Ugryumov M. Compensatory possibilities of the brain. How to restore brain function in violation of its development

“Nerve cells do not recover” - everyone knows this phrase. But not everyone knows that this is actually not true. Nature has given the brain all the possibilities for reparation. The Fleming project tells how nerve cells change their purpose, why a person needs a second hemisphere, and how stroke will be treated in the near future.

Path to change

To the question "Is it possible to restore the nervous tissue?" doctors and scientists from all over the world for a long time with one voice firmly answered "No". However, some enthusiasts did not give up hope to prove the opposite. In 1962, American professor Joseph Altman set up an experiment on the restoration of nervous tissue in a rat. In 1980, the Soviet physiologist and neuroendocrinologist Andrey Polenov discovered in amphibians neuronal stem cells in the walls of the cerebral ventricles, which begin to divide when the nervous tissue is damaged. In the 1990s, Professor Fred Gage used bromdioxyuridine, which accumulated in cells of dividing tissues, to treat brain tumors. Subsequently, traces of this drug were found throughout the cerebral cortex, which allowed him to conclude that there is neurogenesis in the human brain. Today, science has enough data to allow it to assert that the growth and renewal of the functions of nerve cells is possible.

The nervous system is designed to provide communication between the body and the outside world. From the point of view of the structure, the nervous tissue is divided into the nervous tissue proper and neuroglia - a set of cells that ensure the isolation of the parts of the nervous system, their nutrition and protection. Neuroglia also plays a role in the formation of the blood-brain barrier. The blood-brain barrier protects nerve cells from external influences, in particular, it prevents the occurrence of autoimmune reactions directed against one's own cells. In turn, the nervous tissue itself is represented by neurons that have two types of processes: numerous dendrites and a single axon. Approaching, these processes form synapses - the places where the signal passes from one cell to another, and the signal is always transmitted from the axon of one cell to the dendrite of another. The nervous tissue is very sensitive to the influence of the external environment, the supply of nutrients in the neurons themselves is close to zero, therefore, a constant supply of glucose and oxygen is necessary to provide the cells with energy, otherwise degeneration and death of neurons occurs.

Subacute cerebral infarction

Back in 1850, the English physician August Waller studied degenerative processes in injured peripheral nerves and discovered the possibility of restoring nerve function by comparing the ends of the nerve. Waller noticed that the damaged cells are engulfed by macrophages, and the axons from one side of the damaged nerve begin to grow towards the other end. If axons collide with an obstacle, their growth stops and a neuroma is formed - a tumor of nerve cells that causes unbearable pain. However, if the ends of the nerve are very accurately compared, it is possible to completely restore its function, for example, in traumatic amputation of limbs. Thanks to this, microsurgeons now sew cut off legs and arms, which, in case of successful treatment, completely restore their function.

The situation is more complicated with our brain. If in the peripheral nerves the impulse transmission goes in one direction, then in the central organs of the nervous system, neurons form nerve centers, each of which is responsible for a specific, unique function of the body. In the brain and spinal cord, these centers are interconnected and combined into pathways. This feature allows a person to perform complex actions and even combine them into complexes, ensure their synchronism and accuracy.

The key difference between the central nervous system and the peripheral one is the stability of the internal environment provided by glia. Glia prevents the penetration of growth factors and macrophages, and the substances secreted by it inhibit (slow down) cell growth. Thus, axons cannot grow freely, because nerve cells simply do not have the conditions for growth and division, which, even normally, can lead to serious disorders. On top of that, neuroglial cells form a glial scar that prevents axons from sprouting, as is the case with peripheral nerves.

Hit

Stroke, acute stage

Damage to the nervous tissue occurs not only in the periphery. According to the US Centers for Disease Control, more than 800,000 Americans are hospitalized with a diagnosis of stroke, and one patient dies from this disease every 4 minutes. According to Rosstat, in 2014 in Russia, stroke was the direct cause of death in more than 107,000 people.

A stroke is an acute violation of cerebral circulation resulting from hemorrhage with subsequent compression of the brain substance ( hemorrhagic stroke) or poor blood supply to areas of the brain resulting from blockage or narrowing of the vessel ( cerebral infarction, ischemic stroke). Regardless of the nature of a stroke, it leads to a violation of various sensory and motor functions. By what functions are impaired, the doctor can determine the localization of the focus of the stroke and begin treatment and subsequent recovery in the near future. The doctor, focusing on the nature of the stroke, prescribes therapy that ensures the normalization of blood circulation and, thereby, minimizes the consequences of the disease, but even with adequate and timely therapy, less than 1/3 of patients recover.

Retrained Neurons

In the brain, the restoration of nervous tissue can occur in different ways. The first is the formation of new connections in the area of ​​the brain next to the injury. First of all, the area around the directly damaged tissue is restored - it is called the diaschisis zone. With the constant input of external signals normally processed by the affected area, neighboring cells begin to form new synapses and take over the functions of the damaged area. For example, in an experiment with monkeys, when the motor cortex was damaged, the premotor zone took over its role.

In the first months after a stroke, the presence of a second hemisphere in a person also plays a special role. It turned out that in the early stages after brain damage, part of the functions of the damaged hemisphere is taken over by the opposite side. For example, when you try to move a limb on the affected side, that hemisphere is activated, which is normally not responsible for this half of the body. In the cortex, a restructuring of pyramidal cells is observed - they form connections with the axons of motor neurons from the damaged side. This process is active in the acute phase of a stroke; later on, this compensation mechanism comes to naught and some of the connections are broken.

There are also areas in the adult brain where stem cells are active. This is the so-called. dentate gyrus of the hippocampus and subventricular zone. The activity of stem cells in adults, of course, is not the same as in the embryonic period, but nevertheless, cells from these zones migrate to the olfactory bulbs and there they become new neurons or neuroglial cells. In an animal experiment, some cells left their usual migration route and reached the damaged area of ​​the cerebral cortex. There are no reliable data on such migration in humans, due to the fact that this process can be hidden by other phenomena of brain recovery.

brain transplant

Stroke, acute phase

In the absence of natural cell migration, neurophysiologists have proposed artificially replacing damaged areas of the brain with embryonic stem cells. In this case, cells must differentiate into neurons, and the immune system will not be able to destroy them because of the blood-brain barrier. According to one hypothesis, neurons fuse with stem cells, forming binuclear synkaryons; The "old" nucleus subsequently dies, and the new one continues to control the cell, prolonging its life by pushing the limit of cell divisions further.

Experimental operations carried out by an international team of scientists led by the French neurosurgeon Anna-Catherine Baschou-Levy from the Henry Mondor Hospital have already shown the effectiveness of this method in the treatment of Huntington's chorea (a genetic disease that causes degenerative changes in the brain). Unfortunately, in the situation with Huntington's chorea, a functioning graft introduced for replacement purposes cannot resist the progress of neurodegeneration in general, since the cause of the disease is a hereditary genetic defect. However, the autopsy material showed that transplanted nerve cells survive for a long time and do not undergo changes characteristic of Huntington's disease. Thus, intracerebral transplantation of embryonic nervous tissue in patients with Huntington's disease, according to preliminary data, can provide a period of improvement and long-term stabilization during the course of the disease. A positive effect can be obtained only in a number of patients, so careful selection and development of criteria for transplantation is necessary. As in oncology, neurologists and their patients in the future will have to choose between the degree and duration of the expected therapeutic effect and the risks associated with surgery, the use of immunosuppressants, and so on. Similar operations are also performed in the USA, but American surgeons use purified xenografts (taken from organisms of a different species) and are still facing the problem of the occurrence of malignant tumors (30-40% of all operations of this kind).

It turns out that the future of neurotransplantology is not far off: although existing methods do not provide a full recovery and are only experimental in nature, they significantly improve the quality of life, but this is still only the future.

The brain is an incredibly plastic structure that adapts even to damage such as a stroke. In the near future, we will stop waiting for the tissue to rebuild itself and start helping it, which will make the rehabilitation of patients an even faster process.

For the provided illustrations, we thank the portal http://radiopaedia.org/

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The basis for the restoration and compensation of lost functionsis plasticity- the ability of nerve elements to restructure functional properties. The main manifestations of this property are post-tetanic potentiation, the formation of temporary bonds. These phenomena provide a more active involvement in the regulation of impaired function of intact neurons localized in other departments (in addition to the damaged center). The presence of such "scattered" neurons is especially characteristic of the cerebral cortex. In this case, the intensity of the functioning of the neurons preserved in the damaged center also sharply increases, for example, as a result of the degeneration of a significant part of the neurons of the motor center. A particularly important role in compensating for any impaired function (, motor activity, etc.) is played by the possibility of regenerating damaged nerve fibers and restoring broken interneuronal connections and connections with effectors.

A. Activation mechanisms of preserved neurons damaged of the central center and involvement in more active activity of scattered neurons capable of performing the impaired function.

1. Posttetanic potentiation(the phenomenon of relief) is an improvement in conduction in synapses after a short stimulation of the afferent pathways. Short-term activation increases the amplitude of postsynaptic potentials. Relief is also observed during stimulation (at the beginning) - in this case, the phenomenon is called tetanic potentiation. The duration of post-tetanic potentiation depends on the properties of the synapse and the nature of stimulation - after single stimuli, it is weakly expressed, after an irritating series, potentiation (relief) can last from several minutes to several hours. Apparently, the main cause of the facilitation phenomenon is the accumulation of Ca 2+ ions in presynaptic endings, since Ca 2+ ions that enter the nerve ending during PD accumulate there, since the ion pump does not have time to remove them from the nerve ending. Accordingly, the release of the mediator increases with the occurrence of each impulse in the nerve ending, and the EPSP increases. In addition, with frequent use of synapses, the synthesis of the mediator is accelerated, and with their rare use, on the contrary, the synthesis of mediators decreases - this is the most important property of the central nervous system: you need to work actively! Therefore, the background activity of neurons contributes to the emergence in the nerve centers.

The Significance of the Relief Phenomenon when compensating for disturbed functions, it creates the prerequisites for improving the processes of processing information on the remaining neurons of the nerve centers, which begin to work more actively. Repeated occurrences of relief phenomena in the nerve center can cause the center to move from its normal state to the dominant one.

2. Dominant - the dominant focus of excitation in the central nervous system, subordinating the functions of other nerve centers. The dominant state of the preserved neurons of the center and scattered neurons involved in the performance of a particular function ensures a more active and stable activity of these nerve elements. Therefore, post-tetanic potentiation acts as the first stage - more active involvement of preserved and scattered neurons in the regulation of impaired function through the formation of a dominant focus. In this regard, to restore motor functions, more movements are needed, including passive ones.

3. Formation of temporary connections how essential element also contributes to the restoration of impaired functions. First of all, this applies to intellectual activity, and the possibilities of the cerebral cortex are enormous. It is known
that conditioned reflex connections can be developed to virtually any stimulus (any change in the external or internal environment of the body).

B. Regeneration of nerve fibers as a factor contributing to the restoration of impaired function.

1. Well-known clinical observations of patients who, after hemorrhages in the substance of the brain damagedcenters for the regulation of muscle tone and the act of walking. Nevertheless, over time, the paralyzed limb in patients gradually began to be involved in motor activity and the tone of its muscles returned to normal. The disturbed motor function is partially, and sometimes completely restored due to the greater activity of the remaining neurons and the involvement of other CNS neurons in this function, which is facilitated by regular passive and active movements.

The main symptoms of dysfunction are present to a greater or lesser extent with the defeat of each of its three departments, which indicates a functional overlap between individual departments.

The cerebellum does not have direct access to spinal motor neurons, but acts on them through cortical-stem motor centers. This probably explains the high degree of brain plasticity, which is capable of compensating for impaired cerebellar functions.

There are cases of congenital absence of the cerebellum or its slow destruction by a tumor in a person without symptoms of movement disorders.

2. Development and regeneration of neuron processes. After birth, in a child, as in an adult, there is practically no division of neurons and neuroblasts, although individual cases of mitosis are possible. In this regard, the complication of functions in the process of ontogenesis or during functional loads is carried out as a result of the development of nerve processes - an increase in their number and degree of branching. Thus, in an adult, compared with a newborn, the number of branching points of dendrites is 13 times greater, and the total length of the dendrites of cortical neurons is 34 times. The number of collaterals and terminal branches of the axon increases. The ultimate goal of the development of nerve fibers is the formation of new synaptic contacts that provide signal transmission to another cell.

During the development, as well as during the regeneration of the damaged process of the neuron, a fiber growth cone is formed - a thickening with many long and thin processes 0.1-0.2 microns thick and up to 50 microns long, extending in different directions. The growth cone is a zone of intense exo- and endocytosis. The membrane material necessary for regeneration is formed in the body of the neuron and is transported by rapid transport in the form of bubbles to the growth cone and, through exocytosis, is incorporated into cell membrane, lengthening it. It was found that actin filaments are necessary for the movement of the growth cone, the destruction of which (for example, by cytocholasin B) stops growth.

To stabilize the structure of an elongating fiber, microtubules are important, the destruction of which (for example, by Colchicine) leads to a shortening of the growing fiber. Proteins necessary for the formation of microtubules and microfilaments (tubulin, actin, etc.) are delivered by slow axon transport.

Two factors of movement of the growth cone are identified. Cell adhesion factor is a glycoprotein that is located on the plasma membrane of neuron processes and provides adhesion between developing processes, grouping them into bundles. Another protein has been named factornerve growth(FRN). It is released into the intercellular fluid by the target cell for the growing one and has a chemotactic effect, directing the movement of the growth cone towards the target cell.

During the regeneration of damaged fibers in the peripheral nervous system, lemmocytes (Schwann) cells of the distal (from the injury zone) section of the fiber play an important role in the direction of growth. As soon as the growth cone reaches the target cell, it transforms into a presynaptic ending, while the processes of exo- and endocytosis ensure the release and subsequent absorption of the mediator, through which the signal is transmitted through the formed synapse.

If some axons are damaged, others - preserved nerve fibers with the same function - due to growth (dichotomous division) can reinnervate neurons, the connection with which was broken.

When damage to the brain, especially its cortex, occurs in early age, the consequences are usually less severe than after similar disorders in adults. This applies to both motor systems and speech. After the removal of areas of the cortex in newborn monkeys, the development of animals during the first year of life is almost the same as the norm.

It is known that in the process of maturation, many of the connections present in the immature brain disappear. These include, for example, "excessive" connections in the corpus callosum, a significant part of which is later lost.

In the early stages of ontogeny, the visual cortex, for example, of rodents, contains neurons that project into, then they disappear. It can be assumed that damage, by suppressing regression processes, allows fibers that are normally doomed to die off to functionally replace degenerated ones. This explains the higher plasticity of the young brain, its increased ability to reorganize "neural circuits" compared to the mature brain. A few days after muscle denervation, significant spontaneous activity of individual muscle fibers develops, manifested in the form of fibrillations. The muscle membrane becomes hyperexcitable; the area of ​​its sensitivity to acetylcholine gradually expands from the end plate to the entire surface of the fiber. Similar processes are characteristic of the central nervous system. It appears that hypersensitivity of denervated structures is a general principle.

In the central nervous system.

General patterns

Compensation processes in the nervous system are more often considered as reactions that occur after injuries, surgical interventions, or certain pathological phenomena. In a significant number of cases, clinicians are faced with a state when a pathological process is already developing in the nervous system, but it still does not cause dysfunction and is not detected without special studies.

Compensatory processes are implemented initially due to intrastructural mechanisms occurring, for example, within one nucleus of the nervous system. This compensation is based on a series of complex rearrangements in the structure itself. It is possible through the use of the available reserves of the structure and through vicaring.

Vicaration in this case should be understood as an increase in the activity and functionality of the preserved structural elements. For example, the transition of monomodal neurons to polymodal, monosensory neurons to polysensory. This mechanism in disorders of the central nervous system is based on the fact that each of its structures is potentially polyfunctional. Intrastructural compensation often depends on the individual characteristics of the organization of analyzers in humans. So, the 17th field of some people can be twice as large as that of others. Some people noted the expansion of the macular zone 17 field or the anterior part of this field - the area of ​​peripheral vision. It is also known that the lateral geniculate bodies in some individuals

exceed the average by 185%. Naturally, in all such cases, compensatory possibilities are much wider.

Another way of compensation is provided intrasystem interactions, for example, within the striopallidar system, when dysfunction of the caudate nucleus in the regulation of motility can be compensated by the putamen.

The third way of compensation is realized intersystem interactions. Compensation, as an intersystem process, is due mainly to the participation in the elimination of the developing pathology of one structure by the functional structures of other systems associated with it. In this case, another system, due to the formation of new temporary connections, can ensure the preservation of the function that the system damaged by the pathological process is primarily called upon to perform.

It should be noted that all ways of compensation are implemented in parallel, but the weight of participation of each of them is different stages pathology development is different. At the initial stages, a large proportion of compensation is carried out due to intrastructural processes, with an increase in pathology, intra-system compensation becomes more important, then inter-system.

Quite often there is no parallel between morphological disorders of the central nervous system and the ability of this structure to perform its inherent function. For example, in case of damage to the cerebellum by a growing tumor, compensation is so perfect that clinical symptoms appear when most of the cerebellum has died. Compensation of functions is more successfully realized with a slowly growing pathological process at a young age.

Thus, it is known that Louis Pasteur in his youth suffered a cerebral hemorrhage, which led to a significant destruction of the cortex of the right hemisphere of his brain. However, this did not prevent Pasteur from maintaining and developing his mental abilities and performing outstanding work in the field of biology.

In another well-known case, a 12-year-old child almost had most of the left cerebellum removed after undergoing quadruple surgery for a brain tumor. Immediately after each operation, the child experienced disorders in the motor sphere, speech and other brain functions. However, these violations were quickly compensated.

The compensatory capabilities of the brain decrease with age, this is due to the weakening of lability in the formation of new functional connections.

Features of the central nervous system

Providing compensation mechanisms

Impaired functions

Physiological mechanisms of compensation for dysfunctions of the CNS formations are based on the specific properties of neurons in the subcortical and cortical structures of the brain.

These properties include:

The multifunctionality of each of the elements
nervous system;

Polysensory neurons;

Relative specialization of neurons
other areas of the brain;

Localization of functions in the cortex;

Parallel (simultaneous) processing of different
sensory information;

Ability to self-regulation, self-organization;

dominant mechanism;

Reflex principle of functioning;

Feedback;

Redundancy is structural and functional;

Reliability;

Functional asymmetry;

The principle of a common final path;

The ability of nerve elements to synchronize
tions of activity;

Plasticity of nerve centers and individual ones
ronov;

Principle of irradiation and concentration actively
sti;

Integration of the nervous system.

Polyfunctionality. The main function of the nervous system is to collect, process, store, reproduce and transmit information in order to organize intellectual, behavioral activities, regulate the functioning of organs, organ systems and ensure their interaction.

Many of these functions are already implemented at the subneuronal level. Thus, microtubules, synapses, dendrites, neuronal membranes have the ability to perform all the informational functions of the nervous system: perception, processing, storage, multiple reproduction and transmission of information. This is the basic principle of the functioning of the nervous system - the principle of multifunctionality.

Polyfunctionality is inherent in most structures of the central nervous system. For example, stimulation of the same structure of the globus pallidus with different impulse frequencies can cause either a motor or autonomic response. The sensorimotor cortex is able to perceive signals from skin, visual, auditory and other types of reception. V

response to these signals in the sensorimotor cortex, reactions are formed that usually occur during the normal activity of the cortical end of the visual, auditory or other analyzers.

Therefore, due to multifunctionality, the same function can be performed by different brain structures. This fundamental point testifies to the almost limitless possibilities of compensating the function in the central nervous system.

The properties of the polyfunctionality of the nerve centers are closely related to the property polysensory neurons.

Polysensory is the ability of one neuron to respond to signals from different afferent systems. Neurophysiologists distinguish monosensory neurons, responding to only one type of signal, bi-sensory - - reacting to two different signals, for example, some neurons of the visual cortex can respond to visual and auditory stimuli. Finally, there are neurons in the cerebral cortex that respond to three or more types of signals. These neurons are called polysensory.

In addition to the ability to respond to stimuli from different sensory systems, neurons in certain areas of the brain are able to respond to only one characteristic of sensory stimulus, for example, to a certain frequency of a sound or only to one color. Such neurons are called monomodal.

Monomodal neurons are highly selective and highly sensitive to certain types of stimuli, i.e. these neurons are specialized. Specialized neurons are localized in the areas of primary projections

analyzers. Such zones are the primary areas of the visual, auditory, skin and other cortical zones.

The predominant location of monosensory neurons determines function localization in the bark. In the history of studying the localization of functions in the cerebral cortex, two ideas can be distinguished: according to one of them, motor and sensory functions are represented by strictly local areas, damage to which should forever exclude one or another function. The opposite view justified equipotentiality cortex in the implementation of sensory and motor skills.

As a result of many years of research into the central nervous system, a compromise view has been formed. At present, it can be considered established that the localization of functions in the cortex is determined primarily by monosensory neurons, which have the lowest thresholds of sensitivity to their adequate stimuli. However, next to these neurons there are always polysensory neurons that provide interaction of the local structure with other brain structures, and thus the possibility of forming a temporary connection, compensating for violations of the functions of their structure and the structures associated with it.

In cases where a neuron responds to two signs of the same sensory stimulus, for example, two colors of visual stimulation or two tones of auditory stimulation, these neurons are classified as bimodal. Neurons that respond to three or more signs of one sensory channel are called polymodal.

Polymodal neurons provide intrasystem compensation for impaired functions.

In parallel with this, another compensation mechanism is also possible - due to the ability of monomodal neurons to become bi- and polymodal.

In experiments with recording the activity of individual neurons, it was shown that monomodal neurons of the auditory cortex responding to a tone with a frequency of 1,000 Hz, when a tone was applied at a frequency of 500 Hz, did not initially respond to this signal, and after a series of combinations of a 500 Hz tone with extracellular depolarization of a monomodal neuron through a microelectrode, the latter was trained to respond to a tone of 500 Hz. Consequently, the neuron became bimodal and, due to this, could compensate for the disturbances caused by the death of neurons capable of responding to signals with a frequency of 500 Hz.

Fundamentally the same mechanism of temporal connection underlies the training of monosensory neurons to respond to stimuli of different sensibility, i.e. to the signals of different analyzer systems. In this case, we are talking about inter-analyzer, inter-system compensation.

There is no such zone in the cerebral cortex that would be associated with the implementation of only one function. Different parts of the brain have different numbers of polysensory and polymodal neurons. The largest number of such neurons is located in the associative and secondary, tertiary zones of the cortical end of the analyzers. A significant part of the motor cortex neurons (about 40%) is also polysensory; they respond to skin irritations, to sound, and light. In the 17th field of the visual cortex, about 15% of neurons belong to the polysensory, and in the 18-19th fields of the same cortex, more than 60% of such neurons. In the cranked bodies, up to 70% of neurons respond to sound and light irritation, and 24% to skin irritation. Non-specialized neurons also have the property of polysensory

physical nuclei of the thalamus, the red nucleus of the midbrain, the caudate nucleus, the putamen, the nuclei of the auditory system of the brain stem, the reticular formation.

The number of polysensory neurons in brain structures varies depending on the functional state of the nervous system and on the task being performed at a given time. Thus, during the period of learning with the participation of visual and motor analyzers, the number of polysensory neurons in these areas of the cortex increases. Consequently, directed learning creates conditions for an increase in polysensory neurons and, thereby, the compensatory capabilities of the nervous system increase.

The presence of polysensory neurons, an increase in their number under functional loads on the nervous system determine the dynamic possibilities of compensating its structures in various dysfunctions.

It is also important for clinical medicine that some neurons of the cerebral cortex, as a result of training, are able to become polysensory, i.e. if before applying a combination of conditioned and unconditioned stimuli, the neuron responded only to the unconditioned stimulus, then after a series of combinations this neuron becomes able to respond to the conditioned stimulus as well.

Polymodality and polysensory allow the neuron to simultaneously perceive stimuli from different analyzers or, if from one analyzer, then simultaneously perceive signals with different characteristics. Simultaneous parallel perception of signals implies their simultaneous parallel processing. This is evidenced by conditioned reflex experiments, in which it is shown that as a result of the development of a conditioned reflex to a simultaneous complex of signals,

presented to different analyzers (for example, auditory and visual), it can be caused by any separate signal of this complex.

Polyfunctionality and polysensory are associated with another property of the functioning of the brain - its reliability. Reliability is provided, in addition to polysensory and polyfunctionality, by such mechanisms as redundancy, modularity, co-operation.

Redundancy, as an element of ensuring the reliability of the functioning of the brain, is achieved in different ways. The most common is the reservation of elements. In humans, only fractions of a percent of neurons are constantly active in the cortex, but they are enough to maintain the tone of the cortex, which is necessary for the implementation of its activity. When the functioning of the cortex is disturbed, the number of background-active neurons in it increases significantly.

The redundancy of elements in the CNS ensures the preservation of the functions of its structures even if a significant part of them is damaged. For example, the removal of a significant part of the visual cortex does not lead to visual impairment. Unihemispheric damage to the structures of the limbic system does not cause clinical symptoms specific to the limbic system. The proof that the nervous system has large reserves are the following examples. The oculomotor nerve normally realizes its functions of regulating the movements of the eyeball, while only 45% of neurons are preserved in its nucleus. The abducens nerve normally innervates its muscle with the preservation of 38% of the neurons of its nucleus, and the facial nerve performs its functions with only 10% preservation of the number of neurons located in the nucleus of this nerve.

High reliability in the nervous system is also due to the many connections of its structures, a large number of synapses on neurons. So, cerebellar neurons have up to 60 thousand synapses on their body and dendrites, pyramidal neurons of the motor cortex - up to 10 thousand, alpha motor neurons of the spinal cord - up to 6 thousand synapses.

Redundancy manifests itself in a variety of signaling ways; Thus, a duplicated motor signal coming from the cortex to the motor neurons of the spinal cord can reach them not only from the pyramidal neurons of the 4th field of the cortex, but also from the additional motor zone, from other projection fields, from the basal ganglia, the red nucleus, the reticular formation and other structures . Therefore, damage to the motor cortex should not lead to a complete loss of motor information to the motor neurons of the spinal cord.

Therefore, in addition to redundancy, the reliability of the nervous system is achieved by duplication, which allows you to quickly enter, as needed, additional elements to implement a particular function. An example of such duplication is the multichannel transmission of information, for example, in a visual analyzer.

When the reliability of the functioning of the brain is not ensured due to duplication and redundancy, the mechanism of the probabilistic participation of neurons in the implementation of a given function is activated. The probabilistic mechanism creates an operational redundancy in the participation of nerve cells of various modules for the organization of a particular reaction. The probabilistic principle of the functioning of the nervous system is that neurons do not act in isolation, but in a population. Naturally, a single state of all her

of the population when a signal arrives in it is impossible. The participation of an individual neuron in the organization of the reaction is determined by its state (excitability threshold, impulse generalization, etc.). In this regard, participation in the reaction can be realized or not, i.e. it is probabilistic.

Modularity is the principle of the structural and functional organization of the cerebral cortex, which lies in the fact that in one neural module, local processing of information from receptors of one modality is carried out. There are two types of modules: micromodules and macromodules. Micromodules in the somatosensory cortex are an association of 5-6 neurons, among which there are pyramidal neurons, their apical dendrites form a dendritic bundle. Between the dendrites of this bundle, not only synaptic connections take place, but also electrotonic contacts. The latter ensure the synchronous operation of the micromodule neurons, which increases the reliability of information transmission.

The micromodule also contains stellate cells. They have synapses on the pyramidal neurons of their module and contacts from the ascending thalamo-cortical fibers. Some stellate cells send axons along the surface of the cortex, thus creating conditions for the transfer of information from one cortical module to another and forming an inhibitory environment around the active module.

Micromodules are combined into macromodules - vertically oriented columns (according to Mountcastle), their diameter reaches 500-1000 microns. Mountcastle found that when the microelectrode is immersed perpendicular to the surface of the cortex, all the neurons recorded in this case respond to stimulation of one sensory stimulus (for example, to light).

When the microelectrode was immersed at an angle to the cortical surface, neurons with different sensitivities were encountered along its path; responding to different signals (for example, to light, sound).

It is believed that in this case the microelectrode penetrates adjacent columns and registers neurons with different sensitivities. Based on the studies of Mountcastle et al., monosensory, monofunctionality of the column is recognized.

This conclusion contradicts the principle of polysensory neurons. In one module, there should be both monosensory or monomodal neurons and polysensory neurons, otherwise the information reliability of the nervous system, its plasticity, and, therefore, the ability to form new functional compensatory connections are sharply reduced.

In the visual cortex, there is an alternation of columns, the neurons of which respond to visual stimuli either only in the right or only in the left eye. Consequently, in the visual cortex of both hemispheres of the brain there are eye-dominant columns, i.e. columns that respond to stimulation of one eye.

In the auditory cortex, columns are distinguished that are capable of differentiating signals coming from both ears, and columns that are not capable of such differentiation.

In the sensorimotor cortex, nearby columns perform multidirectional reactions: for example, some of them excite the motor neurons of the spinal cord, while others inhibit them.

The modular principle of the structural and functional organization of the brain is a manifestation of the cooperative nature of the functioning of brain neurons. Cooperativeness allows the neurons of the module to participate in the implementation of the function according to the probabilistic

mu type, which creates the possibility of relative interchangeability of neurons, and, thereby, increases the reliability of nervous activity. As a result, the functioning of the system becomes little dependent on the state of a single nerve cell. On the other hand, the mobile structure of such working units, formed by the probabilistic participation of nerve cells in them, determines the greater flexibility of interneuronal connections and the ease of their rearrangements, which determine the plasticity properties characteristic of the higher parts of the brain.

Cooperativeness enables the structure to perform functions that are not inherent in its individual elements. So, a single brain neuron is not capable of learning, but being in a network of neurons, it acquires such an ability.

Cooperativeness makes it possible to implement the mechanisms of self-regulation and self-organization inherent in the nervous system from the earliest stages of its organization.

Self-regulation is a property of the structures of the nervous system to automatically establish and maintain its functioning at a certain level. The main mechanism of self-regulation is the feedback mechanism. This mechanism is well illustrated by the example of supporting reverberation during the interhemispheric development of an epileptic convulsive state. Feedback in the nervous system has either an amplifying, or inhibitory, or purely informational value about the results of the activity, the reaction of the system to which the signal was addressed.

Feedback streamlines, narrows the set of options for the passage of the signal, creating an inhibitory environment for the excitation path of inactive neurons.

The mechanism of its self-organization is closely connected with the self-regulation of the nervous system. Self-organizing systems generally have a number of features that are inherent in the central nervous system:

Lots of inputs;

Lots of exits;

High level the complexity of the interaction
their elements;

A large number of functioning elements
Comrade;

The presence of probabilistic and rigid determinants
forged connections;

The presence of a function of transition states;

Lots of features;

The presence of an output function with feedback.
Due to the principle of self-organization of compensation
function in the nervous system is provided by
the changes in the weights of the functioning of connections, forms
formation of new connections based on inclusion in the
activity of potential synapses, using
accumulated experience of the individual.

The development of the nervous system in phylogenesis and ontogenesis leads to a continuous complication of the interaction of its systems. How more shapes, species, the number of conditioned reflexes organized in ontogenesis, the more connections are established between the structures of the nervous system.

An increase in the number of functional connections between the structures of the nervous system has crucial, since in this case the number of signal passage options increases, the possibilities for compensating impaired functions are significantly expanded.

Due to self-organization, the development of clinical signs of the pathology of the nervous system does not manifest itself at a certain stage.

Self-organization leads to qualitative changes in the interaction of systems, which makes it possible to implement a function disturbed by pathology. It is important here that the nervous system, in addition to the possibility of a large choice of ways to achieve the goal, is able to selectively amplify or weaken signals.

In the first case, when the signal is amplified, reliable transmission of information is ensured with partial morphological preservation of the structure.

In the second case, when the signal is weakened, it becomes possible to reduce the interference coming from other sources. Since the nervous system is capable of selectively filtering the desired signal, this allows it, by highlighting the desired, but weak signal, firstly, to directly amplify it, and secondly, to give it an advantage when passing to the perceiving structure by reducing the strength of unnecessary, interfering signals.

The compensatory capabilities of the nervous system are also associated with the specific localization of functions in the cerebral cortex, which is not absolute. First of all, each cortical end of the analyzer has primary, secondary and tertiary fields.

The primary fields of the cortex correspond to the architectonic fields of the cortex, in which the sensory projection pathways terminate. These zones are connected with the peripheral receptor systems in the most direct ways, they have a clear somatotopic localization, and a qualitative analysis of incoming specific signals is carried out in them. The defeat of these zones leads to elementary disorders of sensitivity.

The secondary fields of the crust are close to the primary ones. In the secondary fields associated with the receptor systems directly and indirectly, continuing

signal processing is carried out, its biological significance is determined, connections are established with other analyzers and with the executive, more often with the motor system. The defeat of this zone leads to disorders specific to this analyzer of memory and perception.

Tertiary, or associative, zones are located in areas of mutual overlap of analyzers and occupy a large part of the cortical representation of this analyzer in a person.

The neuronal associations of these zones are most adapted to establish communication with other areas of the brain, and thus are most adapted to the implementation of compensatory processes. Damage to the associative areas does not lead to disorders of the specific functions of the analyzers, but manifests itself in the most complex forms of analytical and synthetic activity (gnosis, praxis, speech, purposeful behavior) associated with the function of this analyzer.

Structural localization of functions suggests that the brain has deterministic pathways, systems that implement signal transmission, organization of a particular reaction, etc. However, in addition to rigidly determined connections in the brain, functional connections are realized that develop in ontogeny.

The more strengthened and fixed the connections between the structures of the brain in the process of individual development, the more difficult it is to use compensatory possibilities in pathologies.

Based on the principle of structure, the mechanism of hierarchy is implemented. It consists not so much in subordination as in the organization of compensatory processes. Each overlying structure participates in the implementation of the functions of the underlying one, but de-

it barks when the underlying structure finds it difficult to perform its functions.

The structures of the brain during learning, with dysfunction of one of them, do not localize excitation within their boundaries, but allow it to spread widely throughout the brain - the principle of irradiation.

Irradiation of the state of activity spreads to other brain structures both through direct connections and through indirect pathways. The occurrence of irradiation in case of hypofunction of the structure involved in the implementation of a particular process makes it possible to find ways to compensate for hypofunction and to realize the desired reaction.

Finding a new path is fixed according to the reflex principle and ends with the concentration of activity in certain structures that are interested in performing the reaction.

Convergence and the principle of a common final path are closely related to the concentration of activity in certain brain structures. This principle is implemented on a separate neuron and at the system level. In the first case, the information in the neuron is collected on the dendrites, the soma of the neuron, and is transmitted mainly through the axon. Information from a neuron can be transmitted not only through the axon, but also through dendritic synapses. Information is fed through the axon to the neurons of other brain structures, a through synapses of dendrites only to neighboring neurons.

Having a common final pathway allows the nervous system to have different variants achieving the desired effect through different structures that have access to the same final path.

Difficulties in compensation, noted at older ages, are due not to the fact that the reserves of the brain have been exhausted, but to the fact that a large

the number of optimal ways to implement the function, which, although they are activated in the case of pathology, but because of it, they cannot be implemented. More often, pathology requires the formation of new ways of implementing a particular function.

The formation of new ways, new functions of the brain structure is based on the following principle of its functioning - the principle of plasticity.

Plasticity allows the nervous system, under the influence of various stimuli, to reorganize connections in order to preserve the main function or to implement a new function.

Plasticity allows nerve centers to realize functions that were not previously inherent to them, but thanks to existing and potential connections, these centers become able to participate in compensating for functions disturbed in other structures. Polyfunctional structures have great possibilities of plasticity. In this regard, non-specific systems of the brain, associative structures, secondary projection zones of analyzers, as having a significant number of polyfunctional elements, are more capable of plasticity than the zones of primary projections of analyzers. A clear example of the plasticity of nerve centers is the classic experience of P.K. Anokhin with a change in the connections of the centers of the phrenic and brachial nerves.

In this experiment, the phrenic and brachial nerves were cut and the central end of the phrenic nerve was attached to the peripheral end of the brachial, and, conversely, the central end of the brachial nerve to the peripheral phrenic. After some time after the operation, the correct regulation of breathing and the correct sequence of voluntary movements were restored in the animal.

Consequently, the nerve centers rebuilt their function in such a way as was required by the peripheral muscular system, with which a new connection was established.

In the early stages of ontogeny, restructuring of this type proceeds more completely and dynamically.

The most significant role in compensating dysfunctions of brain structures is played by reflex the principle of its functioning. Each new reflex connection between the structures of the brain is a new state of it, allowing you to implement the required in this moment function.

Neurodegenerative diseases such as Alzheimer's or Parkinson's disease, strokes, injuries lead to the loss of nerve cells and, accordingly, the function of the organ that these cells performed. The ability of the adult mammalian brain, including humans, to compensate for these losses is very limited. Therefore, scientists are exploring the possibilities of transplanting nerve cells, replacing lost neurons with new ones. Until recently, it was not known whether the transplanted neurons could integrate into existing neural circuits enough to restore the functions of the affected area of ​​the brain.

German researchers from the Max Planck Institute for Neurobiology, the Ludwig-Maximilian University of Munich and the Helmholtz Center of Munich set out to find out whether transplanted embryonic mouse nervous tissue cells can integrate into the damaged visual cortex of adult mice. This area of ​​the brain is ideal for such experiments, the scientists say, because enough is known about the structural and functional interconnections of neurons in the visual cortex to easily assess whether new neurons will actually perform the desired function.

Scientists have surgically destroyed cells in the primary visual cortex of mice, the area of ​​the brain where signals from the retina are integrated. A few days later, embryonic, immature mouse neurons were transplanted into the injury site.

Over the following weeks, the "behavior" of the implanted neurons was monitored using two-photon microscopy to see if they differentiated into the type of cells normally found in that region of the brain, the so-called pyramidal neurons. The process of integration of transplanted neurons was similar to the process of normal development, including the order of morphological maturation of cells - the development of axons, dendrites, dendritic spines. Within two months, the introduced neurons acquired the morphology of typical mature pyramidal cells.

As for function, pyramidal cells derived from transplanted immature neurons formed normal functional connections, could respond to visual stimuli, process information and correctly transmit it further. That is, implanted neurons with high precision integrated into neural networks.

Without the intervention of scientists, new nerve cells would never have appeared in the damaged area of ​​the cortex. The brain of an adult mammal can regenerate - but by introducing immature neurons into the site of damage.

Similar experimental operations are also being performed on humans, for example, the transplantation of embryonic stem cells into the affected area of ​​the brain of a patient with Parkinson's disease was first performed more than twenty years ago, and such experiments continue - however, with varying success. Of course, it is still very far from treating people in this way “on the fly” due to the problems of using embryonic cells, both ethical and practical, associated with a high risk of developing a malignant tumor.

Photo: https://www.flickr.com NIH Image Gallery. Credit: Scott Vermilyea, Neuroscience Training Program, School of Medicine and Public Health and neurobiology undergraduate Scott Guthrie, with SCRMC members Ted Golos and Marina Emborg, professors in the School of Medicine and Public Health and Wisconsin National Primate Research Center.

Prepared by Maria Perepechaeva

In cases where there is a "breakdown" of any mechanism of the brain, the process of development and learning is disrupted. "Breakdown" can occur on different levels: information input, its reception, processing, etc. may be violated. For example, damage to the inner ear with the development of hearing loss causes a decrease in the flow of sound information. This leads, on the one hand, to functional and then to structural underdevelopment of the central (cortical) section of the auditory analyzer, on the other hand, to underdevelopment of connections between the auditory cortex and the motor zone of the speech muscles, between the auditory and other analyzers. Under these conditions, phonemic hearing and the phonetic formation of speech are disturbed. Not only the speech, but also the intellectual development of the child is disturbed. As a result, the process of his training and education becomes much more difficult.

Thus, underdevelopment or violation of one of the functions leads to underdevelopment of another or even several functions. However, the brain has significant compensatory capabilities. We have already noted that the unlimited possibilities of associative connections in the nervous system, the absence of a narrow specialization of the neurons of the cerebral cortex, the formation of complex “ensembles of neurons” form the basis of the great compensatory possibilities of the cerebral cortex.

The reserves of compensatory possibilities of the brain are truly grandiose. According to modern calculations, the human brain can hold approximately 1020 units of information; this means that each of us is able to remember all the information contained in the millions of volumes of the library. Of the 15 billion cells in the brain, humans use only 4%. The potential capabilities of the brain can be judged by the extraordinary development of any function in talented people and the ability to compensate for impaired function at the expense of other functional systems. In the history of various times and peoples, a large number of people are known who possessed a phenomenal memory. The great commander Alexander the Great knew by name all his soldiers, of whom there were several tens of thousands in his army. A. V. Suvorov possessed the same memory for faces. Giuseppe Mezzofanti, the chief custodian of the library in the Vatican, was striking in his phenomenal memory. He was fluent in 57 languages. Mozart had a unique musical memory. At the age of 14 in the Cathedral of St. Peter, he heard church music. The notes of this work were the secret of the papal court and were kept in the strictest confidence. The young Mozart “stole” this secret in a very simple way: when he came home, he wrote down the score from memory. When, many years later, it was possible to compare Mozart's notes with the original, there was not a single mistake in them. The artists Levitan and Aivazovsky had exceptional visual memory.

A large number of people are known who have an original ability to memorize and reproduce a long series of numbers, words, etc.

These examples clearly demonstrate the unlimited possibilities of the human brain. In the book “From Dream to Discovery”, G. Selye notes that as much mental energy is contained in the human cerebral cortex as physical energy is contained in the atomic nucleus.

Large reserve capabilities of the nervous system are used in the process of rehabilitation of persons with certain developmental disabilities. With the help of special techniques, a defectologist can compensate for impaired functions at the expense of intact ones. So, in the case of congenital deafness or hearing loss, a child can be taught visual perception oral speech, i.e. lip reading. Tactile speech can be used as a temporary substitute for oral speech. If the left temporal region is damaged, a person loses the ability to understand speech addressed to him. This ability can be gradually restored through the use of visual, tactile and other types of perception of speech components.

Thus, defectology bases its methods of work on the habilitation and rehabilitation of patients with lesions of the nervous system on the use of the enormous reserve capabilities of the brain.