In the activity of the nervous system, the reflex mechanism is the main one. A reflex is a response of the body to an external stimulus, carried out with the participation of the nervous system.

The neural pathway of the reflex is called the reflex arc. The reflex arc consists of: 1) a perceiving formation - a receptor, 2) a sensitive or afferent neuron that connects the receptor with nerve centers, 3) intermediate (or intercalary) neurons of nerve centers, 4) an efferent neuron that connects nerve centers with the periphery, 5) a working the organ that responds to irritation is a muscle or gland.

The simplest reflex arcs include only two nerve cells, but many reflex arcs in the body consist of a significant number of diverse neurons located in various parts of the central nervous system. Performing responses, the nerve centers send commands to the working organ (for example, a skeletal muscle) through efferent pathways that play the role of the so-called channels in a direct connection. In turn, during the implementation of the reflex response or after it, the receptors located in the working organ and other receptors of the body send information about the result of the action to the central nervous system. The afferent pathways of these messages are feedback channels. The information received is used by the nerve centers to control further actions, i.e., the cessation of the reflex reaction, its continuation or change. Therefore, the basis

holistic reflex activity is not a separate reflex arc, but a closed reflex ring formed by direct and feedback connections of the nerve centers with the periphery.

HOMEOSTASIS

The internal environment of the body, in which all its cells live, is blood, lymph, interstitial fluid. It is characterized by relative constancy - the homeostasis of various indicators, since any changes in it lead to disruption of the functions of cells and tissues of the body, especially highly specialized cells of the central nervous system. Such constant indicators of homeostasis include the temperature of the internal parts of the body, maintained within 36-37 ° C, the acid-base balance of the blood, characterized by pH = 7.4-7.35, the osmotic pressure of the blood (7.6-7.8 atm.), the concentration of hemoglobin in the blood - 130-160 ּлֿ¹ and others.

Homeostasis is not a static phenomenon, but a dynamic equilibrium. The ability to maintain homeostasis under conditions of constant metabolism and significant fluctuations in environmental factors is provided by a complex of regulatory functions of the body. These regulatory processes of maintaining dynamic balance are called homeokinesis.

The degree of shift in homeostasis indicators with significant fluctuations in environmental conditions or with hard work in most people is very small. For example, a long-term change in blood pH by only 0.1-0.2 can lead to death. However, in the general population there are individuals who have the ability to tolerate much larger shifts in the indicators of the internal environment. In highly skilled runners, as a result of a large intake of lactic acid from skeletal muscles into the blood during middle and long distance running, blood pH can decrease to values ​​of 7.0 and even 6.9. Only a few people in the world were able to climb to a height of about 8800 m above sea level (to the top of Everest) without an oxygen device, that is, to exist and move in conditions of an extreme lack of oxygen in the air and, accordingly, in the tissues of the body. This ability is determined by the innate characteristics of a person - the so-called genetic norm of the reaction, which even for fairly constant functional indicators of the body has wide individual differences.

2.5. ORIGINATION OF EXCITATION AND ITS CONDUCT 2.5.1. MEMBRANE POTENTIALS

The cell membrane consists of a double layer of lipid molecules, turned "heads" outward, and "tails" to each other. Clumps of protein molecules float freely between them. Some of them penetrate the membrane through. Some of these proteins have special pores or ion channels through which the ions involved in the formation of membrane potentials can pass (Fig. I-A).

Two special proteins play the main role in the emergence and maintenance of the resting membrane potential. One of them plays the role of a special sodium-potassium pump, which, using the energy of ATP, actively pumps sodium out of the cell and potassium into the cell. As a result, the concentration of potassium ions becomes higher inside the cell than in the fluid surrounding the cell, and sodium ions are higher outside.

Rice. 1. Membrane of excitable cells at rest (A) and during excitation (B).

(According to: B. Alberte et al., 1986)

a - a double layer of lipids, b - membrane proteins.

On A: "potassium leak" channels (1), "sodium-potassium pump" (2)

and a resting closed sodium channel (3).

On B: the sodium channel (1) open upon excitation, the entry of sodium ions into the cell and the change of charges on the outer and inner sides

membranes.

The second protein serves as a potassium leakage channel, through which potassium ions, due to diffusion, tend to leave the cell, where they are found in excess. Potassium ions, leaving the cell, create a positive charge on the outer surface of the membrane. As a result, the inner surface of the membrane is negatively charged with respect to the outer one. Thus, the membrane at rest is polarized, that is, there is a certain potential difference on both sides of the membrane, called the resting potential. It is equal to approximately minus 70 mV for a neuron, and minus 90 mV for a muscle fiber. The resting membrane potential is measured by inserting the thin tip of the microelectrode into the cell, and placing the second electrode into the surrounding liquid. At the moment the membrane is punctured and the microelectrode enters the cell, the beam shift is observed on the oscilloscope screen, which is proportional to the value of the resting potential.

The basis of the excitation of nerve and muscle cells is an increase in the permeability of the membrane for sodium ions - the opening of sodium channels. External stimulation causes the movement of charged particles inside the membrane and a decrease in the initial potential difference on both sides or depolarization of the membrane. Small amounts of depolarization lead to the opening of part of the sodium channels and a slight penetration of sodium into the cell. These reactions are subthreshold and cause only local (local) changes.

With an increase in stimulation, changes in the membrane potential reach the threshold of excitability or the critical level of depolarization - about 20 mV, while the value of the resting potential decreases to approximately minus 50 mV. As a result, a significant part of the sodium channels open. An avalanche-like entry of sodium ions into the cell occurs, causing a sharp change in the membrane potential, which is recorded as an action potential. The inner side of the membrane at the site of excitation is positively charged, while the outer side is negatively charged (Fig. 1-B).

This whole process is extremely short. It only takes about

1-2 ms, after which the sodium channel gates close. By this time, the permeability for potassium ions, which slowly increases upon excitation, reaches a large value. Potassium ions leaving the cell cause a rapid decrease in the action potential. However, the final recovery of the initial charge continues for some time. In this regard, in the action potential, a short-term high-voltage part is distinguished - a peak (or with soldering) and long-term small fluctuations - trace potentials. Motor neuron action potentials have a peak amplitude of about

100 mV and a duration of about 1.5 ms, in skeletal muscles - the amplitude of the action potential is 120-130 mV, the duration is 2-3 ms.

In the process of recovery after a potential action, the operation of the sodium-potassium pump ensures that excess sodium ions are "pumped out" and the lost potassium ions are "pumped" inside, i.e., the return to the original asymmetry of their concentration on both sides of the membrane. About 70% of the total energy required by the cell is spent on the operation of this mechanism.

The emergence of excitation (action potential) is possible only if a sufficient amount of sodium ions is maintained in the environment surrounding the cell. Large losses of sodium by the body (for example, with sweat during prolonged muscular work in conditions of high air temperature) can disrupt the normal activity of nerve and muscle cells, reducing a person's performance. Under conditions of oxygen starvation of tissues (for example, in the presence of a large oxygen debt during muscular work), the excitation process is also disrupted due to the defeat (inactivation) of the mechanism of entry of sodium ions into the cell, and the cell becomes unexcitable. The process of inactivation of the sodium mechanism is affected by the concentration of Ca ions in the blood. With an increase in the content of Ca, cellular excitability decreases, and with a deficiency of Ca, excitability increases, and involuntary muscle cramps appear.

Physiology of the central nervous system (CNS).

The CNS is a system that regulates almost all functions in the body. The central nervous system connects all the cells and organs of our body into a single whole. With its help, the most adequate changes in the work of various bodies occur, aimed at ensuring one or another of its activities. In addition, the central nervous system communicates the body with the external environment by analyzing and synthesizing the information coming to it from receptors and forms a response aimed at maintaining homeostasis.

The structure of the CNS.

The structural and functional unit of the nervous system is nerve cell(neuron). Neuron - a specialized cell capable of receiving, encoding, transmitting and storing information, organizing the body's responses to stimuli, and establishing contacts with other neurons.

A neuron consists of a body (soma) and processes - numerous dendrites and one axon (Fig. 1).

Fig.1. The structure of a neuron.

Dendrites usually branch strongly and form many synapses with other nerve cells, which determines their leading role in the perception of information by the neuron. The axon starts from the cell body with the axon mound, the function of which is to generate a nerve impulse, which is carried along the axon to other cells. The length of the axon can reach one meter or more. The axon branches strongly, forming many collaterals (parallel paths) and terminals. Terminal - the end of an axon, with the help of which a synapse is formed with another cell. In the CNS, terminals form neuro-neuronal synapses; in the periphery (outside the CNS), axons form either neuromuscular or neurosecretory synapses. The end of the axon is often called not the terminal, but the synaptic plaque (or synaptic button). A synaptic plaque is a terminal (terminal) thickening of an axon that serves to deposit a neurotransmitter (see lectures on the synapse). The membrane of the endings contains a large number of voltage-dependent calcium channels through which calcium ions enter the ending when it is excited.

In most central neurons (i.e. CNS neurons), AP primarily occurs in the region of the axon colliculus membrane, and from here excitation spreads along the axon to the synaptic plaque. Thus, the unique features of a neuron are the ability to generate electrical discharges and transmit information using specialized endings - synapses.

Each neuron performs 2 main functions: conducts impulses and processes impulses (see below "transformation of the excitation rhythm"). Any part of a neuron has a conduction. Conducting impulses (information) from one cell to another, the neuron carries out thanks to its processes: the axon and dendrites. Each neuron has one axon and many dendrites.

Processing of impulsation (processing of information, transformation of impulsation) - this is the most significant function of the neuron, which is carried out on the axon colliculus.

In addition to neurons in the CNS, there are glial cells, which occupy half the volume of the brain. Peripheral axons (peripheral - meaning outside the CNS) are also surrounded by a sheath of glial cells. They are capable of dividing throughout their lives. Dimensions are 3-4 times smaller than neurons. With age, their number increases.

The functions of glial cells are diverse:

1) they are the supporting, protective and trophic apparatus for neurons;

2) maintain a certain concentration of calcium and potassium ions in the intercellular space;

3) actively absorb neurotransmitters, thus limiting the duration of their action.

Classification of neurons

Dependences on the departments of the central nervous system: vegetative and somatic

By the type of mediator that is released by the endings of the neuron: adrenergic (NA), etc.

By influence, there are excitatory and inhibitory

According to the specificity of perceiving sensory information, the neurons of the higher parts of the CNS are mono and polymodal.

According to the activity of neurons, there are: phonoactive, silent - which are excited only in response to irritation.

According to the source or direction of information transfer: afferent, intercalary, efferent

The reflex principle of the activity of the central nervous system.

The main mechanism of the activity of the central nervous system is the reflex. Reflex - This is the response of the body to the actions of the stimulus, carried out with the participation of the central nervous system. For example, withdrawing a hand during an injection, closing the eyelids when irritating the cornea is also a reflex. The separation of gastric juice when food enters the stomach, defecation when filling the rectum, reddening of the skin when exposed to heat, knee, elbow, Babinsky, Rosenthal - these are all examples of reflexes. The number of reflexes is unlimited. Common to all of them is the mandatory participation in their implementation of the central nervous system.

Another definition of reflex, also emphasizing the role of the CNS, is the following: reflex is a centrifugal response to centripetal stimulation. (In the examples given, determine for yourself what is a centrifugal response and what is irritation. Irritation is always centripetal, that is, the stimulus acting on the receptors causes an impulse that enters the central nervous system).

The structural basis of the reflex, its material substrate is reflex arc(fig.2 ).

Rice. 2.Reflex arc

The reflex arc consists of 5 links:

1) receptor;

2) afferent (sensitive, centripetal) link;

3) insert link (central);

4) efferent (motor, centrifugal) link;

5) effector (working body).

The part of the body containing receptors, upon stimulation of which a certain reflex occurs, is called receptive field of the reflex.

The reflex can be carried out only when the integrity of all links of the reflex arc is preserved.

H nerve center.

Nerve center (CNS center or nucleus) is a set of neurons involved in the implementation of a specific reflex. Those. each reflex has its own center: there is a center for the knee reflex, its own center for the elbow reflex, its own - nictitating, has cardiovascular, respiratory, food centers, centers of sleep and wakefulness, hunger and thirst, etc. In the whole organism, during the formation of complex adaptive processes, there is a functional association of neurons located at different levels of the CNS, i.e. complex association of a large number of centers.

The union of nerve centers (nuclei) is carried out by the CNS pathways with the help of neuro-neuronal (interneuronal) synapses. There are 3 types of neuron connections: sequential, divergent and convergent.

Nerve centers have a number of characteristic functional properties, which are largely due to these three types of neural networks, as well as the properties of interneuronal synapses.

The main properties of the nerve centers:

1. Convergence (convergence) ( Fig.3). In the CNS, excitations from various sources can converge to one neuron. This ability of excitations to converge to the same intermediate and final neurons is called convergence of excitations.

Fig.3. Convergence of excitation.

2. Divergence (divergence) - divergence of impulses from one neuron to many neurons at once. On the basis of divergence, irradiation of excitation occurs and it becomes possible to quickly involve many centers located at different levels of the central nervous system in the response.

Fig.4. Excitation divergence.

3. Excitation in the nerve centers spreads unilaterally - from the receptor to the effector, which is due to the property of chemical synapses to unilaterally conduct excitation from the presynaptic membrane to the postsynaptic one.

4. Excitation in the nerve centers is carried out slower, than along a nerve fiber. This is due to the slow conduction of excitation through synapses (synaptic delay), which are numerous in the nucleus.

5. In the nerve centers, summation of excitations. Summation is the addition of pre-threshold pulses. There are two types of summation.

Temporary or sequential, if excitatory impulses come to the neuron along the same path through one synapse with an interval less than the time of complete repolarization of the postsynaptic membrane. Under these conditions, local currents on the postsynaptic membrane of the perceiving neuron are summed up and bring its depolarization to a level Ek sufficient to generate an action potential by the neuron. This summation is called temporal, because a series of impulses (irritations) comes to the neuron over a certain period of time. It is called sequential because it is implemented in a serial connection of neurons.

Spatial or Simultaneous - observed when excitatory impulses arrive at the neuron simultaneously through different synapses. This summation is called spatial because the stimulus acts on some space of the receptive field, i.e. several (at least 2) receptors of different parts of the receptive field. (While temporal summation can be realized under the action of a series of stimuli on the same receptor). It is called simultaneous, because information arrives to the neuron simultaneously through several (at least 2) communication channels, i.e. simultaneous summation is realized by convergent connection of neurons.

6.Transformation of the rhythm of excitation - change in the number of excitatory impulses leaving the nerve center, compared with the number of impulses coming to it. There are two types of transformation:

1) step down transformation, which is based on the phenomenon of summation of excitations, when in response to several pre-threshold excitations that have come to the nerve cell, only one threshold excitation occurs in the neuron;

2) up transformation, it is based on multiplication (multiplication) mechanisms that can dramatically increase the number of excitation pulses at the output.

7. Reflex aftereffect - is that the reflex reaction ends after the cessation of the stimulus. This phenomenon is due to two reasons:

1) prolonged trace depolarization of the neuron membrane, against the background of the arrival of powerful afferentation (strong sensitive impulses), causing the release of a large amount (quanta) of the mediator, which ensures the emergence of several action potentials on the postsynaptic membrane and, accordingly, a short-term reflex aftereffect;

2) prolongation of the output of excitation to the effector as a result of the circulation (reverberation) of excitation in the neural network of the "neural trap" type. Excitation, getting into such a network, can circulate in it for a long time, providing a long reflex aftereffect. Excitation in such a chain can circulate until some external influence slows down this process or fatigue occurs in it. An example of an aftereffect is a well-known life situation, when even after the cessation of a strong emotional stimulus (after the quarrel has ended), general excitement continues for some more or less long time, blood pressure remains elevated, facial hyperemia persists, tremor of the hands.

8. Nerve centers have high sensitivity to lack of oxygen. Nerve cells are characterized by intensive consumption of O 2 . The human brain absorbs about 40-70 ml of O 2 per minute, which is 1/4-1/8 of the total amount of O 2 consumed by the body. Consuming a large amount of O 2 , nerve cells are highly sensitive to its deficiency. Partial cessation of the blood circulation of the center leads to severe disorders in the activity of its neurons, and a complete cessation - to death within 5-6 minutes.

9. Nerve centers, like synapses, have high sensitivity to the action of various chemicals in, especially poisons. One neuron can have synapses that have different sensitivity to different chemicals. Therefore, it is possible to choose such chemicals that will selectively block some synapses, leaving others in working condition. This makes it possible to correct the states and reactions of both healthy and diseased organisms.

10. Nerve centers, like synapses, have fatigue in contrast to the nerve fibers, which are considered practically untiring. This is due to a sharp decrease in the mediator reserves, a decrease in the sensitivity of the postsynaptic membrane to the mediator, a decrease in its energy reserves, which is observed during prolonged work and is the main cause of the development of fatigue.

11. Nerve centers, like synapses, have low lability, the main cause of which is synaptic delay. The total synaptic delay observed in all neuro-neuronal synapses during the conduction of impulses through the central nervous system, or in the nerve center, is called the central delay.

12. Nerve centers have tone, which is expressed in the fact that even in the absence of special irritations, they constantly send impulses to the working bodies.

13. Nerve centers have plasticity - the ability to change their own functionality and expand their functionality. Plasticity can also be defined as the ability of some neurons to take on the function of the affected neurons of the same center. Namely, the ability to restore the motor activity of the limbs, for example, the legs, lost as a result of spinal cord injuries, is associated with the phenomenon of plasticity. However, this is possible only if part of the neurons of this center is damaged or if part of the CNS pathways is preserved intact. With a complete rupture of the spinal cord, the restoration of motor activity is impossible. In addition, neurons of one center, for example, flexors, cannot take on the function of neurons of another center. - extensors. Those. the phenomenon of plasticity of the centers of the central nervous system is limited.

14. occlusion (blocking) (fig.5) - this is the sum of the threshold impulses. Occlusion is carried out (as well as spatial summation) in the convergent system of connection of neurons. Simultaneous activation of several (at least two) receptors by strong or superstrong stimuli to one neuron will converge several threshold or superthreshold impulses. Occlusion will occur on this neuron, i.e. he will respond to these two stimuli with the same maximum force as to each of them separately. The phenomenon of occlusion is that the number of excited neurons with simultaneous stimulation of the afferent inputs of both nerve centers is less than the arithmetic sum of the excited neurons with a separate stimulation of each afferent input separately.

Fig.6. The phenomenon of occlusion in the CNS.

The phenomenon of occlusion leads to a decrease in the strength of the response. Occlusion has a protective value, preventing overstrain of neurons under the action of superstrong stimuli.

Similar information.

Textbook for grade 8

Higher nervous activity

Higher nervous activity (HNA) is understood as all those nervous processes that underlie human behavior, ensuring the adaptation of each person to rapidly changing and often very difficult and unfavorable conditions of existence. The material basis of higher nervous activity is the brain. It is in the brain that all the information about what is happening in the world around us flows. Based on a very fast and accurate analysis of this information, the brain makes decisions that lead to changes in the activity of body systems, ensuring optimal (best under these conditions) interaction between a person and the environment, maintaining the constancy of his internal environment.

Reflex activity of the nervous system

The idea that mental activity is carried out with the participation of the nervous system arose in ancient times, but how this happens remained unclear for a very long time. Even now it cannot be said that the mechanisms of the brain are fully disclosed.

The first scientist to prove the involvement of the nervous system in shaping human behavior was the Roman physician Galen (2nd century AD). He discovered that the brain and spinal cord are connected to all other organs by nerves, and that the rupture of the nerve connecting the brain and muscle leads to paralysis. Galen also proved that when the nerves coming from the sense organs are cut, the body ceases to perceive stimuli.

The origin of the physiology of the brain as a science is associated with the work of the French mathematician and philosopher Rene Descartes (XVII century). It was he who laid the idea of ​​​​the reflex principle of the body. True, the term “reflex” itself was proposed in the 18th century. Czech scientist I. Prochazka. Descartes believed that the activity of the brain, as well as the entire human body, is based on the same principles that underlie the operation of the simplest mechanisms: clocks, mills, bellows, etc. Explaining the simple movements of a person from a completely materialistic position, R. Descartes recognized that he had a soul that governs the complex and diverse behavior of man.

What is a reflex? Reflex is the most correct, most common reaction of the body to external stimuli, which is carried out through the nervous system. For example, a child touched a hot stove with his hand and instantly felt pain. The only correct decision that the brain always makes in this situation is to withdraw the hand in order not to get burned.

At a higher level, the doctrine of the reflex principle of the body's activity was developed by the great Russian physiologist Ivan Mikhailovich Sechenov (1829-1905). The main work of his life - the book "Reflexes of the Brain" - was published in 1863. In it, the scientist proved that a reflex is a universal form of interaction between the body and the environment, that is, not only involuntary, but also voluntary - conscious movement. They begin with irritation of some sense organs and continue in the brain in the form of certain nervous phenomena, leading to the launch of behavioral programs. I. M. Sechenov was the first to describe the inhibitory processes that develop in the central nervous system. In a frog with destroyed cerebral hemispheres, the scientist studied the reaction to irritation of the hind leg with an acid solution: in response to a painful stimulus, the leg bent. Sechenov discovered that if a crystal of salt is first applied to the surface of the midbrain in an experiment, the time to a response will increase. Based on this, he concluded that reflexes can be inhibited by some strong influences. A very important conclusion made by scientists at the end of the 19th - beginning of the 20th centuries was the conclusion that any response of the organism to a stimulus is always expressed by movement. Any sensation, consciously or unconsciously, is accompanied by a response motor reaction. By the way, it is precisely on the fact that any reflex ends with muscle contraction or relaxation (ie movement) that the work of lie detectors is based, capturing the smallest, unconscious movements of an agitated, anxious person.

The assumptions and conclusions of I. M. Sechenov were revolutionary for their time, and not all scientists at that time immediately understood and accepted them. Experimental evidence of the truth of the ideas of I. M. Sechenov was obtained by the great Russian physiologist Ivan Petrovich Pavlov (1849 1936). It was he who introduced the term "higher nervous activity" into the scientific language. He believed that higher nervous activity is equivalent to the concept of "mental activity".

Indeed, both sciences - GNI physiology and psychology study the activity of the brain; they are also united by a number of general research methods. At the same time, GNI physiology and psychology explore different aspects of the brain: GNI physiology - the mechanisms of the activity of the entire brain, its individual structures and neurons, the connections between structures and their influence on each other, as well as the mechanisms of behavior; psychology - the results of the work of the central nervous system, manifested in the form of images, ideas, ideas and other mental manifestations. The scientific research of GNI psychologists and physiologists has always been interdependent. In recent decades, a new science has even emerged - psychophysiology, the main task of which is to study the physiological foundations of mental activity.

All reflexes that occur in the body of an animal or a person, I. P. Pavlov divided into unconditional and conditional.

unconditioned reflexes. Unconditioned reflexes ensure that the body adapts to constant environmental conditions. In other words, it is the reaction of the body to strictly defined external stimuli. All animals of the same species have a similar set of unconditioned reflexes. Therefore, unconditioned reflexes are classified as species traits.

An example of unconditioned reflexes is the occurrence of a cough when foreign bodies enter the respiratory tract, withdrawal of a hand when pricked by rose thorns.

Already in a newborn child, unconditioned reflexes are observed. This is understandable, because it is impossible to live without them, and there is no time to learn: to breathe, eat, avoid dangerous influences is necessary from the very first moments of life. One of the important reflexes of newborns is the sucking reflex - the unconditioned food reflex. An example of a protective unconditioned reflex is pupil constriction in bright light.

The role of unconditioned reflexes is especially important in the life of those creatures whose existence lasts only a few days, or even just one day. For example, the female of one of the species of large solitary wasps emerges from the chrysalis in the spring and lives only for a few weeks. During this time, she must have time to meet with the male, catch the prey (spider), dig a mink, drag the spider into the mink, lay eggs. All these actions she does several times during her life. The wasp comes out of the chrysalis already "adult" and is immediately ready to perform its activities. This does not mean that she is incapable of learning. She, for example, can and should remember the location of her mink.

More complex forms of behavior - instincts - are a chain of reflex reactions sequentially connected to each other, which follow one after another. Here, each individual reaction serves as a signal for the next one. The presence of such a chain of reflexes allows organisms to adapt to a particular situation, environment.

A vivid example of instinctive activity is the behavior of ants, bees, birds when building a nest, etc.

In highly organized vertebrates, the situation is different. For example, a wolf cub is born blind and completely helpless. Of course, at birth, he has a number of unconditioned reflexes, but they are not enough for a full life. In order to adapt to existence in constantly changing conditions, it is necessary to develop a wide range of conditioned reflexes. Conditioned reflexes, being developed as a superstructure over innate reflexes, greatly increase the chances of the organism to survive.

Conditioned reflexes. Conditioned reflexes are reactions acquired during the life of each person or animal, with the help of which the organism adapts to changing environmental influences. For the formation of a conditioned reflex, the presence of two stimuli is necessary: ​​a conditioned (indifferent, signal, indifferent to the reaction being developed) and an unconditioned one, causing a certain unconditioned reflex. The conditioned signal (a flash of light, the sound of a bell, etc.) must be somewhat ahead of the time of the unconditioned reinforcement. Usually, a conditioned reflex is developed after several combinations of conditioned and unconditioned stimuli, but in some cases one presentation of a conditioned and unconditioned stimulus is enough to form a conditioned reflex.

For example, if a light bulb is turned on several times before giving food to a dog, then, starting at some point, the dog will come up to the feeder and salivate each time the light is turned on before food is presented to it. Here, the light becomes a conditioned stimulus, signaling that the body must prepare for an unconditioned reflex food reaction. A temporary functional connection is formed between the stimulus (the light of a light bulb) and the food reaction. The conditioned reflex is developed in the learning process, and the connection between the sensory (in our case, visual) system and the effector organs that ensure the implementation of the food reflex is formed on the basis of a combination of a conditioned stimulus and its unconditional reinforcement with food. So, for the successful development of a conditioned reflex, three conditions must be met. First, the conditioned stimulus (in our example, light) must precede the unconditioned reinforcer (in our example, food). Second, the biological significance of the conditioned stimulus must be less than that of the unconditioned reinforcer. For example, for a female of any mammal, the cry of her cub is obviously a stronger stimulus than food reinforcement. Thirdly, the strength of both conditioned and unconditioned stimuli must have a certain value (the law of strength), since very weak and very strong stimuli do not lead to the development of a stable conditioned reflex.

A conditioned stimulus can be any event that occurred in the life of a person or animal, which coincided several times with the action of reinforcement.

The brain, capable of developing conditioned reflexes, considers conditioned stimuli as signals indicating the imminent appearance of reinforcement. So, an animal that has only unconditioned reflexes can only eat the food that it accidentally stumbled upon. An animal that is capable of developing conditioned reflexes associates a previously indifferent smell or sound with the presence of nearby food. And these irritants become a clue that makes him search for prey more actively. For example, pigeons can sit quietly on the eaves and window sills of some architectural landmark, but as soon as a bus with tourists approaches them, the birds immediately begin to sink to the ground, expecting to be fed. Thus, the sight of a bus, and especially tourists, is a conditioned irritant for pigeons, indicating that it is necessary to take a comfortable seat and start fighting with rivals for food.

As a result, an animal capable of rapidly developing conditioned reflexes will be more successful in obtaining food than one that lives using only a set of innate unconditioned reflexes.

Braking. If unconditioned reflexes are practically not inhibited during life, then the developed conditioned reflexes can lose their significance when the conditions of the organism's existence change. The extinction of conditioned reflexes is called inhibition.

There are external and internal inhibition of conditioned reflexes. If, under the influence of a new strong external stimulus, a focus of strong excitation arises in the brain, then the previously developed conditioned reflex connection does not work. For example, the food conditioned reflex in a dog is inhibited by strong noise, fright, the action of a painful stimulus, etc. This type of braking is called external. If the salivation reflex developed to the bell is not reinforced by feeding, then gradually the sound ceases to play the role of a conditioned stimulus; the reflex will begin to fade and soon slow down. The temporary connection between the two centers of excitation in the cortex will be destroyed. This type of inhibition of conditioned reflexes is called internal.

Skills. In an independent category of conditioned reflexes, motor conditioned reflexes developed during life, that is, skills, or automated actions, are distinguished. A person learns to walk, swim, ride a bike, type on a computer keyboard. Learning takes time and perseverance. However, gradually, when the skills are already fixed, they are performed automatically, without consciousness control.

During his life, a person masters many special motor skills related to his profession (working on a machine tool, driving a car, playing a musical instrument).

Skills are good for a person because they save time and energy. Consciousness and thinking are freed from control over operations that have become automated and become habits in everyday life.

Works by A. A. Ukhtomsky and P. K. Anokhin

At every moment of life, a person is affected by many external and internal stimuli - some of them are very important, while others can be neglected at the moment. After all, the body cannot ensure the simultaneous implementation of many reflexes. You should not even try to satisfy the need for food while running away from the dog. You have to choose one thing. According to the great Russian physiologist Prince A. A. Ukhtomsky, a single focus of excitation temporarily dominates in the brain, as a result of which the fulfillment of one reflex that is vital at the moment is ensured. A. A. Ukhtomsky called this focus of excitation the dominant (from the Latin “dominance” - dominant). Dominants constantly replace each other as the main needs at some point are satisfied and new ones arise. If the need for food after a hearty meal has passed, the need for sleep may arise, and a completely different dominant will appear in the brain, aimed at finding a sofa and pillow. The dominant focus inhibits the work of neighboring nerve centers and, as it were, subordinates them to itself: when you want to eat, your sense of smell and taste become aggravated, and when you want to sleep, the sensitivity of the sense organs weakens. The dominant underlies such mental processes as attention, will, and makes a person's behavior active and selectively aimed at satisfying the most important needs.

Since the body of an animal or a person cannot fully respond to several different stimuli at the same time, something like a “queue” has to be established. Academician P.K. Anokhin believed that in order to satisfy the most important need at the moment, various systems and organs are combined into a so-called “functional system”, consisting of many sensitive and working links. This functional system "works" until the desired result is achieved. For example, feeling hungry, a person is full. Now the same systems that were involved in the search, extraction, absorption of food can be combined into a different functional system and participate in the satisfaction of other needs.

Sometimes previously developed conditioned reflexes persist for a long time, even if they no longer receive unconditional reinforcement.

• In the English cavalry of the middle of the XIX century. horses have been taught for years to attack in close formation. Even if the rider was knocked out of the saddle, his horse had to ride in the general formation side by side with other horses and make a U-turn with them. During the Crimean War, in one of the attacks, the cavalry unit suffered very heavy losses. But the surviving part of the horses, turning around and maintaining the system as much as possible, went to their original position, rescuing those few wounded cavalrymen who were able to stay in the saddles. As a token of gratitude, these horses were sent from the Crimea to England and kept there in excellent conditions, without being forced to walk under the saddle. But every morning, as soon as the stable doors opened, the horses ran out into the field and lined up. Then the leader of the herd gave a signal with a neigh, and the line of horses rushed in perfect order across the field. At the edge of the field, the line unfolded and returned to the stable in the same order. And this was repeated day after day ... This is an example of a conditioned reflex that persisted for a long time without unconditional reinforcement.

Test your knowledge

1. What are the merits of I.M. Sechenov and I. P. Pavlov in the development of the doctrine of higher nervous activity?
2. What is an unconditioned reflex?
3. What unconditioned reflexes do you know?
4. What underlies the innate form of behavior?
5. How is a conditioned reflex different from an unconditioned reflex?
6. What is instinct?
7. What conditions are necessary for the development of a conditioned reflex?
8. What forms of behavior can be classified as acquired?
9. Why can a conditioned reflex fade over time?
10. What is the purpose of conditional inhibition?

Think

As a result, the conditioned reflex fades away? What is the biological meaning of this phenomenon?

Reflex is the basis of nervous activity. Distinguish between innate and acquired behavior. They are based on unconditioned and conditioned reflexes. A complex form of acquired behavior is rational activity, this is the beginning of thinking. Conditioned reflexes may fade. Distinguish between unconditional and conditional inhibition.

The concept of "reflex" as an act of nervous activity was introduced in the 17th century by Descartes. However, the term itself appeared in the 18th century and belongs to Prohaska (a Czech scientist). The reflex theory was most developed in our country in the works of Sechenov and Pavlov.

A reflex is a response of the body to irritation, carried out with the participation of the central nervous system.

The structural elements involved in the implementation of the reflex reaction form a reflex arc, that is, a reflex arc is a series-connected chain of nerve cells that provides appropriate responses to irritation. It consists of a receptor, an afferent fiber, a nerve center, an efferent nerve fiber of the executive organ - an effector.

There are simple and complex reflex arcs: 1) monosynaptic arc - a reflex arc consisting of two neurons: sensory and motor with one synapse between them; 2) polysynaptic arc - contains sensitive, intercalary and motor neurons. In this case, there are one or more intercalary neurons between sensory and motor neurons.

Receptors are specialized formations intended for the perception by cells or the nervous system of various stimuli and irritations in nature. All types of receptors are divided into: exteroreceptors (perceiving information from the external environment) and interoreceptors.

Usually, receptors are not located alone, but form clusters of various densities. These clusters of receptors are called reflexogenic zones or receptor fields.

The time that elapses from the moment the stimulus begins to act until the moment the reaction occurs is called the reflex time.

In recent years, the doctrine of the reflex has been enriched with the concept of reverse afferentation (Fig. 1), that is, the reflex arc is considered as a closed formation in the form of a ring with feedback. The developed theory of Anokhin's functional systems showed that the adaptive activity of the body is based on the formation of functional systems in humans and animals in the process of individual development of functional systems not only receives, but also executes commands from the nerve center (direct connection), but also continuously sends impulses about its functional state ( feedback), on the basis of which the center makes adjustments to its commands.

Rice. 1. Diagram of a reflex arc with feedback.

1. Receptor.

afferent neuron.

Intermediate neuron (located in the CNS).

efferent neuron.

Effector.

Feedback neuron.

Classification of reflexes

Reflexes can be classified depending on what signs are taken as the basis. I.P. Pavlov took the higher nervous activity as the basis for the division of reflexes and divided it according to the basis of their formation into:

Unconditional (congenital, stereotypical forms of behavior);

Conditional (acquired, adaptive nature, responses to environmental factors).

Depending on the location of the receptors, reflexes are divided into exteroceptive, that is, caused by irritation of the receptor on the surface of the body, into interoreceptive, or visceral, arising from irritation of the receptors of internal organs and blood vessels, and proprioceptive, caused by irritation of the receptors of the joints, skeletal muscles, tendons.

Depending on the level of location of the nerve centers, reflexes are distinguished:

spinal (nerve centers are located in segments of the spinal cord);

bulbar (in the medulla oblongata);

mesencephalic (in the midbrain);

diencephalic (in the diencephalon);

cortical (in various areas of the cerebral cortex).

According to the nature of the response, reflexes are divided:

motor (reflexes of skeletal muscles, cardiac, vasomotor, oculomotor);

secretory (salivary, sweating);

trophic (expressed in a change in cellular metabolism).

Biological focus:

defensive;

1. indicative;

Properties of nerve centers

Nerve impulses travel along afferent pathways to the nerve centers. It is necessary to distinguish between the anatomical and physiological understanding of the nerve center. The nerve center from an anatomical point of view is a collection of neurons located in a specific section of the central nervous system. From a physiological point of view, the nerve center is a complex, functional association of several anatomical centers located at different stages of the central nervous system - from the spinal cord to the cerebral cortex - and causing complex reflexes due to their activity. In the process of functioning, the neurons located on the lower levels of the central nervous system are subject to the corrective influences of the higher-lying nerve centers according to the principle of subordination.

The properties of the nerve centers are due to:

The structure of the neurons that form the center.

Features of the conduction of nerve impulses by the synapse.

At present, the following features of the conduction of excitation in the nerve centers have been identified:

In nerve fibers, impulses are conducted in both directions. In the CNS, excitation can spread in only one direction: from an afferent neuron to an efferent one. Unilateral conduction of excitation due to the fact that the transmission of excitation is possible through the synapse in only one direction - from the nerve ending that secretes the mediator to the postsynaptic membrane. In the opposite direction, the excitatory postsynaptic potential does not propagate.

synaptic delay conduction of excitation - it is due to a slower conduction of nerve impulses through synapses, since time is spent on the following processes: the release of the mediator by the endings of the axon in response to the incoming nerve impulse; diffusion of the mediator through the synaptic cleft to the postsynaptic membrane; the emergence of an excitatory postsynaptic potential under the action of a mediator. Therefore, the more complex the reflex and the more synapses in its reflex arc, the longer the reflex time.

Summation excitation in the nerve centers: discovered in 1863 by Sechenov. There are two types of summation - temporal and spatial. If a single impulse of a small value arrives at a neuron, then an excitatory postsynaptic potential (EPSP) of a subthreshold value arises, which is insufficient to evoke a response. If the neuron receives a series of such successive fast impulses and the excitatory postsynaptic potential (EPSP) from the previous impulses is superimposed by the EPSP excitatory postsynaptic potential from the subsequent ones - they are summed up, reaching the threshold level and cause an action potential, excitation of the neuron and a response - temporal summation. Spatial summation is observed when various receptive fields are simultaneously stimulated by a subthreshold stimulus, when simultaneously impulses from these fields arrive along the axons to one neuron or nerve center, the neuron is formed and an excitatory postsynaptic potential of threshold strength arises, capable of causing a response.

Transformation of rhythm and strength of excitation- strengthening or weakening of the rhythm or strength of excitation coming from the periphery.

Aftereffect- in response to a single volley of afferent impulses, a series of impulses run through the efferent neurons, that is, the duration of the response exceeds the duration of the stimulation. The ability to remain aroused for some time after the cessation of the stimulus.

Relief- after each stimulus, excitability increases in the nerve centers.

smashing- the ability of one nerve center to increase the excitability of other centers.

Plastic- the functions of the nerve centers can change when conditions change. A change in the functions of the centers occurs if the working body with which the given center is connected is replaced by another (opened in 1827 by Flurence).

inertia- the nerve centers have the property to come into a state of excitation only with relatively prolonged stimulation.

Tone- the state of slight constant excitation, in which all the nerve centers are located, has a reflex character due to the ring interaction between the nerve centers and the periphery.

Fatigue- is the result of a violation of the transmission of excitation in interneuronal synapses due to a decrease in the stores of the mediator and a decrease in the sensitivity of the postsynaptic membrane to it, as well as a decrease in the energy resources of the nerve cell.

12.Braking- is the process of weakening or cessation of any activity. Inhibition in the central nervous system was discovered by Sechenov. It is understood as an independent, active nervous process caused by excitation and manifested in the suppression or complete shutdown of another excitation. Normal inhibition is inextricably linked with excitation, is its derivative, accompanies the excitable process, limiting and preventing the spread of excitation. Inhibition is an innate process that constantly takes place during the individual life of the organism. Motor reactions can be inhibited if excitations coming from two receptive fields occur in the centers.

The reflex of pulling the frog's leg to irritation with a weak solution of hydrochloric acid is inhibited by strong squeezing of the other leg. Inhibition is observed when a twist is applied to the lip of a horse or forceps to the nasal septum of a bull. In this case, strong pain stimulation inhibits the motor reactions of animals. Currently, it is customary to distinguish two forms of inhibition: primary and secondary.

For the emergence of primary inhibition, the presence of specialized inhibitory structures (inhibitory neurons and synapses) is necessary. In this case, inhibition occurs primarily, without prior excitation. An example of primary inhibition is pre- and postsynaptic inhibition. Presynaptic inhibition develops at axoaxonal synapses formed at the presynaptic endings of a neuron. It is based on the development of a slow and prolonged depolarization of the presynaptic ending, which leads to a decrease or blockade of further conduction of excitation. Postsynaptic inhibition is associated with hyperpolarization of the postsynaptic membrane under the influence of mediators that are released during excitation of inhibitory neurons. It occurs on the postsynaptic membrane of axosomatic or axodendrial synapses under the influence of activation of inhibitory neurons.

Secondary inhibition does not require special structures. It develops as a result of a change in the functional activity of ordinary excitable neurons. Secondary inhibition was discovered by Vvedensky. He discovered pessimal and parabiotic inhibition.

Pessimal inhibition occurs if the frequency of incoming impulses to the nerve centers exceeds their lability.

Parabiotic inhibition occurs in pathological conditions when the lability of the nerve centers is significantly reduced and the usual excitation for the centers is frequent and superstrong.

There is also a third type of secondary inhibition - inhibition following excitation. It develops in neurons after the end of excitation as a result of strong trace hyperpolarization of the membrane.

Significance of braking processes. Inhibition, along with excitation, takes an active part in the adaptation of the body to the environment. It plays an important role in the formation of conditioned reflexes, frees the central nervous system from processing less significant information, and ensures the coordination of reflex reactions. Inhibition limits the spread of excitation to other nervous structures, preventing disruption of their normal functioning, therefore, inhibition performs a protective function, protecting the nerve centers from fatigue and exhaustion.

The interaction of nerve cells forms the basis of the purposeful activity of the nervous system and, above all, the implementation of reflex acts. Thus, nervous regulation is reflex in nature.

Reflex called the response of the body to irritation of receptors, carried out through the central nervous system (CNS). The main provisions of the reflex principle of the activity of the central nervous system have been developed over two and a half centuries. Scientists identify five stages in the development of this concept.

First stage. Associated with the formation in the 11th century of the foundations for understanding the reflex principle of the central nervous system. The principle of reflex (reflective) activity of the nervous system was put forward in the 17th century by the French philosopher and mathematician Rene Descartes, who believed that all things and phenomena can be explained by natural science. This starting position allowed R. Descartes to formulate two important provisions of the reflex theory:

1) the activity of the organism under external influence is reflected (later it was called reflex - from lat reflexus - reflected);

2) the response to irritation is carried out with the help of the nervous system.

According to the theory of R. Descartes, nerves are tubes through which animal spirits, material particles of an unknown nature, move at great speed. They travel along the nerves to the muscle, which swells (contracts) as a result.

Second phase. Associated with the experimental substantiation of materialistic ideas about the reflex (ХУ11 - ХУ111 centuries). In particular, it was found that the reflex reaction can be carried out on one frog metamere ( metame p - segment of the spinal cord associated with the "piece of the body"). A significant contribution to the development of ideas about the reflex activity of the nervous system was made by the 18th century Czech physiologist I. Prochazka, who proceeded from the recognition of the unity of the organism and the environment, and also asserted the leading role of the nervous system in the regulation of body functions. It was I. Prokhazka who proposed the very term “reflex”. In addition, he introduced the law of force into physiology (an increase in the strength of a stimulus increases the strength of the reflex reaction of the body; there are not only external stimuli, but also internal ones); first gave a description of the classical reflex arc. In this time period, as a result of clinical experimental studies, scientists established the role of the posterior (sensory) and anterior (motor) roots of the spinal cord (Bell-Magendie law). Ch. Sherrington is actively studying reflex activity (in particular, segmental reflexes). As a result of his scientific research, the scientist describes the principle of afferent innervation of antagonist muscles, introduces the concept of "synapse", the principle of a common nerve pathway, the concept of the integrative activity of the nervous system.

Third stage. Materialistic ideas about mental activity are affirmed (I.M. Sechenov, 60s of the 19th century). Observing the development of children, the scientist comes to the conclusion that the basis of the formation of mental activity is precisely the principle of the reflex. He expressed this statement of his in the following phrase: "All acts of conscious and unconscious life, by their mode of origin, are reflexes." In the study of reflexes, he substantiated the adaptive nature of the variability of the reflex, discovered the mechanism of inhibition of reflexes, as well as the mechanism of summation of excitation in the central nervous system.

Fourth stage. Associated with the development of the foundations of the doctrine of higher nervous activity (research by I.P. Pavlov, early twentieth century). IP Pavlov discovered conditioned reflexes and used them as an objective method in the study of mental activity (higher nervous activity). The scientists formulated three basic principles of the reflex theory:

The principle of determinism (principle of causality), according to which any reflex reaction is causally conditioned. I.P. Pavlov argued: "There is no action without a reason." Every activity of the organism, every act of nervous activity is caused by a certain cause, an influence from the external world or the internal environment of the organism. The expediency of the reaction is determined by the specificity of the stimulus, the sensitivity to them (irritants) of the body.

The principle of structure. Its essence lies in the fact that the reflex reaction is carried out with the help of certain structures. The more structures, structural elements involved in the implementation of this reaction, the more perfect it is. There are no processes in the brain that do not have a material basis. Each physiological act of nervous activity is confined to a specific structure.

The principle of the unity of the processes of analysis and synthesis as part of a reflex reaction. The nervous system analyzes, i.e. distinguishes, with the help of receptors, all acting external and internal stimuli and, on the basis of this analysis, forms a holistic response - synthesis. Analysis and synthesis of both incoming information and responses occur continuously in the brain. As a result, the body extracts useful information from the environment, processes it, fixes it in memory and forms response actions in accordance with circumstances and needs.

Fifth stage. It is characterized by the creation of the doctrine of functional systems (research by P.K. Anokhin, mid-twentieth century). A functional system is a dynamic set of various organs and tissues that is formed to achieve a useful (adaptive) result. A useful result is maintaining the constancy of the internal environment of the body by regulating the functions of internal organs and behavioral somatic regulation (for example, the search for and consumption of water in case of its lack in the body and the appearance of thirst - a biological need). Satisfaction of social needs (achievement of high results of educational activity) can also be a useful result.

Investigating the reflex basis of the life of living organisms, scientists came to the conclusion that the basic reflexes are innate (unconditioned), since it is these reflexes, formed over millions of years of evolution, that are the same for all representatives of a particular type of animal organisms and little depend on the situational conditions for the existence of that or another specific representative of this animal species. With a sharp change in environmental conditions, the unconditioned reflex can lead to the death of the organism.

Unconditioned reflexes- the body's response to irritation of sensory receptors, carried out with the help of the nervous system. I.P. Pavlov singled out, first of all, unconditioned reflexes aimed at self-preservation of the organism (the main ones here are food, defensive, orienting and some others). These reflexes make up large groups of various innate reactions.

Unconditioned reflex activity was studied by P.V. Somonov. According to the scientist, the development of each sphere of the environment corresponds to three different classes of unconditioned reflexes:

vital unconditioned reflexes that ensure the individual and species preservation of the organism (food, drink, sleep regulation, defensive and orienting reflex, energy saving reflex, etc.). The criteria for these reflexes are: the physical death of an individual as a result of the failure to satisfy the corresponding need, the realization of an unconditioned reflex without the participation of another individual of the same species;

role-playing (zoosocial). They can only be realized through interaction with other individuals of their species. These reflexes underlie the territorial, parental, etc. behavior. In addition, they are of great importance for the phenomenon of emotional resonance, “empathy” and the formation of a group hierarchy, where each individual invariably acts in one role or another (marriage partner, parent or cub, owner of the territory or alien, leader or follower, etc.). d.);

unconditioned reflexes of self-development. They are focused on the development of new space-time environments, facing the future. These include exploratory behavior, the unconditioned reflex of resistance (freedom), imitation (imitative) and play.

Among the unconditioned reflexes, scientists also include the orienting reflex. Orienting reflex- unconditioned reflex involuntary sensory attention, accompanied by an increase in muscle tone, caused by an unexpected or new stimulus for the body. Scientists often call this reaction a reflex of alertness, anxiety, surprise, and I.P. Pavlov defined it as a “what is it?” reflex. The orienting reflex is characterized by the manifestation of a whole complex of reactions. Scientists distinguish three phases in the development of this reflex.

First phase. It is characterized by the cessation of current activity and fixation of the posture. According to P.V.Simonov, this is a general (preventive) inhibition that occurs on the appearance of any extraneous stimulus with an unknown signal value.

Second phase. It begins when the state of "stop reaction" changes into an activation reaction. At this phase, the entire body is transferred to a state of reflex readiness for a possible meeting with an emergency, which manifests itself, is expressed in a general increase in the tone of the entire skeletal muscles. At this phase, the orienting reflex manifests itself in the form of a multicomponent reaction, which includes turning the head and eyes in the direction of the stimulus.

Third phase. It begins with the fixation of the stimulus field to deploy the process of differentiated analysis of external signals and make a decision about the body's response.

The polycomponent composition of the orienting reflex indicates its complex morphological and functional organization.

The orienting reflex is included in the structure of orienting behavior (orienting research activity), which is especially pronounced in a new environment. Research activity here can be aimed both at the development of novelty, the satisfaction of curiosity, and at the search for an irritant, an object that can satisfy this need. In addition, the orienting reflex is also aimed at determining the "significance" of the stimulus. At the same time, an increase in the sensitivity of analyzers is observed, which facilitates the perception of stimuli affecting the body and the determination of their significance.

The mechanism for the implementation of the orienting reflex is the result of a dynamic interaction between many different formations of specific and nonspecific systems of the central nervous system. Thus, the phase of general activation is associated mainly with the activation of the stem reticular formation and generalized excitation of the cortex. In the development of the stimulus analysis phase, cortical-limbic-thalamic integration occupies a leading position. The hippocampus plays an important role in this. This ensures the specialization of the processes of analysis of the "novelty" and "significance" of the stimulus.

Along with unconditioned reflexes, which can be attributed to lower nervous activity, in higher animals and humans, on the basis of this lower nervous activity, new mechanisms of adaptation to constantly changing environmental conditions have been formed - higher nervous activity. With its help, and more specifically, with the help of conditioned reflexes, these living organisms acquired the ability to respond not only to the direct impact of biologically significant agents (food, defensive, etc.), but also to their remote signs.

At the turn of the 19th and 20th centuries, the famous Russian physiologist I.P. Pavlov, who studied the functions of the digestive glands for a long time (for these studies, the scientist was awarded the Nobel Prize in 1904), discovered in experimental animals a regular increase in the secretion of saliva and gastric juice, not only when food into the oral cavity, and then into the stomach, but also with only one expectation of eating. At that time, the mechanism of this phenomenon was unknown and was explained by "mental stimulation of the salivary glands." As a result of further scientific research in this direction, this phenomenon was named by scientists as conditioned reflexes. According to I.P. Pavlov, conditioned reflexes are developed on the basis of unconditioned ones and are acquired in the process of life. In addition, conditioned reflexes are unstable, that is, they can appear and disappear throughout a person's life, depending on the changing conditions of existence. The acquisition of conditioned reflexes occurs throughout a person's life. It is due to the immediate, constantly changing environment. The newly acquired conditioned reflexes greatly increase and expand the range of adaptive reactions of animals and humans.

To develop a conditioned reflex, it is necessary to coincide in time with two stimuli acting on an animal (or a person). One of these stimuli under any circumstances causes a natural reflex reaction, classified as an unconditioned reflex. Such a stimulus itself is defined as a conditioned reflex. Another stimulus used to develop a conditioned reflex, due to its routine, as a rule, does not cause any reaction and is defined as indifferent (indifferent). Stimuli of this kind evoke a certain orienting response only at the first presentation, which, for example, can manifest itself in turning the head and eyes in the direction of the acting stimulus. With repeated actions of the stimulus (stimulus), the orienting reflex weakens, and then completely disappears as a result of the habituation mechanism, and then the stimulus that caused it becomes indifferent.

As shown by numerous studies by I.P. Pavlov and his colleagues, the conditioned reflex is developed subject to the following rules:

The indifferent stimulus must act a few seconds earlier than the unconditioned stimulus. I.P. Pavlov’s research on dogs showed that if, for example, an indifferent stimulus (various sound signals) begins to act directly during feeding, and not before it begins, then a conditioned reflex is not formed.

The biological significance of an indifferent stimulus should be less than that of an unconditioned stimulus. Again, referring to the research carried out in the laboratory of I.P. Pavlov, it should be noted that if, for example, too loud, frightening sound signals are used, giving the animal food immediately after that, a conditioned reflex is not formed.

The formation of a conditioned reflex should not be interfered with by extraneous stimuli that distract the attention of the animal.

We can talk about a developed conditioned reflex if a previously indifferent stimulus begins to cause the same reaction as the unconditioned stimulus used in combination with it. So, if the feeding of an animal was preceded several times by the inclusion of some kind of sound signal, and as a result of this combination, salivation began to occur only at the sound signal, then this reaction should be considered a manifestation of a conditioned reflex. The action of an unconditioned stimulus following an indifferent one is defined as reinforcement, and when the previously indifferent stimulus begins to cause a reflex reaction, it becomes a conditioned stimulus (conditioned signal).

There are several approaches to the classification of conditioned reflexes.

First of all, scientists divide all conditioned reflexes (as well as unconditioned ones) into the following groups.

By biological significance they are distinguished into food, defensive, etc.

By type of receptor conditioned reflexes are divided into exteroceptive, proprioceptive, interoreceptive. In the studies of V.M. Bykov and V.N. Chernigovsky with their colleagues, the connection of the cerebral cortex with all internal organs was shown. Interoreceptive conditioned reflexes are usually accompanied by vague sensations, which I.M. Sechenov defined as “dark feelings” that affect mood and performance. Proprioceptive conditioned reflexes underlie the learning of motor skills (walking, production operations, etc.). Exteroreceptive conditioned reflexes form the adaptive behavior of animals in obtaining food, avoiding harmful effects, procreation, etc. For a person, exteroceptive verbal stimuli that form actions and thoughts are of paramount importance.

According to the function of the department of the nervous system and the nature of the efferent response There are conditioned reflexes somatic (motor) and vegetative (cardiovascular, secretory, excretory, etc.).

In relation to the signal stimulus to the unconditioned (reinforcing) stimulus all conditioned reflexes are divided into natural and artificial (laboratory). Natural conditioned reflexes are formed to signals that are natural signs of a reinforcing stimulus (smell, color, a certain time, etc.). For example, eating at the same time leads to the release of digestive juices and some other body reactions (for example, leukocytosis at the time of eating). Artificial (laboratory) are called conditioned reflexes to such signal stimuli that in nature are not related to the unconditioned (reinforced) stimulus. The main of these conditioned reflexes are the following:

according to complexity, they distinguish: simple conditioned reflexes developed to single stimuli (classical conditioned reflexes discovered by I.P. Pavlov); complex conditioned reflexes (reflexes formed on the impact of several signals acting either simultaneously or sequentially); chain reflexes - reflexes to a chain of stimuli, each of which causes its own conditioned reflex (a typical example here may be a dynamic stereotype),

according to the ratio of the time of action of the conditioned and unconditioned stimuli, there are cash and trace reflexes. The development of conditioned cash reflexes is characterized by the coincidence of the action of conditioned and unconditioned stimuli. Trace reflexes are developed under conditions when the unconditioned stimulus is connected somewhat later in time (after 2-3 minutes) than the conditioned one. THOSE. the development of a conditioned reflex occurs on the trail of a signal stimulus,

according to the development of a conditioned reflex on the basis of another conditioned reflex, conditioned reflexes of the first, second, third and other orders are distinguished. Reflexes of the first order are conditioned reflexes developed on the basis of unconditioned reflexes (classical conditioned reflexes). Second-order reflexes are developed on the basis of first-order conditioned reflexes, in which there is no unconditioned stimulus. A third-order reflex is formed on the basis of a second-order reflex, and so on. The higher the order of the conditioned reflex, the more difficult it is to develop it. So, in dogs it is possible to develop only conditioned reflexes of the third order (not higher),

conditioned reflexes for a while can be not only natural, but also artificial. With repeated application of an unconditioned stimulus with a constant interval between applications, a time reflex is formed. That is, some time before the reinforcement is given, a conditioned effector reaction occurs.

Depending on the signaling system distinguish conditioned reflexes to the signals of the first and second signal systems, i.e. on external influences and on speech.

Besides, conditioned reflexes can be positive and negative .

Many scientists define conditioned reflexes as reactions to future events. biological meaning conditioned reflexes lies in their preventive role. For the body, they have an adaptive value, preparing the body for future beneficial behavioral activities and helping it to avoid harmful effects, subtly and effectively adapt to the natural and social environment. It should also be noted that conditioned reflexes are formed due to the plasticity of the nervous system.

General characteristics of unconditioned and conditioned reflexes are presented in Table 1.

Table 1

General characteristics of unconditioned and conditioned reflexes

Unconditional	Conditional
1. Congenital, inherited (salivation, swallowing, breathing, etc.) 2. Species. 3. Have permanent reflex arcs. 4. Relatively constant, little changing (when food gets on the root of the tongue, a swallowing movement occurs). 5. Carried out in response to adequate stimulation. 6. Carried out at the level of the spinal cord and brain stem.	Acquired by the body during life. Individual Reflex arcs are formed only under certain conditions (they are not ready-made) The impermanent can arise and disappear. They are carried out on any irritation perceived by the body; are formed on the basis of unconditioned reflexes. They are carried out due to the activity of the cerebral cortex.

The path along which excitation spreads during the implementation of the reflex is called reflex arc ( Fig 2) .

The reflex arc consists of five main links:

Receptor.

Sensitive way.

Central nervous system.

Motor path.

Working body.

Fig.2. Reflex arc:

a - two-neuron; b - three-neurton

1 - receptor; 2 - sensitive (centripetal) nerve; 3 - sensitive neuron in the spinal glia; 4 - axon of a sensitive neuron; 5 - posterior roots of the spinal nerves; 6 - intercalary neuron; 7 - axon of the intercalary nerve; 8 - motor neuron in the horns of the spinal cord; 9 - spinal cord; 10 - axon of a motor (centrifugal) neuron; 11 - working body.

The reflex arc is a chain of nerve cells, including afferent (sensitive) and effector (motor, or secretory) neurons, along which the nerve impulse moves from its place of origin (from the receptor) to the working organ (effector). Most reflexes are carried out with the participation of reflex arcs, which are formed by neurons of the lower parts of the central nervous system - neurons of the spinal cord.

The simplest reflex arc consists of only two neurons - afferent (receptor) and effector (efferent). The body of the first neuron (afferent) is located outside the CNS. As a rule, this is the so-called unipolar neuron, the body of which is located in the spinal node or in the sensory node of the cranial nerves. The peripheral process of this cell is located in the composition of the spinal nerves or those having sensory fibers of the cranial nerves and their branches and ends with a receptor that perceives external (from the external environment) or internal (in the organs, tissues of the body) irritation. This irritation is transformed by the receptor into a nerve impulse that reaches the body of the nerve cell, and then along the central process (the totality of such processes forms the posterior, sensory roots of the spinal nerves) is sent to the spinal cord or along the corresponding cranial nerves to the brain. In the gray matter of the spinal cord or in the motor nucleus of the brain, this process of the sensory cell forms a synapse with the tol of the second (efferent) neuron. In the interneuronal synapse, with the help of mediators, the nervous excitation is transmitted from the sensitive (afferent) neuron to the motor (efferent) neuron, the process of which leaves the spinal cord as part of the anterior roots of the spinal nerves or motor (secretory) nerve fibers of the cranial nerves and goes to the working organ, causing muscle contraction, or inhibition, or increased secretion of the gland.

Complex reflex arc. As a rule, the reflex arc does not consist of two neurons and is much more complex. Between two neurons - receptor (afferent) and effector (efferent) - there is one or more closing (intercalary) neurons. In this case, the excitation from the receptor neuron through its central process is not transmitted directly to the effector nerve cell, but to one or more intercalary neurons. The role of intercalary neurons in the spinal cord is performed by cells located in the gray matter of the posterior columns. Some of these cells have an axon (neuritis), which goes to the motor cells of the anterior horns of the spinal cord of the same level and closes the reflex arc at the level of this segment of the spinal cord. The axon of other cells in the spinal cord may preliminarily divide in a T-shape into descending and ascending branches, which are directed to the motor cells of the anterior horns of adjacent, superior and underlying segments. On the way, each of the marked ascending or descending branches can give off collaterals to the motor cells of these and other neighboring segments. In this regard, it should be noted that irritation of even the smallest number of receptors can be transmitted not only to the nerve cells of a certain segment of the spinal cord, but also spread to the cells of several neighboring segments. As a result, the response is not a contraction of one muscle or one group of muscles, but several groups at once. Thus, in response to irritation, a complex reflex movement arises - a reflex.

As we noted above, I.M. Sechenov in his work “Reflexes of the Brain” put forward the idea of ​​causality (determinism), noting that every phenomenon in the body has its own cause, and the reflex effect is a response to this cause. These ideas were continued and confirmed in the works of I.P. Pavlov and S.P. Botkin. It was I.P. Pavlov who extended the doctrine of the reflex to the entire nervous system, starting from its lower sections and ending with its higher sections, and experimentally proved the reflex nature of all forms of the body's vital activity without exception. According to I.P. Pavlov, a simple form of activity of the nervous system, which is constant, innate, specific, and for the formation of structural prerequisites for which special conditions are not required, is an unconditioned reflex. Temporary connections acquired in the process of vital activity, which allow the body to establish rather complex and diverse relationships with the environment, are, according to I.P. Pavlov’s definition, conditionally reflex. The place of closure of conditioned reflexes is the cerebral cortex. Thus, the brain and its cortex are the basis of higher nervous activity.

Another scientist - P.K. Anokhin and his students confirmed the presence of the so-called feedback of the working organ with the nerve centers (this phenomenon is called "reverse afferentation"). At the moment when efferent impulses reach the executive organs from the central nervous system, a response (movement or secretion) is produced in them. This working effect irritates the receptors of the executive organ itself. The impulses that have arisen as a result of these processes are sent along afferent paths back to the centers of the spinal cord or brain in the form of information about the performance of a certain action by the organ at each given moment. Thus, it is possible to accurately take into account the correct execution of commands in the form of nerve impulses entering the working organs from the nerve centers, and their constant correction is carried out. The existence of two-way signaling through closed, circular or ring reflex nerve chains of "reverse afferentation" allows for constant, continuous, momentary corrections of any reactions of the body to any changes in the conditions of the internal and external environment. Without feedback mechanisms, the adaptation of living organisms to the environment would be impossible.

Thus, with scientific progress, the old ideas that the activity of the nervous system is based on an “open” (non-closed) reflex arc have been replaced by the idea of ​​a closed, annular arc, which is a chain of reflexes.

The process of formation of a classical conditioned reflex goes through three main stages.

Pregeneralization stage. It is characterized by a pronounced concentration of excitation (mainly in the projection zones of the cortex of conditioned and unconditioned stimuli) and the absence of conditioned behavioral reactions.

The stage of generalization of the conditioned reflex, which is based on the process of "diffuse" propagation (irradiation) of excitation. Conditioned reactions occur to signal and other stimuli (phenomenon of afferent generalization), as well as in the intervals between presentations of a conditioned signal (intersignal reactions). During this period, various bioelectrical shifts (blockade of the alpha rhythm, desynchronization, etc.) are widespread in the cortex and subcortical structures.

Specialization stage when intersignal reactions fade away and a conditioned response occurs only to a signal stimulus. Changes in biocurrents are more limited and are confined mainly to the action of the conditioned stimulus. This process ensures differentiation, subtle discrimination of stimuli, specialization of the conditioned reflex skill. In the process of specialization, the sphere of distribution of biopotentials is significantly narrowed and the conditioned reflex response increases.

According to the results of research by I.P. Pavlov, a temporary connection is formed between the cortical center of the unconditioned reflex and the cortical center of the analyzer, the receptors of which are affected by the conditioned stimulus, i.e. connection closes in the cerebral cortex). The closure of a temporary connection is based on dominant interaction process between excited centers. Impulses caused by an indifferent (conditioned) signal from any part of the skin and other sense organs (eye, ear, etc.) enter the cerebral cortex and provide the formation of a focus of excitation there. If, after an indifferent signal, food reinforcement (feeding) is given, then a more powerful second focus of excitation arises in the cerebral cortex, to which the excitation that has arisen and radiates through the cortex is directed. The repeated combination of an indifferent (conditioned) signal and an unconditioned stimulus (reinforcement) facilitates the passage of impulses from the cortical center of the indifferent signal to the cortical representation of the unconditioned reflex.

I.P. Pavlov called the formation of a temporary connection in the cerebral cortex the closing of a new conditioned reflex arc.

Also, scientists' research has proved that in parallel with the formation of a conditioned reflex, another conditioned reflex connection is being formed, which specifically changes the state of neurons, which is expressed in an increase in their background activity. If for some reason a conditioned reflex change in the state of a given neuron does not occur, then the reflex developed in it is not detected. This enabled scientists to conclude that the associative response includes the formation of a state that is qualitatively specific for each temporary connection. This phenomenon is considered by physiologists as another of the leading mechanisms for the formation of conditioned reflex behavior.

Thus, according to I.P. Pavlov, there are two mechanisms of conditioned reflex activity:

tuning, regulating the state of the brain and creating a certain level of excitability and performance of the nerve centers:

trigger, which initiates one or another conditional reaction.

The modern explanation of the mechanism of formation of conditioned reflexes is based on the idea of ​​modification of the activity of synapses that exist in those conditional points of the neural network that are capable of associating sensory signals that coincided in time.

Also, research scientists have proven that the process of formation of conditioned reflexes is directly related to memory. At the beginning of the development of a conditioned reflex, the connection is carried out only with the help of the mechanisms of short-term memory - the spread of excitation is carried out between two excited cortical centers. As the action of the conditioned and unconditioned stimuli is repeated and the corresponding centers are repetitively stimulated, short-term memory passes into long-term memory, that is, significant structural changes occur in neurons.

Conditioned reflexes, as shown by numerous studies, are changeable (variable), they can be inhibited.

Two types of inhibition of conditioned reflexes can be distinguished, which are fundamentally different from each other: congenital and acquired (Fig. 3). Moreover, each type of braking has its own variations.

Unconditional Conditional (internal)

1. External 1. Fading

3. Differential

4. Conditional brake

Rice. 3. Inhibition of conditioned reflexes

Unconditional (innate) inhibition conditioned reflexes is divided into external and transcendental. External braking manifests itself in the weakening or complete cessation of the present (currently occurring) conditioned reflex under the action of some extraneous stimulus. For example, turning on the light during the current conditioned reflex causes the appearance of an orienting-exploratory reaction that weakens or stops the existing conditioned reflex activity. I.P. Pavlov called this reaction, which arose to a change in the external environment (a reflex to novelty), the “what is it?” reflex. With the repetition of the action of an additional stimulus, the reaction to this signal weakens and disappears, since the body does not need to take any action. IP Pavlov also studied the mechanism of this type of inhibition of conditioned reflexes. According to his theory, an extraneous signal is accompanied by the appearance in the cerebral cortex of a new focus of excitation, which, with an average strength of the stimulus, has a depressing effect on the current conditioned reflex activity by the dominant mechanism. External inhibition is unconditioned reflex. This type of inhibition was called external because in these cases the excitation of the cells of the orienting-exploratory reflex arising from an extraneous stimulus is outside the arc of the present conditioned reflex. External inhibition contributes to the emergency adaptation of the body to changing conditions of the external and internal environment and provides the ability to switch to another activity in accordance with the situation.

Extreme braking occurs with prolonged nervous excitation of the body, under the action of an extremely strong conditioned signal or several weak ones. There is a certain correspondence between the strength of the conditioned stimulus and the magnitude of the response - the "law of force": the stronger the conditioned signal, the stronger the conditioned reflex reaction. However, this law can be preserved only up to a certain value (threshold), above which the effect begins to decrease, despite the continued increase in the strength of the conditioned signal. These facts allowed I.P. Pavlov to conclude that cortical cells have a working capacity limit.

Conditional (internal, acquired) inhibition conditioned reflexes is an active nervous process that requires development, like the reflex itself. It is no coincidence that this type of inhibition of the conditioned reflex is called conditioned reflex inhibition. It is acquired, individual. According to the theory of I.P. Pavlov, it is localized within (“inside”) the nerve center of this conditioned reflex. There are the following types of conditioned inhibition: fading, retarded, differential and conditioned inhibition.

Fading braking occurs when the conditioned signal is repeatedly applied and its further non-reinforcement. In this case, at first the conditioned reflex weakens, and then completely disappears. However, it may recover after some time. The rate of extinction depends on the intensity of the conditioned signal and the biological significance of the reinforcement. The more significant they are, the more difficult is the extinction of the conditioned reflex. It is extinction inhibition that can explain the forgetting of previously received information, which is not repeated for a long time.

delayed braking occurs when reinforcements lag behind by 1-3 minutes relative to the start of the action of the conditioned signal. Gradually, the appearance of the conditioned response is shifted to the moment of reinforcement. This type of inhibition of the conditioned reflex is also characterized by the phenomenon of disinhibition.

Differential braking is produced with the additional inclusion of a stimulus close to the conditioned one, and not reinforcing it.

Conditional brake occurs when another stimulus is added to the conditioned signal and this combination is not reinforced. So, if you develop a conditioned salivary reflex to light, then connect an additional stimulus (sound) to this signal, and do not reinforce this combination, then the conditioned reflex to it will gradually fade away.

The significance of all types of conditioned (internal) inhibition of conditioned reflexes lies in the elimination of unnecessary activity at a given time, that is, a very subtle adaptation of the organism to the environment.

A fixed system of conditioned and unconditioned reflexes, combined into a single functional complex, is commonly called dynamic stereotype. A dynamic stereotype is formed under the influence of stereotypically repeated changes and influences of the external and internal environment of the organism. Repeated in the same sequence stimuli acting on the body are external stereotype. It corresponds to the stereotypical dynamics of the cortical processes of excitation and inhibition, which, as a result of multiple repetitions of the external stereotype, begins to be reproduced in the same sequence as a whole. After that, the stereotype sequence of cortical processes can be evoked not only by the action of an external stereotype (that is, a complex of stimuli), but also by the action of any one stimulus from this complex.

The concept of "dynamic stereotype" was introduced in the early 30s of the twentieth century, when I.P. Pavlov, proving his position on the reflex theory of the functioning of the nervous system. The opponents of the domestic scientist were mainly foreign researchers who argued that the reflex theory had ceased to contribute to understanding the functions of the brain and had become an obstacle to progress in this field of knowledge. Defending and explaining his approach to the theory of reflexes, I.P. Pavlov singled out “three basic principles of exact scientific research” in reflex activity:

the principle of determinism, that is, the reason, the reason for any given action, effect;

the principle of analysis and synthesis, that is, the primary decomposition of the whole into parts that make up units and then again the gradual addition of the whole from units, individual elements;

the principle of structurality, that is, the location of the actions of force in space. IP Pavlov comments on this principle as follows. When any stimulus causes excitation or inhibition of cells in the cortex and the nearest subcortex, the excited and inhibited cells located in its different parts form a dynamic combination with each other. Since the number of stimuli and variants of their combination is incalculable, the dynamic combinations of excited and inhibited cells also cannot be taken into account. Such combinations can become stable and exist during the action of the stimulus. At the same time, they can be preserved as "imprints of reality" even after the cessation of external influence. This means that the trace of previous influences can influence the nature of responses in the future, which, therefore, will depend not only on the immediate stimulus, but also on previously acquired experience.

I.P. Pavlov considered the formation and preservation of a dynamic stereotype as "serious nervous work, different, depending on the complexity of the stereotype and the individuality of the animal."

In the laboratory of I.P. Pavlov, various schemes for the development of dynamic stereotypes were used, some of which were relatively simple and consisted, for example, of only two positive reflexes. Others were complex combinations of positive, that is, excitatory, and inhibitory stimuli. The rearrangement of the active stimuli of the complex, the change in the value of individual stimuli from excitatory to inhibitory or vice versa made it possible to reveal the individual characteristics of the behavior of animals. In the process of changing the dynamic stereotype, all animals became hyperexcited, stopped responding to the previous conditioned stimuli, sometimes refused food, and resisted being brought into the laboratory. I.P. Pavlov called such a state for the animal “painful” and explained it as “intense nervous labor”, which he considered not only as an associative activity, but also as a mental activity (labor).

Questions for self-control:

Define a reflex.

Expand the main provisions of the reflex principle of the central nervous system.

What types of reflexes exist?

What are the specific features of unconditioned reflexes.

Open the mechanism of formation of conditioned reflexes.

Classification of conditioned reflexes.

What is the role of reflexes in the life of living organisms?

What is a reflex arc?

What is the structure of the reflex arc?

Describe the simplest reflex arc?

Open the mechanism of functioning of a complex reflex arc.

What is "reverse afferentation"?

What is the essence and significance of feedback mechanisms?

Expand the stages of formation of the classical conditioned reflex.

The mechanism of inhibition of conditioned reflexes.

What is the "law of power"?

What is the significance of inhibition of the conditioned reflex?

What is a dynamic stereotype?