The human nervous system can be represented as a nervous network, i.e. systems of neural circuits that transmit excitatory and inhibitory signals. Neural networks are built from three main components: input fibers, interneurons and efferent neurons. The simplest and most elementary neural circuits are local networks, or microgrids(Fig. 69). Often a certain type of micronetwork is repeated throughout the entire layer of a neural structure, such as the cerebral cortex, and acts as module for a special way of processing information.

Local networks exist in different parts of the brain. They serve: 1) to enhance weak signals; 2) reducing and filtering too intense activity; 3) highlighting contrasts; 4) maintaining rhythms or maintaining the working state of neurons by adjusting their inputs. Micronetworks can have an excitatory or inhibitory effect on target neurons.

Local networks can be compared to integrated circuits in electronics, i.e. standard elements that perform the most frequently repeated operations and can be included in the circuits of a wide variety of electronic devices.

One of the varieties local networks As a rule, they consist of neurons with short axons (Fig. 69, A). Therefore, the tasks and spheres of influence of such neurons are very limited. The second type of local network is formed by neurons that are sufficiently distant from each other, but belonging to the same nerve area. The main functions of these networks are to spread activity beyond a single module or to provide antagonistic interactions between neighboring modules within a given neural region.

More complex are networks with remote connections, connecting two or more areas of the nervous system with local networks. Networks with distant connections can be either specific (Fig. 69, B) or diffuse (Fig. 69, C). A specific sequential connection of several areas performs the function of transmitting information from the periphery to the central nervous system (for example, the conductive sections of the analyzers) or from the central sections to the periphery (for example, the motor system). In such cases, networks with distant connections are usually called upstream and downstream pathways, or systems. The neural structures included in the ascending pathways are united according to the principle of an ascending hierarchy, and those forming the descending pathways are united according to the principle of a descending hierarchy.

The highest level of organization is a system of connections between a number of areas that control some behavior in which the entire organism participates. Such networks are called distributed systems(Fig. 69, D). They may be located in different parts of the brain and may be connected by hormonal influences or long neural pathways. Distributed systems are involved in the implementation of higher functions of motor and sensory systems, as well as many other central systems that provide complex behavioral acts, abstract thinking, speech and other psychophysiological processes.

In the process of evolution, neural networks became more complex. In invertebrate animals with a poorly integrated nervous system, neural networks are usually organized either in the form ganglia, or in the form records(Fig. 70, A). Ganglia are a structure with a concentrated arrangement of synaptic contacts between input and output elements, and lamina are a structure with a two-layer organization of such contacts.

In higher invertebrate animals, integration of signals at a higher level occurs in the nerve centers, as exemplified by mushroom bodies insect brain. Mushroom bodies are hidden deep in the brain, rather than on its surface, where they could grow wider.

In vertebrates and humans, some of the neural networks are grouped into ganglia. The centers located deep in the brain increase due to the formation of bends, like the mushroom bodies of insects. However, a fundamentally new feature of higher vertebrates and

in humans is a grouping of a huge number of neurons into layers lying on the surface of the brain, i.e. education bark(Fig. 70, B).

The cortex is located in such a way that the neurons of all its layers are accessible to any input signals. Together with local networks formed by branches of neuronal processes and interneurons, the cortex has enormous capabilities for integrating, storing and combining information. In each area, or field of the cortex, similar modules (local networks) are repeated many times, thanks to which this field is able to carry out specific operations involving certain input and output connections (visual field, auditory field). When moving to the neighboring field of the cortex, all these three elements, i.e. local networks, inputs and outputs, change slightly. Functional properties also change. Thus, each of the cortical fields is a site adapted to perform certain functions in the distributed system of which it is a part.

5.11. Basic laws of the functioning of nerve networks

5.11.1. Divergence and convergence of neural pathways

In all studied neural networks, divergence and convergence of pathways were found. Divergence called the ability of a neuron to establish numerous synaptic connections with many other nerve cells (Fig. 71, A). For example, the axon of a sensory neuron enters the dorsal horn of the spinal cord as part of the dorsal roots and in the spinal cord branches into many branches (collaterals) that form synapses on many interneurons and motor neurons. Through the process of divergence, the same nerve cell can participate in different neural reactions and control a large number of other neurons. This expansion of the scope and propagation of the signal in nerve networks is called irradiation. Both excitation and inhibition can radiate.

The convergence of many nerve pathways to the same neuron is called convergence(Fig. 71, B). For example, on each motor neuron of the spinal cord, thousands of processes of sensory, as well as excitatory and inhibitory interneurons from different parts of the central nervous system form synapses. Due to the convergence of many nerve pathways to one neuron, this neuron carries out integration excitatory and inhibitory signals simultaneously arriving through different pathways. If, as a result of the algebraic addition of EPSPs and IPSPs arising on the membrane of the neuron, excitation prevails, then the neuron will become excited and send a nerve impulse to the second cell. If a sufficient IPSP value prevails, the neuron will slow down. This addition of postsynaptic potentials is called spatial, or simultaneous summation.

There are approximately 5 times more afferent neurons in the nervous system than efferent ones. In this regard, many afferent impulses arrive at the same intercalary and efferent neurons, which are for impulses common final paths to the working bodies.

The patterns of common terminal pathways were first studied at the beginning of the 20th century by the English physiologist C. Sherrington. The morphological basis of the common terminal tracts is the convergence of nerve fibers. Thanks to common final pathways, the same reflex response of a certain group of motor neurons can be obtained when stimulating different nerve structures. For example, motor neurons innervating the muscles of the pharynx are involved in the reflexes of swallowing, coughing, sucking, and breathing, forming a common final path for various reflex arcs.

Reflexes, the arcs of which have a common final path, are divided into allied And antagonistic. Meeting on common final paths, allied reflexes mutually reinforce each other, and antagonistic reflexes inhibit each other, as if competing for the capture of the common final path. The predominance of one or another, including behavioral, reflex reaction on the final paths is due to its significance for the life of the organism in this moment.

Neural circuits are neurons connected in a suitable manner, most often in series, that perform a specific task. Neural networks are a union of neurons that contains many parallel and interconnected sequential chains of neurons. Such associations perform complex tasks. For example, sensor networks perform the task of processing sensory information. The principle of subordinate behavior of neurons in a network assumes that a set of interconnected elements has great potential for functional rearrangements, that is, at the level of a neural network, not only the transformation of input information occurs, but also the optimization of interneuron relationships, which leads to the implementation of the required functions of the information control system. By nature organizations in the nervous system distinguish three types of networks - hierarchical, local and divergent. In this way, these networks can simultaneously influence the activity of many elements that can be associated with different hierarchical levels. Neural ensembles are usually called a group of neurons with a diameter of 300-500 micrometers, including pyramidal and stellate neurons of the cerebral cortex, which generate single-frequency patterns. The main function of the nervous system system is associated with the processing of information, on the basis of which the perception of the external environment occurs, interaction with it, control of motor activity, and also, together with the endocrine system, control of the work of all internal organs. In humans, the nervous system ensures higher nervous activity and its essential component- mental activity.

21. Inhibition as one of the forms of neuron activity. Modern ideas about braking mechanisms, its types. Inhibition in the central nervous system is necessary for the integration of neurons into a single nerve center. The following inhibitory mechanisms are distinguished in the central nervous system: 1.

Postsynaptic. It occurs in the postsynaptic membrane of the soma and dendrites of neurons, i.e. after the transmitting synapse. In these areas, specialized inhibitory neurons form axo-dendritic or axosomatic synapses. These synapses are glycinergic. As a result of the effect of NLI on the glycine chemoreceptors of the postsynaptic membrane, its potassium and chloride channels open. Potassium and chloride ions enter the neuron, and IPSP develops. The role of chlorine ions in the development of IPSP: small. As a result of the resulting hyperpolarization, the excitability of the neuron decreases. The conduction of nerve impulses through it stops. The alkaloid strychnine can bind to glycerol receptors of the postsynaptic membrane and turn off inhibitory synapses. This is used to demonstrate the role of inhibition. After the administration of strychnine, the animal develops convulsions of all muscles. 2. Presynaptic inhibition. In this case, the inhibitory neuron forms a synapse on the axon of the neuron approaching the transmitting synapse. Those. such a synapse is axo-axonal. The mediator of these synapses is GABA. Under the influence of GABA, chloride channels of the postsynaptic membrane are activated. But in this case, chlorine ions begin to leave the axon. This leads to a small local but long-lasting depolarization of its membrane. A significant part of the sodium channels of the membrane is inactivated, which blocks the conduction of nerve impulses along the axon, and consequently the release of the neurotransmitter at the transmitting synapse. The closer the inhibitory synapse is located to the axon hillock, the stronger its inhibitory effect. Presynaptic inhibition is most effective in information processing, since the conduction of excitation is not blocked in the entire neuron, but only at its one input. Other synapses located on the neuron continue to function. 3. Pessimal inhibition.

22. Reflex as the main act of nervous activity. General diagram of the reflex arc, its parts. Classification of reflexes. The basic principle of the nervous system is the reflex. A reflex (reflexes - reflection) is a natural response of the body to influence from the external or internal environment of the body with the obligatory participation of the central nervous system. All irritations acting on the body from the environmental or internal environment are perceived by the sensitive peripheral endings of the nervous system by receptors. Excitation from receptors along afferent nerve fibers is sent to the central nervous system, where the received information is processed and impulses are formed that are sent along efferent nerve fibers to organs, causing or changing their activity. The path along which excitation spreads from the receptor to the working organ (effector) is called a reflex arc. The reflex arc includes: 1) receptor - perceives irritation and converts the energy of irritation into excitation (nerve impulses) - this is the primary processing of the information received. Receptors are the branches of the dendrites of afferent neurons or specialized cells (cones, rods in the visual sensory system, auditory hair and vestibular cells). 2) afferent pathway - the path from the receptor to the central nervous system, is represented by an afferent (sensitive or centripetal) neuron, the processes of which form the afferent nervous fiber; 3) nerve center - a set of neurons in the central nervous system in which information is processed and a response is formed; 4) efferent (motor or centrifugal) path - the path from the central nervous system to the periphery, represented by an efferent neuron, the axon of which forms an efferent nerve fiber that conducts excitation to an organ; 5) executive organ or effector (muscle, gland, internal organ)

If the integrity of at least one link of the reflex arc is violated, the reflex does not occur. Depending on the number of neurons included in the reflex arc, simple and complex reflexes are distinguished. In a simple reflex, the arc consists of 2 neurons (sensitive and motor) and one synapse; it is called a monosynaptic arc. Simple reflexes are carried out with the participation of the spinal cord and are manifested in a single reflex act, for example, withdrawing a hand during painful stimulation, or in tendon reflexes. In most cases, reflex arcs have 3 or more neurons connected to each other by many synapses; such reflexes are called complex, and the arcs are called multineuron or polysynaptic. These reflex arcs include a significant number of interneurons and are carried out with the participation of the brain stem and cortex. These include instincts that ensure adequate behavior of humans and animals under changing environmental conditions. The concept of "reflex arc" was later replaced by the concept of "reflex ring". The ring, unlike the arc, includes an additional link - feedback. When an organ functions, nerve impulses are sent from it along afferent pathways to the central nervous system, informing it about the execution of a response and the correspondence of this reaction to environmental conditions at the moment. The central nervous system analyzes and synthesizes the information received and makes amendments to the performing reflex act. Reflexes are classified according to a number of characteristics:

1) according to biological significance - nutritional, sexual, protective, indicative, etc.;

2) by the nature of the response - motor, secretory, vegetative;

3) according to the level of closure of reflex arcs in parts of the brain - spinal, bulbar (closed in the medulla oblongata), mesencephalic (in the midbrain), etc.

The neurons of the nerve center, due to structural and functional connections (branching of processes and the establishment of many synapses between different cells), are combined into nerve networks. In this case, connections between nerve cells are genetically determined.

There are three main types of neural networks: hierarchical, local and divergent with a single input. Hierarchical networks ensure the gradual inclusion of higher-level neural structures due to the fact that each nerve cell is capable of establishing numerous synaptic connections with various nerve cells, as a result of which afferent impulses are supplied to an increasing number of neurons. This principle is called divergence. Thanks to this, one nerve cell can participate in several different reactions, transmit excitation to a significant number of other neurons, which, in turn, can excite a larger number of neurons, thus ensuring a wide irradiation of the excitatory process in the central nervous system. If, on the contrary, impulses from many excited neurons converge to a smaller number of nerve cells, this principle of signal propagation is called convergence. Convergence is most characteristic in the effector part of motor spinal reflexes, when a small number of spinal cord motor neurons receive excitation impulses from various efferent pathways of many reflex arcs. On the motor neurons of the spinal cord, in addition to the primary afferent fibers, fibers of various descending tracts from the centers of the brain and the spinal centers themselves, as well as from excitatory and inhibitory interneurons, converge. Studying this mechanism at the level of the spinal cord, Charles Sherrington formulated the principle of the common final pathway, according to which the motor neurons of the spinal cord are the common final pathway of numerous reflexes. Thus, the motor neurons that control the flexors of the right hand are involved in numerous motor reflex reactions - scratching, gestures during speech, transferring food to the mouth and others. At the level of multiple synapses of convergent pathways, competition for a common final pathway occurs. Nerve networks ensure the implementation of the principle of subordination, when the activity of lower located neural structures is subordinated to higher ones.

Local networks contain neurons with short axons that communicate within one level. An example of such a local network is the circular neural chains of Lorento de No, in which excitation circulates in a vicious circle. The return of excitation to the same neuron is called reverberation of excitation. Local networks ensure system reliability by duplicating elements, since many neurons of local networks have the same synaptic connections and function alternately, that is, they are interchangeable.

Divergent single-input networks are neural ensembles in which one neuron forms output connections with big amount other cells of different hierarchical levels and, most importantly, different nerve centers. The most pronounced divergence of connections between different nerve centers indicates that these nerve networks are not specific for the implementation of certain reflexes, but provide the integration of various reflex acts and the general state of activity of numerous neurons in different parts of the brain.