At the heart of all activities nervous system are reflex acts.
Reflex- this is the body's response to stimuli from the external or internal environment, carried out with the obligatory participation of the central nervous system.
Any reflex is based on the successive propagation of an excitation wave through the elements of the nervous system, which form the so-called reflex arc.
The reflex arc of any reflex includes five consecutive links (Fig. 4.1):
1. Receptor(lat. receptor- receiving) - a special sensitive formation, represented by a nerve ending or a specialized cell, which perceives stimuli from the external or internal environment and converts their energy into nerve impulses.
2. Afferent (sensitive) neuron- a neuron that carries out the perception and transmission of excitation in the form of nerve impulses from receptors to CNS neurons.
3. Intercalary (associative, contact) neuron, wnu interneuron,- a neuron located within the CNS that processes information from afferent neurons and transmits it to efferent or other intercalary neurons.
4. Efferent (motor) neuron- a neuron that carries out the transfer of excitation from the central nervous system to the executive structure, the effector.
5. Effector- a muscle or gland that performs a certain type of activity in response to nerve impulses of an efferent neuron.
According to the theory of IP Pavlov, these five elements make up three parts of the reflex arc: analyzer, contact and executive.
Efferent, (motor) nerve fiber
Rice. 4.1. Scheme of the reflex arc of the spinal reflex
Analyzer part includes receptor and sensory nerve
cell. G
Receptors are specific, that is, they perceive a certain stimulus. Stimulus is a factor with a certain amount of energy that is able to cause tissue excitation. So, the action of chemical energy is perceived by chemoreceptors, thermal - by thermoreceptors, mechanical - by mechano-receptors, electromagnetic oscillations with a certain wavelength (light) - by photoreceptors, etc.
In relation to receptors, all stimuli can be divided into adequate and inadequate. Adequate stimulus A stimulus that acts on a receptor specially adapted to interact with it. Inappropriate stimulus- an irritant that acts on a receptor that is not specially adapted for its perception. The threshold intensity of an adequate stimulus is much lower than that of an inadequate one. For example, the sensation of light during the action of a light stimulus on the receptors of the retina of the eyes occurs at a power of 10~17 10 -18 watts. Inadequate mechanical action on the eyeball also causes a sensation of a flash of light, however, the stimulus power is at least 10~4 W, i.e., 13-14 times more than the power of an adequate stimulus.
Irritants are also classified according to the strength (value) of the applied energy; distinguish: " subthreshold- weak stimuli that do not cause a visible response; " threshold- minimum strength stimuli that cause a minimum response; ■ suprathreshold- stimuli of different strength, causing a reaction corresponding to their strength;
" maximum - the strongest stimuli that elicit the greatest possible response.
Depending on the location of the receptors, they can be divided into extero-, intero- and proprioreceptors. Exteroreceptors sensitive to various environmental factors, interoreceptors- fluctuations in the parameters of the internal environment, proprioreceptors(own receptors) - to changes in the state of muscles, ligaments and tendons.
contact part reflex arc is represented by intercalary neurons of the spinal cord or brain.
The simplest reflex arc consists of two neurons - sensitive and motor, and impulses are transmitted immediately from centripetal to centrifugal nerve fiber. Most reflex arcs include many intercalary neurons. The more complex the reflex, the more associative cells are included in the contact part of the reflex arc.
There are also so-called reflex arcs with a humoral link. They differ in that information from the central nervous system, causing a change in the state of the working organ, is transmitted not through nerve conductors, but through the release of hormones into the blood, that is, in a humoral way.
Executive part reflex arc consists of a motor neuron and executive body, or an effector. When stimulated, effectors perform a specific job that can be measured: muscles contract, glands secrete.
The reflex act does not end with the activity of the executive body. Each effector has its own sensitive devices - receptors, which in turn signal to the central nervous system about the work carried out. Information from the receptors, the excitation of which caused the reflex, is compared with the flow of impulses coming from the receptors of the executive organ. Thanks to this comparison, the response of the organism is specified. The connection of effector receptors with the CNS is called feedback. Therefore, it is more correct to speak not about the reflex arc, but about the reflex ring(Fig. 4.2).
The propagation of a nerve impulse from the receptor to the working organ occurs at a certain speed, depending on many factors: the state of nerve cells, the type of nerve fibers (somatic, vegetative), their thickness, the number of intercalary neurons in the reflex arc. The time from the beginning of the action of the stimulus on the receptor until the appearance of the body's response is called reflex time. The reflex time is the sum of the time:
■ excitation of afferent and efferent formations;
■ conducting excitation along afferent and efferent fibers;
■ switching of the nerve impulse from one neuron to another in the central structures of the brain involved in. implementation of the reflex.
The more complex the reflex arc, the longer the reflex time.
Result of action "
Rice. 4.2. Reflex ring diagram: A- information that causes the actions of the body; B f- information about the implementation of the action (feedback); a, 6 - afferent and efferent nerve fibers, respectively
To understand how the reflex is carried out and what the reflex arc is, you can consider the reaction of a person when a hot object is exposed to his hand. At the moment of exposure to the skin of the hand, excitation occurs in the thermoreceptors, which is transmitted in the form of nerve impulses along the dendrite of the sensitive nerve cell (along the afferent, centripetal fiber) to its body. From it, along the axon, excitation is transmitted to the central nervous system to the intercalary neurons of the spinal cord and brain, in which complex processes of processing the information received take place. From them, excitation is transmitted to the motor nerve cells and along their axon (efferent, centrifugal fiber) spreads to the muscle (biceps), which, contracting, causes the hand to be withdrawn.
I. P. Pavlov established that any reflex act, regardless of its complexity, is subject to three universal principles of reflex activity.
1. The principle of determinism, or causality. According to this principle, a reflex act can be carried out only under the action of a stimulus, i.e., every process that occurs in the body is causally conditioned. The stimulus acting on the receptor is the cause, and the reflex response is the effect.
2. The principle of structure, or integrity,- the reflex act is carried out only under the condition of the structural and functional integrity of the material basis of the reflex - the reflex arc, or the reflex ring.
5 Age anatomy
The structural integrity of the reflex arc can be violated by mechanical damage to any of its parts: receptors, afferent or efferent nerve pathways, parts of the central nervous system, effectors. For example, normally, when a substance with a pungent odor (ammonia) is inhaled, there is a reflex delay in breathing or a change in its depth. After a burn of the nasal mucosa, which is accompanied by damage to the olfactory receptors, sharp-smelling substances no longer cause changes in breathing. Damage to the medulla oblongata of the respiratory center with a fracture of the base of the skull can lead to respiratory arrest. If the nerves innervating the respiratory muscles (diaphragm, intercostal) are cut, then respiratory movements will also be impossible.
The absence of a reflex due to a violation of its functional integrity can be caused by a blockade of the conduction of nerve impulses in the structure of the reflex arc. For example, substances used for local anesthesia block the transmission of a nerve impulse from a receptor along a nerve fiber. Therefore, after local anesthesia, the dentist's manipulations with the diseased tooth do not cause a motor response. In the application general anesthesia excitation is blocked in the central part of the reflex arcs, at the level of the brain.
The functional integrity of the reflex structure is also violated when inhibition processes (unconditioned or conditioned) occur in the central part of the reflex arc. In this case, the absence or cessation of the response to the stimulus is also observed. For example, a child stops drawing when he sees a new bright toy.
3. The principle of analysis and synthesis. Any reflex act occurs on the basis of the processes of analysis and synthesis. During the implementation of the reflex, the stimulus is subjected to analysis, i.e., “decomposition”, during which individual qualitative and quantitative characteristics. The analysis of the stimulus begins at the periphery (in the receptor), but it occurs more subtly in the cells of the central nervous system, especially in the cortex hemispheres brain. Simultaneously with the analysis, synthetic processes take place, i.e., the processes of cognition of the stimulus as a whole on the basis of generalization and comparison of its characteristics identified in the analysis. As a result of the analytical and synthetic activity of the nervous system, a response adequate to the strength and quality of the stimulus arises. For example, after analyzing the properties of a visual stimulus (shape, color, nature of the surface, distance, direction of movement, etc.), as a result of synthesis, it can be determined that this is a large, round, yellow-red, even apple that rolls on the table, and then stretch out your hand to him.
An example of an impact that disrupts the analytical and synthetic activity of the brain is the use of alcohol. In a state of intoxication, a person’s coordination of movements is disturbed, an inadequate assessment of the surrounding reality is observed, etc.
The higher the level of organization of the central nervous system, the more complex the analytical and synthetic activity of the brain. Processes of analysis and synthesis are improved as
individual development of the organism. It is these processes that determine the accuracy of reflex reactions and, consequently, the ability of the organism to interact with the environment, while maintaining its integrity and biological reliability.
EXCITATION AND BRAKING.
Similar information.
The body's response to irritation from the external or internal environment, carried out with the participation of the central nervous system, is called a reflex. The path along which the nerve impulse passes from the receptor to the effector (acting organ) is called the reflex arc.
In the reflex arc (Fig. 104), five links are distinguished: 1) receptor; 2) a sensitive fiber that conducts excitation to the centers; 3) the nerve center, where the excitation switches from sensory to motor cells; 4) a motor fiber that transmits nerve impulses to the periphery; 5) acting organ - muscle or gland.
Rice. 104. Scheme of the reflex arc. A - somatic reflex; B - autonomic reflex; 1 - receptor; 2 - sensitive neuron; 3 - central nervous system; 4 - motor neuron; 5 - working body - muscle, gland; 6 - associative (intercalary) neuron; 7 - vegetative node (ganglion)
Any irritation: mechanical, light, sound, chemical, temperature, perceived by the receptor, is transformed (converted) or, as they say now, encoded, by the receptor into a nerve impulse and in this form is sent to the central nervous system through sensory fibers. Here this information is processed, selected and transmitted to the motor nerve cells, which send nerve impulses to the working organs - muscles, glands and cause one or another adaptive act - movement or secretion.
During the response, the receptors of the working organ are excited and impulses are sent from them to the central nervous system - information about the result achieved. A living organism, like any self-regulating system, works on the principle feedback. Afferent impulses that carry out feedback either strengthen and refine the reaction if it has not reached the goal, or stop it. Thus, the reflex is carried out not by a reflex arc, but by a reflex ring (P.K. Anokhin); the reflex ends when the result is reached.
The reflex provides a subtle, precise and perfect balancing of the body with the environment, as well as control and regulation of functions within the body. This is its biological significance. The reflex is a functional unit of nervous activity.
All nervous activity consists of reflexes of varying degrees of complexity, that is, it is reflected, caused by an external reason, an external push. The reflex principle of nervous activity was discovered by the great French philosopher, physicist and mathematician Rene Descartes in the 17th century.
The reflex theory was developed in the fundamental works of the Russian scientists I. M. Sechenov and I. P. Pavlov. In 1863, in the book "Reflexes of the Brain", I.M. Sechenov suggested that not only the spinal cord, as Descartes believed, but also the brain works on the principle of a reflex: "... without external sensory stimulation, it is impossible even for a moment mental activity and its expression is muscular movement.
IM Sechenov wrote: "... if all receptors are turned off, then a person should fall into a dead sleep and never wake up." This theoretical position has found its justification in clinical practice. S. P. Botkin observed a patient in whom one eye and one ear functioned out of all the receptors of the body. As soon as the patient's eyes were closed and his ears plugged, he fell asleep.
In the experiments of V. S. Galkin, dogs in which visual, auditory and olfactory receptors were simultaneously turned off by surgery slept 20–23 hours a day. They awakened only under the influence of internal needs or energetic effects on skin receptors. Consequently, the central nervous system works on the principle of a reflection reflex, on the principle of a stimulus-response.
IP Pavlov discovered conditioned reflexes - a qualitatively new, higher form of nervous activity, characteristic of the brain. He created the reflex theory in its modern form.
For the implementation of any reflex, the integrity of all links of the reflex arc is necessary. Violation of at least one of them leads to the disappearance of the reflex. If the frog's foot is lowered into a weak solution of sulfuric acid, a defensive reflex will occur - the foot will pull back. However, if you remove the skin and thereby remove the skin receptors, then sulfuric acid will not have an effect.
The same can be observed with the destruction of any other link: the central nervous system, sensory or motor nerve fibers. The strongest irritation will not cause a response, there will be no nervous activity.
This is widely used by surgeons, using during the operation novocaine for anesthesia of peripheral nerves or a ganglionic blocker that interrupts the conduction of excitation in synapses. Narcotic substances of central action turn off the function of CNS neurons.
Reflex time. The time elapsed from the moment the stimulus was applied to the response to it is called the reflex time (latent period). It consists of the time required for the excitation of receptors, the conduction of excitation through sensory fibers, the central nervous system, motor fibers, and, finally, the latent (latent) period of excitation of the working organ. Most of the time is spent on conducting excitation through the nerve centers - the central time of the reflex. This is due to the fact that in the synapses of the central nervous system there is a slowdown in the conduction of excitation, the so-called synaptic delay. The fewer neurons included in the reflex arc, the shorter the reflex time. Therefore, the tendon reflexes that occur when the tendon is stretched, having a two-neuron arc, are the fastest. Their time is only 19 - 23 ms, while the time of the blink reflex that occurs when the eye is irritated is 50 - 200 ms. The greatest is the time of vegetative reflexes.
The time of the reflex depends on the strength of the stimulation and the excitability of the central nervous system. With strong irritation, it is shorter; with a decrease in excitability caused, for example, by fatigue, the time of the reflex increases; with an increase in excitability, it decreases significantly.
The receptive field of the reflex. Each reflex can only be evoked from a specific receptive field. The anatomical region, upon stimulation of which this reflex is evoked, is called the receptive field of the reflex. For example, the sucking reflex occurs when the child's lips are irritated, the pupil constriction reflex - when the retina is illuminated, the knee reflex (leg extension) - with a light blow to the tendon below the patella (Fig. 105).
Rice. 105. The technique of reproduction of proprioceptive reflexes and the scheme of the reflex arc of the knee jerk
Nerve center. Each reflex has its own localization in the central nervous system, that is, that part of it that is necessary for its implementation. For example, the center of urination is located in the sacral spinal cord, the center of the knee-jerk reflex is in the lumbar, the center of pupil dilation is in the upper thoracic segment of the spinal cord. When the corresponding area is destroyed, the reflex is absent. However, it turned out that for the regulation of the reflex, its accuracy, the primary, or main, center is not enough, but the participation of the higher parts of the central nervous system, including the cortex, is also necessary. big brain.
Only with the integrity of the central nervous system is the perfection of nervous activity preserved. The nerve center is a collection of nerve cells located in various parts of the central nervous system, necessary for the implementation of the reflex and sufficient for its regulation. So, if the cerebral cortex is removed from an animal, then breathing is preserved, since the primary respiratory center is located in the medulla oblongata. However, during work, there will not be an exact correspondence between the ventilation of the lungs and the body's need for oxygen, since for the fine regulation of the activity of the respiratory center, not only the brain stem, but also the cerebral cortex is needed.
Classification of reflexes. There are the following types of reflexes.
1. By biological significance, reflexes are divided into food, defensive, indicative (acquaintance with changing environmental conditions), sexual (reproduction).
2. According to the type of receptors from which they arise, reflexes are divided into exteroceptive, arising from receptors that perceive irritation from the external environment: light, sound, taste, tactile, etc .; interoceptive, arising from receptors of internal organs: mechano-, thermo-, osmo- and chemoreceptors of vessels and internal organs, and proprioceptive - from receptors located in muscles, tendons, ligaments.
3. Depending on the working organ involved in the response, reflexes are divided into motor, secretory, vascular.
4. According to the location of the main nerve center necessary for the implementation of the reflex, they are divided into spinal, for example, urination, defecation; bulbar (medulla oblongata): coughing, sneezing, vomiting; mesencephalic ( midbrain): straightening the body, walking; diencephalic (midbrain) - thermoregulatory; cortical - conditioned reflexes.
5. Depending on the duration, phase and tonic reflexes are distinguished. Tonic reflexes are long, lasting for hours, such as the standing reflex. Any animal can stand for hours due to prolonged muscle contraction. All postural reflexes are tonic. They fix a certain position of the body, and against their background other, short, phasic reflexes are played out, providing all types of working, sports and other movements.
6. By complexity, reflexes can be divided into simple and complex. Pupil dilation in response to darkening of the eye, extension of the leg in response to a light blow to the tendon - these are simple reflexes. Examples of complex reflexes are the regulation of the cardiovascular system, the process of digestion. In these cases, the end of one reflex serves as an irritant for the emergence of another. There are so-called chain reflexes, the course of which can be very demonstratively traced on the example of the process of digestion. Arbitrary pushing of a lump of food to the back of the pharynx causes irritation of its receptors - a swallowing reflex occurs. Food enters the esophagus and causes it to contract, pushing the food bolus to the entrance to the stomach. Irritation of the lower part of the esophagus leads to the opening of the cardinal sphincter of the stomach and the entry of food into the stomach, and the latter causes the separation of gastric juice, etc. The entire process of digestion is a complex chain of reflexes.
7. According to the principle of effector innervation, reflexes can be divided into skeletal-motor, or somatic (providing motor acts of skeletal muscles), and vegetative (functions of internal organs).
8. Depending on whether reflexes are innate or acquired in the process of individual life, IP Pavlov divided them into unconditioned (innate) and conditional (acquired).
The mechanism of transmission of excitation in synapses. Nerve cells that form reflex arcs are interconnected through contacts - synapses, in which excitation is transferred from one neuron to another. Synapses are located on the body of the nerve cell, on the dendrites, at the peripheral endings of the axon. There are thousands of synapses on each neuron, most of them on dendrites (Fig. 106).
Rice. 106. Synaptic plaques (1) of the endings of presynaptic axons form connections on the dendrites (2) and the body (3) of the neuron [Sterki P., 1984]
Synapses are divided into chemical and electrical synapses according to the mechanism of transmission of excitation. The latter are found in the heart muscle, smooth muscles and glandular tissue; in the CNS, their presence is only assumed.
The synapse, with chemical transmission, consists of a synaptic plaque, a presynaptic membrane, a synaptic cleft 30 nm wide, and a postsynaptic membrane (Fig. 107).
Rice. 107. Interneuronal synapse [Sterki P., 1984]. 1 - synaptic vesicles; 2 - synaptic target; 3 - postsynaptic receptors; 4 - postsynaptic membrane; 5 - synaptic plaque; 6 - mitochondrion
In the synaptic plaque, the neurotransmitter is stored in small vesicles, of which there are about 3 million. Under the influence of a nerve impulse, depolarization of the axon endings occurs, which causes an increase in the Ca 2+ concentration in it, and the contents of the synaptic vesicles are ejected into the synaptic cleft. The role of the trigger mechanism in the release of the mediator is played by an increase in the concentration of Ca 2+ . The mediator diffuses through the synaptic cleft and binds to the receptor proteins of the postsynaptic membrane, causing either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP) in it.
The mediators that cause excitation in neurons are acetylcholine, norepinephrine, serotonin, dopamine. Inhibition in a neuron is caused by an inhibitory mediator - gamma-aminobutyric acid.
In electrical synapses, the synaptic cleft is very narrow (1 - 2 nm), it is crossed by channels through which ions are easily transmitted to the postsynaptic membrane. The action potential is freely, without delay, carried from one cell to another. There is no chemical mediator here; conduction of excitation according to the mechanism is similar to conduction along a nerve fiber.
Features of the nerve centers. Characteristic features of the nerve centers that distinguish them from nerve fibers are fatigue, a very high metabolism, that is, a high need for oxygen and nutrients, and selective sensitivity to certain poisons. Due to these features, circulatory disorders and changes in body temperature primarily affect the function of the central nervous system: stopping the blood supply to the brain for 20 seconds causes fainting - loss of consciousness; an increase in body temperature to 40 - 42 ° C - delirium, impaired consciousness. Resuscitation is possible if clinical death (cardiac and respiratory arrest) lasted no more than 5-6 minutes. After a longer period, it is possible to restore the activity of the heart and even breathing, but the organ of consciousness - the cerebral cortex, which is most sensitive to changes in the internal environment of the body, will not function.
The essence of the work of the nervous system is the organization of reactions in response to external and internal influences. The degree of complexity of such reactions is very different - from automatic constriction of the pupil in bright light to a multifaceted behavioral act that mobilizes all body systems. Nevertheless, in all cases, the same principle of activity remains - reflex. A reflex is an active response that connects the characteristics of the organism and environmental conditions. Consequently, the reflex is not a mechanical, not a passive response, such as the formation of a dent from a blow, but is expedient for given organism reaction necessary for normal life.
The emergence and development of the nervous system in the process of evolution meant, first of all, the appearance and improvement of reflex mechanisms. These mechanisms, regardless of their degree of complexity, have a number of fundamentally common features. To implement a reflex, at least two elements are necessary: a perceiving (receptor) and an executive (effector). Receptors can respond to a very wide range of stimuli and occupy large areas (reflexogenic zone). These include, for example, pain receptors, receptors of internal organs. Other perceiving elements, on the contrary, are extremely specialized and have a limited reflex zone. Examples include taste buds located on the surface of the tongue, or visual rods and cones.
In the same way, the executive apparatus of a reflex can be an isolated muscle and have a rigid connection with a limited group of receptors. A classic example of this is the knee reflex (narrow reflex zone and elementary reaction). In other cases, the executive apparatus includes an ensemble of acting units and has connections with various types receptors. An example of this is the so-called "starting" reflex. It is expressed in the form of general alertness, fading or flinching at a sharp sound or bright light, an unexpected visual image. Thus, a huge number of motor units are involved in the implementation of the “starting” reflex and it is caused by various stimuli, the main feature of which is surprise.
The “starting” reflex is one of many reactions that require the coordinated work of various body systems. Such an interest is impossible in the presence of rigid direct connections with receptors and effectors, since this would lead to the appearance of reflex mechanisms independent of each other and not amenable to coordination.
In the process of evolution, another element was formed that provides reflex reactions - intercalary neurons. Thanks to these neurons, the impulses from the receptors do not reach the effector apparatus immediately, but after intermediate processing, during which consistency is established in various reactions. Broadly interacting with each other and forming clusters, intercalary neurons create an opportunity to combine all reflex mechanisms into a single whole. An integral nervous activity is formed, which is something more than the sum of individual reactions.
Each individual reaction is subject to central influences; it can be boosted, slowed down, completely blocked, or put on high alert. Moreover, on the basis of innate automatisms, new ways of responding, new actions are formed. So, the child learns to walk, stand on one leg, complex manual manipulations.
Integral nervous activity does not yet mean higher nervous activity. The unification of an organism into a single whole and the organization of complex behavioral programs can be carried out on the basis of innate mechanisms fixed in the nervous system by evolution. These mechanisms are called unconditioned reflexes because they are genetically embedded in the nervous system and do not require training. On the basis of unconditioned reflexes, the most complex actions can be formed. As an example, it suffices to mention the construction activities of beavers or the long-distance flights of birds.
However, unconditioned reflex activity inevitably suffers from limitations, because it is almost impossible to correct and thus prevents the accumulation of individual experience. Each individual from birth is almost completely ready for certain actions, monotonously repeated from generation to generation. If environmental conditions suddenly change. then the superbly debugged response mechanism turns out to be unsuitable.
A much greater flexibility of behavior is observed in organisms that are capable of individual learning. This becomes possible due to the emergence of temporary nerve connections in the nervous system. The most studied type of such a neural connection is the conditioned reflex. With the help of this reflex, the stimulus, which was previously indifferent, acquires the value of a vital signal and causes a certain reaction. The mechanisms of the conditioned reflex contain the prerequisites for individual memory, without which, as you know, learning is impossible.
As the cerebral cortex evolves, huge zones of nerve cells arise that do not have any innate program, but are intended only for the formation of connections in the process of individual learning. Since the work of the nervous system is based on the reflex principle, then training extends to three main links of the reflex mechanism: analysis of information coming from receptors, integral processing in intermediate links, and the creation of new activity programs.
Personal experience influences both the perception and processing of information from the external and internal environment, and the formation of activity programs - short-term or long-term. As a result of the perception of many stimuli, recognition occurs, i.e. information about the stimulus is compared with the information stored in the memory. In the same way, when organizing a response, not only the needs of the moment are taken into account, but also the past experience of successful or unsuccessful responses in a similar situation.
When performing the intended action, unexpected interference may occur. Therefore, it is necessary to keep the final goal of the reaction until its full implementation, which requires special mechanisms.
The processes of recognition of incoming signals, the development of action programs that take into account past experience, and control over their implementation constitute the content of higher nervous activity. This activity, while remaining reflex in its essence, differs from innate automatisms in much greater flexibility and selectivity. The same stimulus can cause different reactions depending on the state at the moment, the general situation, individual experience, because much depends not on the characteristics of the stimulus, but on the processing that it undergoes in the intermediate links of the reflex apparatus.
Higher nervous activity creates the prerequisites for the mind. Reason means first of all the ability to find a solution in a new unusual situation. Let's take an example. The monkey sees a bunch of bananas hanging from the ceiling and boxes scattered on the floor. Without prior training, she solves the practical and intellectual problem that has arisen before her - she puts one box on top of another and takes out bananas. With the advent of speech, the possibilities of the intellect expand infinitely, since the essence of the things around us is reflected in words.
Higher nervous activity is the neurophysiological basis of mental processes. But she doesn't exhaust them. For such mental phenomena as feeling, will, imagination, thinking, of course, appropriate brain activity is necessary. However, the specific content of mental processes is determined by the social environment, and not by the processes of excitation or inhibition in neurons. Whether a scientist is solving the most complex intellectual problem or a first-grader is considering a simple school problem, their brain activity can be approximately the same. The direction of brain activity is set not by the physiology of nerve cells, but by the meaning of the work performed.
However, what has been said does not mean that higher nervous activity is something secondary in relation to “truly mental” processes. On the contrary, the general patterns of interaction between neurons and the general principles of the organization of nerve centers determine many characteristics of mental activity, for example, the pace of intellectual work, stability of attention, and memory capacity. These and other indicators are of great importance for pedagogical work, especially if children have defects in the central nervous system.
The most complex brain mechanisms that ensure the processing of information coming from many receptor zones and intermediate centers at once are of great interest for both physiology and psychology. There is an increasing interpenetration of these two disciplines, which is reflected in the doctrine of higher nervous activity.
In the doctrine of higher nervous activity, two main sections can be distinguished. The first of them is closer to neurophysiology and considers the general patterns of interaction of nerve centers, the dynamics of the processes of excitation and inhibition. The second section considers the specific mechanisms of individual brain functions, such as speech, memory, perception, voluntary movements, emotions. This section is closely related to psychology and is often referred to as psychophysiology. In addition, there was a separation of an independent direction - neuropsychology. Neuropsychology is largely a clinical discipline. She not only studies the mechanisms of higher cortical functions, but also develops methods for the accurate diagnosis of cortical lesions and the principles of corrective measures. One of the founders of neuropsychology is the outstanding Russian scientist A. R. Luria.
These sections are closely interconnected, since the brain works as a whole. However, for a better understanding of the general patterns of higher nervous activity, it is advisable to consider separately the principles of higher neurodynamics and the neuropsychological mechanisms of individual cortical functions.
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 appropriate changes in work occur. various bodies aimed at supporting 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 terminal membrane contains big number 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) support certain concentration 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, Babinski, 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, a functional association of neurons located at different levels of the CNS occurs, 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 the 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 the well-known life situation when, even after the cessation of the action of a strong emotional stimulus (after the cessation of the quarrel), general excitement continues for some more or less long time, arterial pressure remains elevated, hyperemia of the face, tremor of the hands persists.
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, one can choose chemical substances, which 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.
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 being 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). I.P. Pavlov discovered conditioned reflexes and used them as objective method in the study of mental activity (higher nervous activity). The scientists formulated three basic principles of the reflex theory:
1. 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.
2. 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.
3. 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 from the environment useful information, 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). Functional system is a dynamic combination of various organs and tissues, which is formed to achieve a useful (adaptive) result. A useful result is the maintenance of the constancy of the internal environment of the body through the regulation of 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 the self-preservation of the body (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, which provide individual and species preservation of the body (food, drink, sleep regulation, defensive and orienting reflex, energy saving, 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 whole organism is transferred to a state of reflex readiness for a possible meeting with 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 activities here it 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 research in this direction This phenomenon has been termed 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 active 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:
1. An indifferent stimulus must act a few seconds earlier than an 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.
2. 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.
3. 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. 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 for 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),
By 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 unconditioned and conditioned reflexes are presented in Table 1.
Table 1
General characteristics of unconditioned and conditioned reflexes
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:
1. Receptor.
2. Sensitive way.
3. Central nervous system.
4. Motor way.
5. 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 part of the spinal nerves or 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 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 emerges from 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 can 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 you to make constant, continuous, moment-to-moment 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 environment it would be impossible.
Thus, with the progress of 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.
1. 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.
2. 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.
3. 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:
1. tuning, regulating the state of the brain and creating a certain level of excitability and performance of the nerve centers:
2. launcher, which initiates one or another conditional reaction.
The modern explanation of the mechanism of formation of conditioned reflexes is based on the concept 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 this, 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 some 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:
1. Define a reflex.
2. Expand the main provisions of the reflex principle of the central nervous system.
3. What types of reflexes exist?
4. What are the specific features of unconditioned reflexes.
5. Open the mechanism of formation of conditioned reflexes.
6. Classification of conditioned reflexes.
7. The mechanism of inhibition of conditioned reflexes.
8. What is the role of reflexes in the life of living organisms?
9. What is a reflex arc?
10. What is the structure of the reflex arc?
11. Describe the simplest reflex arc?
12. Expand the mechanism of functioning of a complex reflex arc.
13. What is "reverse afferentation"?
14. What is the essence and significance of feedback mechanisms?
15. Expand the stages of the formation of a classical conditioned reflex.
16. The mechanism of inhibition of conditioned reflexes.
17. What is the "law of power"?
18. What is the meaning of inhibition of the conditioned reflex?
19. What is a dynamic stereotype?
Loading...