106 A.D.

We are now entering the Christian era and can henceforth not mention “before” and “after” the Nativity of Christ, as we have done until now, in order to avoid confusion.

In 106 the emperor Trajan conquered Dacia. This country roughly corresponds to modern Romania. It was located north of the Danube - the border of the empire - and included the Carpathian mountain range.

The bas-reliefs of Trajan's Column in Rome depict the main episodes of this victorious campaign.

New province of Dacia will be partially colonized by settlers from all parts of the empire, they will take Latin as their language of communication, and it will give rise to Romanian language- the only Latin-based language of the eastern half of the empire. And this despite the fact that Greek culture predominated here.

Critical date

Why did we choose this date?

In the first century AD, the emperors continued the aggressive policy of the Republic, although not on such a scale as before.

Augustus captured Egypt, completed the conquest of Spain, and subdued the rebellious populations of the Alps, making the Danube the frontier of the empire.

To protect Gaul from barbarian invasions, he planned to conquer Germany, the territory between the Rhine and Elbe. At first he succeeds thanks to the defeat of his sons-in-law Drusus and Tiberius.

However, in 9 AD, the Germans rebelled under the leadership of Arminius (Hermann) and destroyed the legions of the legate Varus in the Teutoburg Forest. This catastrophe greatly agitated Augustus (they say that he cried, repeating: "Var, give me back my legions"), forced him, like his heirs, to refuse to move the border along the Rhine. For more than two centuries, the Rhine and Danube (linked in the upper reaches between Mainz and Rotisbon by a fortified wall) formed the border of the empire in continental Europe. In 43, Emperor Claudius annexed Britannia (modern England), which became a Roman province.

The conquest of Dacia in 106 was the last major territorial acquisition of the Roman emperors. After this date, the boundaries remained unchanged for more than a century.

Roman world

The first two centuries of the empire, corresponding approximately to the first two centuries of our era, were a period inner world and prosperity.

Limes- systems of border fortifications, along which legions stood, ensured security, which made it possible to develop trade relations and the economy.

New cities are built and developed according to the model of Rome: they have an autonomous administration with a Senate and elected magistrates. But in reality, as in Rome, power belongs to the rich, not without certain responsibilities on their part. So, they must build water pipelines at their own expense, public buildings: temples, baths, circuses or theaters - and also pay for circus performances.

This roman world cannot be idealized; brutally exploited provinces often rebel. We saw this in Judea. But these uprisings are constantly suppressed by the Roman army.

While wealth and slaves flocked to Rome through conquest or forays on the borders, a certain economic and social balance was maintained.

When the conquests stopped and attacks by “barbarians” (those who lived outside the empire) on Roman lands became more frequent An economic and social crisis is unfolding.

The “middle class” is supplying fewer and fewer citizen soldiers, so the Roman army is increasingly replenished with mercenaries, often these are barbarian immigrants who receive Roman citizenship or a piece of land at the end of their service.

After the reign of Augustus, imperial power became a stake in the struggle of rival armies located on various frontiers (on the Rhine, Danube and in the East), all too often called upon to march on Rome in order to install their commander on the throne. Due to these internal turmoil the borders are often left defenseless and subject to attacks by barbarians.

Crisis of the 3rd century

Difficulties begin during the reign of Marcus Aurelius (161–180), a philosopher-emperor who expounds a humanistic philosophy in his Pensées. The peace-loving emperor is forced to spend most of his time repelling attacks on the borders of the state.

After his death, attacks from outside and internal unrest become more frequent.

In the 3rd century. begins a period called Late Empire.

The edict of Emperor Caracalla (212), according to which all free inhabitants of the empire received Roman citizenship, becomes the starting point in the evolution of the gradual merging of the “provincials” and the Romans.

Between 224 and 228 The Parthian Empire fell under the blows of the Sassanids, the founders of the new dynasty of the Persian Empire. This state would become a dangerous enemy for the Romans - Emperor Valerian would be captured by the Persians in 260 and die in captivity.

At the same time, due to internal rebellions and political instability (from 235 to 284, i.e. in 49 years, there were 22 emperors) barbarians enter the empire for the first time.

In 238 goths, Germanic tribe, first crossed the Danube and invaded the Roman provinces of Moesia and Thrace. From 254 to 259 another Germanic tribe, Alemanni, penetrates into Gaul, then into Italy and reaches the gates of Milan. Previously open, Roman cities build protective walls, including Rome, where Emperor Aurelian begins in 271 the construction of a fortress wall, the first after the one that once existed in the Rome of the kings.

The economic crisis manifests itself in a monetary crisis: due to a shortage of silver Emperors mint coins of low standard, in which the noble metal content is sharply reduced. As the value of such money falls, it happens price inflation.

Diocletian(284–305) tries to save the empire by reorganizing it. Considering that one person cannot ensure the defense of all borders, he divides the empire into four parts: in Milan and Nicomedia two emperors and their two assistants - “Caesars” appear, they are the deputies and heirs of the emperors.

End of the Roman Empire

In 326 the emperor Konstantin moves to Byzantium, a Greek city that controls the Bosphorus Strait, which connects the Black Sea to the Mediterranean. He gives this city his name, baptizing Constantinople(the city of Constantine), and makes it a “second Rome”.

In 395 the Roman Empire was finally divided into Western Roman Empire, which will disappear in 476 under the blows of the barbarians, and Eastern Roman Empire, which will exist for another thousand years (until the capture of Constantinople by the Turks in 1453). However, the latter will very soon become a country Greek culture, and it will begin to be called the Byzantine Empire.

In 454, Emperor Valentinian III executed his brilliant but capricious commander Aetius, and a year later he himself was killed. The next twenty years proved to be a period of political chaos: no less than eight emperors were enthroned and deposed - either on the initiative of the Roman Senate aristocracy, or at the instigation of the Eastern emperor. On August 23, 476, German troops in Italy (which now made up the bulk of the Roman army) elected their commander Odoacer as king and deposed the last Western emperor, Romulus Augustulus (Augustulus’s government refused to allocate a third of the lands to the soldiers - that’s exactly how much the Roman “allies” in Gaul received) .

This event marked the end of the Roman Empire in the West. Formally, the entire territory of the empire was now ruled by the eastern emperor Zeno. In fact, Odoacer, hated by the Roman aristocracy and not recognized by Constantinople, became the independent ruler of Italy.

Ostrogoths in Italy

Zeno did not have the opportunity to reconquer Italy, but he still took revenge on Odoacer. The Ostrogoths, defeated and enslaved by the Huns, eventually, like the Visigoths, moved into the Balkan provinces of the empire. In 488, Zeno convinced their leader, Theodoric, to march from Moesia (modern Serbia) to Italy. This was a clever move on the part of the emperor: whoever won in Italy, the Eastern Empire would at least get rid of the last tribe of barbarians that was still in its provinces.

By 493, the Ostrogoths occupied Italy, Odoacer was dead (according to stories, Theodoric himself killed him). Formally, Theodoric, as the emperor's viceroy, received the title of patrician, but in reality he remained as independent as the other barbarian leaders.

Roman Empire in the East: Justinian

The departure of the Ostrogoths to Italy liberated the eastern part of the Roman Empire from the last barbarian tribe that invaded its territory in the 5th century. In the next, VI century. Graeco-Roman civilization once again demonstrated its vitality, and the military and administrative organization of the empire proved remarkably flexible and capable of responding effectively to the demands of the situation. The great cities of the empire - Alexandria, Antioch, Caesarea and Jerusalem - did not lose their power. The merchants of these cities continued to outfit ships throughout the Mediterranean and down the Red Sea to East Africa, Ceylon and even further.

Byzantine (i.e. Roman) gold coin- solidus (on which the image of the emperor was minted) - went throughout the civilized world, from Ireland to China. Caravans crossed the vast Asian continent along a route equipped with numerous inns. One of these caravans smuggled silkworms out of China, and soon their own silk production flourished in Cyprus and other parts of the empire. For the rich townspeople, life remained much the same as it had been for many centuries. Young people received both classical and religious education at academies and universities. Christianity, which had been under the protection and patronage of the state for three centuries, showed its wealth in hundreds of churches, decorated with luxurious lamps, sculptures and mosaics.

However, Constantinople, the capital of the empire, became the largest and richest city. Mindful of the fate that befell Rome in 410, the emperors surrounded Constantinople with a system of defensive walls with towers that protected it from both land and sea. These walls successfully withstood all attacks until 1204, when the Crusaders treacherously broke into the city and captured it. As before in Rome, so now in Constantinople, the emperors had to pursue a certain policy towards the inhabitants of the huge capital. As before, “bread and circuses” meant a public demonstration of the authorities’ interest in supporting the poorest masses. The fans at the hippodrome (a huge stadium for horse racing, chariot races and baiting of wild animals) were divided into “green” and “blue”. However, these were not just supporters of different teams, but also original parties that differed in political and religious views and were usually at odds. In 532, they united during anti-government riots and terrorized the city for several days. Justinian's advisers strongly recommended that he go into hiding. However, Justinian's wife, Theodora, convinced him to restore order, and the professional soldiers of the commander Belisarius mercilessly dealt with the rebels.

These riots were the last internal crisis of Justinian's reign. He went on to rule the empire as effectively as his predecessors, and even more autocratically, largely thanks to the advice of Empress Theodora. Justinian had complete control over the imperial bureaucracy and imposed taxes at his discretion. As supreme legislator and judge, he initiated the compilation of a code of imperial laws, the famous Corpus juris civilis(Vault civil law). In the first of its three parts, Codex Justinianus(Code of Justinian), all the decrees of the emperors from the time of Hadrian (117–138) to 533 were collected. Later edicts were introduced under the name novel lae(New laws). It was this last part of the “corpus” that contained the justification for the absolute power of the emperor. The second part, the Digests, or Pandects, in 50 books, included excerpts from the works and opinions of Roman jurists related to civil and criminal legislation. The third part, Institutions, was an abridged version of the first two parts, that is, a kind of law textbook. Probably no text of a secular nature had such a wide and lasting influence in Europe as Corpus juris civilis. In the subsequent period of the history of the Eastern Empire, it served as a comprehensive and rationally constructed system of legislation and the study of law. But the Code played a much more important role in the West, becoming the basis of canon and ecclesiastical law of the Roman Catholic Church. From the 12th century Justinian's legislation gradually began to dominate secular courts and law schools and eventually almost replaced common law in most European countries. Thanks to Roman law, Justinian's autocracy served as the intellectual basis for the absolutism of Western monarchies in the 16th, 17th, and 18th centuries. Even in countries like England, where customary local law has survived, the development of systematic and rational jurisprudence, legal science and legal philosophy would probably have been impossible without the historical model - Corpus juris civilis .

A visible expression of the greatness of the emperor and the Christian church (which was actually headed by the emperor) was the reconstruction of the Church of St. Sophia (Divine Wisdom), which burned down during the riots of 532. Justinian invited the best architects, mathematicians and craftsmen from all over the empire to the capital, who erected the most grandiose and the magnificent temple of Christendom. Even now, its huge flat dome dominates the panorama of Istanbul (the current name of Constantinople). Justinian's court historian Procopius of Caesarea left us a description of the stunning interiors of the temple, written in the characteristic rhetorical style of the time; it allows us to understand the specifics of Byzantine religiosity in the 6th century.

An unusual amount of sunlight penetrates into it, which is also reflected from the marble walls. Indeed, one could say that it is not so much illuminated by the sun from the outside as it shines from the inside - its altar is bathed in such an abundance of light... Its entire ceiling is entirely trimmed with pure gold - which makes its beauty majestic. However, most of all, the light is reflected from the stone surfaces, competing with the shine of gold... Who has enough words to adequately describe the galleries of the female side and the colonnades of the side chapels that surround the temple? Who can describe all the beauty of the columns and colored stones that adorn it? You can imagine that you are in the middle of a meadow, replete with the most beautiful flowers: some of them are distinguished by an amazing purple color, others are green, others glow crimson, others are dazzling white, and others, like an artist’s palette, sparkle with the most different colors. And when a person enters this temple to offer prayer, he immediately realizes that it was not through human strength or human skill, but through the care of God that this creation was born so beautiful. And then his spirit rushes to God and rises, feeling that He cannot be far away, but must willingly remain in the dwelling that He has chosen for Himself 24.

Majestic splendor, softened by beauty, light and divine love - such was the legacy of the emperor, who considered himself God's vicegerent on earth. This largely explains the long existence of the Roman Empire in the East.

Candidate of Biological Sciences L. Chailakhyan, researcher at the Institute of Biophysics of the USSR Academy of Sciences

Magazine reader L. Gorbunova (village of Tsybino, Moscow region) writes to us: “I am interested in the mechanism of signal transmission through nerve cells.”

Laureates Nobel Prize 1963 (from left to right): A. Hodgkin, E. Huxley, D. Eccles.

Scientists' ideas about the mechanism of nerve impulse transmission have recently undergone significant changes. Until recently, Bernstein's views dominated science.

The human brain is, without a doubt, the highest achievement of nature. In kilogram nerve tissue contains the quintessence of the whole person, starting from the regulation of vital functions - the work of the heart, lungs, digestive tract, liver - and ending with the spiritual world. Here are our thinking abilities, our entire perception of the world, memory, reason, our self-awareness, our “I”. Knowing the mechanisms of how the brain works is knowing yourself.

The goal is great and tempting, but the object of research is incredibly complex. Just kidding, this kilogram of fabric represents the most complex system connections between tens of billions of nerve cells.

However, the first significant step towards understanding how the brain works has already been taken. It may be one of the easiest, but it is extremely important for everything that follows.

I mean the study of the mechanism of transmission of nerve impulses - signals running along the nerves, as if through wires. It is these signals that are the alphabet of the brain, with the help of which the sensory organs send to the central nervous system information-dispatches about events in the outside world. The brain encodes its orders to muscles and various things with nerve impulses. internal organs. Finally, individual nerve cells and nerve centers speak the language of these signals.

Nerve cells - the main element of the brain - are varied in size and shape, but in principle they have a single structure. Each nerve cell consists of three parts: a body, a long nerve fiber - an axon (its length in humans ranges from several millimeters to a meter) and several short branched processes - dendrites. Nerve cells are isolated from each other by membranes. But the cells still interact with each other. This happens at the junction of cells; this junction is called a “synapse”. At a synapse, the axon of one nerve cell and the body or dendrite of another cell meet. Moreover, it is interesting that excitation can be transmitted only in one direction: from the axon to the body or dendrite, but in no case back. A synapse is like a kenotron: it transmits signals in only one direction.

In the problem of studying the mechanism of a nerve impulse and its propagation, two main questions can be distinguished: the nature of the conduction of a nerve impulse or excitation within one cell - along a fiber, and the mechanism of transmission of a nerve impulse from cell to cell - through synapses.

What is the nature of the signals transmitted from cell to cell along nerve fibers?

People have been interested in this problem for a long time; Descartes assumed that the propagation of the signal was associated with the transfusion of fluid through the nerves, as if through tubes. Newton thought it was a purely mechanical process. When the electromagnetic theory appeared, scientists decided that a nerve impulse is analogous to the movement of current through a conductor at a speed close to the speed of propagation of electromagnetic oscillations. Finally, with the development of biochemistry, a point of view emerged that the movement of a nerve impulse is the propagation along a nerve fiber of a special biochemical reaction.

Yet none of these ideas came to fruition.

Currently, the nature of the nerve impulse has been revealed: it is a surprisingly subtle electrochemical process, which is based on the movement of ions through the cell membrane.

The work of three scientists made a major contribution to the discovery of this nature: Alan Hodgkin, professor of biophysics at the University of Cambridge; Andrew Huxley, Professor of Physiology, University of London, and John Eccles, Professor of Physiology, University of Canberra, Australia. They were awarded the Nobel Prize in Medicine for 1963.

The famous German physiologist Bernstein was the first to suggest the electrochemical nature of the nerve impulse at the beginning of this century.

By the early twentieth century, quite a lot was known about nervous excitation. Scientists already knew that a nerve fiber can be excited by electric current, and the excitation always occurs under the cathode - under the minus. It was known that the excited area of ​​the nerve is charged negatively in relation to the non-excited area. It was found that the nerve impulse at each point lasts only 0.001-0.002 seconds, that the magnitude of excitation does not depend on the strength of the irritation, just as the volume of the bell in our apartment does not depend on how hard we press the button. Finally, scientists have found that carriers electric current in living tissues are ions; Moreover, inside the cell the main electrolyte is potassium salts, and in the tissue fluid - sodium salts. Inside most cells, the concentration of potassium ions is 30-50 times higher than in the blood and in the intercellular fluid that washes the cells.

And based on all this data, Bernstein suggested that the membrane of nerve and muscle cells is a special semi-permeable membrane. It is permeable only to K + ions; for all other ions, including negatively charged anions inside the cell, the path is closed. It is clear that potassium, according to the laws of diffusion, will tend to leave the cell, an excess of anions appears in the cell, and a potential difference will appear on both sides of the membrane: outside - plus (excess cations), inside - minus (excess of anions). This potential difference is called the resting potential. Thus, at rest, in an unexcited state, the inside of the cell is always negatively charged compared to the outer solution.

Bernstein suggested that at the moment of excitation of the nerve fiber, structural changes occur in the surface membrane, its pores seem to increase, and it becomes permeable to all ions. In this case, naturally, the potential difference disappears. This causes a nerve signal.

Bernstein's membrane theory quickly gained recognition and existed for over 40 years, until the middle of our century.

But already at the end of the 30s, Bernstein's theory encountered insurmountable contradictions. It was dealt a major blow in 1939 by the subtle experiments of Hodgkin and Huxley. These scientists were the first to measure the absolute values ​​of the membrane potential of a nerve fiber at rest and during excitation. It turned out that upon excitation, the membrane potential did not simply decrease to zero, but crossed zero by several tens of millivolts. That is, the inner part of the fiber changed from negative to positive.

But it is not enough to overthrow a theory, we must replace it with another: science does not tolerate a vacuum. And Hodgkin, Huxley, Katz in 1949-1953 propose a new theory. It is called sodium.

Here the reader has the right to be surprised: until now there has been no talk about sodium. That's the whole point. Scientists have established with the help of labeled atoms that not only potassium ions and anions are involved in the transmission of nerve impulses, but also sodium and chlorine ions.

There are enough sodium and chlorine ions in the body; everyone knows that blood tastes salty. Moreover, there is 5-10 times more sodium in the intercellular fluid than inside the nerve fiber.

What could this mean? Scientists have suggested that upon excitation, at the first moment, the permeability of the membrane only to sodium sharply increases. The permeability becomes tens of times greater than for potassium ions. And since there is 5-10 times more sodium outside than inside, it will tend to enter the nerve fiber. And then the inside of the fiber will become positive.

And after some time - after excitation - equilibrium is restored: the membrane begins to allow potassium ions to pass through. And they go outside. Thus, they compensate for the positive charge that was introduced into the fiber by sodium ions.

It was not at all easy to come to such ideas. And here's why: the diameter of the sodium ion in solution is one and a half times larger than the diameter of potassium and chlorine ions. And it is completely unclear how a larger ion passes where a smaller one cannot pass.

It was necessary to radically change the view on the mechanism of ion transition through membranes. It is clear that reasoning about pores in the membrane alone is not sufficient here. And then the idea was put forward that ions could cross the membrane in a completely different way, with the help of secret allies for the time being - special organic carrier molecules hidden in the membrane itself. With the help of such a molecule, ions can cross the membrane anywhere, not just through the pores. Moreover, these taxi molecules distinguish their passengers well; they do not confuse sodium ions with potassium ions.

Then the general picture of the propagation of a nerve impulse will look like this. At rest, carrier molecules, negatively charged, are pressed to the outer boundary of the membrane by the membrane potential. Therefore, the permeability for sodium is very small: 10-20 times less than for potassium ions. Potassium can cross the membrane through pores. As the excitation wave approaches, the pressure of the electric field on the carrier molecules decreases; they throw off their electrostatic “shackles” and begin to transfer sodium ions into the cell. This further reduces the membrane potential. There is a kind of chain process of recharging the membrane. And this process continuously spreads along the nerve fiber.

Interestingly, nerve fibers spend only about 15 minutes a day on their main job - conducting nerve impulses. However, the fibers are ready for this at any second: all elements of the nerve fiber work without interruption - 24 hours a day. Nerve fibers in this sense are similar to interceptor aircraft, whose motors are continuously running for instant departure, but the departure itself can only take place once every few months.

We have now become acquainted with the first half of the mysterious act of passing a nerve impulse along one fiber. How is excitation transmitted from cell to cell, through junctions - synapses? This question was explored in the brilliant experiments of the third Nobel laureate, John Eccles.

Excitation cannot directly transfer from the nerve endings of one cell to the body or dendrites of another cell. Almost all of the current flows through the synaptic cleft into the outer fluid, and a tiny fraction of it enters the neighboring cell through the synapse, unable to cause excitation. Thus, in the region of synapses, the electrical continuity in the propagation of the nerve impulse is disrupted. Here, at the junction of two cells, a completely different mechanism comes into force.

When excitation approaches the end of the cell, the site of the synapse, physiologically active substances - mediators, or intermediaries - are released into the intercellular fluid. They become a link in the transfer of information from cell to cell. The mediator chemically interacts with the second nerve cell, changes the ionic permeability of its membrane - as if punching a hole into which many ions rush, including sodium ions.

So, thanks to the work of Hodgkin, Huxley and Eccles, the most important states of a nerve cell - excitation and inhibition - can be described in terms of ionic processes, in terms of structural and chemical rearrangements of surface membranes. Based on these works, it is already possible to make assumptions about the possible mechanisms of short-term and long-term memory, and about the plastic properties of nervous tissue. However, this is a conversation about mechanisms within one or more cells. This is just the ABC of the brain. Apparently, the next stage, perhaps much more difficult, is the discovery of the laws by which the coordinating activity of thousands of nerve cells is built, the recognition of the language that the nerve centers speak among themselves.

In our knowledge of how the brain works, we are now at the level of a child who has learned the letters of the alphabet, but does not know how to connect them into words. However, the time is not far when scientists, using the code - elementary biochemical acts occurring in a nerve cell, will read the most fascinating dialogue between the nerve centers of the brain.

Detailed description of illustrations

Scientists' ideas about the mechanism of nerve impulse transmission have recently undergone significant changes. Until recently, Bernstein's views dominated science. In his opinion, in a state of rest (1) the nerve fiber is charged positively on the outside and negatively on the inside. This was explained by the fact that only positively charged potassium ions (K +) can pass through the pores in the fiber wall; Large negatively charged anions (A –) are forced to remain inside and create an excess of negative charges. Excitation (3) according to Bernstein is reduced to the disappearance of the potential difference, which is caused by the fact that the pore size increases, anions come out and equalize the ionic balance: the number of positive ions becomes equal to the number of negative ones. The work of 1963 Nobel Prize winners A. Hodgkin, E. Huxley and D. Eccles changed our previous ideas. It has been proven that nervous stimulation also involves positive ions sodium (Na +), negative chlorine (Cl –) and negatively charged carrier molecules. The resting state (3) is formed in principle in the same way as was previously thought: an excess of positive ions is outside the nerve fiber, an excess of negative ones is inside. However, it has been established that during excitation (4) it is not the equalization of charges that occurs, but a recharging: an excess of negative ions is formed outside, and an excess of positive ions inside. This is explained by the fact that when excited, carrier molecules begin to transport positive sodium ions through the wall. Thus, the nerve impulse (5) is a recharge of the electrical double layer moving along the fiber. And from cell to cell, excitation is transmitted by a kind of chemical “battering ram” (6) - an acetylcholine molecule, which helps ions break through the wall of the neighboring nerve fiber.

NERVOUS IMPULSE

NERVOUS IMPULSE

A wave of excitation, edges, spreads along the nerve fiber and serves to transmit information from the peripheral. receptor (sensitive) endings to the nerve centers, inside the center. nervous system and from it to the executive apparatus - muscles and glands. Passage of N. and. accompanied by transitional electrical processes that can be recorded with both extracellular and intracellular electrodes.

Generation, transmission and processing of N. and. carried out by the nervous system. Basic structural element The nervous system of higher organisms is a nerve cell, or neuron, consisting of a cell body and numerous. processes - dendrites (Fig. 1). One of the processes in non-riferiforms. neurons have 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 non-pulphate. The pulp fibers have myelin, formed by special fibers. membrane, the edges, like insulation, are wound onto the axon. The length of the sections of the continuous myelin sheath ranges from 200 µm to 1 mm, they are interrupted by the so-called. nodes of Ranvier 1 µm wide. The myelin sheath plays an insulating role; the nerve fiber in these areas is passive, electrically active only in the nodes of Ranvier. Non-pulp fibers are not insulated. plots; their structure is uniform along the entire length, and the membrane is electrically activity over the entire surface.

Nerve fibers end on the bodies or dendrites of other nerve cells, but are separated from them intermediately.

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 membrane of dendrites or muscles is post-synaptic (see. Cellular structures).

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

Speed ​​of spread 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 nerve cell body or nerve fiber. N. and. always has the same characteristics (shape and speed) regardless of the strength of stimulation, i.e., with subthreshold stimulation of N. and. does not occur at all, but when above the threshold it has full amplitude.

After excitation, a refractory period begins, during which the excitability of the nerve fiber is reduced. There are abs. the refractory period, when the fiber cannot be excited by any stimuli, and refers. refractory period, when possible, but its threshold is higher than normal. Abs. the refractory period limits from above the frequency of transmission of N. and. The nerve fiber has the property of accommodation, that is, it gets used to constant stimulation, which is expressed in a gradual increase in the threshold of excitability. This leads to a decrease in the frequency of N. and. and even to their complete disappearance. If stimulation increases slowly, then arousal may not occur even after reaching the threshold.

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

Along the nerve fiber N. and. spreads in the form of electricity. potential. At the synapse, the propagation mechanism changes. When N. and. reaches presynaptic. endings, in synaptic. the gap releases an active chemical. - M e d i a t o r. The transmitter diffuses through the synaptic. gap and changes the permeability of postsynaptic. membrane, as a result of which it appears on it, again generating spreading. This is how chem works. synapse. There is also electric. synapse when . the neuron is excited electrically.

Excitement N. and. Phys. ideas about the appearance of electricity. potentials in cells are based on the so-called. membrane theory. Cell membranes separate electrolyte of different concentrations and have a birate. permeability for certain ions. Thus, the axon membrane is thin layer lipids and proteins ~7 nm thick. Her electric Resistance at rest ~ 0.1 Ohm. m 2, and the capacity is ~ 10 mf/m 2. Inside the axon there is a high concentration of K + ions and a low concentration of Na + and Cl - ions, and in environment- vice versa.

In the resting state, the axon membrane is permeable to K + ions. Due to the difference in concentrations C 0 K . in ext. and C in internal solutions, the potassium membrane potential is established on the membrane

Where T - abs. temp-pa, e - electron charge. A resting potential of ~ -60 mV is indeed observed on the axon membrane, corresponding to the indicated value.

Na + and Cl - ions penetrate the membrane. To maintain the necessary non-equilibrium distribution of ions, the cell uses an active transport system, which consumes cellular energy for work. Therefore, the resting state of the nerve fiber is not thermodynamically equilibrium. It is stationary due to the action of ion pumps, and the membrane potential under open-circuit conditions is determined from the equality to zero of the total electric current. current

The process of nervous excitation develops as follows (see also Biophysics). If you pass a weak current pulse through the axon, leading to depolarization of the membrane, then after removing the external. impact, the potential monotonically returns to its original level. Under these conditions, the axon behaves as a passive electrical current. circuit consisting of a capacitor and DC. resistance.

Rice. 2. Development of action potential in the nervous systemlocke: A- subthreshold ( 1 ) and suprathreshold (2) irritation; b-membrane response; with above-threshold stimulation, full sweat occursaction cial; V- 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 resting level, and at first it even jumps a little (hyperpolarization region, Fig. 2). The response of the membrane does not depend on the disturbance; this impulse is called action potential. At the same time, an ionic current flows through the membrane, directed first inward and then outward (Fig. 2, V).

Phenomenological interpretation of the mechanism of occurrence of N. and. was given by A. L. Hodgkin and A. F. Huxley in 1952. The total ion current is composed of three components: potassium, sodium and leakage current. When the membrane potential shifts by a threshold value j* (~ 20 mV), 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, potassium begins to develop (and at the same time sodium decreases). As a result, the sodium current is replaced by a potassium current directed outward. This current corresponds to a decrease in the action potential.

Established empirically. equation for describing sodium and potassium currents. The behavior of the membrane potential during spatially uniform excitation of the fiber is determined by the equation:

Where WITH - membrane capacity, I- ion current, consisting of potassium, sodium and leakage current. These currents are determined by the post. emf j K , j Na and j l and conductivities g K, g Na and gl:

Size 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 linear equations

Dependence of coefficient a . and b from the membrane potential j (Fig. 3) are selected from the best fit condition

Rice. 3. Dependence of coefficientsa. Andbfrom membranesgreat potential.

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

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

Distribution of N. and. N. and. can propagate along the fiber without attenuation and with DC. speed. This is due to the fact that the energy necessary for signal transmission does not come from a single center, but is drawn locally, at each point of the fiber. In accordance with the two types of fibers, there are two ways of transmitting N. and.: continuous and saltatory (spasmodic), when the impulse moves from one node of Ranvier to another, jumping over areas of myelin insulation.

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

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

Type of functionality I specific for a biologically excitable environment. However, equation (5), if we ignore the type I, has more general character and describes many physical phenomena, for example combustion process. Therefore, N.’s transmission and. likened to the burning of a gunpowder cord. If in a running flame the ignition process 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 that ensure propagationloss of nerve impulse.

Hodgkin-Huxley equation for the dissemination of N. and. were solved numerically. The obtained solutions together with the accumulated experiments. data showed that the spread of N. and. does not depend on the details of the excitation process. Quality picture of the spread of N. and. can be obtained using simple models, reflecting only general properties excitement. This approach made it possible to calculate the shape of N. and. in a homogeneous fiber, their change in the presence of inhomogeneities, and even complex regimes of excitation propagation in active media, for example. in the heart muscle. There are several math. models of this kind. The simplest of them is this. The ionic current flowing through the membrane during the passage of nitrogen is alternating in sign: 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 shifts by a threshold value j*. At this moment, a current appears, directed into the fiber and equal in magnitude j". After t" the current changes to the opposite, equal to j". This continues for a time ~ t ". A self-similar solution to equation (5) can be found as a function of the variable t = x/ u , where u - speed of spread of N. and. (Fig. 2, b).

In real fibers, the time t" is quite long, so only it determines the speed u , for this type the following formula is valid: . Considering that j" ~ ~d, R~d 2 and WITH~ d, Where d- fiber diameter, we find, in agreement with experiment, that u ~d 1/2 . Using piecewise constant approximation, the shape of the action potential is found.

Equation (5) for spreading N. and. actually allows two solutions. The second solution turns out to be unstable; it gives N. and. with a significantly lower speed and potential amplitude. The presence of a second, unstable solution has an analogy in the theory of combustion. When a flame propagates with a lateral heat sink, an unstable mode may also occur. Simple analytical model N. and. can be improved, taking into account additional details.

When the cross-section changes and when nerve fibers branch, N.’s passage and. may be difficult or even completely blocked. In an expanding fiber (Fig. 6), the pulse speed decreases as it approaches expansion, and after expansion it begins to increase until it reaches a new stationary value. Slowing down N. 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 the fiber, which delays N. and.

With the reverse movement of N. and. (from wide fiber to narrow) blocking does not occur, but the change in speed is of the opposite nature. When approaching the narrowing, the speed of N. and. increases and then begins to decrease to a new stationary value. On the speed graph (Fig. 6 A) a kind of hysteresis loop is obtained.

Rie. 6. The passage of nerve impulses expandsto the 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. At the branch node, different types are possible. options for passing and blocking impulses. With a non-synchronous approach, N. and. the blocking condition depends on the time offset. If the time between pulses is small, then they help each other 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 approached first, but failed to excite the third fiber, partially transfers the node to a refractory state. In addition, a synchronization effect occurs: as N. approaches and. towards the node their lag relative to each other decreases.

Interaction N. and. Nerve fibers in the body are combined into bundles or nerve trunks, forming something like a multi-core cable. All fibers in the 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. , which affects the membrane potential of neighboring fibers. Typically, such an influence is negligible and communication lines operate without mutual interference, but it manifests itself pathologically. and arts. conditions. By treating nerve trunks with special chem. substances, it is possible to observe not only mutual interference, but also the transfer of excitation to neighboring fibers.

There are known experiments on the interaction of two nerve fibers placed in a limited external volume. solution. If N. and. runs along one of the fibers, then the excitability of the second fiber simultaneously changes. Change goes through three stages. Initially, the excitability of the second fiber decreases (the excitation threshold increases). This decrease in excitability precedes the action potential traveling along the first fiber and lasts approximately until the potential in the first fiber reaches a maximum. Then the excitability increases; this stage coincides in time with the process of decreasing 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 passing N. and. using two fibers it was sometimes possible to achieve their synchronization. Despite the fact that own speed N. and. in different fibers are different, when they are simultaneously. excitement 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.

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

Where R 1 and R 2 - longitudinal resistance of the first and second fibers, R 3 - longitudinal resistance of the external environment, g = R 1 R 2 + R 1 R 3 . + R 2 R 3 . Ionic currents I 1 and I 2 can be described by one or another model of nervous excitation.

When using a simple analytical model solution leads to the following. picture. When one fiber is excited, an alternating membrane potential is induced in the neighboring one: first the fiber is hyperpolarized, then depolarized, and finally hyperpolarized again. These three phases correspond to a decrease, an increase, and a new decrease in fiber excitability. At normal parameter values, the shift of the membrane potential in the second phase towards depolarization does not reach the threshold, so transfer of excitation to the neighboring fiber does not occur. At the same time excitation of two fibers, system (6) allows a joint self-similar solution, which corresponds to two N. and., moving with the same speed at the station. distance from each other. If there is a slow N.I. ahead, then it slows down the fast impulse without releasing it forward; both move at relatively low speeds. If there is a fast II ahead. and., then it pulls a slow impulse behind it. The collective speed turns out to be close to the intrinsic speed. fast impulse speed. In complex neural structures, the appearance of auto-will.

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

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

An excitable medium can be three-dimensional, although more often it is considered as two-dimensional. The excitement that arose in the k.-l. point on the surface, propagates in all directions in the form of a ring wave. An excitation wave can bend around obstacles, but cannot be reflected from them, nor is it reflected from the boundary of the medium. When waves collide with each other, they are mutually destroyed; 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 the cardiac neuromuscular syncytium - the union of nerve and muscle fibers into a single conductive system capable of transmitting excitation in any direction. Neuromuscular syncytia contract synchronously, obeying a wave of excitation sent by a single control center - the pacemaker. The uniform rhythm is sometimes disrupted and 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 such a regime to occur, the perimeter of the obstacle must exceed the excitation wavelength, which is ~ 5 cm in the human atrium. With flutter, periodic movement occurs. atrial contraction with a frequency of 3-5 Hz. A more complex mode of excitation is fibrillation of the ventricles of the heart, when the department. elements of the heart muscle begin to contract without external influence. commands and without communication with neighboring elements with a frequency of ~ 10 Hz. Fibrillation leads to cessation of blood circulation.

The emergence and maintenance of spontaneous activity in an excitable environment is 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 heart pacemaker works.

Sources of excitation can also arise due to complex spaces. organizing the excitation mode, for example. reverberator of the type of rotating spiral wave, appearing in the simplest excitable medium. Another type of reverberator occurs in a medium consisting of two types of elements with different excitation thresholds; The reverberator 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 a medium that is heterogeneous in 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 modes 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., Problem of excitability, L., 1969; Tasaki I., Nervous excitement, trans. from English, M., 1971; Markin V.S., Pastushenko V.F., Chizmadzhev Yu.A., Theory of excitable media, M., 1981. V. S. Markin.

NERNST'S THEOREM- the same as Third law of thermodynamics.

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

NERNST-ETTINGSHAUSEN EFFECT- appearance of electricity fields E ne in a conductor in which there is a temperature gradient T , in a direction perpendicular to the magnet. field N . There are 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 N And T . In the absence of magnetic thermoelectric fields the field compensates for the flow of charge carriers created by the temperature gradient, and compensation occurs 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, 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 mag. 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 energy. In this case, the currents of cold and hot carriers in the transverse direction ( | T And | N ) cannot compensate each other. This results in a field E | ne , the value of which is determined from the condition that the total current is equal to 0 j = 0.

Field size E | ne depends on T, N and properties of the substance, characterized by coefficient. Nernsta-Ettingsha-uzena N | :

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

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

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

Measurements N.-E. e. in semiconductors make it possible to determine r, i.e. restore the function t(). Usually at high temps in the property area. semiconductor conductivity N | < 0 due to scattering of carriers by optical devices. phonons. When the temperature decreases, an area appears with N | > 0, corresponding to impurity conductivity and scattering of carriers Ch. arr. on phonons ( r< < 0). При ещё более низких T ionization scattering dominates. impurities with N | < 0 (r > 0).

In weak mag. fields (w with t<< 1, где w с - cyclotron frequency carriers) N | does not depend on H. In strong fields (w c t >> 1) coefficient N | proportional 1/ H 2. In anisotropic conductors, coefficient. N | - tensor. By the amount N | affect the entrainment of electrons by photons (increases N | ), anisotropy of the Fermi surface, etc.

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

Magn. the field, bending the trajectories of electrons (see above), reduces their mean free path l in the direction T . Since the free travel time (relaxation time t) depends on the electron energy, then the decrease l is not the same for hot and cold carriers: it is less for that group, for a certain type it is less. Thus, mag. the field changes the role of fast and slow carriers in energy transfer, and thermoelectric. the field ensuring 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 (when carriers are scattered by acoustic phonons), or decreases if m increases with increasing (when scattered 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 transfer processes is small (~ kT), N || small: In a semiconductor with two types of carriers N ||~ ~ g/kT. At low temps N|| may also increase due to the influence of electron drag by phonons. In strong magnetic fields complete thermoelectric. field in magnetic the field is “saturated” and does not depend on the carrier scattering mechanism. In ferromagnetic metals N.-E. e. has features associated with the presence of spontaneous magnetization.

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