home Consultations Nerve cells contact each other. How cells communicate with each other. The concept of the nerve center

Nerve cells contact each other. How cells communicate with each other. The concept of the nerve center

Humans have more than one hundred billion neurons. Each neuron consists of a body and processes - usually one long axon and several short branched dendrites. Thanks to these processes, neurons contact each other and form networks and circles through which nerve impulses circulate. Throughout life, the human brain loses neurons. This cell death is genetically programmed, but unlike cells of other tissues, neurons are not able to divide. In this case, a different mechanism operates: the functions of dead nerve cells are taken over by their “colleagues,” which increase in size and form new connections, compensating for the inactivity of the dead cell.

According to popular belief, nerve cells do not regenerate. However, this is not true: neurons - the cells of the nervous system - indeed cannot divide like cells of other tissues, but they arise and develop even in the brain of an adult. In addition, neurons are able to restore lost processes and contacts with other cells.
The human nervous system consists of a central part and a peripheral part. The central one includes the brain and spinal cord. The brain contains the largest collection of neurons. Numerous processes extend from the body of each, which form contacts with neighboring neurons. The peripheral part is formed by spinal, vegetative and cranial nodes, nerves and nerve endings that ensure the conduction of nerve impulses to the limbs, internal organs and tissues. In a healthy state, the nervous system is a well-coordinated mechanism; if one of the links in a complex chain does not fulfill its functions, the whole body suffers. For example, severe brain damage after strokes, Parkinson's disease, and Alzheimer's disease lead to accelerated death of neurons. For several decades, scientists have been trying to understand whether it is possible to stimulate the restoration of lost nerve cells.

And yet they regenerate

The first scientific publications confirming the birth of new neurons in the brain of adult mammals belong to the American researcher Joseph Altman. In 1962, the journal Science published his article “Are New Neurons Forming in the Brain of Adult Mammals?”, in which Altman described the results of his experiment. Using an electric current, he destroyed one of the structures of the rat's brain (the lateral geniculate body) and injected it with a radioactive substance that penetrates into new cells. A few months later, Altman discovered new radioactive neurons in the thalamus and cerebral cortex. In subsequent years, Altman published several more works proving the existence of neurogenesis in the brain. For example, in 1965, his article was published in the journal Nature. Despite this, Altman had many opponents in the scientific community; only several decades later, in the 1990s, his work received recognition, and the phenomenon of the birth of new neurons - neurogenesis - became one of the most fascinating areas of neurophysiology.
Today it is already known that neurons can arise in the brain of an adult mammal from so-called neuronal stem cells. So far, it has been established that this occurs in three areas of the brain: the dentate gyrus of the hippocampus, the subventricular region (in the lateral walls of the lateral ventricles of the brain) and the cerebellar cortex. Neurogenesis is most active in the cerebellum. This area of the brain is responsible for acquiring and storing information about unconscious automated skills - for example, when learning a dance, we gradually stop thinking about the movements and perform them automatically; information about these parameters is stored precisely in the cerebellum. Perhaps the most intriguing thing for researchers remains neurogenesis in the dentate gyrus. This is where our emotions are born, spatial information is stored and processed. It is not yet possible to understand how newly formed neurons affect already formed memories and interact with mature cells of this part of the brain.

Experiments with rats in mazes of various designs help scientists understand what happens to new neurons in the brain and how they are integrated into the smooth functioning of existing cells of the nervous system.

Labyrinth for memory

In order to understand how new neurons interact with old ones, the learning process of animals in the Morris water maze is being actively studied. During the experiment, the animal is placed in a pool 1.2–1.5 m in diameter, 60 cm deep. The walls of the pool are different, and in a certain place of the pool a platform is hidden a few millimeters under the water. A laboratory rat immersed in water strives to quickly feel solid ground under its feet. While swimming in the pool, the animal learns where the platform is and finds it faster the next time.
By training rats in the Morris water maze, it was possible to prove that the formation of spatial memory leads to the death of the youngest neurons, but actively supports the survival of cells that were formed about a week before the experiment, that is, during the process of memory formation, the volume of new neurons is regulated. At the same time, the emergence of new neurons makes it possible to form new memories. Otherwise, animals and humans would not be able to adapt to changing environmental conditions.
It has been noted that encountering familiar objects activates different groups of hippocampal neurons. Apparently, each group of such neurons carries a memory of a specific event or place. Moreover, living in a diverse environment stimulates neurogenesis in the hippocampus: mice that live in cages with toys and mazes have more newly formed neurons in the hippocampus than their relatives from standard empty cages.
It is noteworthy that neurogenesis actively occurs only in those areas of the brain that are directly responsible for physical survival: orientation by smell, orientation in space, and the formation of motor memory. Learning abstract thinking actively takes place at a young age, when the brain is still growing and neurogenesis affects all zones. But after reaching maturity, mental functions develop due to the restructuring of contacts between neurons, but not due to the appearance of new cells.
Despite several unsuccessful attempts, the search for previously unknown foci of neurogenesis in the adult brain continues. This direction is considered relevant not only for fundamental science, but also for applied research. Many diseases of the central nervous system are associated with the loss of a specific group of neurons in the brain. If it were possible to grow a replacement for them, then Parkinson's disease, many manifestations of Alzheimer's disease, the negative consequences of epilepsy or stroke would be defeated.

Brain Patches

Another interesting method adopted by neuroscientists in their research is the implantation of embryonic stem cells into the brain of an adult animal to restore lost functions. So far, such experiments lead to rejection of the introduced tissue or cells due to a strong immune response, but if stem cells take root in some cases, they develop into glial cells (accompanying tissue), and not into neurons at all. Even if in the future neurogenesis can be activated in any area of the brain, it is unclear how newly formed neurons will form connections within an already established network of nerve cells and whether they will be able to do this at all. If the hippocampus is ready for such a process, then the emergence of new neurons in other areas of the brain can disrupt the networks that have been established over the years; Instead of the expected benefit, perhaps only harm will be caused. Nevertheless, scientists continue to actively study the possibilities of neurogenesis in other parts of the brain.

The figure shows the process of formation of new neurons in the hippocampus of an adult mammal when exposed to low doses of radiation. New neurons are red, glia are green.

More recently, in February 2010, a group of Canadian researchers from the University of Toronto and the University of Waterloo published the results of experiments using cyclosporine A as a neurogenesis stimulator. In cell culture, cyclosporine A was shown to increase the growth and number of cells in the colony, and administration of this substance to adult mice led to an increase in neuronal stem cells in the brain.
Along with artificial substances, the properties of endogenous molecules that can enhance neurogenesis are also being studied. The neurotrophic factors that are produced by the animal body deserve the most attention here. These are nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophins-1, -3 and -4.
Neurotrophic factors belong to a group of proteins that support the growth, development and survival of nerve cells. If you deliver a neurotrophic factor to a damaged area of the brain, you can significantly slow down the death of neurons and support their vital activity. Although neurotrophic factors are not able to activate the formation of new nerve cells in the brain, they have the unique property of activating the restoration of nerve cell processes (axons) after damage or loss. The length of some axons reaches a meter, and it is the axons that conduct nerve impulses from the brain to our limbs, internal organs and tissues. The integrity of these pathways is disrupted by spinal fractures and vertebral displacement. Axonal regeneration is the hope of restoring the ability to move arms and legs in such cases.

Sprouts and shoots

The first works proving the possibility of axon regeneration were published in 1981. Then an article appeared in the journal Science, which proved that such regeneration is possible. Typically, several reasons interfere with axon regeneration, but if the obstacle is removed, the axons actively grow and create new contacts to replace the lost ones. With the beginning of the study of axonal regeneration, a new era in medicine was opened, now people with spinal cord damage have hope that motor abilities can be restored. These studies received widespread support, and not only from various research centers. Thus, the famous actor Christopher Reeve, who played the main role in the film “Superman” and became disabled after a spinal fracture, founded, together with his wife, a foundation to support such research - the Christopher and Dana Reeve Paralysis Foundation.

Recent research by neuroscientists offers some hope for disabled people confined to wheelchairs due to damage to the nervous system.

The main obstacle to axonal regeneration is the formation of scar tissue, which isolates damage to the spinal cord or peripheral nerves from surrounding cells. It is believed that such a scar saves nearby areas from possible penetration of toxins from the damaged area. As a result, axons cannot break through the scar. It has been shown that the basis of scar tissue is protein glycans (chondroitin sulfate).
Research conducted in 1998 in the laboratory of Professor David Muir at the University of Florida Brain Institute showed that it is possible to destroy protein glycans using the bacterial enzyme chondroitinase ABC. But even when the mechanical obstacle is removed, axon growth is still slowed down. The fact is that at the site of damage there are substances that interfere with regeneration, such as MAG, OMgp, Nogo. If you block them, you can achieve a significant increase in regeneration.
Finally, maintaining high levels of neurotrophic factors is important for successful axonal growth. Although neurotrophins have a positive effect on the regeneration of the nervous system, clinical trials have revealed significant side effects, such as weight loss, loss of appetite, nausea, and psychological problems. To enhance regeneration, stem cells could be injected into the site of injury, but there is evidence that implantation of stem cells into the spinal cord can trigger the appearance of tumors.
Even if the axon has grown and become capable of conducting nerve impulses, this does not mean that the limbs will begin to function normally. For this to happen, there must be many contacts (synapses) between the axons of nerve cells and muscle fibers, which set the human body in motion. Restoring such contacts takes a long time. Of course, recovery can be accelerated if you perform special physical exercises, but in a few months or even years it is impossible to completely recreate the picture of nerve contacts that has been formed over decades, from the very first day of the birth of human life. The number of such contacts cannot be counted; it is probably comparable to the number of stars in the Universe.
But there is also a positive point - after all, in recent years we have managed to move forward, and now it is at least clear in what ways we can try to speed up neuroregeneration.

The activity of cells in the body of multicellular animals is coordinated by “chemical messengers” and nerve cells. Over the past few years, it has been possible to largely elucidate the nature of the origin and transmission of nerve impulses.

The higher the place an organism occupies in the animal kingdom, the more important becomes the role of the system of cells designed to coordinate its activities. Nature has created two different coordinating systems. One of them is based on the release and distribution throughout the body of “chemical messengers” - hormones produced by certain specialized cells and capable of regulating the activity of cells located in other parts of the body. The second system, capable of much more rapid and also selective action, is a specialized system of nerve cells, or neurons, whose function is to receive and transmit orders by means of electrical impulses propagating along certain paths. Both of these coordinating systems arose in the process of evolution a very long time ago, and the second of them, namely the nervous system, underwent a particularly significant evolutionary development, culminating in the creation of an amazing and mysterious organ - the human brain.

Our knowledge of the workings of the millions of cells in our brain is in its infancy. However, this knowledge is generally sufficient to complete the task set here - to describe, and partly to explain, how individual cells (neurons) generate and transmit electrical impulses that constitute the main element of the code by which the internal communication system of the human body operates.

Most of the nerve cells are made up of two types of neurons - sensory and motor. Sensory neurons collect and transmit to the higher centers of the nervous system impulses arising in special receptor areas, the function of which is to inspect the external and internal environment of the body. Motor neurons transmit impulses from higher centers to “working” cells (usually muscle cells), i.e., cells on which the body’s response to changes in both of these environments directly depends. In simple reflex reactions, the transmission of signals from sensory to motor neurons occurs automatically and is ensured by relatively simple systems of synapses that are quite well studied.

During embryonic development, from the body of a nerve cell - be it a sensory or a motor cell - a long axonal extension grows, which in some unknown way grows to a destined point in the periphery in order to come into contact with muscle or skin. In an adult, the length of the axon can reach 1-1.5 meters with a thickness of less than 0.025 millimeters. The axon forms a kind of miniature telegraph wire for transmitting messages from the periphery to the body of the nerve cell, which lies in the spinal cord or in the brain under the protection of the spine or skull. Isolated peripheral nerve fibers have probably been studied more intensively than any other tissue, despite the fact that these fibers represent only fragments of cells, cut off both from their cell nuclei and from their peripheral endings. Nevertheless, such isolated nerve fibers retain the ability to transmit nerve impulses for quite a long time and can usually transmit tens of thousands of impulses before they cease to function. This observation, together with a number of others, convinces us that the body of the nerve cell and the nucleus contained in it, apparently, somehow “takes care” of its process, controls its growth and, if necessary, repairs damage, although not are directly involved in signal transmission.

For many years there have been debates about whether the idea of a cell as a basic structural unit is applicable to the nervous system and its functional connections. Some researchers believed that a developing nerve cell literally grows into the cytoplasm of all those cells with which it enters into functional interaction. This issue could not be finally resolved until the advent of an electron microscope with high resolution. It turned out that the nerve cell on most of its surface, including the surface of all its processes, is indeed tightly wrapped in other cells, but the cytoplasm of these cells is separated from the cytoplasm of the nerve cell by clearly defined membranes. In addition, there is a small gap between the membranes of the nerve cell and the other cells surrounding it, usually 100-200 angstroms thick.

Some of these cellular contacts are synapses - points at which signals are transmitted from one cell to the next link in the chain. However, synapses occur only on or near the neuron body and at the peripheral axon terminals. Most of the covering cells, in particular the cells that envelop the axon, are not nerve cells at all. Their function is still a mystery. Some of these companion cells are called Schwann cells, others are called glial cells. These cells apparently do not play any role in the process of impulse transmission: it is possible that they participate in it only indirectly, influencing the electric field around the axon. It is very significant, for example, that on the surface of isolated muscle fibers (which are very close to nerve fibers in their ability to transmit electrical impulses) there are very few such satellite cells.

One of the functions of the axon satellites is to form the so-called pulpal sheath - a segmented insulating sheath that covers the peripheral nerve fibers of vertebrates and improves their conductive ability. Thanks to electron microscopic studies by B. Ben-Guerin-Uzman and F. Schmitt, we now know that each segment of the pulp membrane is formed by a Schwann cell, which contains the nucleus; the cytoplasm of the Schwann cell tightly twists into a spiral around the axon, forming a multilayer sheath. The individual segments of the shell are separated by gaps, the so-called nodes of Ranvier, in which the electrical signal is regenerated.

There are other types of nerve fibers that lack a pulpy sheath, but even these fibers are covered by a single layer of Schwann cells. Perhaps it is precisely because the axon extends so far from the nucleus of the nerve cell that it needs this close contact with satellite cells that have a nucleus. Muscle fibers, in contrast to isolated axons, are completely independent cells, the cytoplasm of which contains nuclei; Their ability to do without satellite cells is probably related to the presence of a nucleus. Whatever the function of these satellites, in any case they cannot maintain the life of the axon for any significant time after it has been cut off from the cell body; After a few days, such a severed process invariably collapses and dies. How the nucleus of a nerve cell throughout life serves as a center that repairs damage, and how exactly it spreads its influence to the most distant parts of the axon, still remains a mystery (after all, if, for example, this influence spread due to ordinary diffusion, then for covering such a distance would take years).

The methods of experimental physiology turned out to be much more fruitful when applied to the study of the processes of direct conduction of impulses along the nerve than to the study of no less important, but much more difficult to study long-term processes. We know very little about the chemical interaction between a nerve and its satellites or about the forces that direct a growing nerve along a particular path and induce it to form synaptic connections with other cells. We also know nothing about how cells accumulate information, that is, what the memory mechanism is. Therefore, we will devote the rest of this article almost exclusively to nerve impulses and the way they are transmitted through the narrow synaptic clefts that separate one nerve cell from another.

Most of our information about the nerve cell comes from studying the giant squid axon, which reaches a thickness of almost a millimeter. It is very easy to apply microelectrodes to this fiber or to monitor the entry and exit of substances labeled with radioactive isotopes. The fiber cladding separates two aqueous solutions that have almost the same electrical conductivity and contain approximately the same number of electrically charged particles, or ions. However, the chemical composition of these two solutions is completely different. In the external solution, more than 90% of the charged particles are sodium ions (positively charged) and chlorine ions (negatively charged). In the solution inside the cell, the totality of these ions makes up less than 10% of the dissolved substances; here the bulk of the positively charged ions are formed by potassium ions, and the negative ions are represented by a variety of organic particles (which, undoubtedly, are synthesized in the cell itself), too large to diffuse through the axon membrane. Therefore, the concentration of sodium ions outside is approximately 10 times higher than inside the axon; the concentration of potassium ions, on the contrary, inside the axon is 30 times higher than outside. Although the permeability of the axon membrane for all these ions is small, it is nevertheless not the same for different ions; Potassium and chlorine ions pass through this membrane much more easily than sodium ions and large organic ions. As a result, a potential difference occurs that reaches 60-90 millivolts, and the internal contents of the cell turn out to be negatively charged relative to the external solution.

To maintain these differences in ion concentration, the nerve cell has a kind of pump that pumps sodium ions out through the membrane at the same rate as they enter the cell in the direction of the electrochemical gradient. The permeability of the surface of a resting cell to sodium is usually so low that the penetration of sodium ions into the cell is very small; therefore, only a small part of the energy that is continuously released in the process of cell metabolism is expended to perform the work associated with the pumping process. We do not know the details of the operation of this pump, but it appears to involve the exchange of sodium ions for potassium ions; that is, for every sodium ion released across the membrane, the cell accepts one potassium ion. Once inside the axon, potassium ions move in it as freely as ions usually move in any simple saline solution. When the cell is at rest, potassium ions leak out through the membrane, but rather slowly.

The axon membrane is similar to the membranes of other cells. It is approximately 50-100 angstroms thick and is equipped with a thin insulating layer consisting of fatty substances. Its specific resistance to the passage of electric current is approximately 10 million times higher than the resistance of the saline solutions that wash it outside and inside. However, the axon would be completely useless if it were used simply as an electrical wire. The resistance of the fluid inside the axon is about 100 million times higher than the resistance of copper wire, and its membrane allows a million times more current leakage than the winding of a good wire. If you stimulate an axon with an electrical current that is too weak to cause a nerve impulse, the electrical signal becomes vague and attenuates after traveling only a few millimeters along the fiber.

How does the axon transmit the primary impulse over a distance of over a meter without attenuation and without distortion?

If you increase the intensity of the electrical signal applied to the membrane of a nerve cell, then at some point a level is reached at which the signal no longer fades or disappears. In this case (if the voltage of the desired sign is taken), a certain threshold is overcome and the cell becomes “excited”. The cell's axon no longer behaves like a passive wire, but generates its own impulse, which amplifies the originally applied impulse. The impulse, or peak, thus amplified, is transmitted from one point to another without losing its strength, and spreads at a constant speed throughout the entire axon. The speed of impulse propagation along the nerve fibers of vertebrates ranges from several meters per second (for thin non-pulp fibers) to approximately 100 meters per second (for the thickest pulp fibers). We find the highest conduction speed - more than 300 kilometers per hour - in sensory and motor fibers that control the maintenance of body balance and fast reflex movements. After transmitting an impulse, the nerve fiber loses its ability to excite for a short time, falling into a refractory state, but after 1-2 thousandths of a second it is again ready to generate impulses.

The electrochemical processes underlying the nerve impulse, or action potential as it is called, have been largely elucidated over the past 15 years. As we have seen, the potential difference between the inner and outer surfaces of the membrane is determined mainly by the different permeability of the membrane to ions; sodium and potassium. Such selective permeability is characteristic of many membranes, both natural and artificial. However, the peculiarity of the nerve fiber membrane is that the degree of its permeability depends, in turn, on the potential difference between its inner and outer surfaces, and the basis of the entire process of conducting impulses is, in essence, this extremely peculiar mutual influence.

A. Hodgkin and A. Huxley found that an artificial decrease in the potential difference between the inner and outer surfaces of the membrane immediately causes an increase in the permeability of the membrane to sodium ions. We do not know why this specific change in membrane permeability occurs, but the consequences of this change are extremely significant. When positively charged sodium ions penetrate the membrane, they cause local extinction of some of the excess negative charge within the axon, which leads to a further decrease in the potential difference. Thus, it is a self-reinforcing process, because the penetration of a few sodium ions through the membrane allows other ions to follow suit. When the potential difference between the inner and outer surfaces of the membrane is reduced to a threshold value, sodium ions penetrate inside in such quantities that the negative charge of the internal solution changes to positive; a sudden “ignition” occurs, resulting in a nerve impulse, or action potential. This impulse, recorded by an oscilloscope in the form of a peak, changes the permeability of the axon membrane in the area lying in front of the point through which the impulse is currently passing, and creates conditions that ensure the penetration of sodium into the axon; Thanks to this, the process, repeated many times, spreads along the axon until the action potential passes along its entire length.

Immediately behind the moving impulse, other events take place. The "sodium door" that opened during the rise of the peak is closed again, and now the "potassium door" is briefly unlocked. This causes a rapid leakage of positively charged potassium ions, resulting in the restoration of the original negative charge within the axon. Within a few thousandths of a second after the potential difference between the inner and outer surfaces of the membrane has returned to its original level, it is difficult to shift this potential difference and cause a new impulse to occur. However, the permeability of the membrane for various ions quickly returns to its original level, after which the cell is ready to generate the next impulse.

The entry of sodium ions into the axon and the subsequent exit of potassium ions out occurs so briefly and affects such a small number of particles that these processes can hardly affect the composition of the contents of the axon as a whole. Even without replenishment, the supply of potassium ions inside the axon is large enough to ensure the passage of dozens of impulses. In a living organism, the enzyme system that controls the sodium pump easily maintains cells in a state of readiness to generate impulses.

This complex process of conducting a signal (which would have to decay very quickly due to leakage in the circuit) through numerous amplifiers along the transmission line provides the conditions necessary for our nervous system to communicate over relatively long distances within the body. It creates a well-known stereotypical coding system for our communication channels - short impulses, almost constant in strength and following each other at various intervals, the magnitude of which depends solely on the duration of the refractory period of the nerve cell. To compensate for the shortcomings of this simple coding system, the body has numerous communication channels (axons) located parallel to each other, each of which is a process of a separate nerve cell. For example, the optic nerve trunk, which extends from the eye, contains more than a million canals that are in close contact with each other; all of them are capable of transmitting various impulses to the higher centers of the brain.

Let us now return to the question of what happens at the synapse - at the point where the impulse reaches the end of one cell and collides with another nerve cell. The self-reinforcing process of impulse transmission, operating within each individual cell, does not have the ability to automatically “jump” across the boundaries of a given cell to neighboring cells. And this is quite natural. After all, if signals traveling along individual channels in a nerve bundle could jump from one channel to another, then this entire communication system would simply be of no use. True, at the site of functional synaptic contacts, the gap between cell membranes is usually no more than several hundred angstroms. However, based on all that we know about the size of the area of \u200b\u200bcontact and the insulating properties of cell membranes, it is difficult to imagine that effective telegraphic communication existed between the end of one nerve cell and the internal contents of another. Convincing experience in this

In this sense, there may be an attempt to transmit a subthreshold impulse - that is, an impulse that does not cause a peak - through the synapse separating one of the motor nerves from the muscle fiber. If a weak current is applied to such a motor nerve near the synapse, then the lead electrode inserted directly into the muscle fiber will not register any impulses. Obviously, at the synapse the telegraphic communication carried out by the nerve fiber is interrupted, and further transmission of messages occurs through some other process.

The nature of this process was discovered approximately 25 years ago by G. Dale and his colleagues. In some respects it resembles the hormonal mechanism mentioned at the beginning of our article. The endings of the motor nerve act like glands, secreting a certain chemical factor (mediator, or mediator). In response to the impulse transmitted to them, these endings release a special substance - acetylcholine, which quickly and efficiently diffuses through the narrow synaptic cleft. Acetylcholine molecules bind to receptor molecules in the area of contact with the muscle fiber and somehow open the “ion doors” of this fiber, allowing sodium to penetrate inside and cause the generation of an impulse. The same results can be achieved by experimentally applying acetylcholine to the area of contact with the muscle fiber. It is possible that similar chemical mediators are involved in creating most of the contacts between cells in our central nervous system. However, one can hardly think that acetylcholine serves as a universal mediator acting in all these cases; Therefore, numerous scientists are conducting intensive research in search of other natural chemical mediators.

The problem of transmission at synapses falls into two sets of questions: 1) how exactly does a nerve impulse cause the secretion of a chemical transmitter? 2) what are the physicochemical factors that determine the ability of a chemical mediator to stimulate a neighboring cell to generate an impulse in some cases or inhibit this generation in others?

So far we have said nothing about inhibition, although it is widespread in the nervous system and represents one of the most interesting manifestations of nervous activity. Inhibition occurs in cases where a nerve impulse serves as a brake for a nearby cell, preventing its activation under the influence of excitatory signals entering it at the same time through other channels. An impulse passing along an inhibitory axon is indistinguishable in its electrical characteristics from an impulse passing along an excitatory axon. However, in all likelihood, the physicochemical effect it has on the synapse is of a different nature. It is possible that inhibition occurs as a result of a process that to some extent stabilizes the membrane potential (electrification) of the receiving cell and prevents that cell from reaching the threshold of instability or the “flash point.”

There are several processes that could lead to such stabilization. We have already mentioned one of them: it occurs during the refractory period, observed immediately after the generation of the impulse. During this period, the membrane potential stabilizes at a high level (the negative charge of the internal contents of the cell is 80-90 millivolts), because the “potassium door” is wide open and the “sodium door” is tightly closed. If a mediator can cause one of these states or even both, then its action is undoubtedly inhibitory. It can be rightfully assumed that it is in this way that impulses coming from the vagus nerve reduce the heart rate; By the way, the transmitter produced by the vagus nerve is the same acetylcholine, as was discovered by V. Levy 40 years ago. Similar effects are observed in various inhibitory synapses located in the spinal cord, but the chemical nature of the mediators involved has not yet been established.

Inhibition can also occur if two “antagonistic” axons from two different cells meet in the same area of a third cell and release chemicals that compete with each other. Although examples of such inhibition have not yet been discovered in nature, the phenomenon of competitive inhibition is well known in chemistry and pharmacology. (For example, the paralyzing effect of curare poison is based on its competition with acetylcholine. Curare molecules have the ability to attach to that area of the muscle fiber that is usually free and interacts with acetylcholine.) The opposite is also possible, i.e. that some substance , secreted by the ending of the inhibitory nerve, acts on the ending of the excitatory nerve, reducing its secretory function, and thereby the amount of excitatory mediator released.

So, we again come up against the same question: how does a nerve impulse cause the release of a transmitter? Recent experiments have shown that the action of nerve impulses at the junction of the nerve with the muscle is not to cause the process of secretion of the transmitter, but, by changing the membrane potential, to change the rate of this process, which occurs continuously. Even in the absence of any stimulation, certain portions of the nerve endings release bursts of acetylcholine at irregular intervals, each burst containing many—perhaps thousands—of molecules.

Whenever a portion of transmitter molecules is spontaneously released in a muscle fiber lying on the other side of the synapse, a sudden small local reaction can be recorded. After one thousandth of a second, the muscle membrane potential decreases by 0.5 millivolts, and then the potential is restored within 20 thousandths of a second. By systematically changing the membrane potential of the nerve ending, it was possible to identify a certain relationship between this membrane potential and the rate of secretion of individual portions of the mediator. The rate of secretion appears to increase approximately 100-fold for every 30 millivolt decrease in membrane potential. At rest, one portion of the transmitter per second is released to each synapse. However, with a short-term change in potential by 120 millivolts during the passage of a nerve impulse, the frequency of release of portions of the transmitter for a short time increases almost a million times, as a result of which several hundred portions of the transmitter are simultaneously released within a fraction of a millisecond.

It is extremely important that the mediator is always released in the form of multimolecular portions of a certain size. This is probably explained by some features of the microscopic structure of the nerve endings. These nerve endings contain a peculiar cluster of so-called vesicles with a diameter of about 500 angstroms each, which possibly contain the mediator, already “packaged” and ready for release. It can be assumed that when these vesicles collide with the axon membrane, as they probably often do, such a collision sometimes leads to the splashing of the contents of the vesicles into the synaptic cleft. Such assumptions have yet to be confirmed by direct data, but they provide a reasonable explanation for everything that we know regarding the spontaneous release of discrete portions of acetylcholine and the acceleration of this release under various natural and experimental conditions. In any case, these assumptions make it possible to bring together the functional and morphological approaches to the same problem.

Due to the paucity of information we have, we have not touched upon many of the most interesting problems of long-term interactions and adaptive modifications that undoubtedly occur in the nervous system. To study these problems of physiology, it will probably be necessary to develop completely new methods, unlike previous ones. It is possible that our adherence to the methods that have made it possible to so successfully study the short-term reactions of excitable cells has prevented us from delving deeper into the problems of learning, memory, the development of conditioned reflexes, and the structural and functional interactions between nerve cells and their neighbors.

Nervous system regulates the activity of all organs and systems, determining their functional unity, and ensures the connection of the body as a whole with the external environment. The structural unit of the nervous system is a nerve cell with processes -
neuron. The entire nervous system is a collection of neurons that contact each other using special devices —synapses . Based on structure and function, there are three types of neurons:

receptor, or sensitive;

insertion, closing (conductor);

effector, motor neurons, from which the impulse is sent to the working organs (muscles, glands).

The nervous system is conventionally divided into two large sections - somatic , or animal, nervous system and vegetative , or autonomic nervous system. The somatic nervous system primarily carries out the functions of connecting the body with the external environment, providing sensitivity and movement causing contraction of skeletal muscles. Since the functions of movement and feeling are characteristic of animals and distinguish them from plants, this part of the nervous system is called animal (animal). The autonomic nervous system influences the processes of so-called plant life, common to animals and plants (metabolism, respiration, excretion, etc.), which is where its name comes from (vegetative - plant). Both systems are closely related to each other, but the autonomic nervous system has a certain degree of independence and does not depend on our will, as a result of which it is also called the autonomic nervous system.

In the nervous system there are central part - brain and spinal cord - central nervous system and peripheral , represented by nerves extending from the brain and spinal cord, is the peripheral nervous system.

1.

The nervous system controls the activities of various organs, systems and apparatuses that make up the body. It regulates the functions of movement, digestion, respiration, blood supply, metabolic processes, etc. The nervous system establishes the relationship of the body with the external environment, unites all parts of the body into a single whole.

The nervous system is divided according to topographic principle into central and peripheral (Fig.). The central nervous system (CNS) includes the brain and spinal cord.

The peripheral parts of the nervous system include spinal and cranial nerves with their roots and branches, nerve plexuses, nerve ganglia, and nerve endings.

In addition, the nervous system consists of two special parts: somatic (animal) and autonomic (autonomic).

The somatic nervous system innervates primarily the organs of the soma (body): striated (skeletal) muscles (face, torso, limbs), skin and some internal organs (tongue, larynx, pharynx). The somatic nervous system primarily carries out the functions of connecting the body with the external environment, providing sensitivity and movement, causing contraction of skeletal muscles. Since the functions of movement and feeling are characteristic of animals and distinguish them from plants, this part of the nervous system is called animal (animal). The actions of the somatic nervous system are controlled by human consciousness.

The autonomic nervous system innervates the viscera, glands, smooth muscles of organs and skin, blood vessels and the heart, and regulates metabolic processes in tissues. The autonomic nervous system influences the processes of so-called plant life, common to animals and plants (metabolism, respiration, excretion, etc.), which is where its name comes from (vegetative - plant). Both systems are closely related to each other, but the autonomic nervous system has a certain degree of independence and does not depend on our will, as a result of which it is also called the autonomic nervous system. It is divided into two parts, sympathetic and parasympathetic. The identification of these departments is based both on an anatomical principle (differences in the location of centers and the structure of the peripheral parts of the sympathetic and parasympathetic nervous system) and on functional differences. Excitation of the sympathetic nervous system promotes intense activity of the body; stimulation of the parasympathetic, on the contrary, helps to restore the resources expended by the body. The sympathetic and parasympathetic systems have opposite effects on many organs, being functional antagonists. Thus, under the influence of impulses coming through the sympathetic nerves, heart contractions become more frequent and intensified, blood pressure in the arteries increases, glycogen is broken down in the liver and muscles, the glucose content in the blood increases, the pupils dilate, the sensitivity of the senses and the performance of the central nervous system increases, and the bronchi, contractions of the stomach and intestines are inhibited, the secretion of gastric juice and pancreatic juice is reduced, the bladder relaxes and its emptying is delayed. Under the influence of impulses coming through the parasympathetic nerves, heart contractions slow down and weaken, blood pressure decreases, blood glucose levels decrease, contractions of the stomach and intestines are stimulated, the secretion of gastric juice and pancreatic juice increases, etc.

The central nervous system consists of the brain and spinal cord. The brain is divided into the brainstem and forebrain. The brainstem consists of the medulla oblongata and midbrain. The forebrain is divided into diencephalon and telencephalon.

All parts of the brain have their own functions.

Thus, the diencephalon consists of the hypothalamus - the center of emotions and vital needs (hunger, thirst, libido), the limbic system (in charge of emotional-impulsive behavior) and the thalamus (filtering and primary processing of sensory information).

In humans, the cerebral cortex is especially developed - the organ of higher mental functions. It has a thickness of 3 mm, and its total area is on average 0.25 sq.m.

The bark consists of six layers. The cells of the cerebral cortex are interconnected.

There are about 15 billion of them.

Different cortical neurons have their own specific function. One group of neurons performs the function of analysis (crushing, dismembering a nerve impulse), another group carries out synthesis, combines impulses coming from various sense organs and parts of the brain (associative neurons). There is a system of neurons that retains traces of previous influences and compares new influences with existing traces.

Based on the characteristics of the microscopic structure, the entire cerebral cortex is divided into several dozen structural units - fields, and according to the location of its parts - into four lobes: occipital, temporal, parietal and frontal.

The human cerebral cortex is an integrally functioning organ, although its individual parts (regions) are functionally specialized (for example, the occipital region of the cortex performs complex visual functions, the frontotemporal region performs speech, and the temporal region performs auditory functions). The largest part of the motor zone of the human cerebral cortex is associated with the regulation of the movement of the labor organ (hands) and speech organs.

All parts of the cerebral cortex are interconnected; they are also connected to the underlying parts of the brain, which carry out the most important vital functions. Subcortical formations, regulating innate unconditioned reflex activity, are the area of those processes that are subjectively felt in the form of emotions (they, in the words of I.P. Pavlov, are “a source of strength for cortical cells”).

The human brain contains all those structures that arose at various stages of the evolution of living organisms. They contain “experience” accumulated during the entire evolutionary development. This indicates the common origin of humans and animals.

As the organization of animals at various stages of evolution becomes more complex, the importance of the cerebral cortex increases more and more.

If, for example, you remove the cerebral cortex of a frog (it has an insignificant proportion in the total volume of its brain), then the frog almost does not change its behavior. A pigeon deprived of its cerebral cortex flies, maintains balance, but already loses a number of vital functions. A dog with a removed cerebral cortex becomes completely unadapted to its environment.

2. STRUCTURE OF THE NERVOUS SYSTEM: NERVOUS TISSUE, NEURONS, NERVE FIBERS, SYNAPSES, CONCEPT OF THE REFLECTOR ARC

The entire nervous system is built on nervous tissue. Nervous tissue consists of nerve cells (neurons) and anatomically and functionally associated auxiliary neuroglial cells. Neurons perform specific functions, being the structural and functional unit of the nervous system. Neuroglia ensure the existence and specific functions of neurons, perform supporting, trophic (nutritive), delimiting and protective functions.

A neuron (neurocyte) receives, processes, conducts and transmits information encoded in the form of electrical or chemical signals (nerve impulses).

Each neuron has a body, processes and their endings. On the outside, the nerve cell is surrounded by a membrane (cytolemma), which is capable of conducting excitation and also ensuring the exchange of substances between the cell and its environment. The body of a nerve cell contains a nucleus and surrounding cytoplasm (perikaryon). The cytoplasm of neurons is rich in organelles (subcellular formations that perform one or another function). The diameter of neuron bodies varies from 4-5 to 135 µm. The shape of nerve cell bodies is also different - from round, ovoid to pyramidal. Thin processes of two types extend from the body of the nerve cell of varying lengths. One or more tree-like branching processes along which a nerve impulse is brought to the body of a neuron is called a dendrite. In most cells their length is about 0.2 µm. The only, usually long process along which the nerve impulse is directed from the nerve cell body is the axon, or neurite.

Based on the number of processes, neurons are divided into unipolar, bi- and multipolar cells. Unipolar (single-process) neurons have only one process. In humans, such neurons are found only in the early stages of intrauterine development. Bipolar (two-candle) neurons have one axon and one dendrite. Their variety is pseudounipolar (false unipolar) neurons. The axon and dendrite of these cells begin from the general outgrowth of the body and subsequently divide in a T-shape. Multipolar (multi-branch) neurons have one axon and many dendrites; they make up the majority in the human nervous system. Nerve cells are dynamically polarized, i.e. are able to conduct a nerve impulse in only one direction - from dendrites to axon.

Depending on their function, nerve cells are divided into sensory, intercalary and effector.

Sensory (receptor, afferent) neurons. These neurons perceive various types of stimuli with their endings. The impulses generated in the nerve endings (receptors) are carried along the dendrites to the body of the neuron, which is always located outside the brain and spinal cord, located in the nodes (ganglia) of the peripheral nervous system. The nerve impulse is then sent along the axon to the central nervous system, spinal cord, or brain. Therefore, sensory neurons are also called afferent nerve cells. Nerve endings (receptors) differ in their structure, location and functions. There are extero-, intero- and proprioceptors. Exteroreceptors perceive irritation from the external environment. These receptors are located in the outer integument of the body (skin, mucous membranes) and in the sensory organs. Interoreceptors receive irritation mainly when the chemical composition of the internal environment of the body (chemoreceptors) and pressure in tissues and organs (baroreceptors) change. Proprioceptors perceive irritation (tension, tension) in muscles, tendons, ligaments, fascia and joint capsules. In accordance with their function, there are thermoreceptors that perceive temperature changes, and mechanoreceptors that detect various types of mechanical influences (touching the skin, squeezing it). Nociceptors perceive painful stimuli.

Intercalary (associative, conductor) neurons make up up to 97% of the nerve cells of the nervous system. These neurons are usually located within the central nervous system (brain and spinal cord). They transmit the impulse received from the sensory neuron to the effector neuron.

Effector (outflow or efferent) neurons conduct nerve impulses from the brain to the working organ - muscles, glands and other organs. The bodies of these neurons are located in the brain and spinal cord, in the sympathetic or parasympathetic nodes in the periphery.

Nerve fibers are processes of nerve cells (dendrites, axons) covered with membranes. In this case, the process in each nerve fiber is an axial cylinder, and the surrounding neurolemmocytes (Schwann cells), related to neuroglia, form the fiber sheath - neurolemma. Taking into account the structure of the membranes, nerve fibers are divided into non-myelinated (myelin-free) and pulpy (myelinated).

Unmyelinated nerve fibers are found mainly in autonomic neurons. The axial cylinder seems to bend the plasma membrane (shell) of the neurolemmocyte, which closes above it. The double membrane of the neurolemmocyte above the axial cylinder is called mesaxon. Under the Schwann cell there remains a narrow space (10-15 nm) containing tissue fluid involved in the conduction of nerve impulses. One neurolemmocyte envelops several (up to 5-20) axons of nerve cells. The membrane of the nerve cell process is formed by many Schwann cells, located sequentially one after another.

Myelinated nerve fibers are thick, they have a thickness of up to 20 microns. These fibers are formed by a rather thick cell axon - the axial cylinder. Around the axon there is a sheath consisting of two layers. The inner layer, myelin, is formed as a result of the spiral winding of the neurolemmocyte (Schwann cell) onto the axial cylinder (axon) of the nerve cell. The cytoplasm of the neurolemmocyte is squeezed out of it, similar to what happens when twisting the peripheral end of a tube of toothpaste. Thus, myelin is a multiply twisted double layer of the plasma membrane (shell) of the neurolemmocyte. The thick, dense myelin sheath, rich in fats, insulates the nerve fiber and prevents nerve impulses from leaking out of the axolemma (axon sheath). Outside the myelin layer there is a thin layer formed by the cytoplasm of the neurolemmocytes itself. Dendrites do not have a myelin sheath. Each neurolemmocyte (Schwann cell) envelops the length of only a small section of the axial cylinder. Therefore, the myelin layer is not continuous, but discontinuous. Every 0.3-1.5 mm there are so-called nodes of the nerve fiber (interceptions of Ranvier), where the myelin layer is absent. In these places, neighboring neurolemmocytes (Schwann cells) approach the axial cylinder directly with their ends. The nodes of Ranvier facilitate the rapid passage of nerve impulses along myelinated nerve fibers. Nerve impulses along myelin fibers are carried out as if in jumps - from the node of Ranvier to the next node.

The speed of nerve impulses along unmyelinated fibers is 1-2 m/s, and through pulpy (myelinated) fibers - 5-120 m/s. As the neuron moves away from the body, the speed of impulse conduction decreases.

Neurons of the nervous system come into contact with each other and form chains through which nerve impulses are transmitted. The transmission of nerve impulses occurs at the points of contact between neurons and is ensured by the presence of special zones between neurons - synapses. Synapses are distinguished between axosomatic, axodendritic and axoaxonal. At axosomatic synapses, the axon terminals of one neuron contact the body of another neuron. Axodendritic synapses are characterized by contact between an axon and the dendrites of another neuron, while axoaxonal synapses are characterized by contact between two axons of different nerve cells. At synapses, electrical signals (nerve impulses) are converted into chemical signals and vice versa. The transfer of excitation is carried out using biologically active substances - neurotransmitters, which include norepinephrine, acetylcholine, some dopamines, adrenaline, serotonin, etc. and amino acids (glycine, glutamic acid), as well as neuropeptides (enkephalin, neurotensin, etc.). They are contained in special vesicles located at the endings of axons - the presynaptic part. When a nerve impulse reaches the presynaptic part, neurotransmitters are released into the synaptic cleft, they come into contact with receptors located on the body or processes of the second neuron (postsynaptic part), which leads to the generation of an electrical signal - the postsynaptic potential. The magnitude of the electrical signal is directly proportional to the amount of neurotransmitter. After the release of the transmitter ceases, its remnants are removed from the synaptic cleft and the receptors of the postsynaptic membrane return to their original state. Each neuron forms a huge number of synapses. From all postsynaptic potentials, the neuron potential is formed, which is transmitted further along the axon in the form of a nerve impulse.

The nervous system functions according to reflex principles. A reflex is the body’s response to an external or internal influence and spreads along a reflex arc. Reflex arcs are circuits made up of nerve cells.

The simplest reflex arc includes sensitive and effector neurons, along which the nerve impulse moves from the place of origin (from the receptor) to the working organ (effector) (Fig. 4). The body of the first sensory (pseudo-unipolar) neuron is located in the spinal ganglion or in the sensory ganglion of one or another cranial nerve. The dendrite begins with a receptor that perceives external or internal stimulation (mechanical, chemical, etc.) and converts it into a nerve impulse that reaches the body of the nerve cell. From the body of the neuron along the axon, the nerve impulse through the sensory roots of the spinal or cranial nerves is sent to the spinal cord or to the brain, where it forms synapses with the bodies of effector neurons. In each interneuron synapse, impulse transmission occurs with the help of biologically active substances (mediators). The axon of the effector neuron leaves the spinal cord as part of the anterior roots of the spinal nerves (motor or secretory nerve fibers) or cranial nerves and is directed to the working organ, causing muscle contraction and increased (inhibited) gland secretion.

More complex reflex arcs have one or more interneurons. The body of the interneuron in three-neuron reflex arcs is located in the gray matter of the posterior columns (horns) of the spinal cord and is in contact with the axon of the sensory neuron that comes as part of the posterior (sensitive) roots of the spinal nerves. The axons of interneurons are directed to the anterior columns (horns), where the bodies of effector cells are located. The axons of effector cells are directed to muscles and glands, affecting their function. The nervous system contains many complex multineuronal reflex arcs, which have several interneurons located in the gray matter of the spinal cord and brain.

Neuroglial cells in the nervous system are divided into two types. These are gliocytes (or macroglia) and microglia.

Among gliocytes, ependymocytes, astrocytes and oligodendrocytes are distinguished.

Ependymocytes form a dense layer lining the central canal of the spinal cord and all ventricles of the brain. They participate in the formation of cerebrospinal fluid, transport processes, brain metabolism, and perform supporting and delimiting functions. These cells have a cubic or prismatic shape, they are located in one layer. Their surface is covered with microvilli.

Astrocytes form the supporting apparatus of the central nervous system. They are small cells with numerous processes diverging in all directions. There are fibrous and protoplasmic astrocytes. Fibrous astrocytes have 20-40 long, weakly branching processes and predominate in the white matter of the central nervous system. The processes are located between the nerve fibers. Some shoots reach blood capillaries. Protoplasmic astrocytes are located predominantly in the gray matter of the central nervous system, have a stellate shape, and short, highly branched, numerous processes extend from their bodies in all directions. The processes of astrocytes serve as supports for the processes of neurons and form a network in the cells of which neurons lie. The processes of astrocytes that reach the surface of the brain connect with each other and form a continuous superficial limiting membrane on it.

Oligodendrites are the most numerous group of neuroglial cells. They surround the bodies of neurons in the central and peripheral nervous system and are part of the sheaths of nerve fibers and nerve endings. Oligrdendrocytes are small ovoid cells with a diameter of 6-8 microns with a large nucleus. The cells have a small number of cone-shaped and trapezoidal processes. The processes form the myelin layer of nerve fibers. The myelin-forming processes spiral onto the axons. Along the axon, the myelin sheath is formed by the processes of many oligodendrocytes, each of which forms one segment. Between the segments there is a nodal interception of the nerve fiber devoid of myelin (interception of Ranvier). Oligodendrocytes that form the sheaths of nerve fibers of the peripheral nervous system are called neurolemmocytes (Schwann cells).

Microglia make up about 5% of neuroglial cells in the white matter of the brain and 18% in the gray matter. Microglia are represented by small elongated cells of angular or irregular shape, scattered in the white and gray matter (Ortega cells). Numerous branches of different shapes extend from the body of each cell, resembling bushes, which end in blood capillaries. The cell nuclei have an elongated or triangular shape. Microgliocytes have mobility and phagocytic ability. They act as a kind of “cleaners”, absorbing particles of dead cells.

CONCLUSION

The entire nervous system is divided into central and peripheral. The central nervous system includes the brain and spinal cord. From them, nerve fibers radiate throughout the body - the peripheral nervous system. It connects the brain with the senses and with the executive organs - muscles and glands.

All living organisms have the ability to respond to physical and chemical changes in the environment.

Stimuli from the external environment (light, sound, smell, touch, etc.) are converted by special sensitive cells (receptors) into nerve impulses - a series of electrical and chemical changes in the nerve fiber. Nerve impulses are transmitted along sensory (afferent) nerve fibers to the spinal cord and brain. Here, appropriate command impulses are generated, which are transmitted along motor (efferent) nerve fibers to the executive organs (muscles, glands). These executive organs are called effectors.

The main function of the nervous system is the integration of external influences with the corresponding adaptive reaction of the body.

The structural unit of the nervous system is the nerve cell - neuron. It consists of a cell body, a nucleus, branched processes - dendrites - along which nerve impulses travel to the cell body - and one long process - an axon - along which a nerve impulse travels from the cell body to other cells or effectors.

The peripheral nervous system is a conditionally distinguished part of the nervous system, the structures of which are located outside the brain and spinal cord.

The nervous system consists of cells - neurons, whose function is to process and disseminate information. Neurons contact each other through connections - synapses. One neuron transmits information to another through synapses using chemical carriers - mediators. Neurons are divided into 2 types: excitatory and inhibitory. The neuron body is surrounded by densely branching processes - dendrites, which are designed to receive information. The extension of a nerve cell that transmits nerve impulses is called axon. Its length in humans can reach 1 meter.

The peripheral nervous system is divided into autonomic nervous system, responsible for the constancy of the internal environment of the body, and somatic nervous system, innervating (supplying nerves) muscles, skin, ligaments.

The peripheral nervous system (or peripheral part of the nervous system) includes nerves that extend from the brain - cranial nerves and from the spinal cord - spinal nerves, as well as nerve cells that have moved outside the central nervous system. Depending on what type of nerve fibers are predominantly included in the nerve, nerves are divided into motor, sensory, mixed and autonomic (vegetative).

Nerves appear on the surface of the brain as motor or sensory roots. In this case, the motor roots are axons of motor cells located in the spinal cord and brain, and reach the innervated organ without interruption, and the sensory roots are axons of nerve cells of the spinal ganglia. To the periphery of the nodes, sensory and motor fibers form a mixed nerve.

All peripheral nerves, based on their anatomical features, are divided into cranial nerves - 12 pairs, spinal nerves - 31 pairs, autonomic (autonomic) nerves.

Cranial nerves arise from the brain and include:

Through the peripheral nerve, the spinal ganglion and the dorsal root, nerve impulses enter the spinal cord, that is, the central nervous system.

Ascending fibers from a limited area of the body are collected together and form peripheral nerve. Fibers of all types (superficial and deep sensitivity, fibers innervating skeletal muscles, and fibers innervating internal organs, sweat glands and vascular smooth muscles) are combined into bundles surrounded by 3 connective tissue membranes (endoneurium, perineurium, epineurium) and form the nerve cable.

After the peripheral nerve enters the spinal canal through the intervertebral foramen, it bifurcates into the anterior and posterior spinal roots.

The anterior roots leave the spinal cord, the posterior roots enter it. Within the nerve plexuses located outside the spinal canal, the fibers of the peripheral nerves intertwine in such a way that ultimately the fibers from one individual nerve end up at different levels within different spinal nerves.

The peripheral nerve contains fibers from several different root segments.

Spinal nerves 31 pairs are distributed into:

cervical nerves - 8 pairs
thoracic nerves -12 pairs
lumbar nerves - 5 pairs
sacral nerves - 5 pairs
coccygeal nerve - 1 pair

Each spinal nerve is a mixed nerve and is formed by the fusion of 2 roots belonging to it: the sensory root, or posterior root, and the motor root, or anterior root. In the central direction, each root is connected to the spinal cord using radicular filaments. The dorsal roots are thicker and contain the spinal ganglion. The anterior roots do not have nodes. Most spinal nodes lie in the intervertebral foramina.

Externally, the spinal ganglion looks like a thickening of the posterior root, located slightly closer to the center from the junction of the anterior and posterior roots. There are no synapses in the spinal ganglion itself.

The human nervous system consists of a central part and a peripheral part. The central one includes the brain and spinal cord. The brain contains the largest collection of neurons. Numerous processes extend from the body of each, which form contacts with neighboring neurons. The peripheral part is formed by spinal, vegetative and cranial nodes, nerves and nerve endings that ensure the conduction of nerve impulses to the limbs, internal organs and tissues. In a healthy state, the nervous system is a well-coordinated mechanism; if one of the links in a complex chain does not fulfill its functions, the whole body suffers. For example, severe brain damage after strokes, Parkinson's disease, and Alzheimer's disease lead to accelerated death of neurons. For several decades, scientists have been trying to understand whether it is possible to stimulate the restoration of lost nerve cells.

And yet they regenerate

Today it is already known that neurons can arise in the brain of an adult mammal from so-called neuronal stem cells. So far, it has been established that this occurs in three areas of the brain: the dentate gyrus of the hippocampus, the subventricular region (in the lateral walls of the lateral ventricles of the brain) and the cerebellar cortex. Neurogenesis is most active in the cerebellum. This area of the brain is responsible for acquiring and storing information about unconscious automated skills - for example, when learning a dance, we gradually stop thinking about the movements and perform them automatically; information about these parameters is stored precisely in the cerebellum. Perhaps the most intriguing thing for researchers remains neurogenesis in the dentate gyrus. This is where our emotions are born, spatial information is stored and processed. It is not yet possible to understand how newly formed neurons affect already formed memories and interact with mature cells of this part of the brain.

Labyrinth for memory

By training rats in the Morris water maze, it was possible to prove that the formation of spatial memory leads to the death of the youngest neurons, but actively supports the survival of cells that were formed about a week before the experiment, that is, during the process of memory formation, the volume of new neurons is regulated. At the same time, the emergence of new neurons makes it possible to form new memories. Otherwise, animals and humans would not be able to adapt to changing environmental conditions.

It has been noted that encountering familiar objects activates different groups of hippocampal neurons. Apparently, each group of such neurons carries a memory of a specific event or place. Moreover, living in a diverse environment stimulates neurogenesis in the hippocampus: mice that live in cages with toys and mazes have more newly formed neurons in the hippocampus than their relatives from standard empty cages.

It is noteworthy that neurogenesis actively occurs only in those areas of the brain that are directly responsible for physical survival: orientation by smell, orientation in space, and the formation of motor memory. Learning abstract thinking actively takes place at a young age, when the brain is still growing and neurogenesis affects all zones. But after reaching maturity, mental functions develop due to the restructuring of contacts between neurons, but not due to the appearance of new cells.

Despite several unsuccessful attempts, the search for previously unknown foci of neurogenesis in the adult brain continues. This direction is considered relevant not only for fundamental science, but also for applied research. Many diseases of the central nervous system are associated with the loss of a specific group of neurons in the brain. If it were possible to grow a replacement for them, then Parkinson's disease, many manifestations of Alzheimer's disease, the negative consequences of epilepsy or stroke would be defeated.

Brain Patches

Another interesting method adopted by neuroscientists in their research is the implantation of embryonic stem cells into the brain of an adult animal to restore lost functions. While such experiments lead to rejection of the introduced tissue or cells due to a strong immune response, but if stem cells take root in some cases, they develop into glial cells (accompanying tissue), and not into neurons. Even if in the future neurogenesis can be activated in any area of the brain, it is unclear how newly formed neurons will form connections within an already established network of nerve cells and whether they will be able to do this at all. If the hippocampus is ready for such a process, then the emergence of new neurons in other areas of the brain can disrupt the networks that have been established over the years; Instead of the expected benefit, perhaps only harm will be caused. Nevertheless, scientists continue to actively study the possibilities of neurogenesis in other parts of the brain.

Along with artificial substances, the properties of endogenous molecules that can enhance neurogenesis are also being studied. The neurotrophic factors that are produced by the animal body deserve the most attention here. These are nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophins-1, -3 and -4.

Neurotrophic factors belong to a group of proteins that support the growth, development and survival of nerve cells. If you deliver a neurotrophic factor to a damaged area of the brain, you can significantly slow down the death of neurons and support their vital activity. Although neurotrophic factors are not able to activate the formation of new nerve cells in the brain, they have the unique property of activating the restoration of nerve cell processes (axons) after damage or loss. The length of some axons reaches a meter, and it is the axons that conduct nerve impulses from the brain to our limbs, internal organs and tissues. The integrity of these pathways is disrupted by spinal fractures and vertebral displacement. Axonal regeneration is the hope of restoring the ability to move arms and legs in such cases.

Sprouts and shoots

The main obstacle to axonal regeneration is the formation of scar tissue, which isolates damage to the spinal cord or peripheral nerves from surrounding cells. It is believed that such a scar saves nearby areas from possible penetration of toxins from the damaged area. As a result, axons cannot break through the scar. It has been shown that the basis of scar tissue is protein glycans (chondroitin sulfate).

Research conducted in 1998 in the laboratory of Professor David Muir at the University of Florida Brain Institute showed that it is possible to destroy protein glycans using the bacterial enzyme chondroitinase ABC. But even when the mechanical obstacle is removed, axon growth is still slowed down. The fact is that at the site of damage there are substances that interfere with regeneration, such as MAG, OMgp, Nogo. If you block them, you can achieve a significant increase in regeneration.

Finally, maintaining high levels of neurotrophic factors is important for successful axonal growth. Although neurotrophins have a positive effect on the regeneration of the nervous system, clinical trials have revealed significant side effects, such as weight loss, loss of appetite, nausea, and psychological problems. To enhance regeneration, stem cells could be injected into the site of injury, but there is evidence that implantation of stem cells into the spinal cord can trigger the appearance of tumors.

Even if the axon has grown and become capable of conducting nerve impulses, this does not mean that the limbs will begin to function normally. For this to happen, there must be many contacts (synapses) between the axons of nerve cells and muscle fibers, which set the human body in motion. Restoring such contacts takes a long time. Of course, recovery can be accelerated if you perform special physical exercises, but in a few months or even years it is impossible to completely recreate the picture of nerve contacts that has been formed over decades, from the very first day of the birth of human life. The number of such contacts cannot be counted; it is probably comparable to the number of stars in the Universe.

But there is also a positive point - after all, in recent years we have managed to move forward, and now it is at least clear in what ways we can try to speed up neuroregeneration.

Partner news