Allergy (definition). General etiology and general pathogenesis. Types of hypersensitivity

Anatomy of the upper respiratory tract of a dog with brachiocephalic syndrome. Pink marks indicate areas of narrowing of the airways.

Neural networks l

Neural networks are a branch of artificial intelligence in which phenomena similar to those occurring in the neurons of living beings are used to process signals. The most important feature of the network, testifying to its wide capabilities and enormous potential, is the parallel processing of information by all links. With a huge number of interneuron connections, this can significantly speed up the process of information processing. In many cases, real-time signal conversion becomes possible. In addition, with a large number of interneuron connections, the network becomes resistant to errors that occur on some lines. The functions of damaged links are taken over by healthy lines, as a result of which the network activity does not undergo significant disturbances.

l Another equally important property is the ability to learn and generalize accumulated knowledge. A neural network has the features of artificial intelligence. A network trained on a limited set of data is able to generalize the information received and show good results on data that was not used in the training process.

l Feature The network also lies in the possibility of its implementation using ultra-high degree of integration technology. The differences in network elements are small, but their repeatability is enormous. This opens up the prospect of creating a universal processor with a homogeneous structure capable of processing a variety of information.

l The use of these properties against the backdrop of the development of devices with a very large degree of integration (VLSI) and the widespread use of computing technology has caused a huge increase in interest in neural networks and significant progress in their research in recent years. A basis has been created for the development of new technological solutions related to perception, artificial recognition and generalization of video information, control of complex systems, processing of speech signals, etc. Artificial neural networks in practical applications are usually used as a control or decision-making subsystem that transmits an executive signal to other subsystems that have a different methodological basis.

l The functions performed by networks are divided into several groups: approximation; classification and pattern recognition; forecasting; identification and assessment; associative management. l The approximating network plays the role of a universal approximator of a function of several variables, which implements a nonlinear function of the form y = f(x), where x is the input vector, and y is the implemented function of several variables. Many problems of modeling, identification, and signal processing can be formulated in an approximation formulation. l To classify and recognize images, the network accumulates, during the learning process, knowledge about the basic properties of these images, such as the geometric display of the image structure, principal component distribution (PCA), or other characteristics. When generalizing, the differences between images from each other are emphasized, which form the basis for developing classification decisions. Functions of neural networks l In the field of forecasting, the network task is formulated as predicting the future behavior of the system based on the existing sequence of its previous states. Based on information about the values ​​of the variable at points in time preceding the prediction, the network makes a decision about what the estimated value of the sequence under study should be equal to at the current point in time. l

In problems of controlling dynamic processes, a neural network usually performs several functions. Firstly, it presents a nonlinear model of this process and identifies its main parameters necessary to generate the appropriate control signal. Secondly, the network performs the functions of a tracking system, monitors changing conditions environment and adapts to them. It can also play the role of a neuroregulator, replacing traditional devices. Of great importance, especially when controlling robots, are the classification of the current state and making decisions about the further development of the process. Functions of neural networks l

In association problems, the neural network acts as an associative memory device. Here we can distinguish auto-associative type memory, in which interdependencies cover only specific components of the input vector, and hetero-associative type memory, with the help of which the network determines the relationships of various vectors. Even if a vector distorted by noise or devoid of individual fragments of data is supplied to the network input, the network is able to restore the complete and noise-free original vector by generating the corresponding output vector. l

Various ways of combining neurons with each other and organizing their interaction led to the creation of networks different types. Each type of network, in turn, is closely related to the corresponding method for selecting the weights of interneuron connections (i.e., learning). Functions of neural networks l It seems interesting to combine various types neural networks among themselves, especially networks with self-organization and supervised learning. Such combinations are called “hybrid networks”. The first component is a network with self-organization based on competition, operating on a variety of input signals and grouping them into clusters based on the coincidence of properties. It plays the role of a data preprocessor. The second component, in the form of a network trained with a teacher (for example, a perceptron), matches input signals assigned to specific clusters with the corresponding specified values ​​(post-processing). Such a network structure makes it possible to divide the training phase into two parts: first, the component with self-organization is trained, and then the network with a teacher. An additional advantage of this approach is that it reduces the computational complexity of the learning process, as well as better interpretation of the results obtained.

Excitation in the central nervous system spreads along various configurations of nerve chains. In all studied nerve networks the following were found:

1) convergence of paths transmitting information to higher as well as executive centers;

2) divergence of pathways transmitting certain signals;

3) reverberation, or neural traps.

Convergence is the convergence of several nerve pathways to the same neurons or nerve centers.

The convergence of many neural pathways to a single neuron makes that neuron (or nerve center) an integrator of the corresponding signals. Its state (impulse or inhibition) at each moment of time is determined by the algebraic addition of the mass of exciting and inhibitory inputs. In other words, the sum of all its EPSPs and IPSPs arriving at a given neuron. If we are talking about a motor neuron, i.e., the final link of the nervous path to the muscles, we talk about the principle of a common final path. For example, in vertebrates, on each motor neuron of the spinal cord and brain stem, thousands of sensory, as well as excitatory and inhibitory interneurons of different levels form synoptic endings. Powerful convergence is also found on neurons of the reticular formation of the brain stem. Another area of ​​“application” of convergence is “sensory funnels”. The essence of funnels is that the number of inputs to it is less than the number of outputs. Thanks to convergence, “compression” occurs, reducing the amount of information coming from the receptors to the central nervous system. Convergence is also involved in the processes of spatial facilitation and occlusion.

Divergence is the contact of one neuron or nerve center with many neurons or nerve centers. Thus, there is a division of the axon of a sensory neuron in the spinal cord into many collaterals. The resulting branches are directed to different segments of the spinal cord and to the brain.

Divergence of the signal path is observed in many interneurons and command cells. Thanks to divergence, parallel computing processes can be formed, which ensures a high level of performance of the central nervous system. Path divergence provides expansion of the signal propagation area. This forms the spread of the excitation process to other nerve centers, i.e. irradiation, excitation or inhibition.

The process of irradiation plays a positive role in the formation of new reactions of the body, since the activation of a large number of different nerve centers makes it possible to select from among them those most necessary for subsequent activities and to form functional connections between them, i.e., to improve the body’s response. Thanks to this process, conditioned reflexes arise between different nerve centers.

The irradiation of excitation can also have a negative impact on the state and behavior of the body. Thus, the irradiation of strong excitation in the central nervous system disrupts the subtle relationships that have developed between the processes of excitation and inhibition in the nerve centers and leads to a disorder of motor activity. For example, in epilepsy, excitation from the pathological focus radiates to a large number of nerve centers in the cerebral cortex.

All features of the spread of excitation in the central nervous system are explained by its neural structure: the presence of chemical synapses, multiple branching of neuron axons, and the presence of closed neural pathways. These features are the following.

1. Irradiation (divergence) of excitation in the central nervous system. It is explained by the branching of neuron axons, their ability to establish numerous connections with other neurons, and the presence of interneurons, the axons of which also branch (Fig. 4.4, a).

The irradiation of excitation can be observed in an experiment on a spinal frog, when weak stimulation causes flexion of one limb, and strong stimulation causes energetic movements of all limbs and even the torso. Divergence expands the scope of each neuron. One neuron, sending impulses to the cerebral cortex, can participate in the excitation of up to 5000 neurons.

Rice. 4.4. Divergence of afferent dorsal roots onto spinal neurons, the axons of which, in turn, branch, forming numerous collaterals (c), and convergence of efferent pathways from various parts of the central nervous system onto the α-motoneuron of the spinal cord (6)

1. Convergence of excitation(the principle of a common final path) - the convergence of excitation of different origins along several paths to the same neuron or neural pool (the Sherrington funnel principle). The convergence of excitation is explained by the presence of many axon collaterals, intercalary neurons, and also by the fact that there are several times more afferent pathways than efferent neurons. One CNS neuron can have up to 10,000 synapses. The phenomenon of convergence of excitation in the central nervous system is widespread. An example is the convergence of excitation on a spinal motor neuron. Thus, primary afferent fibers approach the same spinal motor neuron (Fig. 4.4, b), as well as various descending pathways many overlying centers of the brain stem and other parts of the central nervous system. The phenomenon of convergence is very important: it ensures, for example, the participation of one motor neuron in several different reactions. The motor neuron innervating the muscles of the pharynx is involved in the reflexes of swallowing, coughing, sucking, sneezing and breathing, forming common final path for numerous reflex arcs. In Fig. 4.4, I show two afferent fibers, each of which sends collaterals to 4 neurons in such a way that 3 neurons out of a total of 5 form connections with both afferent fibers. On each of these 3 neurons, two afferent fibers converge.

Many axon collaterals, up to 10,000-20,000, can converge on one motor neuron, so the generation of APs at each moment depends on the total sum of excitatory and inhibitory synaptic influences. APs arise only if excitatory influences predominate. Convergence may facilitate the process of excitation on common neurons as a result of spatial summation of subthreshold EPSPs or block it due to the predominance of inhibitory influences (see section 4.8).

3. Circulation of excitation along closed neural circuits. It can last for minutes or even hours (Fig. 4.5).

Rice. 4.5. Circulation of excitation in closed neural circuits according to Lorento de No (a) and according to I.S. Beritov (b). 1,2,3- excitatory neurons

Circulation of excitation is one of the causes of the phenomenon aftereffects, which will be discussed further (see section 4.7). It is believed that the circulation of excitation in closed neural circuits is the most likely mechanism for the phenomenon of short-term memory (see section 6.6). Circulation of excitation is possible in a chain of neurons and within one neuron as a result of contacts of the branches of its axon with its own dendrites and body.

4. One-sided propagation of excitation in neural circuits and reflex arcs. The propagation of excitation from the axon of one neuron to the body or dendrites of another neuron, but not vice versa, is explained by the properties of chemical synapses, which conduct excitation in only one direction (see section 4.3.3).

5. Slow spread of excitation in the central nervous system in comparison with its spread along the nerve fiber is explained by the presence of many chemical synapses along the paths of excitation propagation. The time it takes for excitation to pass through the synapse is spent on the release of the transmitter into the synaptic cleft, its propagation to the postsynaptic membrane, the occurrence of EPSP and, finally, AP. The total delay in the transmission of excitation at the synapse reaches approximately 2 ms. The more synapses in a neuronal chain, the lower the overall speed of excitation propagation along it. Using the latent time of the reflex, or more precisely, the central time of the reflex, you can roughly calculate the number of neurons in a particular reflex arc.

6. The spread of excitation in the central nervous system is easily blocked by certain pharmacological drugs, which is widely used in clinical practice. Under physiological conditions, restrictions on the spread of excitation throughout the central nervous system are associated with the activation of neurophysiological mechanisms of neuronal inhibition.

The considered features of the propagation of excitation make it possible to approach the understanding of the properties of nerve centers.

PROPERTIES OF NERVE CENTERS

The properties of nerve centers discussed below are explained by certain features of the propagation of excitation in the central nervous system, the special properties of chemical synapses and the properties of nerve cell membranes. The main properties of nerve centers are the following.

A. Background activity of nerve centers (tone) is explained as follows:

Spontaneous activity of CNS neurons;

The humoral influence of biologically active substances circulating in the blood (metabolites, hormones, mediators, etc.), affecting the excitability of neurons;

Afferent impulses from various reflexogenic zones;

The summation of miniature potentials arising as a result of the spontaneous release of transmitter quanta from axons forming synapses on neurons;

Circulation of excitation in the central nervous system.

The significance of the background activity of nerve centers is to ensure a certain initial level of the active state of the center and effectors. This level can increase or decrease depending on fluctuations in the total activity of neurons in the nerve center-regulator.

B. Transformation of the rhythm of excitation - this is a change in the number of impulses arising in the neurons of the center at the output, relative to the number of impulses arriving at the input of this center.

Transformation of the rhythm of excitation is possible both in the direction of increase and decrease. An increase in the number of impulses arising in the center in response to afferent impulses is facilitated by the irradiation of the excitation process (see section 4.6) and the aftereffect. The decrease in the number of impulses in the nerve center is explained by a decrease in its excitability due to the processes of pre- and postsynaptic inhibition, as well as an excess flow of afferent impulses. With a large flow of afferent influences, when all the neurons of the center or neuronal pool are already excited, a further increase in afferent inputs does not increase the number of excited neurons.

B. Inertia- a relatively slow onset of excitation of the entire complex of neurons of the center when impulses arrive to it and a slow disappearance of excitation of the neurons of the center after the cessation of input impulses. The inertia of the centers is associated with the summation of excitation and aftereffect.

I. The phenomenon of summation excitation in the central nervous system was discovered by I.M. Sechenov (1868) in an experiment on a frog: irritation of a frog’s limb with weak, rare impulses does not cause a reaction, and more frequent irritations with the same weak impulses are accompanied by a response - the frog makes a jump. Distinguish temporary (sequential) sulilation and spatial summation(Fig. 4.6).

Temporal summation. In Fig. 4.6 on the left shows a diagram for experimental testing of the effects caused in a neuron by rhythmic stimulation of the axon. The recording above allows us to see that if EPSPs quickly follow each other, they add up due to their relatively slow time course (a few milliseconds), eventually reaching the threshold level. Temporal summation is due to the fact that the EPSP from the previous pulse is still ongoing when the next pulse arrives. Therefore, this type of summation is also called sequential summation. It plays an important physiological role because many neural processes are rhythmic in nature and, thus, can be summed up, giving rise to suprathreshold excitation in neural associations of nerve centers.

Spatial summation (see Fig. 4.6, b). Separate stimulation of each of the two axons produces a subthreshold EPSP, whereas simultaneous stimulation of both axons produces an AP, which cannot be achieved by a single EPSP. Spatial summation is associated with such a feature of the propagation of excitation as convergence.

2. Aftereffect - this is the continuation of excitation of the nerve center after the cessation of impulses reaching it along the afferent nerve pathways. The reasons for the aftereffect are:

Long-term existence of EPSP, if the EPSP is polysynaptic and high-amplitude; in this case, several APs occur with one EPSP;

Repeated occurrences of trace depolarization, which is characteristic of neurons of the central nervous system; if the trace depolarization reaches Ecr, then AP occurs;

Circulation of excitation along closed neural circuits (see section 4.6).

The first two reasons do not last long - tens or hundreds of milliseconds, the third reason - circulation of excitation - can last minutes or even hours. Thus, the peculiarity of the spread of excitation (its circulation) provides another phenomenon in the central nervous system - aftereffect. The latter plays a crucial role in learning processes - short-term memory.

D. Greater sensitivity of the central nervous system to changes in the internal environment: for example, to changes in blood glucose levels, blood gas composition, temperature, and various pharmacological drugs administered for therapeutic purposes. Neuron synapses react first. CNS neurons are especially sensitive to a lack of glucose and oxygen. When glucose levels drop 2 times below normal, seizures may occur. Severe consequences for the central nervous system are caused by a lack of oxygen in the blood. Stopping blood flow for just 10 seconds leads to obvious disturbances in brain function: a person loses consciousness. If blood flow stops for 8-12 minutes, then irreversible disturbances in brain activity occur; many neurons die, primarily cortical ones, which leads to serious consequences.

D. Fatigue of nerve centers demonstrated by N.E. Vvedensky in an experiment on a frog preparation with repeated reflex stimulation of contraction of the gastrocnemius muscle using irritation of the tibial ( P. tibialis) and fibular ( P. peroneus) nerves. In this case, rhythmic stimulation of one nerve causes a rhythmic contraction of the muscle, leading to a weakening of the force of its contraction until the complete absence of contraction. Switching stimulation to another nerve immediately causes a contraction of the same muscle, which indicates the localization of fatigue not in the muscle, but in the central part of the reflex arc (Fig. 4.7).

At the same time, it develops postsynaptic depression(habituation, habituation) - weakening of the center’s reaction to stimulation (afferent impulses), expressed in a decrease in postsynaptic potentials during prolonged stimulation or after it. This weakening is explained by the consumption of the mediator, the accumulation of metabolites, and the acidification of the environment during long-term conduction of excitation along the same neural circuits.

E. Plasticity of nerve centers- the ability of nerve elements to rearrange functional properties. The main manifestations of this property are the following: post-tetanic potentiation and depression, dominance, the formation of temporary connections, and in pathological cases - partial compensation of impaired functions.

1. Post-tetanic potentiation(synaptic facilitation) is an improvement in conduction at synapses after a short stimulation of afferent pathways. Short-term activation increases the amplitude of postsynaptic potentials. Relief is also observed during irritation (in the beginning); in this case, the phenomenon is called tetanic potentiation. The degree of relief increases with increasing pulse frequency; relief is greatest when the pulses arrive at intervals of a few milliseconds.

Rice. 4.7. Diagram of N.E. Vvedensky’s experiment, illustrating the localization of fatigue in the reflex arc.

The processes of processing information entering the nerve center (if it is sensory), or the formation of commands to the executive organs (in the efector center) are determined by the interaction of neurons through synaptic contacts. In this case, you can detect phenomena that are called divergence and convergence (Fig. 37).

Divergence is the ability of a neuron to establish numerous connections with other neurons. As a result, the same cell can participate in various nervous processes and reactions, control a large number of other neurons, that is, each neuron can ensure the spread of impulses - the irradiation of excitation. Divergence processes are more typical for the afferent parts of the central nervous system.

Convergence - the convergence of different paths of neuronal impulses to the same nerve cell is more characteristic of the nerve centers of the efferent sections.

Most nerve centers are represented by a cluster of various neurons. Among them there are both excitatory and inhibitory neurons, sensory and motor neurons (afferent or efferent). their rather complex interaction ensures the performance of the corresponding functions.

Interaction of reflexes

In the process of regulation of the majority complex functions of the body, the organization of a reflex response very often involves several nerve centers, which can even be located on different floors of the central nervous system. This is due to the phylogenetic features of the formation of the central nervous system. The appearance of the “junior” department was accompanied by the formation of new centers of regulation in it. But the “old” nerve centers located in the lower sections retained their characteristic functions. At the same time, the absolute autonomy of individual segments of the central nervous system was lost, and an increasing part of the functions were “transferred” to higher departments. This process is called encephalization of functions. Since the brain was formed in stages, from the hindbrain to the forebrain with its cerebral hemispheres, then with the formation of the cerebral cortex, other parts of the central nervous system are subordinated to it, that is corticalization of functions.

Since each of the nerve centers is responsible for certain reflexes, during their interaction we can talk about the interaction of different reflexes. This interaction is carried out on the basis of certain patterns that allow the central nervous system to solve its functional tasks both with the targeted regulation of various body systems and with the organization of its behavior in specific, constantly changing environmental conditions.

The following principles of coordination of central nervous system functions can be identified.

1. Inhibition in the central nervous system.

An important part of the neural circuits that form reflex arcs is the presence of inhibitory neurons (Fig. 38). As a result, the intense process of excitation is weakened or completely stopped, which mainly ensures the ordering of the manifestation of the reflex. Braking example - reciprocal no inhibition antagonist muscles at the level of spinal cord motor neurons (Fig. 38, A). The process of inhibitory influence is triggered through special inhibitory Renshaw cells contained in the spinal cord. Upon receipt of an afe

Rice. 38. Inhibition in the central nervous system: A- participation of inhibitory interneurons of the spinal cord (D) in the regulation of the activity of antagonist muscles: inhibition (-) of the motor neuron of the extensor muscle (MR) during excitation of the (+) motor neuron of the flexor muscle (MZ); b- rotary (postsynaptic) inhibition (MN - motor neuron, G - Renshaw inhibitory cell; M - muscle); V- inhibition of neurons diencephalon with the participation of the inhibitory basket cell (D); G- presynaptic inhibition (G - inhibitory cell; N - neuron; Pr - presynaptic fiber; behind Eccles)

rent impulses, they are activated simultaneously with neurons that are excited, providing a reciprocal relationship in the implementation of motor reflexes: motor neurons of some muscles are excited, and their antagonists are inhibited.

The second, quite common, type of primary braking is return braking(Fig. 38, b). Renshaw cells are also located in such a way that, through the collaterals of the excited motor neuron, they cause its inhibition. This is a typical example of negative feedback when excessive impulses are suppressed.

2. Irradiation and concentration of nervous processes.

Excitation that arises in one of the centers can spread through collaterals and synapses to other centers. The process of irradiation most often develops in the case of a strong stimulus. For example, during strong pressure on the frog's leg, not one, but all limbs contract. After some time, the irradiation changes to the phenomenon of concentration of excitation in the required center. This is due to the action of inhibitory synaptic connections. The processes of irradiation and concentration are based on the properties of convergence and divergence.

3. Phenomena of summation and occlusion

(Fig. 39). Summation (facilitation) occurs during exposure to several subthreshold stimuli (from different receptors), each of which, acting separately, does not cause a response. And their summation (provided there are nearby synaptic fields) contributes to the manifestation of the response of the nerve center (the phenomenon of relief).

Rice. 39. Diagram illustrating the phenomenon of facilitation (1) and occlusion (2) nerve impulse: A- in the central circles are depicted neurons that are excited both during isolated and simultaneous stimulation of nerve fibers (B, 2); dotted lines outline neurons that are excited only by simultaneous stimulation of both nerve fibers; b- in the central part, formed by intersecting circles, there are neurons that are excited both by isolated and simultaneous stimulation of nerve fibers

The opposite phenomenon is occlusion(jamming) - develops under the same conditions of location of synaptic fields, but with the simultaneous action of several stimuli of supra-border force. The total response may be less than the arithmetic sum of responses to each of the stimuli separately, which occurs due to “overlap” both at the level of the receptor and the general central neurons.

4. The principle of "common final path"

(Fig. 40). It is based on the phenomenon of convergence. There are significantly more afferent inputs to the central nervous system than efferent outputs. Consequently, the same reflex can be evoked by stimulating different reflex fields.

5. The principle of dominant focus.

The content of the principle is that in the case of simultaneous excitation of several nerve centers, one of the centers can become dominant. As a result, excitations from other foci can be actively attracted to it (irradiate), which will lead to the summation of excitation, enhancing the dominant excitation. High excitability of neurons is caused by corresponding afferent impulses (for example, from a crowded Bladder), humoral influences. As a result, it turns out that for the body the function of this center in a specific time period becomes the most important.

The main signs of a dominant focus are as follows:

1) persistence of excitation over time;

2) increased excitability;

3) ability to summation. The dominant is the physiological basis for the emergence of relationships between individual nerve centers during the formation of conditioned reflexes, the basis of attention.

Rice. 40. A- spinal cells

ganglion; b- intermediate neurons; V- motor neurons; G- muscle (crossed out are the bodies of neurons that inhibit nerve impulses; after Sherrington)

Conversion

(Latin converqere - bring together, converge) - the convergence of two or more excitations from sensory stimuli (for example, sound, light) to one neuron. There are several types of convergence.

Convergence nerve impulses sensory-biological - the convergence to one neuron of two or more excitations from sensory and biological stimuli simultaneously (for example, sound, hunger, light and thirst). This type of convergence is one of the mechanisms of learning, the formation of conditioned reflexes and afferent synthesis of functional systems.

Convergence of nerve impulses is multibiological - the convergence to one neuron of two or more excitations from biological stimuli (for example, hunger and pain, thirst and sexual arousal).

Efferent-afferent convergence of nerve impulses is the convergence of two or more afferent and efferent excitations simultaneously to one neuron. Efferent excitation leaves the neuron, then through several interneurons returns to the neuron and interacts with the afferent excitation coming to the neuron at that moment. This type of convergence is one of the mechanisms of the acceptor of the result of an action (prediction of a future result), when afferent excitation is compared with efferent excitation.

Divergence

(Latin diverqere - directed in different directions) - the ability of a single neuron to establish numerous synaptic connections with various nerve cells.

Thanks to the process of divergence, the same cell can participate in organizing different reactions and control a larger number of neurons. At the same time, each neuron can provide a wide redistribution of impulses, which leads to irradiation of excitation.

Irradiation (from Latin irradio - I shine, emit rays) in physiology, the spread of the process of excitation or inhibition in the central nervous system.

Irradiation plays an important role in the activity of the cerebral cortex. The irradiation of excitation is especially clearly manifested during strong irritation, when the reflex response involves nerve centers that are usually not involved in it.

Thus, the animal responds to moderate painful irritation of the skin of the foot by flexing the paw at the ankle joint; an increase in the force of irritation leads to bending of the leg at the knee and hip joints. When studying the effect of an inhibitory conditioned stimulus, I. P. Pavlov showed that inhibition can also spread (irradiate) in the cells of the cerebral cortex.

Reverberation- circulation of excitation by closed neurons and their circuits in the central nervous system.

The excitation of one of the neurons included in this chain is transmitted to another (or others), to axon collaterals and again returns to the nerve cell, etc.

Reverberation of excitation is observed in the so-called reflex aftereffect, when the reflex act does not end immediately after termination, but after a certain (sometimes long) period, and also plays a certain role in the mechanisms of short-term (working) memory. This also includes cortical-subcortical reverberation, which plays an important role in higher nervous activity (behavior) of humans and animals.

Unilateral conduction

Excitation impulses can travel along nerve fibers in both directions from the site of irritation. In the central nervous system, they usually spread only in one direction - only from afferent neurons to efferent ones. This means that in the central nervous system impulses are transmitted only from the axon of one neuron to the cell body and dendrites of other neurons and are not transmitted from the dendrites and from the body of the nerve cell to the axon branches approaching them.

This pattern was first established in 1823 simultaneously by two researchers - the Scot I. Bell and the French physiologist F. Magendie - and was called

Bell-Magendie's law, according to which afferent fibers enter the spinal cord through the dorsal roots, and efferent fibers leave the spinal cord through the ventral roots.

The one-sided conduction of excitation in nerve centers is due to the structure of synapses: mediators are released only by the terminal apparatus of axons and only the postsynaptic membrane of the synapse is sensitive to mediators, on which an action potential (exciting or inhibitory) arises. Thus, excitation at the synapse spreads from the axon terminals through the transmitter to the postsynaptic membrane of the nerve cell body, dendrite or interneuron. In the opposite direction, the transfer of excitation is possible only in an electrical synapse, in which excitation from the presynaptic membrane is transmitted to the postsynaptic membrane electrically.

9. Basic principles of the coordination activity of the central nervous system: reciprocity, facilitation, occlusion, feedback, common “final” path, dominant.

Coordination- this is the unification of the reflex activity of the central nervous system into a single whole, which ensures the implementation of all functions of the body.

Principles of coordination

1. The principle of irradiation of excitations. Neurons of different centers are interconnected by interneurons, so impulses arriving during strong and prolonged stimulation of receptors can cause excitation not only of the neurons of the center of a given reflex, but also of other neurons. For example, if you irritate one of the hind legs of a spinal frog by gently squeezing it with tweezers, it contracts (defensive reflex); if the irritation is increased, then both hind legs and even the front legs contract. Irradiation of excitation ensures that, under strong and biologically significant stimuli, a greater number of motor neurons are included in the response.

2. The principle of a common final path. Impulses arriving in the central nervous system through different afferent fibers can converge (converge) to the same intercalary, or efferent, neurons. Sherrington called this phenomenon the “common final path principle.” The same motor neuron can be excited by impulses coming from different receptors (visual, auditory, tactile), i.e. participate in many reflex reactions (be included in various reflex arcs).

3. The principle of dominance. It was discovered by A.A. Ukhtomsky, who discovered that irritation of the afferent nerve (or cortical center), usually leading to contraction of the muscles of the limbs when the animal’s intestines are full, causes an act of defecation. In this situation, the reflex excitation of the defecation center suppresses and inhibits the motor centers, and the defecation center begins to react to signals that are foreign to it.

A.A. Ukhtomsky believed that at every given moment of life a defining (dominant) focus of excitation arises, subordinating the activity of all nervous system and the determining nature of the adaptive reaction. Excitations from various areas of the central nervous system converge to the dominant focus, and the ability of other centers to respond to signals coming to them is inhibited. Thanks to this, conditions are created for the formation of a certain reaction of the body to the stimulus that has the greatest biological significance, i.e. satisfying a vital need.

4. Feedback principle. The processes occurring in the central nervous system cannot be coordinated if there is no feedback, i.e. data on the results of function management. Feedback allows you to correlate the severity of changes in system parameters with its operation. The connection between a system's output and its input with a positive gain is called positive feedback, and with a negative gain is called negative feedback. Positive feedback is mainly characteristic of pathological situations.

Negative feedback ensures the stability of the system (its ability to return to its original state after the influence of disturbing factors ceases). There are fast (nervous) and slow (humoral) feedbacks. Feedback mechanisms ensure the maintenance of all homeostasis constants. For example, maintaining a normal level of blood pressure is achieved by changing the impulse activity of the baro-receptors of the vascular reflexogenic zones, which change the tone of the vagus and vasomotor sympathetic nerves.

5. The principle of reciprocity. It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limbs), and lies in the fact that the neurons of one center, when excited, inhibit the neurons of the other and vice versa.

6. The principle of subordination (subordination). The main trend in the evolution of the nervous system is manifested in the concentration of regulation and coordination functions in the higher parts of the central nervous system - cephalization of the functions of the nervous system. There are hierarchical relationships in the central nervous system - the highest center of regulation is the cerebral cortex, the basal ganglia, middle, medulla and spinal cord obey its commands.

7. The principle of compensation of functions. The central nervous system has a huge compensatory capacity, i.e. can restore some functions even after the destruction of a significant part of the neurons that form the nerve center. If individual centers are damaged, their functions can transfer to other brain structures, which is carried out with the obligatory participation of the cerebral cortex. In animals in which the cortex was removed after restoration of lost functions, their loss occurred again.

occlusion

(Lat. occlusum - close, close) - interaction of two streams of impulses with each other.

The phenomenon of occlusion was described by C. Sherrington. Its essence lies in the mutual inhibition of reflex reactions, in which the total result turns out to be significantly less than the sum of interacting reactions. According to Ch. Sherrington, the phenomenon of occlusion is explained by the overlap of synaptic fields formed by the afferent links of interacting reflexes. Therefore, with the simultaneous arrival of two afferent influences, the excitatory postsynaptic potential is caused by each of them partly in the same motor neurons of the spinal cord.

Relief

After each, even the weakest irritation, excitability increases in the nerve center. With the phenomenon of summation, when two streams of impulses enter the central nervous system separated by a short time interval, they cause a significantly greater effect than could be expected as a result of simple summation. One stream of impulses seems to “pave the way” for another.

Nerve center- this is a set of neurons necessary for the implementation of a certain reflex or regulation of a certain function.

The main cellular elements of the nerve center are numerous, the accumulation of which forms the nerve nuclei. The center may include neurons scattered outside the nuclei. The nerve center can be represented by brain structures located at several levels of the central nervous system (for example, blood circulation, digestion).

Any nerve center consists of a core and periphery.

Nuclear part The nerve center is a functional association of neurons, which receives basic information from afferent pathways. Damage to this area of ​​the nerve center leads to damage or significant impairment of this function.

Peripheral part the nerve center receives a small portion of afferent information, and its damage causes a limitation or reduction in the volume of the function performed (Fig. 1).

The functioning of the central nervous system is carried out thanks to the activity of a significant number of nerve centers, which are ensembles of nerve cells united through synaptic contacts and characterized by a huge variety and complexity of internal and external connections.

Rice. 1. Diagram of the general structure of the nerve center

The following hierarchical departments are distinguished in the nerve centers: working, regulatory and executive (Fig. 2).

Rice. 2. Scheme of hierarchical subordination of different departments of nerve centers

Working department of the nerve center is responsible for the implementation of this function. For example, the working section of the respiratory center is represented by the centers of inhalation, exhalation and pneumotaxis, located in the pons; disruption of this department causes respiratory arrest.

Regulatory department of the nerve center - this is a center located in and regulating the activity of the working section of the nerve center. In turn, the activity of the regulatory section of the nerve center depends on the state of the working section, which receives afferent information, and on external environmental stimuli. Thus, the regulatory department of the respiratory center is located in the frontal lobe of the cerebral cortex and allows you to voluntarily regulate pulmonary ventilation (depth and frequency of breathing). However, this voluntary regulation is not unlimited and depends on the functional activity of the working part, afferent impulses, reflecting the state of the internal environment (in this case, blood pH, concentrations of carbon dioxide and oxygen in the blood).

Executive department of the nerve center - This is a motor center located in the spinal cord and transmits information from the working part of the nerve center to the working organs. The executive branch of the respiratory nerve center is located in the anterior horns thoracic spinal cord and transmits orders from the work center to the respiratory muscles.

On the other hand, the same neurons in the brain and spinal cord can be involved in the regulation of different functions. For example, the cells of the swallowing center are involved in the regulation of not only the act of swallowing, but also the act of vomiting. This center provides all the successive stages of the act of swallowing: movement of the muscles of the tongue, contraction of the muscles of the soft palate and its elevation, subsequent contraction of the muscles of the pharynx and esophagus during the passage of the bolus. These same nerve cells ensure contraction of the muscles of the soft palate and its elevation during vomiting. Consequently, the same nerve cells enter both the swallowing center and the vomiting center.

Properties of nerve centers

The properties of nerve centers depend on their structure and mechanisms of transmission of excitation to. The following properties of nerve centers are distinguished:

• One-sidedness of excitation
• Synaptic delay
• Excitation summation
• Rhythm transformation

• Fatigue
• Convergence
• Divergence
• Irradiation of excitation
• Excitation concentration
• Tone
• Plastic
• Relief
• Occlusion
• Reverberation
• Prolongation

Unilateral conduction of excitation in the nerve center. Excitation in the central nervous system is carried out in one direction from the axon to the dendrite or cell body of the next neuron. The basis of this property is the features of the morphological connection between neurons.

The unilateral conduction of excitation depends on the humoral nature of the transmission of the impulse in it: the transmitter that transmits the excitation is released only in the presynaptic terminal, and the receptors that perceive the mediator are located on the postsynaptic membrane;

Deceleration of excitation conduction (central delay). In the reflex arc system, excitation is carried out most slowly at the synapses of the central nervous system. In this regard, the central time of the reflex depends on the number of interneurons.

The more complex the reflex reaction, the longer the central time of the reflex. Its value is associated with the relatively slow conduction of excitation through sequentially connected synapses. The slowdown in the conduction of excitation is created due to the relative duration of the processes occurring in the synapses: the release of the transmitter through the presynaptic membrane, its diffusion through the synaptic cleft, excitation of the postsynaptic membrane, the emergence of an excitatory postsynaptic potential and its transition to the action potential;

Transformation of the rhythm of excitation. Nerve centers are capable of changing the rhythm of impulses arriving at them. They can respond to single stimuli with a series of impulses or to stimuli of low frequency with the occurrence of more frequent action potentials. As a result, the central nervous system sends a number of impulses to the working organ that is relatively independent of the frequency of stimulation.

This is due to the fact that a neuron is an isolated unit of the nervous system; at any moment a lot of irritations come to it. Under their influence, a change in the membrane potential of the cell occurs. If a small but long-lasting depolarization is created (long excitatory postsynaptic potential), then one stimulus causes a series of impulses (Fig. 3);

Rice. 3. Scheme of transformation of the excitation rhythm

Aftereffect - the ability to maintain arousal after the end of the stimulus, i.e. there are no afferent impulses, but efferent impulses continue to act for some time.

The aftereffect is explained by the presence of trace depolarization. If the trailing depolarization is prolonged, then against its background action potentials (rhythmic activity of the neuron) may arise within a few milliseconds, as a result of which the response is maintained. But this gives a relatively short aftereffect.

A longer aftereffect is associated with the presence of circular connections between neurons. In them, the excitation seems to support itself, returning along collaterals to the initially excited neuron (Fig. 4);

Rice. 4. Scheme of ring connections in the nerve center (according to Lorento de No): 1 - afferent pathway; 2-intermediate neurons; 3 - efferent neuron; 4 - efferent pathway; 5 - recurrent branch of axon

Making it easier to navigate or pave the way. It has been established that after excitation that arose in response to rhythmic stimulation, the next stimulus causes a greater effect, or to maintain the previous level of response, a smaller force of subsequent stimulation is required. This phenomenon is called "relief".

It can be explained by the fact that with the first stimuli of a rhythmic stimulus, the transmitter vesicles move closer to the presynaptic membrane and with subsequent stimulation the transmitter is more quickly released into the synaptic cleft. This, in turn, leads to the fact that, due to the summation of the excitatory postsynaptic potential, the critical level of depolarization is reached more quickly and a propagating action potential arises (Fig. 5);

Rice. 5. Facilitation scheme

Summation, first described by I.M. Sechenov (1863) and consists in the fact that weak stimuli that do not cause a visible reaction, with frequent repetition, can be summed up, create a suprathreshold force and cause an excitation effect. There are two types of summation - sequential and spatial.

• Sequential summation at synapses occurs when several subthreshold impulses arrive at the centers along the same afferent path. As a result of the summation of local excitation caused by each subthreshold stimulus, a response occurs.
• Spatial summation consists in the appearance of a reflex reaction in response to two or more subthreshold stimuli arriving at the nerve center along different afferent pathways (Fig. 6);

Rice. 6. Property of the nerve center - spatial (B) and sequential (A) summation

Spatial summation, like sequential summation, can be explained by the fact that with subthreshold stimulation coming through one afferent pathway, an insufficient amount of transmitter is released to cause depolarization of the membrane to a critical level. If impulses arrive simultaneously through several afferent pathways to the same neuron, a sufficient amount of transmitter is released at the synapses, necessary for threshold depolarization and the occurrence of an action potential;

Irradiation. When a nerve center is excited, nerve impulses spread to neighboring centers and bring them into an active state. This phenomenon is called irradiation. The degree of irradiation depends on the number of interneurons, the degree of their myelination, and the strength of the stimulus. Over time, as a result of afferent stimulation of only one nerve center, the irradiation zone decreases, and a transition to the process occurs concentrations, those. limiting excitation to only one nerve center. This is a consequence of a decrease in the synthesis of mediators during interneurons, as a result of which biocurrents are not transmitted from this nerve center to neighboring ones (Fig. 7 and 8).

Rice. 7. The process of irradiation of excitation in nerve centers: 1, 2, 3 - nerve centers

Rice. 8. The process of concentration of excitation in the nerve center

Expression this process is a precise coordinated motor response in response to stimulation of the receptive field. The formation of any skills (labor, sports, etc.) is due to the training of motor centers, the basis of which is the transition from the process of irradiation to concentration;

Induction. The basis of the relationship between nerve centers is the process of induction - guidance (induction) of the opposite process. A strong excitation process in a nerve center causes (induces) inhibition in neighboring nerve centers (spatial negative induction), and a strong inhibitory process induces excitation in neighboring nerve centers (spatial positive induction). When these processes change within the same center, they speak of sequential negative or positive induction. Induction limits the spread (irradiation) of nervous processes and ensures concentration. The ability to induce largely depends on the functioning of inhibitory interneurons—Renshaw cells.

The degree of development of induction determines the mobility of nervous processes and the ability to perform high-speed movements that require a rapid change of excitation and inhibition.

Induction is the basis dominants- formation of a nerve center of increased excitability. This phenomenon was first described by A.A. Ukhtomsky. The dominant nerve center subjugates the weaker nerve centers, attracts their energy and thereby strengthens itself even more. As a result of this, stimulation of various receptor fields begins to cause a reflex response characteristic of the activity of this dominant center. A dominant focus in the central nervous system can arise under the influence of various factors, in particular strong afferent stimulation, hormonal influences, motivations, etc. (Fig. 9);

Divergence and convergence. The ability of a neuron to establish multiple synaptic connections with different nerve cells within the same or different nerve centers is called divergence. For example, the central axon terminals of a primary afferent neuron form synapses on many interneurons. Thanks to this, the same nerve cell can participate in various nervous reactions and control a large number of others, which leads to irradiation of excitation.

Rice. 9. Formation of a dominant due to spatial negative induction

The convergence of different pathways of nerve impulses to the same neuron is called convergence. The simplest example of convergence is the closure of impulses from several afferent (sensitive) neurons on one motor neuron. In the CNS, most neurons receive information from different sources through convergence. This ensures spatial summation of impulses and enhancement of the final effect (Fig. 10).

Rice. 10. Divergence and convergence

The phenomenon of convergence was described by C. Sherrington and was called Sherrington's funnel, or the common final path effect. This principle shows how, when various nervous structures are activated, the final reaction is formed, which is of paramount importance for the analysis of reflex activity;

Occlusion and relief. Depending on the relative position of the nuclear and peripheral zones of different nerve centers, the phenomenon of occlusion (blockage) or relief (summation) may appear during the interaction of reflexes (Fig. 11).

Rice. 11. Occlusion and relief

If mutual overlap of the nuclei of two nerve centers occurs, then upon stimulation of the afferent field of the first nerve center, two motor responses conditionally arise. When only the second center is activated, two motor responses also occur. However, with simultaneous stimulation of both centers, the total motor response is only three units, not four. This is due to the fact that the same motor neuron belongs to both nerve centers simultaneously.

If there is an overlap of the peripheral parts of different nerve centers, then when one center is irritated, one response occurs, and the same is observed when the second center is irritated. When two nerve centers are simultaneously excited, three responses occur. Because motor neurons that are in the overlap zone and do not respond to isolated stimulation of the nerve centers receive, with simultaneous stimulation of both centers, a total dose of the transmitter, which leads to a threshold level of depolarization;

Fatigue of the nerve center. The nerve center has low lability. It constantly receives from many highly labile nerve fibers a large number of stimuli that exceed its lability. Therefore, the nerve center works at maximum capacity and gets tired easily.

Based on the synaptic mechanisms of excitation transmission, fatigue in the nerve centers can be explained by the fact that as the neuron works, the transmitter reserves are depleted and the transmission of impulses in the synapses becomes impossible. In addition, during the activity of a neuron, a gradual decrease in the sensitivity of its receptors to the transmitter occurs, which is called desensitization;

Sensitivity of nerve centers to oxygen and some pharmacological substances. Nerve cells carry out intense metabolism, which requires energy and a constant flow of the required amount of oxygen.

The nerve cells of the cerebral cortex are especially sensitive to lack of oxygen; after five to six minutes of oxygen starvation they die. A person has even a short-term limitation cerebral circulation leads to loss of consciousness. Insufficient oxygen supply is more easily tolerated by the nerve cells of the brain stem; their function is restored 15-20 minutes after the complete cessation of blood supply. And the function of spinal cord cells is restored even after 30 minutes of lack of blood circulation.

Compared to the nerve center, the nerve fiber is insensitive to lack of oxygen. Placed in a nitrogen atmosphere, it stops excitation only after 1.5 hours.

Nerve centers have a specific reaction to various pharmacological substances, which indicates their specificity and the originality of the processes occurring in them. For example, nicotine and muscarine block the conduction of impulses in excitatory synapses; their action leads to a drop in excitability, a decrease in motor activity and its complete cessation. Strychnine and tetanus toxin turn off inhibitory synapses, which leads to increased excitability of the central nervous system and increased motor activity, up to general convulsions. Some substances block the conduction of excitation in nerve endings: curare - in the end plate; atropine - in the endings of the parasympathetic nervous system. There are substances that act on certain centers: apomorphine - on the emetic; lobelia - for respiratory; cardiazol - on the motor cortex; mescaline - on the visual centers of the cortex, etc.;

Plasticity of nerve centers. Plasticity is understood as the functional variability and adaptability of nerve centers. This is especially pronounced when different parts of the brain are removed. The impaired function can be restored if some parts of the cerebellum or cerebral cortex have been partially removed. The possibility of a complete restructuring of the centers is evidenced by experiments on stitching together functionally different nerves. If you cut the motor nerve that innervates the muscles of the limbs, and its peripheral end is sutured with the central end of the cut vagus nerve, which regulates the internal organs, then after some time the peripheral fibers of the motor nerve degenerate (due to their separation from the cell body), and the fibers of the vagus nerve grow into the muscle . The latter form synapses in the muscle characteristic of the somatic nerve, which leads to gradual recovery motor function. In the first time after restoration of the innervation of the limb, skin irritation causes a reaction characteristic of the vagus nerve - vomiting, since excitation from the skin travels through the vagus nerve to the corresponding centers of the medulla oblongata. After some time, skin irritation begins to cause a normal motor reaction, since a complete restructuring of the activity of the center occurs.