-way connection between the central parts of the nervous system and the organs and systems of the body. Anatomically, the PNS is represented by cranial (cranial) and spinal nerves, as well as a relatively autonomous enteric nervous system localized in the intestinal wall.
All cranial nerves (12 pairs) are divided into motor, sensory or mixed. The motor nerves originate in the motor nuclei of the trunk, formed by the bodies of the motor neurons themselves, and the sensory nerves are formed from the fibers of those neurons whose bodies lie in the 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 foramen from which these nerves emerge. Each spinal nerve has an anterior and a posterior root that merges to form the nerve itself. The back root contains sensory fibers; it is closely related to the spinal ganglion (posterior root ganglion), which consists of the bodies of neurons whose axons form these fibers. The anterior root consists of motor fibers formed by neurons whose cell bodies lie in the spinal cord.
| cranial nerves | |||
| Number | Name | Functional characteristic | Innervated structures |
| I | Olfactory | Special sensory (smell) | Olfactory epithelium of the nasal cavity |
| II | Visual | Special touch (vision) | Rods and cones of the retina |
| III | Oculomotor | Motor | Most of the external muscles of the eye Smooth muscles of the iris and lens |
| IV | Blocky | Motor | Superior oblique muscle of the eye |
| V | ternary | All-sensory Motor |
Skin of the face, mucous membrane of the nose and mouth Chewing muscles |
| VI | diverting | Motor | External rectus eye |
| VII | Facial | Motor visceromotor Special touch |
Mimic muscles Salivary glands Taste buds of the tongue |
| VIII | vestibulocochlear | Special touch Vestibular (balance) Auditory (hearing) |
Semicircular canals and spots (receptor sites) of the labyrinth Auditory organ in the cochlea (inner ear) |
| IX | Glossopharyngeal | Motor visceromotor Viscerosensory |
Muscles of the posterior wall of the pharynx Salivary glands Receptors for taste and general sensitivity in the back parts of the mouth |
| X | Wandering | Motor visceromotor Viscerosensory All-sensory |
Muscles of the larynx and pharynx heart muscle, smooth muscle, lung glands, bronchi, stomach and intestines, including digestive glands Receptors in large blood vessels, lungs, esophagus, stomach, and intestines outer ear |
| XI | Additional | Motor | Sternocleidomastoid and trapezius muscles |
| XII | Sublingual | Motor | Muscles of the tongue |
| The definitions "visceromotor", "viscerosensory" indicate the connection of the corresponding nerve with the internal (visceral) organs. | |||
AUTONOMIC SYSTEM
The autonomic, or autonomic, nervous system regulates the activity of the involuntary muscles, the heart muscle, and various glands. Its structures are located both in the central nervous system and in the peripheral. The activity of the autonomic nervous system is aimed at maintaining homeostasis, i.e. a relatively stable state of the internal environment of the body, such as a constant body temperature or blood pressure corresponding to the needs of the body.
Signals from the CNS arrive at the working (effector) organs through pairs of series-connected neurons. The bodies of neurons of the first level are located in the CNS, and their axons terminate in the autonomic ganglia lying outside the CNS, and here they form synapses with the bodies of neurons of the second level, the axons of which directly contact the effector organs. The first neurons are called preganglionic, the second - postganglionic.
In that part of the autonomic nervous system, which is called the sympathetic, the 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 thoraco-lumbar 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 are in contact with 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 pregangliar neurons are located in the brainstem (intracranial, i.e. inside the skull) and the sacral (sacral) section of the spinal cord. Therefore, the parasympathetic system is also called the craniosacral system. Axons of preganglionic parasympathetic neurons terminate and form synapses with postganglionic neurons in the 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 the cholinergic system.
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 "fright, 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 arises in it, triggering a response action called a reflex act (reflex). Reflexes underlie most of the manifestations of the vital activity 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.
The arc of the reflex that causes contraction of the 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 the neurons of the spinal cord or brain stem, and the motor (lower, or peripheral, motor neuron), whose body is located in gray matter, and the axon terminates in a motor end plate on skeletal muscle fibers.
The reflex arc between the sensory and motor neurons can also include a third, intermediate, neuron located in the gray matter. The arcs of many reflexes contain two or more intermediate neurons.
Reflex actions are carried out involuntarily, many of them are not realized. The knee jerk, for example, is elicited 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 reflex withdrawal of a hand from a hot object: the arc of this reflex includes a sensory neuron, one or more intermediate neurons in the gray matter of the spinal cord, a peripheral motor neuron, and a muscle.
Literature:
Bloom F., Leizerson A., Hofstadter L. Brain, mind and behavior. M., 1988
human physiology, ed. R. Schmidt, G. Tevsa, vol. 1. M., 1996
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 constantly, day and night, receives 20% of the blood circulating in the body and the corresponding amount of oxygen.