Nerve impulse- an electrical impulse traveling along a nerve fiber. Through the transmission of nerve impulses, information is exchanged between neurons and information is transferred from neurons to cells of other tissues of the body. The nerve impulse passes through the central nervous system and from it to the executive apparatus - skeletal muscles, smooth muscles of internal organs and blood vessels, exocrine and internal secretion glands, from peripheral receptor (sensitive) endings to the nerve centers. The emergence and propagation of a nerve impulse is ensured by the electrical properties of the membrane and cytoplasm of nerve cells. “Nerve impulse” is not an unambiguous synonym for the concept of “action potential”. For example, when transmitting information in the retina of the eye, the real action potential occurs only in the third cell of the chain, counting from the receptor cell, and before that the impulse is a gradual potential. Every second, billions of nerve signals rush through our brain. They carry information from the senses, transmit commands to muscles, and determine thoughts, emotions, and memories. Some of these signals can be recorded using an electroencephalograph by placing several electrodes on a person's head. A nerve is the path along which excitation is transmitted. Nerve fibers are divided into: Non-myelinated (non-myelinated) and Pulp (myelinated) Non-myelinated fibers most often work in the periphery, they transmit excitation along the vegetative pathways (heart...kidneys). Excitation is transmitted according to the Bigford cord principle. Slowly, from point to point - speed 1-2 m/s. Excitation is transmitted through the pulpy fibers in jumps, and excitation occurs in those places of the nerve fiber where they are not covered by Schwann cells - in the nodes of Ranvier. The speed is much higher and reaches 120 m/s. Moreover, the thicker the nerve fiber, the greater the length of the interstitial gap, which means the higher the speed of excitation. One nerve can contain several thousand nerve fibers. (sciatic nerve -16,000 nerve fibers). Pulp nerve fibers work in the somatic nervous system. Nerve properties: 1. Excitability 2. Conductivity 3. Refractoriness is the property of a nerve to reduce its excitability to 0 at the moment an excitation passes through it. 4. Nerve fiber lability is the property of a nerve to respond to a gradually increasing frequency of stimulation to a certain limit Laws of conduction of excitation along the nerve: 1. The law of anatomical and physiological continuity of the nerve fiber, i.e. the functions of the nerve fiber must be preserved. 2. The law of two-way conduction of excitation. 3. Law of isolated conduction of excitation. Excitation does not spread to neighboring nerve fibers. The speed of excitation depends on the type of nerve fiber (A, B, C). Type A fibers are thick, diameter = 20 µm. Conduction speed = 20-120 m/s. Type B fibers – diameter from 2-12 µm (-3-20 m/s) Type C fibers – diameter from 0.5-2 µm (up to 3 m/s)

14. The importance of reflexes for the life of the body. The main differences between conditional and unconditional. The adaptation of animals and humans to changing conditions of existence in the external environment is ensured by the activity of the nervous system and is realized through reflex activity. In the process of evolution, hereditarily fixed reactions (unconditioned reflexes) arose that combine and coordinate the functions of various organs and carry out adaptation of the body. In humans and higher animals, in the process of individual life, qualitatively new reflex reactions arise, which I. P. Pavlov called conditioned reflexes, considering them the most perfect form of adaptation. While relatively simple forms of nervous activity determine the reflex regulation of homeostasis and autonomic functions of the body, VND provides complex individual forms of behavior in changing living conditions. The main form of GNI is a reflex act. GNI is realized due to the dominant influence of the cortex on all underlying structures of the central nervous system. The main processes that dynamically replace each other in the central nervous system are the processes of excitation and inhibition. Depending on their ratio, strength and localization, the control influences of the cortex are built. The functional unit of the GNI is the conditioned reflex. GNI is a set of unconditioned and conditioned reflexes, as well as higher mental functions that ensure adequate behavior in changing natural and social conditions. For the first time, the assumption about the reflex nature of the activity of the higher parts of the brain was made by I.M. Sechenov, which made it possible to extend the reflex principle to human mental activity. The ideas of I.M. Sechenov received experimental confirmation in the works of Pavlov, who developed a method for objective assessment of the functions of the higher parts of the brain - the method of conditioned reflexes. Pavlov showed that all reflex reactions can be divided into two groups: unconditioned and conditioned. Differences. Unconditioned reflexes 1. Congenital, hereditary reactions, most of them begin to function immediately after birth2. They are specific, i.e. characteristic of all representatives of this species. 3. Permanent and maintained throughout life. 4. Carried out by the lower parts of the central nervous system (subcortical nuclei, brain stem, spinal cord). 5. They arise in response to adequate stimulation acting on a specific receptive field. Conditioned reflexes 1. Reactions acquired in the process of individual life. 2. Individual. 3. Impermanent - they can appear and disappear. 4. They are primarily a function of the cerebral cortex. 5. Occur in response to any stimuli acting on different receptive fields. Unconditioned reflexes can be simple and complex. Complex innate unconditional reflex reactions are called instincts and have a chain reaction.

15. Unconditioned reflexes - their classification, significance for life. Age characteristics.BR- these are inherited, unchangeable reactions of the body to certain influences of the external or internal environment, regardless of the conditions for the occurrence and course of reactions. BD ensure the organism’s adaptation to constant environmental conditions, i.e. perform a protective function and the function of maintaining homeostasis. The main types of BR: food, protective, indicative, sexual. An example of a defensive reflex is the reflexive withdrawal of the hand from a hot object. Homeostasis is maintained, for example, by a reflex increase in breathing when there is an excess of carbon dioxide in the blood. Almost every part of the body and every organ is involved in reflex reactions. BRs are closed at the level of the spinal cord and brain stem. BRs are carried out through a phylogenetically fixed, anatomically expressed reflex arc. The arcs of unconditioned reflexes are formed at the time of birth and remain throughout life. However, they can change under the influence of illness. Many unconditioned reflexes appear only at a certain age; Thus, the grasping reflex characteristic of newborns fades away at the age of 3-4 months. Many unconditioned reflexes, for example, those associated with locomotion and sexual intercourse, arise in humans and animals a long time after birth, but they necessarily appear under the condition of normal development of the nervous system. BRs are the physiological basis:1. Human species memory, i.e. congenital, inherited, constant, common to the entire human species; 2. Lower nervous activity (LNA) Pavlov's classification:1) simple2) complex3) complex (these are instincts - an innate form of adaptive behavior): a) individual (food activity, passive-defensive, aggressive, freedom reflex, exploratory, play reflex). These reflexes ensure individual self-preservation of the individual. b) species (sexual instinct and parental instinct). These reflexes ensure the preservation of the species. In accordance with the nature of the current stimulus. Pavlov distinguished such types of unconditioned reflexes as: 1) food (swallowing, sucking, etc.); 2) sexual (“tournament fights,” erection, ejaculation, etc.); 3) protective (coughing, sneezing, blinking, etc.); 4) indicative (alertness, listening, turning the head to the source of sound, etc.), etc. The implementation of all these reflexes is due to the presence of corresponding needs that arise as a result of a temporary violation of internal constancy (homeostasis) body or as a result of complex interactions with the outside world. For example, an increase in the amount of hormones in the blood (a change in the internal constancy of the body) leads to the manifestation of sexual reflexes, and an unexpected rustle (impact from the outside world) leads to alertness and the manifestation of an indicative reflex. Therefore, it is possible to believe that the emergence of an internal need is actually a condition for the implementation of an unconditional reflex and, in a certain sense, its beginning.

16. Conditioned reflexes - their classification, significance for life. Age characteristics. Organism is born with a certain fund of unconditioned reflexes. They provide him with the maintenance of vital functions in relatively constant conditions of existence. These include unconditioned reflexes: food (chewing, sucking, swallowing, secretion of saliva, gastric juice, etc.), defensive (pulling a hand away from a hot object, coughing, sneezing, blinking when a stream of air enters the eye, etc.), sexual reflexes (reflexes associated with sexual intercourse, feeding and caring for offspring), thermoregulatory, respiratory, cardiac, vascular reflexes that maintain the constancy of the internal environment of the body (homeostasis), etc. UR ensures a more perfect adaptation of the body to changing living conditions. They help to find food by smell, timely escape from danger, and orientation in time and space. The conditioned reflex separation of saliva, gastric and pancreatic juices by sight, smell, and meal times creates better conditions for digesting food even before it enters the body. Enhancing gas exchange and increasing pulmonary ventilation before starting work, only when seeing the environment in which the work is being done, contributes to greater endurance and better performance of the body during muscular activity. When a conditioned signal is applied, the cerebral cortex provides the body with preliminary preparation for responding to those environmental stimuli that will subsequently have an impact. Therefore, the activity of the cerebral cortex is signal SD is divided according to several criteria By nature of education conditioned reflexes are divided into: Natural UR are formed on the basis of natural unconditioned stimuli (the sight, smell of food, etc.); they do not require a large number of combinations for their formation, are durable, persist throughout life, and thus approach unconditioned reflexes. Natural UR are formed from the first moment after birth. Artificial UR are produced in response to stimuli that have no biological significance, for example, you can develop a food reflex to a flashing light. are produced more slowly than natural ones, and quickly fade away if not reinforced. By type of unconditional reinforcement(according to their biological significance): Food; Defensive; Sexual. According to the nature of the activity caused: positive, causing a certain conditioned reflex reaction; negative or brake , the conditioned reflex effect of which is the active cessation of conditioned reflex activity. By methods of development and type of reinforcement: Reflexes first order – in which an unconditioned reflex is used as reinforcement; Reflexes second order - these are reflexes in which a previously developed strong UR is used as reinforcement. Based on these reflexes, it is possible to develop UR third order, fourth order etc. Higher order reflexes – in which the previously developed strong conditioned reflex of the second (third, fourth) is used as reinforcement etc.) order. It is this type of SD that is formed in children and forms the basis for the development of their mental activity. The formation of higher order reflexes depends on the perfect organization of the nervous system. SDs of higher orders are unstable and easily fade away. According to the nature and complexity of the conditioned stimulus: Simple UR - are produced by the isolated action of single stimuli - light, sound, etc. Complex conditioned reflexes – under the action of a complex of stimuli consisting of several components acting either simultaneously or sequentially, directly one after the other or at short intervals. Chain conditioned reflexes are produced by a chain of stimuli, each component of which acts in isolation after the previous one, not coinciding with it, and causes its own conditioned reflex reaction According to the ratio of the time of action of the conditioned and unconditioned stimuli: Cash conditioned reflexes, when the conditioned signal and reinforcement coincide in time. With matching UR reinforcement is immediately attached to the signal stimulus (no later than 1-3 s), when abandoned UR – within a period of up to 30 s, and in the case delayed reflex, the isolated action of the conditioned stimulus lasts 1-3 minutes. Trace UR when reinforcement is presented only after the end of the conditioned stimulus Trace UR are formed when reinforcement follows after the end of the action of the conditioned stimulus and, therefore, are combined only with trace processes of excitation that arose during the action of the conditioned stimulus. UR for a while – a special type of trace conditioned reflexes. They are formed with regular repetition of an unconditioned stimulus and can be developed for various time intervals - from several seconds to several hours and even days. Apparently, various periodic processes occurring in the body can serve as a guide in counting time. The phenomenon of the body keeping time is often called the “biological clock.” By naturereception highlight: Exteroceptive UR are produced in response to environmental stimuli addressed to exteroceptors (visual, auditory). These reflexes play a role in the relationship between the body and the environment, and therefore are formed relatively quickly. Interoceptive are formed when irritation of internal organs is combined with some unconditioned reflex. They are produced much more slowly and are highly inert. Proprioceptive reflexes occur when stimulation of proprioceptors is combined with an unconditioned reflex (for example, flexing a dog’s paw, reinforced by food). The nature of the efferent response:Somatomotor . A conditioned reflex motor reaction can manifest itself in the form of movements such as blinking, chewing, etc. Vegetative. Conditioned reactions of vegetative UR ​​are manifested in changes in the activity of various internal organs - heart rate, breathing, changes in the lumen of blood vessels, metabolic levels, etc. For example, alcoholics in the clinic are quietly injected with a substance that causes vomiting, and when it begins to act, they are given a sniff of vodka. They start vomiting, and they think it’s from the vodka. After numerous repetitions, they begin to vomit from just one type of vodka without this substance. A special group includes imitative U The peculiarity of which is that they are produced in an animal or a person without his active participation in the production process; they are formed by observing the development of these reflexes in another animal or person. On the basis of the imitative reflex, children develop speech-motor acts and many social skills. L.V. Krushinsky identified a group of conditioned reflexes, which he called extrapolative . Their peculiarity lies in the fact that motor reactions arise not only to a specific conditioned stimulus, but also to the direction of its movement. Prediction of the direction of movement occurs from the first presentation of the stimulus without prior training. Currently, the extrapolation reflex used to study complex forms of behavior not only in animals, but also in humans. This methodological technique has found wide application for studying brain activity in human ontogenesis. Its use on twins makes it possible to talk about the role of genetic factors in the implementation of behavioral reactions. A special place in the system of conditioned reflexes is occupied by temporary connections that are closed between indifferent stimuli (when combined, for example, light and sound), called associations. In this case, the unconditioned reinforcement is the indicative reaction. The formation of these temporary connections occurs in three stages: the stage of the emergence of an orienting reaction to both stimuli, the stage of development of a conditioned orienting reflex, and the stage of extinction of the orienting reaction to both stimuli. After extinction, the connection between these stimuli remains. This type of reaction is of particular importance for humans, since in humans many connections are formed precisely with the help of associations.

17. Factors in the formation of conditioned reflexes. Mechanism of origin.UR are formed when two foci of excitation appear in the cerebral cortex: one in response to the action of a conditioned stimulus, and the other in response to the action of an unconditioned stimulus. When the action of these stimuli is combined, a temporary connection is established between the emerging foci of excitation, which becomes stronger from experience to experience. Pavlov called such a connection in the cerebral cortex a closure and explained to them mechanism of SD formation. The process of forming a classical SD goes through three main stages: 1) Stage pregeneralization - a short-term phase, which is characterized by a pronounced concentration of excitation in the projection zones of the cortex of conditioned and unconditioned stimuli and the absence of conditioned behavioral reactions.2) Stage generalization, which is based on the process of “diffuse” propagation (irradiation) of excitation. This is a phenomenon that occurs in the initial stages of developing a conditioned reflex. The required reaction in this case is caused not only by the reinforced stimulus, but also by others, more or less close to it. During the generalization stage, conditioned reactions occur to signal and other stimuli (the phenomenon of afferent generalization), as well as in the intervals between presentations of the conditioned stimulus. The initial stage of the formation of a UR consists of the formation of a temporary connection not only to this specific conditioned stimulus, but also to all stimuli related to it in nature. The neurophysiological mechanism consists in the irradiation of excitation from the center of the projection of the conditioned stimulus to the nerve cells of the surrounding projection zones, which are functionally close to the cells of the central representation of the conditioned stimulus, to which the conditioned reflex is formed. The farther from the initial initial focus caused by the main stimulus, reinforced by the unconditioned stimulus, the zone covered by the irradiation of excitation is located, the less likely it is to activate this zone. Consequently, at the initial stage of generalization of conditioned excitation, characterized by a generalized generalized reaction, a conditioned reflex response is observed to similar stimuli that are close in meaning as a result of the spread of excitation from the projection zone of the main conditioned stimulus.3) Stage specializations. As the conditioned stimulus is reinforced, the intersignal reactions fade and the conditioned response occurs only to the signal stimulus. The volume of distribution of biopotentials decreases. As the UR strengthens, the processes of irradiation of excitation are replaced by processes of concentration, limiting the source of excitation only to the zone of representation of the main stimulus. As a result, clarification and specialization of SD occurs. At the final stage of strengthened SD, a concentration of conditioned excitation occurs: a conditioned reflex reaction is observed only to a given stimulus, and to secondary stimuli that are close in meaning, it stops. At the stage of concentration of conditioned excitation, the excitatory process is localized only in the zone of the central representation of the conditioned stimulus (a reaction is realized only to the main stimulus), accompanied by inhibition of the reaction to side stimuli. The external manifestation of this stage is the differentiation of the parameters of the current conditioned stimulus - specialization of the SD. Speed ​​of SD formation depends on the individual characteristics of the animal, on the frequency of stimulation, on the functional state of the cortex itself and its areas, on the ratio of the strength of unconditioned and conditioned stimuli, on the environment and the changes occurring in it. Initially, Pavlov assumed that the UR is formed at the level of “cortex-subcortical formations”. In later works, he explained the formation of a conditioned reflex connection by the formation of a temporary connection between the cortical center of the BR and the cortical center of the analyzer. In this case, the main cellular elements of the UR formation mechanism are intercalary and associative neurons of the cerebral cortex, and the closure of the temporary connection is based on the process of dominant interaction between excited centers. To develop SD it is necessary: 1) the presence of two stimuli, one of which is unconditioned (food, painful stimulus, etc.), causing an unconditioned reflex reaction, and the other is conditioned (signal), signaling the upcoming unconditional stimulus (light, sound, type of food, etc. .); 2) multiple combinations of conditioned and unconditioned stimuli (although the formation of a conditioned reflex is possible with their single combination); 3) the conditioned stimulus must precede the action of the unconditioned; 4) any stimulus from the external or internal environment can be used as a conditioned stimulus, which must be as indifferent as possible, not cause a defensive reaction, not have excessive force and be able to attract attention; 5) the unconditioned stimulus must be strong enough, otherwise a temporary connection will not form; 6) excitation from the unconditioned stimulus must be stronger than from the conditioned ;7) it is necessary to eliminate extraneous stimuli, since they can cause inhibition of the conditioned reflex; 8) the organism in which the conditioned reflex is developed must be healthy; 9) when developing a conditioned reflex, motivation must be expressed, for example, when developing a food salivary reflex in an animal should be hungry, but when well-fed - this reflex is not developed.

18. Coordination of reflex activity (irradiation, concentration, induction). Implications for teaching activities. Age-related characteristics: At the beginning of the formation of a positive conditioned reflex, excitation spreads from the immediate point of stimulation in the cerebral cortex to other parts. Pavlov called this distribution the irradiation of the excitatory process. During irradiation, the process of excitation involves neighboring nerve cells in relation to the group of cells directly excited by the incoming signals. Spread occurs along the associative nerve fibers of the cortex, which connect adjacent cells. Subcortical formations and the reticular formation may also participate in the irradiation of excitation. As the conditioned reflex slows down, the excitation is concentrated in an increasingly limited area of ​​the cortex to which the stimulation is addressed. This phenomenon is called the concentration of the excitatory process. In the case of the development of differential inhibition, it limits the irradiation of excitation. Pavlov believed that inhibition is also capable of irradiation and concentration. Inhibition that occurs in the analyzer when using a negative conditioned stimulus radiates through the cerebral cortex, but 4-5 times slower (from 20 seconds to 5 minutes) than excitation. The concentration of inhibition occurs even more slowly. As the negative conditioned reflex is repeated and consolidated, the time of concentration of inhibition is shortened and inhibition is concentrated in a limited area of ​​the cortex. When studying the relationship between excitation and inhibition in the cerebral cortex, it was found that within a few seconds after exposure to an inhibitory stimulus, the effect of positive conditioned stimuli increases. Conversely, after the use of positive conditioned stimuli, the effect of inhibitory stimuli increases. The first phenomenon was called negative induction by Pavlov, the second - positive induction. With positive induction in cells adjacent to those where inhibition was just caused, after the cessation of the inhibitory signal, a state of increased excitability arises. As a result, impulses received by neurons under the action of a positive stimulus cause an increased effect. With negative induction, a process of inhibition occurs in the cortical cells surrounding the excited neurons. Negative induction limits the irradiation of the excitation process in the cerebral cortex. Negative induction can explain the inhibition of conditioned reflexes by stronger extraneous stimuli (external unconditioned inhibition). Such strong irritation causes intense excitation of neurons in the cerebral cortex, around which a wide zone of neuronal inhibition appears, capturing cells excited by the conditioned stimulus. The phenomena of negative and positive induction in the cerebral cortex are mobile and constantly replace each other. In different points of the cerebral cortex, foci of excitation and inhibition, positive and negative induction, can simultaneously arise.

19.Dynamic stereotype. Definition and significance for the life of the body, age characteristics, mechanism of formation. Critical periods of its withdrawal. Dynamic stereotype is a system of conditioned and unconditioned reflexes, which is a single functional complex. In other words, a dynamic stereotype is a relatively stable and long-lasting system of temporary connections formed in the cerebral cortex in response to the implementation of the same types of activities at the same time, in the same sequence from day to day, i.e. . it is a series of automatic actions or a series of conditioned reflexes brought to an automatic state. DS can exist for a long time without any reinforcement. The physiological basis for the formation of the initial stage of a dynamic stereotype is conditioned reflexes for time. But the mechanisms of the dynamic stereotype have not yet been deeply studied. DS plays an important role in the education and upbringing of children . If a child goes to bed and wakes up at the same time every day, has breakfast and lunch, does morning exercises, carries out hardening procedures, etc., then the child develops a time reflex. The consistent repetition of these actions forms in the child a dynamic stereotype of nervous processes in the cerebral cortex. It can be assumed that the reason for what is called student overload has a functional nature and is caused not only by the dosage and difficulty of educational tasks, but also by the negative attitude of teachers towards the dynamic stereotype, as the most important physiological basis of learning. Teachers do not always succeed in constructing a lesson so that it represents a system of dynamic stereotypes. If the content of each new lesson were organically linked with the previous and subsequent ones into a single mobile system, which made it possible, if necessary, to make changes to it, as in a dynamic stereotype, and not as a simple addition, then the work of students would be so facilitated that it would no longer would cause overload. The strengthening of a dynamic stereotype is the physiological basis of human inclinations, which in psychology are designated as habits. Habits are acquired by a person in different ways, but, as a rule, without sufficient motivation and often completely spontaneously. However, according to the mechanism of a dynamic stereotype, not only such, but also purposeful habits are formed. These include the daily routine developed by the schoolchild. Each habit is developed and strengthened through training on the principle of a conditioned reflex. At the same time, external and internal irritations serve as trigger signals for them. For example, we do morning exercises not only because we are used to it, but also because we see sports equipment that in our minds is associated with morning exercises. This habit is reinforced by both the morning exercise itself and the feeling of satisfaction that comes after it. From a physiological point of view, skills are dynamic stereotypes, in other words, chains of conditioned reflexes. A well-developed skill loses connection with the second signaling system, which is the physiological basis of consciousness only if a mistake is made, i.e. a movement is made that does not achieve the desired result, an indicative reflex appears. The resulting excitations disinhibit the inhibited connections of the automatic skill, and it is again carried out under the control of the second signaling system, or, in psychological terms, consciousness. Now the error is corrected and the necessary conditioned reflex movement is carried out. A person’s DS includes not only a large number of different motor skills and habits, but also a habitual way of thinking, beliefs, ideas about surrounding events. Modernity requires a reworking of habitual views, and sometimes even strong beliefs , i.e. a situation is created when it is necessary to move from one dynamic stereotype to another. And this is associated with the appearance of corresponding unpleasant feelings. In this case, our nervous system does not always cope with life’s tasks easily. The difficulty lies in the fact that before developing a new attitude towards reality (a new life stereotype), it is necessary to destroy the old attitude towards it. Therefore, some people find it quite difficult to rebuild any element of their life stereotype, not to mention the restructuring of their ideas and beliefs. Remaking stereotypes is also difficult in childhood. Pavlov came to the conclusion that emotional states may depend on whether a dynamic stereotype is supported or not. When maintaining a dynamic stereotype, positive emotions usually appear, and when changing the stereotype, negative ones. It should be noted that in the implementation of complex stereotypes, adjustment plays an important role, i.e. such a state of readiness for activity, which is formed according to the mechanism of temporary communication. The emergence of a conditioned reflex setting can be seen in students who divide academic subjects into favorite and unloved. The student goes to a lesson with a teacher teaching his favorite subject with desire, and this can be seen in his good mood. A student often goes to a lesson with a teacher of an unloved subject, or perhaps even with an unloved teacher, in a bad, sometimes even depressed, mood. The reason for this behavior of the student lies in the conditioned reflex attunement from the complex classroom environment, the essence of the academic subject, and the behavior of the teacher. Dissimilar situations also cause different settings.

20. Dominant. Definition, properties and types of dominants. Meaning in the pedagogical process. Age characteristics. Dominant - the dominant focus of excitation in the central nervous system, subordinating the functions of other nerve centers. The phenomenon of dominance was discovered by A.A. Ukhtomsky (1923) in experiments with irritation of the motor zones of the cerebrum and observation of the resulting flexion of the animal’s limb. As it turned out, if you irritate the cortical motor area against the background excessive increase in excitability of another nerve center, then normal flexion of the limb does not occur. Instead of flexing the limb, irritation of the motor zone causes a reaction of those effectors whose activity is controlled by the dominant one, i.e. currently dominant in the central nervous system, the nerve center. In an experiment, a dominant can be obtained by repeatedly sending afferent impulses to a certain center, by humoral influences on it. The role of hormones in the formation of a dominant focus of excitation is demonstrated by experiment on a frog: in the spring, irritation of any part of the skin in a male does not cause a protective reflex, but an increase in the hugging reflex. Under conditions of natural behavior, the dominant state of the nerve centers can be caused by metabolic reasons. Dominant focus excitement has a number of special properties, the main ones are the following: inertia, persistence, increased excitability, the ability to “attract” to oneself excitations radiating through the central nervous system, the ability to exert a depressing effect on competing centers and other nerve centers. Meaning The dominant focus of excitation in the central nervous system is that on its basis specific adaptive activity is formed, aimed at achieving useful results. For example, on the basis of the dominant state of the hunger center, food-procuring behavior is realized; Based on the dominant state of the thirst center, behavior aimed at searching for water is triggered. Successful completion of these behavioral acts ultimately eliminates the physiological causes of the dominant state of the hunger and thirst centers. The dominant plays an important role in the coordination activity of the central nervous system (see section 4.9), in remembering and processing information.

NERVOUS IMPULSE

NERVOUS IMPULSE

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

Generation, transmission and processing of N. and. carried out by the nervous system. Basic The structural element of the nervous system of higher organisms is the nerve cell, or neuron, consisting of a cell body and numerous. processes - dendrites (Fig. 1). One of the processes in non-riferiforms. neurons have a large length - this is a nerve fiber, or axon, the length of which is ~ 1 m, and the thickness is from 0.5 to 30 microns. There are two classes of nerve fibers: pulpy (myelinated) and non-pulphate. The pulp fibers have myelin, formed by special fibers. membrane, the edges, like insulation, are wound onto the axon. The length of the sections of the continuous myelin sheath ranges from 200 µm to 1 mm, they are interrupted by the so-called. nodes of Ranvier 1 µm wide. The myelin sheath plays an insulating role; the nerve fiber in these areas is passive, electrically active only in the nodes of Ranvier. Non-pulp fibers are not insulated. plots; their structure is uniform along the entire length, and the membrane is electrically activity over the entire surface.

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

an eerie width of ~10 nm. This area of ​​contact between two cells is called. synapse. The axon membrane entering the synapse is called presynaptic, and the corresponding membrane of dendrites or muscles is post-synaptic (see. Cellular structures).

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

Speed ​​of spread of N. and. u . depends on the type of nerve fiber and its diameter d, u . ~ d 1/2. In the thin fibers of the human nervous system u . ~ 1 m/s, and in thick fibers u . ~ 100-120 m/s.

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

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

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

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

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

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

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

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

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

Rice. 2. Development of action potential in the nervous systemlocke: A- subthreshold ( 1 ) and suprathreshold (2) irritation; b-membrane response; with above-threshold stimulation, full sweat occursaction cial; V- ion current flowing through membrane when excited; G - approximation ion current in a simple analytical model.

If the current pulse exceeds a certain threshold value, the potential continues to change even after the disturbance is turned off; the potential becomes positive and only then returns to the resting level, and at first it even jumps a little (hyperpolarization region, Fig. 2). The response of the membrane does not depend on the disturbance; this impulse is called action potential. At the same time, an ionic current flows through the membrane, directed first inward and then outward (Fig. 2, V).

Phenomenological interpretation of the mechanism of occurrence of N. and. was given by A. L. Hodgkin and A. F. Huxley in 1952. The total ion current is composed of three components: potassium, sodium and leakage current. When the membrane potential shifts by a threshold value j* (~ 20 mV), the membrane becomes permeable to Na + ions. Na + ions rush into the fiber, shifting the membrane potential until it reaches the equilibrium sodium potential:

component ~ 60 mV. Therefore, the full amplitude of the action potential reaches ~120 mV. By the time the max. potential in the membrane, potassium begins to develop (and at the same time sodium decreases). As a result, the sodium current is replaced by a potassium current directed outward. This current corresponds to a decrease in the action potential.

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

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

Size g l considered constant, conductivity g Na and g K is described using parameters m, h And P:

g Na, g K - constants; options t, h And P satisfy linear equations

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

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

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

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

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

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

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

Type of functionality I specific for a biologically excitable environment. However, equation (5), if we ignore the type I, is more general in nature and describes many physical. phenomena, for example combustion process. Therefore, N.’s transmission and. likened to the burning of a gunpowder cord. If in a running flame the ignition process is carried out due to thermal conductivity, then in N. and. excitation occurs with the help of the so-called. local currents (Fig. 5).

Rice. 5. Local currents that ensure propagationloss of nerve impulse.

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

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

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

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

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

Rie. 6. The passage of nerve impulses expandsto the fiber: A - change in pulse speed in depending on its direction; b-schematic image of an expanding fiber.

Another type of heterogeneity is fiber branching. At the branch node, different types are possible. options for passing and blocking impulses. With a non-synchronous approach, N. and. the blocking condition depends on the time offset. If the time between pulses is small, then they help each other penetrate into the wide third fiber. If the shift is large enough, then N. and. interfere with each other. This is due to the fact that N. and., who approached first, but failed to excite the third fiber, partially transfers the node to a refractory state. In addition, a synchronization effect occurs: as N. approaches and. towards the node their lag relative to each other decreases.

Interaction N. and. Nerve fibers in the body are combined into bundles or nerve trunks, forming something like a multi-core cable. All fibers in the bundle are independent. communication lines, but have one common “wire” - intercellular. When N. and. runs along any of the fibers, it creates an electric current in the intercellular fluid. , which affects the membrane potential of neighboring fibers. Typically, such an influence is negligible and communication lines operate without mutual interference, but it manifests itself pathologically. and arts. conditions. By treating nerve trunks with special chem. substances, it is possible to observe not only mutual interference, but also the transfer of excitation to neighboring fibers.

There are known experiments on the interaction of two nerve fibers placed in a limited external volume. solution. If N. and. runs along one of the fibers, then the excitability of the second fiber simultaneously changes. Change goes through three stages. Initially, the excitability of the second fiber decreases (the excitation threshold increases). This decrease in excitability precedes the action potential traveling along the first fiber and lasts approximately until the potential in the first fiber reaches a maximum. Then the excitability increases; this stage coincides in time with the process of decreasing the potential in the first fiber. Excitability decreases again when a slight hyperpolarization of the membrane occurs in the first fiber.

At the same time passing N. and. using two fibers it was sometimes possible to achieve their synchronization. Despite the fact that own speed N. and. in different fibers are different, when they are simultaneously. excitement could arise collective N. and. If own speeds were the same, then the collective impulse had a lower speed. With a noticeable difference in property. speeds, the collective speed had an intermediate value. Only N. and. could synchronize, the speeds of which did not differ too much.

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

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

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

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

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

An excitable medium can be three-dimensional, although more often it is considered as two-dimensional. The excitement that arose in the k.-l. point on the surface, propagates in all directions in the form of a ring wave. An excitation wave can bend around obstacles, but cannot be reflected from them, nor is it reflected from the boundary of the medium. When waves collide with each other, they are mutually destroyed; These waves cannot pass through each other due to the presence of a refractory region behind the excitation front.

An example of an excitable environment is the cardiac neuromuscular syncytium - the union of nerve and muscle fibers into a single conductive system capable of transmitting excitation in any direction. Neuromuscular syncytia contract synchronously, obeying a wave of excitation sent by a single control center - the pacemaker. The uniform rhythm is sometimes disrupted and arrhythmias occur. One of these modes is called. atrial flutter: these are autonomous contractions caused by the circulation of excitation around an obstacle, for example. superior or inferior vein. For such a regime to occur, the perimeter of the obstacle must exceed the excitation wavelength, which is ~ 5 cm in the human atrium. With flutter, periodic movement occurs. atrial contraction with a frequency of 3-5 Hz. A more complex mode of excitation is fibrillation of the ventricles of the heart, when the department. elements of the heart muscle begin to contract without external influence. commands and without communication with neighboring elements with a frequency of ~ 10 Hz. Fibrillation leads to cessation of blood circulation.

The emergence and maintenance of spontaneous activity in an excitable environment is inextricably linked with the emergence of wave sources. The simplest source of waves (spontaneously excited cells) can provide periodic. pulsation of activity, this is how the heart pacemaker works.

Sources of excitation can also arise from complex spaces. organizing the excitation mode, for example. reverberator of the type of rotating spiral wave, appearing in the simplest excitable medium. Another type of reverberator occurs in a medium consisting of two types of elements with different excitation thresholds; The reverberator periodically excites one or the other elements, while changing the direction of its movement and generating plane waves.

The third type of source is the leading center (echo source), which appears in a medium that is heterogeneous in refractoriness or excitation threshold. In this case, a reflected wave (echo) appears on the inhomogeneity. The presence of such wave sources leads to the appearance of complex excitation modes studied in the theory of autowaves.

Lit.: Hodgkin A., Nerve impulse, trans. from English, M., 1965; Katz B., Nerve, muscle and synapse, trans. from English, M., 1968; Khodorov B.I., Problem of excitability, L., 1969; Tasaki I., Nervous excitement, trans. from English, M., 1971; Markin V.S., Pastushenko V.F., Chizmadzhev Yu.A., Theory of excitable media, M., 1981. V. S. Markin.

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

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

NERNST-ETTINGSHAUSEN EFFECT- appearance of electricity fields E ne in a conductor in which there is a temperature gradient T , in a direction perpendicular to the magnet. field N . There are transverse and longitudinal effects.

Transverse H.-E. e. consists in the appearance of electricity. fields E ne | (potential difference V ne | ) in a direction perpendicular to N And T . In the absence of magnetic thermoelectric fields the field compensates for the flow of charge carriers created by the temperature gradient, and compensation occurs only for the total current: electrons with an energy greater than the average (hot) move from the hot end of the sample to the cold, electrons with an energy less than the average (cold) - in the opposite direction. The Lorentz force deflects these groups of carriers in a direction perpendicular to T and mag. field, in different directions; the deflection angle (Hall angle) is determined by the relaxation time t of a given group of carriers, i.e., it differs for hot and cold carriers if t depends on energy. In this case, the currents of cold and hot carriers in the transverse direction ( | T And | N ) cannot compensate each other. This results in a field E | ne , the value of which is determined from the condition that the total current is equal to 0 j = 0.

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

IN semiconductors Under the influence T charge carriers of different signs move in one direction, and in a magnetic direction. the fields are deviated in opposite directions. As a result, the direction of the Nernst-Ettingshausen field created by charges of different signs does not depend on the sign of the carriers. This significantly distinguishes the transverse N.-E. e. from Hall effect, where the direction of the Hall field is different for charges of different signs.

Because coefficient N | is determined by the dependence of the carrier relaxation time t on their energy, then N.-E. e. sensitive to mechanism charge carrier scattering. The scattering of charge carriers reduces the influence of the magnetic field. fields. If t ~ , then at r> 0 hot carriers scatter less often than cold ones and the direction of the field E | ne is determined by the direction of deflection in mag. hot carrier field. At r < 0 направление E | ne is opposite and is determined by cold carriers.

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

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

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

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

Magn. the field, bending the trajectories of electrons (see above), reduces their mean free path l in the direction T . Since the free travel time (relaxation time t) depends on the electron energy, then the decrease l is not the same for hot and cold carriers: it is less for that group, for a certain type it is less. Thus, mag. the field changes the role of fast and slow carriers in energy transfer, and thermoelectric. the field ensuring the absence of charge during energy transfer must change. At the same time, the coefficient N || also depends on the carrier scattering mechanism. Thermoelectric the current increases if m decreases with increasing carrier energy (when carriers are scattered by acoustic phonons), or decreases if m increases with increasing (when scattered by impurities). If electrons with different energies have the same t, the effect disappears ( N|| = 0). Therefore, in metals, where the energy range of electrons involved in transfer processes is small (~ kT), N || few: In a semiconductor with two types of carriers N ||~ ~ g/kT. At low temps N|| may also increase due to the influence of electron drag by phonons. In strong magnetic fields complete thermoelectric. field in magnetic the field is “saturated” and does not depend on the carrier scattering mechanism. In ferromagnetic metals N.-E. e. has features associated with the presence of spontaneous magnetization.

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Nerve fibers have the ability to perceive irritations and conduct emerging nerve impulses, connecting the body with the environment and between its parts.

A nerve impulse arising in receptors and propagating along conductors through neuron bodies to working organs or to other neurons is a wave of activity that causes changes in electrical potential. Each excited section of the nerve becomes electronegative relative to the sections that are at rest. The resulting potential fluctuation is an electrical expression of the passage of an impulse through a given area and is called an action potential.

The propagation of an excitation wave along the nerve is accompanied by the occurrence of physicochemical processes. Simultaneously with electrical phenomena, the passage of a nerve impulse is accompanied by the release of a small amount of heat in the nerve. Thus, the energy for the occurrence and conduction of an impulse arises in the nerve itself, and not in the source of irritation. The irritation itself is an impetus for the release of energy and functionality of the nerve fibers.

After the appearance of the excitation wave, a period of refractory state begins, when for 0.001 seconds the nerve is not capable of conducting an impulse. Then the response occurs only to very strong stimulation (relative refractory period) and, finally, the excitability of the fiber reaches its initial level.

An indicator of the strength of irritation in a nerve fiber is the frequency of periodic impulses. An increase in the number of excited fibers in the nerve ensures strong voluntary muscle contraction.

Metabolism in the nerve is characterized by continuous consumption of oxygen and release of carbon dioxide. When the nerve is excited, the need for oxygen increases and at the same time the amount of carbon dioxide released increases. Glucose and phospholipids play an important role in the metabolism of the nerve, ensuring its vital activity at rest. When excited, phosphocreatine, which contains energy-rich phosphate bonds, is broken down. Lactic acid and ammonia are also released.

An important property of the nerve, independent of the strength of stimulation, is the speed at which the impulse travels through the fibers. Efferent motor fibers have the highest conduction speed, afferent fibers (tactile and temperature sensitivity) have a slightly lower conduction speed, and pain sensitivity fibers have the lowest. In humans, the speed of impulse conduction in the motor fiber ranges from 60 to 120 m/sec. In the fibers that conduct pain sensations and in the fibers of the autonomic nervous system, the conduction speed ranges from 1 to 30 m/sec. All slow impulses propagate along the pulpless fibers.

The speed of impulse conduction depends on the caliber of the fiber conducting the excitation wave. There are three groups of nerve fiber structures within the nerve.

Group A fibers- pulpy, have a diameter of 1 - 20 centimeters and are distinguished by the highest speed of the nerve impulse in mammals (100-5 m/sec). These fibers conduct deep sensitivity, tactile stimulation, a sense of pressure, as well as motor impulses.

Group B fibers- thin pulpy, impulse conduction speed 12-3 m/sec. These fibers transmit precisely localized pain sensations as well as impulses in the preganglionic fibers.

Group C fibers- pulpless (the thinnest pulpy, according to the latest electron microscopy data), have a pulse conduction speed of less than 2 m/sec. These fibers conduct pain and temperature sensitivity and are also postganglionic fibers.

It has been established that there is a direct relationship between the speed of a nerve impulse along a fiber and its excitability: the faster the fiber conducts, the greater its excitability.

Conduction of nerve impulses

The question of the essence of the nerve impulse remains unresolved at present. There are different theories regarding its conduction along the nerve.

Currently, the theory of electrical propagation of a nerve impulse has received the most recognition, according to which the area of ​​the nerve excited by the stimulus acquires a negative electrical charge in relation to the adjacent resting sections. The resulting potential difference leads to an electric current flowing from the unexcited area to the excited one. The resting section turns into an excited one and receives a negative charge, in turn influencing the neighboring unexcited section with a positive charge, from where an electric current flows towards it. Thus, local currents in active areas of the nerve affect neighboring inactive ones.

It is possible that the emergence and conduction of nerve impulses occur according to the gradual theory of excitation, based on the recognition of the electrical mechanism of the nerve impulse and the quantitative dependence of the magnitude of the local electrical reaction on the strength of stimulation.

According to some, excitation along the nerve spreads spasmodically, appearing and giving discharges at the nodes of Ranvier. The transverse membrane in each interception is of particular importance for the conduction of nerve impulses.

The theory of chemical transfer of excitation has become widespread. According to this theory, local currents are factors in conducting the impulse across the synapse, but the development of depolarization depends on the chemical mediation of acetylcholine, which is then quickly destroyed by the enzyme cholinesterase, which restores the original polarized state.

The main conducting structure in the fiber is axoplasm with neurofibrils. The pulpy membrane, closely connected with the axon, determines the nature of excitability and conductivity. The speed of propagation of the nerve impulse directly depends on the thickness of the pulpy membrane. In various pathological conditions, the pulp membrane reacts to an unusual stimulus by changing its composition.

The nodes of Ranvier are considered areas of particular importance for the exchange and conduction of nerve fibers.

The Schwann cell, the cytoplasm of which envelops the myelin and axon, is important for the life of the nerve as an environment in which the processes of exchange of myelin and axon occur, where the products of splitting and disintegration of these most important components of the nerve accumulate.

The endoneural sheath, consisting of collagen and reticular layers, forms a protective sheath for the nerve fiber, protecting it from stretching and compression, and also providing its blood supply.

The article was prepared and edited by: surgeon

Action potential or nerve impulse, a specific response that occurs in the form of an excitatory wave and flows along the entire nerve pathway. This reaction is a response to a stimulus. The main task is to transmit data from the receptor to the nervous system, and after that it directs this information to the desired muscles, glands and tissues. After the passage of the pulse, the surface part of the membrane becomes negatively charged, while its inner part remains positive. Thus, a nerve impulse is a sequentially transmitted electrical change.

The exciting effect and its distribution are subject to physico-chemical nature. The energy for this process is generated directly in the nerve itself. This happens due to the fact that the passage of an impulse leads to the formation of heat. Once it has passed, the attenuation or reference state begins. In which only a fraction of a second the nerve cannot conduct a stimulus. The speed at which the pulse can be delivered ranges from 3 m/s to 120 m/s.

The fibers through which excitation passes have a specific sheath. Roughly speaking, this system resembles an electrical cable. The composition of the membrane can be myelin or non-myelin. The most important component of the myelin sheath is myelin, which plays the role of a dielectric.

The speed of the pulse depends on several factors, for example, on the thickness of the fibers; the thicker it is, the faster the speed develops. Another factor in increasing conduction speed is the myelin itself. But at the same time, it is not located over the entire surface, but in sections, as if strung together. Accordingly, between these areas there are those that remain “bare”. They cause current leakage from the axon.

An axon is a process that is used to transmit data from one cell to the rest. This process is regulated by a synapse - a direct connection between neurons or a neuron and a cell. There is also a so-called synaptic space or cleft. When an irritating impulse arrives at a neuron, neurotransmitters (molecules of a chemical composition) are released during the reaction. They pass through the synaptic opening, eventually reaching the receptors of the neuron or cell to which the data needs to be conveyed. Calcium ions are necessary for the conduction of a nerve impulse, since without this the neurotransmitter cannot be released.

The autonomic system is provided mainly by non-myelinated tissues. Excitement spreads through them constantly and continuously.

The transmission principle is based on the appearance of an electric field, so a potential arises that irritates the membrane of the adjacent section and so on throughout the fiber.

In this case, the action potential does not move, but appears and disappears in one place. The transmission speed through such fibers is 1-2 m/s.

Laws of conduct

There are four basic laws in medicine:

• Anatomical and physiological value. Excitation is carried out only if there is no violation in the integrity of the fiber itself. If unity is not ensured, for example, due to infringement, drug use, then the conduction of a nerve impulse is impossible.
• Isolated conduction of irritation. Excitation can be transmitted along the nerve fiber, without spreading to neighboring ones.
• Bilateral conduction. The path of impulse conduction can be of only two types - centrifugal and centripetal. But in reality, the direction occurs in one of the options.
• Non-decremental implementation. The impulses do not subside, in other words, they are carried out without decrement.

Chemistry of impulse conduction

The irritation process is also controlled by ions, mainly potassium, sodium and some organic compounds. The concentration of these substances is different, the cell is negatively charged inside itself, and positively charged on the surface. This process will be called potential difference. When a negative charge oscillates, for example, when it decreases, a potential difference is provoked and this process is called depolarization.

Stimulation of a neuron entails the opening of sodium channels at the site of stimulation. This may facilitate the entry of positively charged particles into the cell. Accordingly, the negative charge is reduced and an action potential or nerve impulse occurs. After this, the sodium channels close again.

It is often found that it is the weakening of polarization that promotes the opening of potassium channels, which provokes the release of positively charged potassium ions. This action reduces the negative charge on the cell surface.

The resting potential or electrochemical state is restored when potassium-sodium pumps are activated, with the help of which sodium ions leave the cell and potassium ions enter it.

As a result, we can say that when electrochemical processes are resumed, impulses occur that travel along the fibers.

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

Generation, transmission and processing of N. and. carried out by the nervous system. Basic The structural element of the nervous system of higher organisms is the nerve cell, or neuron, consisting of a cell body and numerous. processes - dendrites (Fig. 1). One of the processes in non-riferiforms. neurons have a large length - this is a nerve fiber, or axon, the length of which is ~ 1 m, and the thickness is from 0.5 to 30 microns. There are two classes of nerve fibers: pulpy (myelinated) and non-pulphate. The pulp fibers have a myelin sheath formed by special fibers. membrane, the edges, like insulation, are wound onto the axon. The length of the sections of the continuous myelin sheath ranges from 200 µm to 1 mm, they are interrupted by the so-called. nodes of Ranvier 1 µm wide. The myelin sheath plays an insulating role; the nerve fiber in these areas is passive, only the membrane in the nodes of Ranvier is electrically active. Non-pulp fibers are not insulated. plots; their structure is uniform along the entire length, and the membrane is electrically activity over the entire surface.

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

an eerie width of ~10 nm. This area of ​​contact between two cells is called. synapse. The axon membrane entering the synapse is called presynaptic, and the corresponding membrane of dendrites or muscles is post-synaptic (see. Cellular structures).

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

Speed ​​of spread of N. and. u depends on the type of nerve fiber and its diameter d, u ~ d 1/2. In thin fibers of the human nervous system u ~ 1 m/s, and in thick fibers u ~ 100-120 m/s.

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

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

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

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

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

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

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

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

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

Rice. 2. Development of action potential in the nervous systemlocke: A- subthreshold ( 1 ) and suprathreshold (2) irritation; b-membrane response; with above-threshold stimulation, full sweat occursaction cial; V- ion current flowing through membrane when excited; G- approximation ion current in a simple analytical model.

If the current pulse exceeds a certain threshold value, the potential continues to change even after the disturbance is turned off; the potential becomes positive and only then returns to the resting level, and at first it even jumps a little (hyperpolarization region, Fig. 2). The response of the membrane does not depend on the disturbance; this impulse is called action potential. At the same time, an ionic current flows through the membrane, directed first inward and then outward (Fig. 2, V).

Phenomenological interpretation of the mechanism of occurrence of N. and. was given by A. L. Hodgkin and A. F. Huxley in 1952. The total ion current is composed of three components: potassium, sodium and leakage current. When the membrane potential shifts by a threshold value j* (~ 20 mV), the membrane becomes permeable to Na + ions. Na + ions rush into the fiber, shifting the membrane potential until it reaches the equilibrium sodium potential:

component ~ 60 mV. Therefore, the full amplitude of the action potential reaches ~120 mV. By the time the max. potential in the membrane, potassium conductivity begins to develop (and at the same time sodium conductivity decreases). As a result, the sodium current is replaced by a potassium current directed outward. This current corresponds to a decrease in the action potential.

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

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

Size g l considered constant, conductivity g Na and g K is described using parameters m, h And P:

g Na, g K - constants; options t, h And P satisfy linear equations

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

Rice. 3. Dependence of coefficients a And b from membranesgreat potential.

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

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

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

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

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

Type of functionality I specific for a biologically excitable environment. However, equation (5), if we ignore the type I, is more general in nature and describes many physical. phenomena, for example combustion process. Therefore, N.’s transmission and. likened to the burning of a gunpowder cord. If in a running flame the ignition process is carried out due to, then in N. and. excitation occurs with the help of the so-called. local currents (Fig. 5).

Rice. 5. Local currents that ensure propagationloss of nerve impulse.

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

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

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

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

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

Rie. 6. The passage of nerve impulses expandsto the fiber: A- change in pulse speed in depending on its direction; b-schematic image of an expanding fiber.

Another type of heterogeneity is fiber branching. At the branch node, different types are possible. options for passing and blocking impulses. With a non-synchronous approach, N. and. the blocking condition depends on the time offset. If the time shift between the pulses is small, then they help each other penetrate the wide third fiber. If the shift is large enough, then N. and. interfere with each other. This is due to the fact that N. and., who approached first, but failed to excite the third fiber, partially transfers the node to a refractory state. In addition, a synchronization effect occurs: as N. approaches and. towards the node their lag relative to each other decreases.

Interaction between N. and. Nerve fibers in the body are combined into bundles or nerve trunks, forming something like a multi-core cable. All fibers in the bundle are independent. communication lines, but have one common “wire” - intercellular fluid. When N. and. runs along any of the fibers, it creates an electric current in the intercellular fluid. field, which affects the membrane potential of neighboring fibers. Typically, such an influence is negligible and communication lines operate without mutual interference, but it manifests itself pathologically. and arts. conditions. By treating nerve trunks with special chem. substances, it is possible to observe not only mutual interference, but also the transfer of excitation to neighboring fibers.

There are known experiments on the interaction of two nerve fibers placed in a limited external volume. solution. If N. and. runs along one of the fibers, then the excitability of the second fiber simultaneously changes. Change goes through three stages. Initially, the excitability of the second fiber decreases (the excitation threshold increases). This decrease in excitability precedes the action potential traveling along the first fiber and lasts approximately until the potential in the first fiber reaches a maximum. Then the excitability increases; this stage coincides in time with the process of decreasing the potential in the first fiber. Excitability decreases again when a slight hyperpolarization of the membrane occurs in the first fiber.

At the same time passing N. and. using two fibers it was sometimes possible to achieve their synchronization. Despite the fact that own speed N. and. in different fibers are different, when they are simultaneously. excitement could arise collective N. and. If own speeds were the same, then the collective impulse had a lower speed. With a noticeable difference in property. speeds, the collective speed had an intermediate value. Only N. and. could synchronize, the speeds of which did not differ too much.

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

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

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

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

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

An excitable medium can be three-dimensional, although more often it is considered as a two-dimensional surface. The excitement that arose in the room. point on the surface, propagates in all directions in the form of a ring wave. An excitation wave can bend around obstacles, but cannot be reflected from them, nor is it reflected from the boundary of the medium. When waves collide with each other, they are mutually destroyed; These waves cannot pass through each other due to the presence of a refractory region behind the excitation front.

An example of an excitable environment is the cardiac neuromuscular syncytium - the union of nerve and muscle fibers into a single conductive system capable of transmitting excitation in any direction. Neuromuscular syncytia contract synchronously, obeying a wave of excitation sent by a single control center - the pacemaker. The uniform rhythm is sometimes disrupted and arrhythmias occur. One of these modes is called. atrial flutter: these are autonomous contractions caused by the circulation of excitation around an obstacle, for example. superior or inferior vein. For such a regime to occur, the perimeter of the obstacle must exceed the excitation wavelength, which is ~ 5 cm in the human atrium. With flutter, periodic movement occurs. atrial contraction with a frequency of 3-5 Hz. A more complex mode of excitation is fibrillation of the ventricles of the heart, when the department. elements of the heart muscle begin to contract without external influence. commands and without communication with neighboring elements with a frequency of ~ 10 Hz. Fibrillation leads to cessation of blood circulation.

The emergence and maintenance of spontaneous activity in an excitable environment is inextricably linked with the emergence of wave sources. The simplest source of waves (a group of spontaneously excitable cells) can provide periodic pulsation of activity, this is how the heart pacemaker works.

Sources of excitation can also arise from complex spaces. organizing the excitation mode, for example. reverberator of the type of rotating spiral wave, appearing in the simplest excitable medium. Another type of reverberator occurs in a medium consisting of two types of elements with different excitation thresholds; the reverberator periodically excites one or the other elements, while changing the direction of its movement and generating.

The third type of source is the leading center (echo source), which appears in a medium that is heterogeneous in refractoriness or excitation threshold. In this case, a reflected wave (echo) appears on the inhomogeneity. The presence of such wave sources leads to the appearance of complex excitation modes studied in the theory of autowaves.

Lit.: Hodgkin A., Nerve impulse, trans. from English, M., 1965; Katz B., Nerve, muscle and synapse, trans. from English, M., 1968; Khodorov B.I., Problem of excitability, L., 1969; Tasaki I., Nervous excitement, trans. from English, M., 1971; Markin V.S., Pastushenko V.F., Chizmadzhev Yu.A., Theory of excitable media, M., 1981. V. S. Markin.