Unconditioned salivation reflex. Salivation. Mechanism of saliva secretion
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The main elementary act of higher nervous activity is the formation of a conditioned reflex.
There are countless conditioned reflexes. If the appropriate rules are followed, any perceived stimulus can be made a stimulus that triggers a conditioned reflex (signal), and any activity of the body can be its basis (reinforcement). Depending on the type of signals and reinforcements, as well as on the relationships between them, different classifications of conditioned reflexes have been created. As for studying the physiological mechanism of temporary connections, researchers have a lot of work to do.
The classification of conditioned reflexes was determined according to the following particular characteristics: 1) circumstances of formation, 2) type of signal, 3) composition of the signal, 4) type of reinforcement, 5) relationship in time of the conditioned stimulus and reinforcement.
General signs of conditioned reflexes. The conditioned reflex a) is the highest individual adaptation to changing living conditions; b) carried out by the higher parts of the central nervous system; c) is acquired through temporary neural connections and is lost if the environmental conditions that caused it have changed; d) represents a warning signal reaction.
So, a conditioned reflex is an adaptive activity carried out by the higher parts of the central nervous system through the formation of temporary connections between signal stimulation and the signaled reaction.
Natural and artificial conditioned reflexes. Depending on the nature of the signal stimulus, conditioned reflexes are divided into natural and artificial.
Natural reflexes are conditioned reflexes that are formed in response to the influence of agents that are natural signs of signaled unconditional stimulation.
An example of a natural conditioned food reflex is the salivation of a dog to the smell of meat. This reflex inevitably develops naturally throughout the dog's life.
Artificial are called conditioned reflexes that are formed in response to the influence of agents that are not natural signs of the signaled unconditional irritation. An example of an artificial conditioned reflex is the release of saliva in a dog to a sound, a metronome. In life, this sound has nothing to do with food. The experimenter artificially made it a food intake signal.
Nature develops natural conditioned reflexes from generation to generation in all animals according to their lifestyle. As a result, natural conditioned reflexes are easier to form, are more likely to be strengthened and are more durable than artificial ones.
Exteroceptive, interoceptive and proprioceptive conditioned reflexes. Conditioned reflexes to external stimuli are called exteroceptive, to stimuli from internal organs - interoceptive, to stimuli of the musculoskeletal system - proprioceptive.
Exteroceptive reflexes are divided into reflexes caused by distant (acting at a distance) and contact (acting by direct contact) stimuli. Next, they are divided into groups according to the main types of sensory perception; visual, auditory, etc.
Interoceptive conditioned reflexes can also be grouped by organs and systems that are sources of signaling: gastric, intestinal, cardiac, vascular, pulmonary, renal, uterine, etc. The so-called time reflex occupies a special position. It manifests itself in various vital functions of the body, for example, in the daily frequency of metabolic functions, in the secretion of gastric juice when it is time for lunch, in the ability to wake up at the appointed hour. Apparently, the body “keeps time” mainly based on interoceptive signals. The subjective experience of interoceptive reflexes does not have the figurative objectivity of exteroceptive ones. It gives only vague feelings that make up the overall well-being, which affects mood and performance.
Proprioceptive conditioned reflexes underlie all motor skills. They begin to be developed from the first flaps of the chick’s wings, from the first steps of the child. They are associated with mastery of all types of locomotion. The coherence and accuracy of movement depends on them. The proprioceptive reflexes of the hand and vocal apparatus in humans are receiving a completely new use in connection with labor and speech. The subjective “experience” of proprioceptive reflexes consists mainly in the “muscular feeling” of the position of the body in space and its members relative to each other. At the same time, for example, signals from the accommodative and oculomotor muscles have a visual nature of perception: they provide information about the distance of the object in question and its movements; signals from the muscles of the hand and fingers make it possible to evaluate the shape of objects. With the help of proprioceptive signaling, a person reproduces events happening around him with his movements.
Conditioned reflexes to simple and complex stimuli. A conditioned reflex can be developed to any one of the listed extero-, intero- or proprioceptive stimuli, for example, to turning on a light or to a simple sound. But in life this rarely happens. More often, the signal becomes a complex of several stimuli, for example, smell, warmth, the soft fur of the mother cat becomes an irritant of the conditioned sucking reflex for the kitten. Accordingly, conditioned reflexes are divided into simple and complex, or complex, stimuli.
Natural signals always consist of many components, in other words, they are complex stimuli. To such signals, conditioned reflexes are formed that are more complex and changeable than to simple signals. In a complex signal, each of its components has a different physiological strength and the effect caused by each stimulus corresponds to it.
Simultaneous complex stimuli consist of several components acting simultaneously. Conditioned reflexes to successive complexes of stimuli are formed if individual stimuli follow each other in a certain sequence (such a signal is reinforced by food). Numerous studies have established that as a result of more or less long-term training of a conditioned reflex to a complex stimulus, fusion occurs, the synthesis of individual components of the complex into a single stimulus. Thus, with repeated use of a sequential complex of stimuli consisting of four sounds, they merge into a single stimulus. As a result, each of the four sounds loses its signal meaning, i.e. used alone does not cause a conditioned response.
Conditioned reflexes on a chain of stimuli. If the indifferent stimuli from which a complex signal is formed act sequentially, i.e. do not coincide with each other, and unconditional reinforcement is added to the last of them, then to such a signal it is possible to form a conditioned reflex to a chain of stimuli. The signal value of an individual member of the chain turns out to be greater, the closer it is to the reinforcement, i.e. to the end of the chain. The formation of conditioned reflexes on a chain of stimuli underlies the development of various so-called motor skills by reinforcing random or forced movements. For example, having told the dog “Give me a paw!”, we “raise” its paw ourselves, “rewarding” the dog with a piece of cookie. Soon the dog, having heard these words, “gives its paw” on its own. Analysis of the mechanism of formation of this type of reflex showed that first a temporary connection is formed between three foci of excitation: auditory, motor and food centers. Then the sequence of actions of the chain members is fixed. Finally, the position of its main members of the sound signal “give me a paw”, proprioceptive (limb movement) and natural food (feeding) is clarified.
An important concept in the physiology of higher nervous activity is integrity in conditioned reflex activity. It manifests itself primarily in systematicity, stereotypy, “tuning” and “switching” reactions according to situational signals. As a result, the behavior of animals is determined not by single signals, but by the entire holistic picture of the environment. Conditioned reflex activity covers many aspects of the present and connects it with the experience of the past, and this in turn leads to a subtle adaptation to events of the future.
The real stimuli that the organism deals with form a dynamic stereotype of stimuli. The existing stereotype of stimuli directs the formation of new reflexes along a certain direction. For example, when mastering new hunting objects, a predator uses the most reliable of the hunting techniques already familiar to it. The stereotype allows it to react adequately, despite some changes in the situation. So, for example, having developed a car driving stereotype, you can drive a car, slightly varying the control depending on the nature of the road surface, and at the same time talk with the passenger sitting next to you. An analysis of human activity shows that each of us continuously forms countless everyday, work, sports and other stereotypes throughout our lives. In particular, this manifests itself in the appearance of appetite at a certain time of the day, stereotypical performance of work or sports movements, etc. With age, stereotypes become stronger and more difficult to change. Changing existing stereotypes is always very difficult.
Conditioned reflex tuning. The formation of successive complexes from environmental and basic conditioned stimuli, like a chain with widely spaced links, is a physiological mechanism of the so-called conditioned reflex tuning. The very name “tuning” indicates that we are not talking about the performance of some activity, but only about the state of readiness for this activity caused by the mechanism of temporary communication.
Conditioned reflex switching. The formation of complexes of different signal significance from the same basic signals with the addition of different environmental stimuli is a physiological mechanism of conditioned reflex switching. When considering the physiological mechanisms of a conditioned reflex of any complexity, it should be borne in mind that the process of developing even the most elementary temporary connection is associated with the formation of a conditioned reflex to the experimental situation. It is now obvious that during the development of any conditioned reflex, several types of temporary connections are formed - a situational reflex (the appearance of a given experimental chamber, smells, lighting, etc.), a time reflex, a reflex to a given stimulus, etc. Each conditioned reaction consists of a number of somatic and vegetative components.
To understand the physiological mechanism of environmental conditioned reflexes, E. A. Asratyan introduced the concept of “conditioned reflex switching.” It consists in the fact that the same stimulus can become a conditioned signal for various conditioned reactions. So, for example, a beep in one experimental chamber can be a signal of a food reaction, and in another chamber it can be a signal of a defensive reflex. The same signal in the first half of the day can serve as a defensive conditioned stimulus, and in the second half - a food signal. It is obvious that in both examples the conditioned signal is not the signal itself, but a complex of stimuli consisting of a given signal and the entire experimental setting. While maintaining the experimental setting, any sound or other stimuli can be used, which, like the experimental setting, can serve, in the terminology of E. A. Asratyan, as switches.
Conditioned reflexes of the nth order. The dog has developed a strong food conditioned reflex, for example, to turn on a light bulb. If, after 10 - 15 s, following an indifferent agent, for example, sound, a light bulb is turned on (the conditioned stimulus of a previously developed food conditioned reflex) without subsequent unconditional reinforcement, then a conditioned connection is formed between the foci of excitation caused by the actions of sound and light. Reactions developed in this way are called a conditioned reflex of the 2nd order.
Let's give another example. The dog developed a strong salivary reflex to the metronome. Then they began to show her a black square, but instead of feeding her, they presented her with the sound of a metronome, to which a conditioned reflex had previously been developed. After several combinations of these stimuli without food reinforcement, a conditioned reflex of the 2nd order was formed, i.e. the black square began to cause salivation, although it was never presented on its own in combination with food. Conditioned reflexes of the 2nd order in dogs, as a rule, are unstable and soon disappear. Usually they manage to develop conditioned reflexes no higher than 3rd order. Conditioned reflexes of the nth order are formed more easily with a general increase in the excitability of the cerebral cortex. For example, children with increased excitability quite easily develop conditioned reflexes up to the 6th order, while in balanced healthy children - usually no higher than the 3rd order. In healthy adults, conditioned reflexes up to the 20th order are easily developed, but they are also unstable.
Imitative conditioned reflexes. These reflexes are especially easily developed in animals that lead a group lifestyle. For example, if one monkey from a herd develops a conditioned reflex (for example, food) in full view of the entire herd, then other members will also develop this conditioned reflex (L.G. Voronin). Imitative reflexes, as one of the types of adaptive reactions of animals, are widespread in nature. In its simplest form, this reflex is found in the form of a following reflex. For example, schooling fish follow their relatives or even the silhouettes of fish. Another example was given by Charles Darwin. It is well known that crows do not allow a person with a gun or any long object in their hands to get close. It is quite obvious that this “saving fear” (in the words of Charles Darwin) developed mainly not as a result of personal experience with humans, but through imitation of the behavior of individuals of the same species or even other species. For example, the cry of a jay serves as a danger signal for many forest animals.
Imitation is of great importance in the ontogenesis of the behavior of primates, including humans. For example, “blind” imitation in children gradually turns into purely human abilities.
In their physiological mechanism, imitative conditioned reflexes are obviously similar to conditioned reflexes of the nth order. This is easily seen in the example of the development of a conditioned motor food reflex. The spectator monkey perceives a conditioned stimulus and, although it does not receive food reinforcement, it also perceives natural conditioned stimuli accompanying food intake (the type of food, its smell, etc.). So, on the basis of a natural conditioned reflex, a new conditioned reflex is developed. And if we consider that natural conditioned reflexes, due to their inextricable and long-term connection with unconditioned reflex activity, are very strong, then it will become clear why conditioned reflex reactions are formed so easily and quickly on their basis.
Associations. Associations are formed by combining indifferent stimuli without reinforcement. For the first time such conditioned connections were studied in dogs in the laboratory of I. P. Pavlov. The experiments involved combinations of tone and light without food reinforcement. Already after 20 combinations, the first signs of the formation of a temporary connection between these stimuli appeared: when the light was applied, the dog turned to the sound source (inactive at that time), and when the tone sounded, it looked at the light bulb (which was not lit), as if waiting for it to turn on. Studies have shown that a temporary connection between indifferent stimuli (exteroceptive) is formed in mammals after 10-40 combinations, and between stimuli of the same modality it is formed faster than for signals of different modalities.
Conditioned reflexes to attitude. These conditioned reflexes are developed not to absolute, but to relative signs of stimuli. For example, if an animal is simultaneously presented with a small and a large triangle, and only the small triangle is reinforced with food, then, according to the rules for the formation of a conditioned reflex, a positive conditioned reflex is formed for the small triangle, and a negative conditioned reflex (differentiation) is formed for the large triangle. If we now present a new pair of triangles, in which the small triangle is equal in absolute size to the large triangle, then the animal will “from the spot” display a conditioned food reflex to the smaller triangle in this pair.
Let's give another example. The dolphins were able to learn to choose the middle one from three presented objects, since in preliminary experiments they received reinforcement (fish) only when choosing the middle one. It is important that the animals grasped the sign “middle object” under conditions where, with each new experiment, different objects (balls, cylinders, etc.) were presented and in different parts of space, in order to avoid the formation of a conditioned reflex “to place”.
The biological significance of a conditioned reflex to an attitude, as well as a temporary connection between indifferent stimuli, as a reflex of the nth order, is that if the agents that cause them subsequently coincide with the unconditioned reflex, then they immediately (“from the spot”) become conditioned reflexes - there is a “transfer” of the developed conditioned reflex to a similar situation. There is every reason to believe that the reflex to an attitude, a temporary connection between indifferent stimuli, as well as conditioned reflexes of a higher order, underlie the physiological mechanism of such phenomena as “transfer of experience”, “foresight”, “insight”, etc. arising as if without the preliminary development of a conditioned reflex.
Chain conditioned reflex. The possibility of obtaining a conditioned reflex to a chain of stimuli depends on the phylogenetic level of development of the nervous system of a given animal species. Thus, in monkeys (macaques, baboons, capuchins), after 40–200 applications of a chain stimulus, its components, tested separately, in most cases do not cause a conditioned reflex. In lower vertebrates (fish, reptiles), even after 700 - 1300 applications of the stimulus chain, its components retain their signaling value. In these animals, a conditioned reflex to a chain of stimuli is developed quite easily, but the complex stimulus does not become a single one: each of its components retains its signal value.
There are four known ways of forming chain conditioned reflexes in animals. The first method is to combine single motor reactions into a chain of exteroceptive single stimuli. The second method is to build up the chain of movements from the reinforced end. For example, first an animal (pigeon, rat, etc.) is trained to peck (press) the first shelf in the experimental chamber based on a conditioned signal (turning on a light bulb). Then, having let a sufficiently hungry animal into the chamber, they do not give a conditioned signal, forcing the animal to perform search reactions. The bait is placed on the second shelf. As soon as the animal touches the second shelf, the lamp is immediately turned on (conditioned signal), and after pecking (pressing) the second shelf, the animal receives food reinforcement.
As a result of several such combinations, the animal becomes accustomed to pecking (pressing) the second shelf. After this, another exteroceptive signal is introduced - the activation of the bell, which precedes the pecking (pressing) of the second shelf. Thus, a two-member, three-member, etc. is formed. chain of movements. In contrast to this method, with the third method of forming a chain of motor reflexes, new movements and stimuli are “wedged” in the same way, but between the last link of the chain and reinforcement. Finally, with the fourth method of forming a chain of movements, the animal is not limited in its movements, but only those chains that were “correct” are reinforced. It turned out that under such conditions, for example, the monkeys quickly learned to perform the required chain of movements, and all unnecessary actions gradually disappeared from them.
In animals, chains of movements are developed with varying degrees of difficulty depending on the phylogenetic level of development of the nervous system. In turtles, for example, over a long period of time it is possible with great difficulty to develop a very unstable three-part chain of movements; in pigeons it is possible to form a fairly strong chain of 8-9 movements, and in mammals - from an even larger number of movements. It was concluded that there is a dependence of the rate of formation of individual links and the entire chain of movements as a whole on the level of phylogeny of the animal.
Automation of conditioned reflexes. Many conditioned reflexes in animals and humans become automated after long training and become, as it were, independent of other manifestations of higher nervous activity. Automation tends to develop gradually. Initially, it can be expressed in the fact that individual movements are ahead of the corresponding signals. Then comes a period when the chain of movements is completely carried out in response to the first, “trigger” component of the chain of stimuli. At first glance at the result of training a conditioned reflex, one may get the impression that at first the reflex is “tied” to something that controls it, and then after prolonged exercise it becomes independent to some extent.
Conditioned reflexes developed at different timings of signal and reinforcement. Based on how the signal is located in time relative to the reinforcing reaction, present and trace conditioned reflexes are distinguished.
Conditioned reflexes are called conditioned reflexes, in the development of which reinforcement is used during the action of a signal stimulus. Depending on the timing of the addition of reinforcement, existing reflexes are divided into coinciding, delayed and delayed. A matching reflex is developed when, immediately after the signal is turned on, reinforcement is attached to it.
A delayed reflex is developed in cases where a reinforcing reaction is added only after some time has passed (up to 30 s). This is the most common method of developing conditioned reflexes, although it requires a larger number of combinations than the coincidence method.
A delayed reflex is developed when a reinforcing reaction is added after a long isolated action of the signal. Typically, this isolated action lasts 1-3 minutes. This method of developing a conditioned reflex is even more difficult than the previous two.
Trace reflexes are conditioned reflexes, during the development of which a reinforcing reaction is presented only some time after the signal is turned off. In this case, the reflex is developed in response to the action of the signal stimulus; use short intervals (15-20 s) or long ones (1-5 min). The formation of a conditioned reflex using the trace method requires the largest number of combinations. But trace conditioned reflexes provide very complex acts of adaptive behavior in animals. An example would be hunting for hidden prey.
Conditions for the development of temporary connections. Combination of a signal stimulus with reinforcement. This condition for the development of temporary connections was revealed from the very first experiments with salivary conditioned reflexes. The steps of a servant carrying food only caused “psychic salivation” when they were combined with food.
This is not contradicted by the formation of trace conditioned reflexes. Reinforcement is combined in this case with a trace of excitation of nerve cells from a previously switched on and switched off signal. But if the reinforcement begins to precede the indifferent stimulus, then the conditioned reflex can be developed with great difficulty, only by taking a number of special measures.
Indifference of the signal stimulus. The agent chosen as a conditioned stimulus for the food reflex should not itself have any relation to food. He must be indifferent, i.e. indifferent, for the salivary glands. The signal stimulus should not cause a significant orienting reaction that interferes with the formation of a conditioned reflex. However, each new stimulus evokes an indicative reaction. Therefore, for it to lose its novelty, it must be reused. Only after the indicative reaction is practically extinguished or reduced to an insignificant value does the formation of a conditioned reflex begin.
The predominance of the strength of excitation caused by reinforcement. The combination of the sound of the metronome and the feeding of the dog leads to the rapid and easy formation of a conditioned salivary reflex to this sound. But if you try to combine the deafening sound of a mechanical rattle with food, then such a reflex is extremely difficult to form. For the development of a temporary connection, the ratio of signal strength and reinforcing reaction is of great importance. In order for a temporary connection to form between them, the focus of excitation created by the latter must be stronger than the focus of excitation created by the conditioned stimulus, i.e. a dominant must arise. Only then will there be a spread of excitation from the focus of the indifferent stimulus to the focus of excitation from the reinforcing reflex.
The need for significant intensity of excitation. A conditioned reflex is a warning reaction to a signal about upcoming significant events. But if the stimulus that they want to make a signal turns out to be an event even more significant than those that follow it, then this stimulus itself causes a corresponding reaction in the body.
No extraneous irritants. Each extraneous irritation, for example, an unexpected noise, causes an indicative reaction.
Normal functioning of the nervous system. Full closure function is possible provided that the higher parts of the nervous system are in normal working condition. The performance of nerve cells in the brain sharply decreases due to insufficient nutrition, under the influence of toxic substances, such as bacterial toxins in diseases, etc. Therefore, general health is an important condition for the normal functioning of the higher parts of the brain. Everyone knows how this condition affects a person’s mental functioning.
The formation of conditioned reflexes is significantly influenced by the state of the body. Thus, physical and mental work, nutritional conditions, hormonal activity, the action of pharmacological substances, breathing at high or low pressure, mechanical overload and ionizing radiation, depending on the intensity and timing of exposure, can modify, strengthen or weaken conditioned reflex activity up to its complete suppression.
The study of the final, behavioral manifestations of higher nervous activity was significantly ahead of the study of its internal mechanisms. To date, both the structural basis of the temporal connection and its physiological nature have not yet been sufficiently studied. There are different views on this matter, but the issue has not yet been resolved. However, at the current level of research it is becoming more and more certain that, along with the structural one, it is also necessary to take into account the neurochemical organization of the brain.
Eating food stimulates salivation reflexively. Salivation continues throughout the meal and stops soon after.
From oral receptors, signals are transmitted to the central nervous system through afferent fibers of the trigeminal, facial, glossopharyngeal and vagus nerves. The main salivary center is located in the medulla oblongata. It is here, as well as in the lateral horns of the upper thoracic segments of the spinal cord, that signals from the oral cavity and higher parts of the brain arrive. From here, influences along the efferent parasympathetic and sympathetic nerve fibers are directed to the salivary glands.
Parasympathetic innervation of the salivary glands begins from the nuclei of the medulla oblongata. Sympathetic innervation of the salivary glands is carried out from the lateral horns of the II-IV thoracic segments of the spinal cord.
Salivation begins according to the type of conditioned reflexes - in response to the sight and smell of food.
Reflex influences can also inhibit salivation until it stops. Such inhibition can be caused by painful irritation, negative emotions, mental stress, and dehydration of the body. All these effects reduce the activity of the food center and its part - the salivation center. The causative agents of the latter can be some humoral substances. Thus, copious secretion of saliva is observed during asphyxia due to irritation of the salivary center with carbonic acid.
Acts of chewing and swallowing
If the food is liquid, it is usually swallowed immediately; if it is solid, it is chewed. Chewing is the process of mechanical processing of food in the oral cavity, which consists of grinding its solid components and mixing with saliva. The act of chewing is partly reflexive, partly voluntary. It is regulated by a nerve center located in the medulla oblongata (mastication center). When food enters the oral cavity, the receptors of its mucous membrane (tactile, temperature, taste) are irritated, from where impulses are transmitted along the afferent fibers of the trigeminal nerve to the center of chewing, and then along the motor fibers (mandibular branch of the trigeminal nerve) to the masticatory muscles. Swallowing is a reflex act that occurs as a result of irritation by a bolus of food of the sensitive endings of the soft palate, the base of the tongue and the back wall of the pharynx. This excitation travels along the glossopharyngeal nerves to the center of swallowing (the bottom of the fourth ventricle of the medulla oblongata). Efferent impulses go to the muscles of the oral cavity, pharynx, larynx, and esophagus along the sublingual, trigeminal, glossopharyngeal and vagus nerves.
Chewing phases: rest, introducing food into the mouth, indicative, basic, formation of a food bolus, swallowing.
Swallowing is a complex reflex act caused by irritation of the receptors of the oral cavity and pharynx, stimulation of the swallowing center of the medulla oblongata and representing the movement of a bolus of food from the oral cavity to the esophagus as a result of the coordinated activity of the muscles of the mouth, pharynx and esophagus.
Swallowing phases:
1) oral (voluntary),
2) pharyngeal (fast involuntary),
3) esophageal (slow, involuntary).
During swallowing, the soft palate rises, preventing food from entering the nasal cavity, and the epiglottis, closing the entrance to the larynx, prevents food from entering the respiratory tract. The volume of the food bolus is 5 - 15 ml.
It has long been known that the secretion of saliva occurs not only as a result of the action of chemicals on the receptors of the oral mucosa. The side properties of food - the sight and smell of it, and even just thoughts about food - cause salivation before eating. The nature of this everyday phenomenon was unclear for a long time. Therefore, in the pre-Pavlovian era, scientists distinguished between physiological salivation, on the one hand, and mental salivation, on the other. It was clear that the “physiological” secretion of saliva occurs according to the type of unconditioned reflex, which is caused by chemical substances in food (Fig. 52, 53), but the phenomenon designated by the term “mental salivation” remained incomprehensible. After all, the color, smell, etc. properties of food do not affect the receptors of the oral cavity.
Meanwhile, it turned out that the sight of meat and its smell cause salivation in all normal dogs if they have ever tasted meat. If the dog was raised on a dairy-vegetable diet and is not “familiar” with meat, the sight and smell of meat does not cause it to salivate. This means that the reaction to the appearance and smell of this food is not innate, but is formed as a result of individual experience.
Rice. 52. Scheme of the unconditioned salivary reflex.
1 - taste bud of the tongue; 2 - sensory nerve; 3 - food center of the medulla oblongata; 4 - food center of the cortex; 5 - salivary gland.
Rice. 53. Scheme of the formation of the conditioned salivary reflex.
a - formation of two independent foci of excitation in the cortex: b - formation of a temporary connection between two foci of excitation.
As an example, let us give one of the experiments carried out in Pavlov’s laboratory.
If you turn on the light before feeding the dog and repeatedly combine it with giving food, then after a while the animal begins to salivate only when the light is turned on once. How can this be explained? When exposed to light, the visual center of the cerebral cortex is excited, and under the influence of food chemicals during feeding, the food center of the cortex is excited. When two areas of the cerebral cortex are simultaneously excited, a connection is established between them. In order for such a connection to be established, several combinations of feeding and exposure to light are needed. When this connection has arisen, the excitation of the visual center of the cortex, which perceives light stimulation, is transmitted to the salivary center, causing its excitation and, thus, the mere action of light causes the secretion of saliva. An indifferent stimulus (in our example, light) signals the upcoming action of a specific stimulus, which activates the mechanism of the unconditioned reflex, therefore the system of conditioned reflexes is considered as a signaling system - the first signaling system according to I. P. Pavlov. It is characteristic of both animals and humans.
The secretion of saliva in response to light stimulation can occur only under certain conditions, for example, when an unconditioned food stimulus is combined with the action of light, which is why this reflex is called a conditioned reflex.
Countless other indifferent (nonspecific) stimuli act in a similar way, in particular color, smell, and the general appearance of food. This means that salivation at the sight of food is a conditioned reflex.
Thus, the study of the mechanism of salivation led to the discovery of the most important laws.
Let us consider the most essential features of conditioned reflexes.
Firstly, a conditioned reflex is formed only on the basis of innate reflexes, that is, only when the action of a stimulus that causes an unconditioned reflex is combined with any other. For example, you can get salivation by combining the giving of food with touching the skin, with sound, with the action of electric current, temperature, etc.
In the previous sections, unconditioned reflexes, the mechanism of their implementation and reflex arcs were already considered. For example, the protective flexion reflex that occurs when the skin of the foot in animals is irritated, or the secretion of saliva when food enters the mouth and irritates the taste buds with it are typical unconditioned reflexes.
An example of a conditioned reflex would be the secretion of saliva by animals in response to the smell or sight of food before it enters the mouth and before the taste buds are irritated. This is a natural conditioned reflex. An artificial conditioned reflex, which was studied in the laboratory of I.P. Pavlov, helps to better understand it.
A dog with a salivary gland fistula stands in a special pen. In front of her is a feeder, in which food appears at the right moment. The animal eats it, and saliva begins to flow from the fistula. This is definitely a reflex salivation. But then they light a light bulb in front of the animal and after 15-30 seconds they give food again. After 10-20 such combinations, the dog begins to salivate as soon as the light comes on. This is a conditioned reflex. Each animal can develop a lot of such conditioned reflexes to different stimuli: a bell, metronome beats, scratching the skin, smell, showing certain figures, etc. If the unconditioned stimulus has a great influence on the body (for example, an electric shock), then the conditioned reflex can develop even after a single application of the conditioned stimulus. To develop a conditioned reflex, it is necessary that some indifferent (indifferent) stimulus for the animal be systematically and repeatedly combined with the action of an unconditioned stimulus, and the indifferent stimulus must begin the action before the unconditioned one. Then this indifferent stimulus becomes conditioned stimulus signal of the subsequent action of the unconditioned stimulus (reinforcement) and begins to evoke a conditioned reflex. During the experiment, it is necessary to isolate the animal from extraneous stimuli so that they do not interfere with the development of conditioned reflexes. To do this, experiments are carried out in special soundproof chambers with a control panel and an experimenter in another room.
There are clear differences between conditioned and unconditioned reflexes. The main ones are given in table. 33.
Classification of conditioned reflexes. Conditioned reflexes are classified according to various criteria. According to the receptor characteristic, i.e. according to those receptors on which the conditioned stimulus acts, they distinguish exteroceptive And interoceptive conditioned reflexes.The former are divided into visual, olfactory, gustatory, etc. They play a leading role in the relationship of the organism with the environment, and are easily formed and fixed. Interoceptive conditioned reflexes are reflexes in which the conditioned stimulus is irritation of the receptors of internal organs by a change in the chemical composition, temperature of the internal environment, pressure in hollow organs, and blood vessels. For example, inflating a balloon in the stomach with air can serve as a signal that the paw is irritated by the current and cause a defensive reaction.
According to the effector characteristic, i.e., according to those effectors that respond to conditioned stimulation, they distinguish vegetative And somatomotor conditioned p eflexes. Autonomic ones include food and (for example, salivation), cardiovascular, respiratory and other conditioned reflexes. Somatomotor are called
such conditioned reflexes that manifest themselves in the movements of the entire organism or its individual parts in response to the action of a conditioned stimulus. For example, a rat responds to the action of a conditioned stimulus (bell) by pressing the pedal with its paws and only then receives reinforcement with food.
The mechanism of formation of conditioned reflexes; Conditioned reflexes are carried out with the participation of the cerebral cortex. As already noted, any signals from the external or internal environment enter the cerebral cortex. When the mouth receptors are irritated by food, the stimulation travels along the sensory nerve fibers of the facial nerve to the center in the medulla oblongata, and from there along the motor fibers of the same nerve goes to the salivary gland and causes salivation. But at the same time, excitation from the center in the medulla oblongata enters that part of the cerebral cortex where the representation of the food center is located. When exposed to a light stimulus, excitation from the retina also comes to the cerebral cortex (to the visual center). If the simultaneous excitation of these two cortical centers is repeated often enough, then a temporary connection is established between them, in the formation of which many neurons are involved. As a result of this, the excitation of the cortical visual center, which occurs when a light bulb is applied to the eye, is transmitted to the cortical food center, from there to the salivation center in the medulla oblongata and through it to the salivary glands - a conditioned reflex arises.
Later studies carried out by modern methods, including electrophysiological ones, made it possible to penetrate deeper into the processes underlying the formation of conditioned reflexes. Using the method extirpation(removal) or temporary switching off of the cerebral cortex, it was established that the cortex is necessary for the development of a conditioned reflex, but its reproduction can occur without the participation of the cortex, due to the activity of subcortical formations (caudate and amygdaloid nuclei, diencephalon).
Registration of action potentials of individual neurons of the brain using microelectrodes showed that even before the development of a conditioned reflex, each of the stimuli (conditioned and unconditioned) activates certain systems of neurons located at different levels of the central nervous system. If at lower levels the reception of these stimuli is carried out by different systems of neurons, then in the cerebral cortex and in some subcortical formations the majority of neurons respond to both stimuli. Neurons that respond with excitation to stimulation of various receptors or sensory organs are called polysensory. Apparently, they play the most important role in the formation of a conditioned reflex, but the subtle mechanism of this process is not yet known.
The biological significance of conditioned reflexes is that they give the animal the opportunity to adapt much better and more accurately to the conditions of existence and to survive in these conditions. For example, the smell or voice of a predator signals another animal about danger and gives it the opportunity to escape. The sight or smell of food warns of it and prepares the digestive system to accept it, while the shape or color of other objects, such as a ladybug, indicates their inedible qualities. Animal behavior is various forms of external, mainly motor activity, aimed at establishing vital connections between the organism and the environment. Animal behavior consists of conditioned, unconditioned reflexes and instincts Instincts are complex unconditional reactions that, being innate, appear only during certain periods of life (for example, the instinct of nesting or feeding offspring). Instincts play a leading role in the behavior of lower animals. However, the higher an animal is at the evolutionary level, the more complex and varied its behavior, the more perfect and subtle it adapts to the environment, and the greater the role conditioned reflexes play in its behavior.
The environment in which animals exist is very variable. Adaptation to the conditions of this environment through conditioned reflexes will be subtle and accurate only if these reflexes are also changeable, that is, conditioned reflexes unnecessary in the new environmental conditions will disappear, and new ones will form in their place. The disappearance of conditioned reflexes occurs due to inhibition processes.
- Source-
Bogdanova, T.L. Handbook of biology / T.L. Bogdanov [and others]. – K.: Naukova Dumka, 1985.- 585 p.
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A child is born with functioning salivary glands, which, under the influence of the taste properties of food, immediately begin to secrete their secretion. This ability is formed in the prenatal period and is, therefore, innate. This ability is universal and unchanging in the sense that all children respond by secreting saliva to stimulation of taste buds. The stimulation effect is involuntary and is realized regardless of consciousness. Under normal conditions, a child cannot stop salivation when food enters his mouth, by the way, just like you. Therefore, this is an unconditioned reflex. There are no conditions that could contribute to its manifestation.
The sight or smell of food alone does not initially cause salivation (secretion of saliva). After some time has passed, during which experience appears that suggests that when a certain sight or smell appears, food will soon enter the mouth, the child or cub begins to salivate from these nonspecific stimuli.
We can say that the child learns that if he sees and smells food, this means that he will now taste it, and in anticipation of this pleasant event, the baby begins to salivate (it should be noted, involuntary). Once this connection is established, the response becomes automatic and resembles a normal reflex in every way. However, this new reflex has one peculiarity. It depends on the conditions, on the association with the smell and type of food. If the cub is always fed in the dark, then the sight of food will not cause salivation, since feeding has never been associated with the sight of food. If some dish has never been included in the cub’s diet, then the sight of this particular dish will also not cause salivation when it appears, even if it is some kind of unimaginable delicacy for this biological species. If the puppy has never eaten meat, then he will not develop salivation from the smell of meat.
The reflex that causes a response to such associations is called a conditioned reflex. It seems that the body is able to find a shorter path to close the reflex arc. The body encounters a situation where “a certain smell means a certain taste, and the taste causes salivation.” After this, the neural pathway comes into play and simplifies the situation by saying: “A certain smell causes salivation.” (This is reminiscent of the property of mathematical equalities: if a=b and b=c, then a=c.)
This property of organisms is very valuable for survival, since a response that is useful for a particular stimulus is likely to be useful for other stimuli that invariably or almost invariably accompany the first one. An animal looking for food and guided only by its taste will be forced to taste everything it finds with its tongue. Such an animal will most likely either die of starvation or be poisoned. An animal that has developed a conditioned reflex to the smell of food has great advantages.
A conditioned reflex can be developed to any stimulus, even one that seems to “make no sense.” The development of a conditioned reflex is not subject to logic; it is a purely associative process. The first person to conduct experiments with artificial associations that do not make sense was the Russian physiologist Ivan Petrovich Pavlov. The first stage of his career was devoted to the study of the neural mechanisms that control the secretion of certain digestive glands. In 1889, he performed a very impressive experiment, during which a dog's esophagus was opened, the upper segment of which was brought out into an incision on its neck. The food that was fed to the dog fell out instead of going into the stomach. However, it turned out that stimulation of the taste buds still led to the release of gastric juice. It was an unconditioned reflex. But Pavlov did not stop there, but went further, finding out that when certain nerves are cut, the arc of this reflex is broken. Although the dog continued to eat with appetite, gastric juice was no longer released. For this work, Pavlov was awarded the Nobel Prize in 1904.
By that time, however, a new direction had begun to develop in physiology. In 1902, Baylis and Starling showed that nerve networks are not the only means of eliciting responses from the juice-secreting digestive glands. Indeed, these scientists found that the activity of the pancreas is not impaired when the nerves leading to it are cut, and that there are regulatory mechanisms due to the delivery of chemical regulators through the bloodstream. Pavlov took a different path, obtaining even more fruitful results. Let's assume that the dog is offered food. Submitting to an unconditioned reflex, the dog will begin to salivate, tasting the food. Due to early conditioning, the dog will also salivate in response only to the smell and sight of the food. But suppose further that every time the dog is given food, the bell will ring. This condition will pair the sight and smell of food with the sound of the bell. After this, when the bell was repeated 20 to 40 times, the dog began to salivate at the bell alone.
For the remaining thirty years of his life, Pavlov conducted experiments on developing a wide variety of conditioned reflexes. Such reflexes could be developed for almost any combination of stimuli and responses, although the limit turned out to be not infinite. Experimenters have discovered that some experimental conditions are more effective than others. If a stimulus to which it is desirable to develop a conditioned reflex is presented immediately before a normal stimulus, then the conditioned reflex is developed very quickly. For example, if the bell rings just before feeding. If the bell rings after giving food or long before it, then the development of a conditioned reflex becomes more difficult.
Some responses are difficult to obtain to a conditioned stimulus. For example, salivation is easy to control, and animals that salivate profusely can be easily induced to secrete it in response to any food-related stimulus. In contrast, the response of the iris to increased illumination is very difficult to modify by any stimuli other than the light itself. (This, however, is not without meaning. The response to food must necessarily be flexible, since food can appear in any guise and under different conditions. But light is light, and flexibility in the response to its influence is neither necessary nor desirable.)
Different species of animals differ from each other in the ease with which they develop conditioned reflexes. As a rule, conditioned reflexes are more easily developed in animals with a developed nervous system. They easily make the connection between the bell and the food. In other words, we can say that the facilitation of the emergence of new neural connections is due to the large number of neurons in the nervous system and their complex interactions with each other.
The development of conditioned reflexes differs from imprinting in that the former has greater flexibility. A conditioned reflex can be developed at any time for a large variety of stimuli and responses, while imprinting occurs over a short critical period and involves a specific stimulus and a specific response. The development of a conditioned reflex requires more time than imprinting, and unlike imprinting, a conditioned reflex can fade away.
Let's assume that a dog has developed a conditioned reflex to salivate when a bell rings, and then is not fed for a period of time after the bell rings. In this situation, over time, the salivation in response to the call will become weaker and eventually stop completely. The conditioned reflex will fade away.
It is not surprising that the longer and the more difficult it is to develop a conditioned reflex, the longer and with more difficulty it fades away. It is also not surprising that a conditioned reflex that has been developed and extinguished is more easily developed the second time. We can say that the nervous system, once it has developed a conditioned reflex, keeps it constantly at hand “ready for use.”
The conditioned reflex has proven to be an invaluable tool in the study of animal behavior. The development of conditioned reflexes makes it possible to obtain answers to questions that would otherwise require the ability to communicate with lower animals. And in the previous chapter I said that a bee cannot see red, but can see ultraviolet light. But how was this fact established if the bee is not able to tell us about it directly? The answer lies in the development of a conditioned reflex.
It is impossible to imagine that an animal can develop a conditioned reflex to one stimulus and not to another, only on the condition that it distinguishes between these stimuli. This statement seems self-evident. Now suppose that bees are presented with drops of sugar syrup on cards. Bees will fly to the cards and eat the syrup. Over time, the bees will develop a conditioned reflex, and they will begin to fly to the cards even when there is no syrup on them. Let us further assume that the experiment uses two types of cards, identical in shape, smoothness and size, but differing in color - some cards are blue, and others are gray. Let's assume that syrup was always put on blue cards and never on gray cards. Over time, in the absence of syrup, the bees begin to fly only to blue cards, but not to gray ones. From this we can conclude that the bee can distinguish blue cards from gray ones, since the cards differ from each other only in color. Therefore, the bee distinguishes the color blue.
Let's say that a change was made to the experimental conditions and red and gray cards began to be used. Moreover, food is always present only on red cards. Finally, after sufficient time had passed for the development of a conditioned reflex (based on the data obtained in the previous experiment), the bees were tested using cards that did not contain syrup. It turned out that bees fly indiscriminately to both red and gray cards. This means that bees do not distinguish gray from red, that is, they do not distinguish red.
On the other hand, bees can distinguish from each other cards that appear to us to be exactly the same color, although some of them reflect more ultraviolet rays than others. If the syrup is placed only on cards that reflect ultraviolet rays, and never on others, this leads to the successful development of the corresponding conditioned reflex in the bees. The bee distinguishes cards even in the absence of food, but we cannot. In short, it turned out that the bee sees in the ultraviolet spectrum.
In the same way, we can test how finely a dog distinguishes the pitch of sounds or the shape of any objects, when developing conditioned reflexes to the pitch of a sound or to the shape of objects. At the same time, you can note what sounds and forms the dog remains indifferent to. It turned out that a dog, for example, can distinguish a circle from an ellipse. It distinguishes a circle, the two perpendicular diameters of which are equal to ten units of length, from an ellipse, in which the ratio of the two perpendicular diameters is nine to ten. In addition, the dog distinguishes sounds if their frequencies differ by only three hertz. It has also been shown that dogs "suffer" from absolute color blindness because they cannot condition a response using color differences.