Only physiology holds in its hands the key to a truly scientific analysis of mental phenomena.

I. M. Sechenov

4.1. Reflex as the main form of activity of the nervous system

Structure nervous system and the set of processes taking place in it make it possible to perform regulatory and control functions that ensure:

1. Rapid coordination of body functions.

2. Coordination of the state of the body with different conditions environment.

3. Uniting individual organs and systems of the body into a single whole.

The control apparatus in higher animals and humans is represented by a reflex mechanism, which manifests itself in all parts of the nervous system and is the main form of activity of the nervous system. The first ideas about the reflex principle of the activity of the nervous system, i.e. about the principle of “reflection” and the concept reflex were introduced by R. Descartes in the 17th century. But due to the lack of scientific information about the structure and function of the nervous system, his ideas about the mechanism of the reflex were speculative and mechanistic. Thus, Descartes explained the motor reaction in response to external influence by the fact that under the influence of any stimulus on the sensory organs, “nerve threads” are stretched, running along the “neural tubes” to the brain. The tension of the threads leads to the opening of “valves” through which the “animal spirit” comes out of the brain, rushing along the nerves to the muscles and inflating them.

Currently reflex called every reaction of the body, arising under the action of a stimulus from the external or internal environment and carried out with the obligatory participation of the central nervous system. The basis of any reflex is the sequential propagation of a wave of excitation through the elements of the nervous system, forming the so-called reflex arc (Fig. 4.1).

Rice. 4.1. Diagram of the reflex arc of the spinal reflex:

1 – receptor; 2 – sensitive nerve cell; 3 – sensitive dorsal root; 4 – central (contact) part of the reflex arc; 5 - motor neuron; 6 – efferent (motor) nerve fiber; 7 – muscle; 8 – spinal cord

To understand how the reflex is carried out and what a reflex arc is, consider how the hand withdraws when exposed to a hot object. At this moment, excitation arises in the receptors - sensitive nerve endings, which afferent(centripetal) fiber is transmitted to the sensitive nerve cell. From it, along the axon, excitation is transmitted to the central nervous system to interneurons, in which complex processes of processing incoming information occur. After this, the excitation will be transmitted to the motor nerve cells and along their axon ( efferent, centrifugal fiber) will go to the muscles, which, contracting, will cause the hand to withdraw.

According to the theory of I.P. Pavlov, the reflex arc of any reflex consists of three parts: analyzer, contact and executive.

Analyzer part includes a receptor, an afferent fiber and a sensory nerve cell. The function of the receptor is to perceive irritation and process (transform) it into a nerve impulse.

Receptors are specific: they are adapted to perceive a specific stimulus. Rirritant - This is a factor that has a certain amount of energy, which, when applied to tissue, is capable of causing its excitation. Thus, the action of chemical energy is perceived chemoreceptors, thermal – thermoreceptors, mechanical – mechanoreceptors, electromagnetic vibrations with a certain wavelength (light) – photoreceptors etc. In relation to receptors, all stimuli can be divided into adequate and inadequate. Adequate for a given type of receptor is the stimulus to which they are adapted to perceive. The threshold intensity of an adequate stimulus is much lower than that of an inadequate one. Thus, the sensation of light under the action of a light stimulus occurs when its power is 10 -17 -10 -18 W. But a mechanical, inadequate effect on the eyeball also causes a sensation of a flash of light. In this case, the stimulus power should be at least 10 -4 W, i.e., 13–14 orders of magnitude higher than the power of an adequate stimulus.

In addition, stimuli are classified by the strength or amount of energy applied. The following types of stimuli are distinguished by strength:

a) subthreshold - weak stimuli that do not cause a visible response;

b) threshold - stimuli of minimal strength that cause a minimal response;

c) suprathreshold - stimuli of varying strengths that cause a reaction corresponding to their strength;

d) maximum - strong stimuli that cause the maximum possible reaction.

Depending on the location of the receptors, they can be divided into two groups: exterO- And interoreceptors. The former are excited by various environmental factors, the latter are sensitive to fluctuations in the parameters of the internal environment. And finally, there are the so-called proprioceptors(own receptors) that perceive changes in the condition of muscles, ligaments and tendons.

Contact part The reflex arc is represented by interneurons of the spinal cord or brain.

In the simplest case, the reflex arc includes only two neurons, and impulses are transmitted from the centripetal to the centrifugal nerve fiber. More often, excitation in the central nervous system passes through a number of interneurons. The more complex the reflex, the more associative cells are included in the contact part of the reflex arc.

It should be noted the existence of so-called “reflex arcs with a humoral link.” Such arches 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 humoral route, through the release of hormones into the blood.

Executive level The reflex arc consists of an effector neuron and an executive organ, or effector. These organs include muscles and glands. Effectors are characterized by the fact that, when excited, they perform specific work that can be measured: muscles contract, glands secrete secretions.

However, the reflex act does not end with the activity of the executive body. Each effector has its own sensitive receptor devices, which, in turn, signal to the central nervous system about the work they have carried out. Information from the receptors whose excitation causes the reflex is compared with the flow of impulses coming from the receptors of the executive organ. Thanks to this comparison, the body's response is clarified. The connection between the receptors of the working organ and the central nervous system is called “feedback”. Therefore, it is more correct to speak not about a reflex arc, but about reflex ring .

For many centuries, people have wondered about the amazing adaptability of animal behavior to environmental conditions. Purposeful, reasonable human behavior seemed even more mysterious. The explanation for this was first expressed in 1863 by the great Russian physiologist I.M. Sechenov, who explained behavior and “mental” - mental activity of a person by the principle of operation of the nervous system.

I. P. Pavlov experimentally confirmed, creatively expanded and developed I. M. Sechenov’s position on the reflex principle of brain activity and created new section in science - higher physiology nervous activity animals and humans. Under lower nervous activity I. P. Pavlov meant reflex regulation of the physiological functions of the body, higher nervous activity defined as mental activity that determines the reflex regulation of a person’s relationship with the environment.

Higher nervous activity ensures individual behavioral adaptation of humans and higher animals to changing conditions of the environment and internal environment; it is reflexive in nature, carried out by unconditioned and conditioned reflexes.

Unconditioned reflexes

Unconditioned reflexes- ensure the maintenance of vital functions in relatively constant environmental conditions; they are inherent in a person from birth. For example, the separation of saliva under the direct action of food on the oral mucosa: food acts on sensitive nerve endings oral cavity and causes excitement in them, which rushes along the centripetal nerves to salivary gland and puts it into action. This reflex, like all unconditioned reflexes, has a certain reflex arc, ready at the moment of birth. Unconditioned reflexes are congenital, hereditary, species-specific and always arise under constant conditions (obligatory, unconditional) and persist throughout the life of the organism.

Unconditioned reflexes include food, defensive, sexual and orientation reflexes, thanks to which the integrity of the body is preserved, the constancy of the internal environment is maintained and reproduction occurs. From the section “Animals” you know the instinctive behavior of many animals. These are also unconditioned reflexes. Instincts are a system of innate unconditioned reflex behavioral reactions associated with the continuation and preservation of the species.

Conditioned reflexes

In an endlessly complex and changing environment adaptability without help conditioned reflexes is insufficient and the organism may die if it does not prepare in advance for new environmental changes. Thus, an animal has an incomparably greater chance of saving itself if it detects signs of a predator approaching in advance. Consequently, everything that signals, warns of the approach of a predator - noise, smell, appearance, etc., acquires vital importance for the animal and causes appropriate reactions in it, in accordance with the prevailing environmental conditions.

Similarly, the sight, smell of familiar food, everything that signals, warns a hungry person about the possibility of eating food soon, causes his salivary separation reflex, the preliminary release of digestive juices, which allows him to quickly and fully process food when it enters the digestive system.

These reflexes allow you to adapt to a future event that has not yet occurred. I. P. Pavlov called conditioned reflexes, because they are formed under certain conditions: a repeated coincidence in time of the action of two stimuli is necessary - the future signal, or conditional, and the unconditional, that is, the causing unconditioned reflex. The conditioned stimulus must somewhat precede the unconditioned stimulus, since it signals it. Thus, a conditioned reflex is a reflex acquired by the body during life and formed as a result of a combination of conditioned stimuli with an unconditioned one. In mammals and humans, the arcs of conditioned reflexes pass through the cortex cerebral hemispheres brain.

IP Pavlov also called the conditioned reflex a temporary connection, because this reflex manifests itself only while the conditions under which it was formed are in effect; acquired individual, since it is formed in the individual life of the organism. Conditioned reflexes can be formed by any stimulus on the basis of any unconditioned reflex.

Conditioned reflexes form the basis of skills, habits, training and education, the development of speech and thinking in a child, labor, social and creative activities.

Research has established that the basis for the formation of conditioned reflexes is the establishment of temporary connections in the cerebral cortex between nerve centers unconditioned reflex and conditioned stimulus.

Excitation and inhibition

Along with excitation, inhibition of the active state occurs in the cerebral cortex, a delay in some reactions, which makes it possible to carry out others. With the help of the formation of conditioned reflexes and their inhibition, a deeper adaptation of the body to specific conditions of existence is carried out.

Excitation and inhibition are two interrelated processes that continuously occur in the cerebral cortex and determine its activity. IP Pavlov divided the phenomenon of inhibition in the cerebral cortex into 2 types: external and internal.

External braking occurs due to the emergence of another focus of excitation in the cerebral cortex. It is caused by an additional stimulus, the action of which causes another reflex act.

Internal inhibition occurs as a result of reinforcement of a conditioned stimulus with an unconditioned one, which leads to the gradual disappearance of the conditioned reflex. This got the name extinction of the conditioned reflex. Internal inhibition is characteristic only of the higher parts of the central nervous system and is very important for the body.

CONDITIONED REFLEX ACTIVITY OF THE ORGANISM

Reflex. Reflex arc. Types of reflexes

The main form of nervous activity is the reflex. Reflex is a causally determined reaction of the body to changes in the external or internal environment, carried out with the participation of the central nervous system in response to irritation of receptors. This is how the emergence, change or cessation of any activity of the body occurs.

Reflex arcs can be simple or complex. A simple reflex arc consists of two neurons - a perceiver and an effector, between which there is one synapse.

An example of a simple reflex arc is the tendon reflex reflex arc, such as the knee reflex reflex arc.

The reflex arcs of most reflexes include not two, but a larger number of neurons: a receptor, one or more intercalary and an effector. Such reflex arcs are called complex, multineuron.

It has now been established that during the response of the effector, numerous nerve endings present in the working organ are excited. Nerve impulses now from the effector again enter the central nervous system and inform it about the correct response of the working organ. Thus, reflex arcs are not open, but circular formations.

Reflexes are very diverse. They can be classified according to a number of characteristics: 1) by biological significance, (food, defensive, sexual);

2) depending on the type of irritated receptors:

exteroceptive, interoceptive and proprioceptive;

3) according to the nature of the response: motor or motor (executive organ - muscle), secretory (effector - gland), vasomotor (constriction or dilation of blood vessels).

All reflexes of the whole organism can be divided into two large groups: unconditional and conditional.

From the receptors, nerve impulses travel along afferent pathways to the nerve centers. It is necessary to distinguish between anatomical and physiological understanding of the nerve center.

From an anatomical point of view, the nerve center is a set of neurons located in a certain part of the central nervous system. Due to the work of such a nerve center, simple reflex activity is carried out, for example, the knee reflex. The nerve center of this reflex is located in lumbar region spinal cord(II–IV segments):

From a physiological point of view, the nerve center is a complex functional union of several anatomical nerve centers located at different levels of the central nervous system and, due to their activity, determining the most complex reflex acts. For example, many organs (glands, muscles, circulatory and lymphatic vessels etc.). The activity of these organs is regulated by nerve impulses coming from nerve centers located in various parts of the central nervous system. A. A. Ukhtomsky called these functional associations “constellations” of nerve centers.

Physiological properties of nerve centers. Nerve centers have a number of characteristic functional properties, depending on the presence of synapses and large quantity neurons included in their composition. The main properties of nerve centers are:

1) unilateral conduction of excitation;

2) delay in excitation;

3) summation of excitations;

4) transformation of the rhythm of excitations;

5) reflex aftereffect;

6) fatigue.

The unilateral conduction of excitation in the central nervous system is due to the presence of synapses in the nerve centers, in which the transfer of excitation is possible only in one direction - from the nerve ending that secretes the mediator to the postsynaptic membrane.

The delay in the conduction of excitation in nerve centers is also associated with the presence of a large number of synapses. The release of the transmitter, its diffusion through the synaptic cleft, and the excitation of the postsynaptic membrane require more time than the propagation of excitation along the nerve fiber.

The summation of excitations in nerve centers occurs either with the application of weak but repeated (rhythmic) stimulation, or with the simultaneous action of several subthreshold stimulations. The mechanism of this phenomenon is associated with the accumulation of the mediator on the postsynaptic membrane and an increase in the excitability of the cells of the nerve center. An example of the summation of excitation is the sneezing reflex. This reflex occurs only with prolonged stimulation of the receptors of the nasal mucosa. The phenomenon of summation of excitations in nerve centers was first described by I.M. Sechenov in 1863.

The transformation of the rhythm of excitations lies in the fact that the central nervous system responds to any rhythm of stimulation, even a slow one, with a volley of impulses. The frequency of excitations coming from the nerve centers to the periphery of the working organ ranges from 50 to 200 per second. This feature of the central nervous system explains that all contractions of skeletal muscles in the body are tetanic.

Reflex acts do not end simultaneously with the cessation of the irritation that caused them, but after a certain, sometimes relatively long, period. This phenomenon is called reflex aftereffect.

Two mechanisms have been identified that cause the aftereffect. or short-term memory. The first is due to the fact that excitation in nerve cells does not disappear immediately after the cessation of stimulation. For some time (hundredths of a second), the nerve cells continue to produce rhythmic discharges of impulses. This mechanism can only cause a relatively short-term aftereffect. The second mechanism is the result of the circulation of nerve impulses along closed neural circuits of the nerve center and provides a longer aftereffect.

The excitation of one of the neurons is transmitted to another, and along the branches of its axon it returns again to the first nerve cell. This is also called reverberation of signals. The circulation of nerve impulses in the nerve center will continue until one of the synapses becomes fatigued or the activity of neurons is suspended by the arrival of inhibitory impulses. Most often, this process involves not one, but many synapses of the excitation profile from the perceived one, and this area remains excited for a long time. This is very important point. With each act of perception, such pockets of memory about what was perceived appear in the brain, which can accumulate more and more throughout the day. Consciousness can leave this area and this picture will not be perceived, but it continues to exist and if consciousness returns here it will “remember” it. This leads not only to general exhaustion, but, combined with boundaries, makes it difficult to distinguish between images. During sleep, general inhibition extinguishes these foci.

Nerve centers are easily fatigued, unlike nerve fibers. With prolonged stimulation of afferent nerve fibers, fatigue of the nerve center is manifested by a gradual decrease and then complete cessation of the reflex response.

This feature of the nerve centers is proven as follows. After termination muscle contraction in response to irritation of the afferent nerves, the efferent fibers innervating the muscle begin to irritate. In this case, the muscle contracts again. Consequently, fatigue did not develop in the afferent pathways, but in the nerve center.

Reflex tone of nerve centers. In a state of relative rest, without causing additional irritation, discharges of nerve impulses arrive from the nerve centers to the periphery of the corresponding organs and tissues. At rest, the discharge frequency and the number of simultaneously working neurons are very small. Rare impulses continuously coming from the nerve centers cause tone (moderate tension) of skeletal muscles, smooth muscles of the intestines and blood vessels. This constant stimulation of the nerve centers is called the tone of the nerve centers. It is supported by afferent impulses continuously coming from receptors (especially proprioceptors) and various humoral influences (hormones, CO2, etc.).

Inhibition (as well as excitation) – active process. Inhibition occurs as a result of complex physicochemical changes in tissues, but outwardly this process is manifested by a weakening of the function of any organ.

In 1862, classical experiments were carried out by the founder of Russian physiology I.M. Sechenov, which were called “central inhibition”. I.M. Sechenov placed a crystal of sodium chloride ( table salt) and observed inhibition of spinal reflexes. After the stimulus was removed, the reflex activity of the spinal cord was restored.

The results of this experiment allowed I.M. Sechenov to conclude that in the central nervous system, along with the process of excitation, the process of inhibition also develops, capable of inhibiting the reflex acts of the body.

Currently, it is customary to distinguish two forms of inhibition: primary and secondary.

For primary inhibition to occur, the presence of special inhibitory structures (inhibitory neurons and inhibitory synapses) is necessary. In this case, inhibition occurs primarily without previous excitation.

Examples of primary inhibition are pre- and postsynaptic inhibition. Presynaptic inhibition develops in axo-axonal synapses formed at the presynaptic terminals of a neuron. Presynaptic inhibition is based on the development of slow and prolonged depolarization of the presynaptic terminal, which leads to a decrease or blockade of further excitation. Postionaptic inhibition is associated with hyperpolarization of the postsynaptic membrane under the influence of mediators that are released when inhibitory neurons are excited.

Primary inhibition plays a large role in limiting the flow of nerve impulses to effector neurons, which is essential in coordinating the work of various parts of the central nervous system.

No special braking structures are required for secondary braking to occur. It develops as a result of changes in the functional activity of ordinary excitable neurons.

The importance of the braking process. Inhibition, along with excitation, takes an active part in the adaptation of the organism to the environment; Inhibition plays an important role in the formation of conditioned reflexes: it frees the central nervous system from processing less essential information; ensures coordination of reflex reactions, in particular, motor acts. Inhibition limits the spread of excitation to other nervous structures, preventing disruption of their normal functioning, that is, inhibition performs a protective function, protecting nerve centers from fatigue and exhaustion. Inhibition ensures the extinction of the unwanted, unsuccessful result of an action, and excitation enhances the desired one. This is ensured by the intervention of a system that determines the importance of the result of an action for the body.

The coordinated manifestation of individual reflexes that ensure the implementation of integral work acts is called coordination.

The phenomenon of coordination plays an important role in the activity of the motor system. Coordination of motor acts such as walking or running is ensured by the interconnected work of nerve centers.

Due to the coordinated work of the nerve centers, the body perfectly adapts to the conditions of existence.

Principles of coordination in the activity of the central nervous system

This occurs not only due to the activity of the motor system, but also due to changes in the vegetative functions of the body (respiration processes, blood circulation, digestion, metabolism, etc.).

A number of general principles have been established - principles of coordination: 1) the principle of convergence; 2) the principle of excitation irradiation; 3) the principle of reciprocity; 4) the principle of sequential change of excitation by inhibition and inhibition by excitation; 5) the phenomenon of “recoil”; 6) chain and rhythmic reflexes; 7) the principle of a common final path; 8) feedback principle; 9) the principle of dominance.

The principle of convergence. This principle was established by the English physiologist Sherrington. Impulses arriving in the central nervous system through various afferent fibers can converge (convert) to the same intercalary and effector neurons. The convergence of nerve impulses is explained by the fact that there are several times more afferent neurons than effector neurons. Therefore, afferent neurons form numerous synapses on the bodies and dendrites of effector and intercalary neurons.

The principle of irradiation. Impulses entering the central nervous system with strong and prolonged stimulation of the receptors cause excitation not only of this reflex center, but also of other nerve centers. This spread of excitation in the central nervous system is called irradiation. The process of irradiation is associated with the presence in the central nervous system of numerous branching axons and especially dendrites nerve cells and chains of interneurons that connect various nerve centers with each other.

The principle of reciprocity(conjugacy). This phenomenon was studied by I.M. Sechenov, N.E. Vvedensky, Sherrington. Its essence is that when some nerve centers are excited, the activity of others may be inhibited. The principle of reciprocity was shown in relation to the nerve centers of the antagonists of the flexor and extensor muscles of the limbs. It manifests itself most clearly in animals with the brain removed and the spinal cord preserved (spinal animal). If the skin of a limb in a spinal animal (cat) is irritated, a flexion reflex of this limb is noted, and at this time an extension reflex is observed on the opposite side. The described phenomena are associated with the fact that when the center of flexion of one limb is excited, a reciprocal inhibition of the center of extension of the same limb occurs. On the symmetrical side there is an inverse relationship: the extensor center is excited and the flexor center is inhibited. Only with such mutually combined (reciprocal) innervation is walking possible.

The reciprocal relationships between the centers of the brain determine a person’s ability to master complex labor processes and no less complex special movements performed during swimming, acrobatic exercises, etc.

The principle of a common final path. This principle is associated with the structural features of the central nervous system. This feature, as already indicated, is that there are several times more afferent neurons than effector neurons, as a result of which various afferent impulses converge to common outgoing pathways. Quantitative relationships between neurons can be schematically represented as a Funnel: excitation flows into the central nervous system through a wide socket (afferent neurons) and flows out of it through a narrow tube (effector neurons). Common ways There can be not only final effector neurons, but also intercalary ones.

Feedback principle. This principle was studied by I.M. Sechenov, Sherrington, P.K. Anokhin and a number of other researchers. During reflex contraction of skeletal muscles, proprioceptors are excited. From proprioceptors, nerve impulses again enter the central nervous system. This controls the accuracy of the movements performed. Similar afferent impulses that arise in the body as a result of the reflex activity of organs and tissues (effectors) are called secondary afferent impulses or “feedback”.

Feedback can be: positive and negative. Positive feedback enhances reflex reactions, while negative feedback inhibits them.

The principle of dominance was formulated by A. A. Ukhtomsky. This principle plays an important role in the coordinated work of nerve centers. Dominant is the temporarily dominant focus of excitation in the central nervous system, which determines the nature of the body’s response to external and internal stimuli. In fact, this is a neurophysiological manifestation of the most common, dominant emotion.

The dominant focus of excitation is characterized by the following basic properties: 1) increased excitability; 2) persistence of excitation; 3) the ability to sum up excitation; 4) inertia - the dominant in the form of traces of excitation can persist for a long time even after the cessation of the irritation that caused it.

The dominant focus of excitation is capable of attracting (attracting) nerve impulses from other nerve centers that are less excited in this moment. Due to these impulses, the activity of the dominant increases even more, and the activity of other nerve centers is suppressed.

Dominants can be of exogenous and endogenous origin. Exogenous dominance occurs under the influence of environmental factors. For example, when reading an interesting book, a person may not hear music playing on the radio at that time.

Endogenous dominant occurs under the influence of factors of the internal environment of the body, mainly hormones and other physiological factors. active substances. For example, when the content decreases nutrients in the blood, especially glucose, the food center is excited, which is one of the reasons for the food installation in the body of animals and humans.

The dominant may be inert (persistent), and for its destruction the emergence of a new, more powerful source of excitation is necessary.

The dominant is the basis coordination activities organism, ensuring the behavior of humans and animals in the environment, emotional states, attention reactions. The formation of conditioned reflexes and their inhibition is also associated with the presence of a dominant focus of excitation.

Reflex is the main form of activity of the nervous system.

The assumption about the completely reflex nature of the activity of the higher parts of the brain was first developed by the scientist-physiologist I.M. Sechenov. Before him, physiologists and neurologists did not dare to raise the question of the possibility of physiological analysis mental processes, which were left to psychology to solve.

Further, the ideas of I. M. Sechenov were developed in the works of I. P. Pavlov, who opened the ways of objective experimental research functions of the cortex, developed a method for developing conditioned reflexes and created the doctrine of higher nervous activity. Pavlov in his works introduced the division of reflexes into unconditioned, which are carried out by innate, hereditarily fixed nerve pathways, and conditioned, which, according to Pavlov’s views, are carried out through nerve connections formed in the process of individual life of a person or animal.

Charles S. Sherrington (Nobel Prize in Physiology or Medicine, 1932) made a great contribution to the formation of the doctrine of reflexes. He discovered coordination, mutual inhibition and facilitation of reflexes.

The meaning of the doctrine of reflexes

The doctrine of reflexes has given a lot to understanding the very essence of nervous activity. However, he himself reflex principle could not explain many forms of goal-directed behavior. Currently, the concept of reflex mechanisms has been supplemented by the idea of the role of needs in the organization of behavior; it has become generally accepted that the behavior of animals, including humans, is active in nature and is determined not only by certain stimuli, but also by plans and intentions that arise under influenced by certain needs. These new ideas were expressed in physiological concepts " functional system"P.K. Anokhin or "physiological activity" N.A. Bernstein. The essence of these concepts boils down to the fact that the brain can not only adequately respond to stimuli, but also foresee the future, actively make behavioral plans and implement them in action. The idea of an “acceptor of action”, or a “model of the required future”, allows us to talk about “ahead of reality”.

General mechanism of reflex formation

Neurons and the pathways of nerve impulses during a reflex act form a so-called reflex arc:

Stimulus - receptor - neuron - effector - response.

In humans, most reflexes are carried out with the participation of at least two neurons - sensitive and motor (motoneuron, executive neuron). In the reflex arcs of most reflexes, interneurons (interneurons) are also involved - one or more. Any of these neurons in humans can be located both inside the central nervous system (for example, reflexes with the participation of central chemo- and thermoreceptors) and outside it (for example, reflexes of the metasympathetic division of the ANS).

Classification

Based on a number of characteristics, reflexes can be divided into groups.

By type of education: conditioned and unconditioned reflexes.
By type of receptors: exteroceptive (skin, visual, auditory, olfactory), interoceptive (from receptors internal organs) and proprioceptive (from receptors of muscles, tendons, joints)
By effector: somatic or motor (skeletal muscle reflexes), for example flexor, extensor, locomotor, statokinetic, etc.; vegetative - digestive, cardiovascular, sweating, pupillary, etc.
According to biological significance: defensive, or protective, digestive, sexual, orientation.
According to the degree of complexity of the neural organization of reflex arcs, a distinction is made between monosynaptic, the arcs of which consist of afferent and efferent neurons (for example, knee), and polysynaptic, the arcs of which also contain one or more interneurons and have two or more synaptic switches (for example, flexor pain).
According to the nature of the influences on the activity of the effector: excitatory - causing and enhancing (facilitating) its activity, inhibitory - weakening and suppressing it (for example, reflex increase heart rate sympathetic nerve and its reduction or cardiac arrest - vagus).
Based on the anatomical location of the central part of the reflex arcs, spinal reflexes and cerebral reflexes are distinguished. Neurons located in the spinal cord are involved in the implementation of spinal reflexes. An example of the simplest spinal reflex is the withdrawal of a hand from a sharp pin. Brain reflexes are carried out with the participation of brain neurons. Among them there are bulbar, carried out with the participation of neurons of the medulla oblongata; mesencephalic - with the participation of midbrain neurons; cortical - with the participation of neurons in the cerebral cortex. There are also peripheral reflexes carried out by the metasympathetic division of the ANS without the participation of the brain and spinal cord.

Unconditional

Unconditioned reflexes are hereditarily transmitted (innate) reactions of the body, inherent to the entire species. They perform a protective function, as well as the function of maintaining homeostasis (constancy of the internal environment of the body).

Unconditioned reflexes are inherited, unchangeable reactions of the body to certain influences of the external or internal environment, regardless of the conditions for the occurrence and course of reactions. Unconditioned reflexes ensure the body's adaptation to constant environmental conditions. The main types of unconditioned reflexes: food, protective, orientation, sexual.

An example of a defensive reflex is the reflexive withdrawal of the hand from a hot object. Homeostasis is maintained, for example, by a reflex increase in breathing when there is an excess of carbon dioxide in the blood. Almost every part of the body and every organ is involved in reflex reactions.

Neural organization of the simplest reflex

The simplest reflex in vertebrates is considered monosynaptic. If the arc of the spinal reflex is formed by two neurons, then the first of them is represented by a cell of the spinal ganglion, and the second is a motor cell (motoneuron) of the anterior horn of the spinal cord. The long dendrite of the spinal ganglion goes to the periphery, forming a sensitive fiber of a nerve trunk, and ends with a receptor. The axon of a neuron of the spinal ganglion is part of the dorsal root of the spinal cord, reaches the motor neuron of the anterior horn and, through a synapse, connects with the body of the neuron or one of its dendrites. The axon of the anterior horn motor neuron is part of the anterior root, then the corresponding motor nerve and ends in a motor plaque in the muscle.

Pure monosynaptic reflexes do not exist. Even the knee reflex, which is a classic example of a monosynaptic reflex, is polysynaptic, since the sensory neuron not only switches to the motor neuron of the extensor muscle, but also sends an axonal collateral that switches to the inhibitory interneuron of the antagonist muscle, the flexor muscle.

Conditional

Conditioned reflexes arise during individual development and the accumulation of new skills. The development of new temporary connections between neurons depends on environmental conditions. Conditioned reflexes are formed on the basis of unconditioned ones with the participation of higher parts of the brain.

The development of the doctrine of conditioned reflexes is associated primarily with the name of I. P. Pavlov. He showed that a new stimulus can initiate a reflex response if it is presented for some time together with an unconditioned stimulus. For example, if you give a dog a sniff of meat, then it secretes gastric juice (this is an unconditioned reflex). If you ring a bell at the same time as the meat, the dog’s nervous system associates this sound with food, and gastric juice will be released in response to the bell, even if no meat is presented. Conditioned reflexes are the basis acquired behavior. These are the simplest programs. The world is constantly changing, so only those who quickly and expediently respond to these changes can live successfully in it. As you purchase life experience A system of conditioned reflex connections is formed in the cerebral cortex. Such a system is called dynamic stereotype. It underlies many habits and skills. For example, having learned to skate or bicycle, we subsequently no longer think about how we should move so as not to fall.

Axon reflex

The axon reflex is carried out along the branches of the axon without the participation of the neuron body. The reflex arc of the axon reflex does not contain synapses and cell bodies of neurons. With the help of axon reflexes, the regulation of the activity of internal organs and blood vessels can be carried out (relatively) independently of the central nervous system.

Pathological reflexes

Pathological reflexes are a neurological term that refers to reflex reactions that are unusual for a healthy adult. In some cases, more early stages phylo- or ontogeny.

There is an opinion that mental dependence from something caused by the formation of a conditioned reflex. For example, mental dependence on drugs is due to the fact that taking a certain substance is associated with a pleasant state (a conditioned reflex is formed that persists for almost the entire life).

Candidate of Biological Sciences Kharlampiy Tiras believes that “the idea of conditioned reflexes that Pavlov worked with is completely based on forced behavior, and this gives incorrect registration [of results in experiments].” “We insist: an object must be studied when it is ready for it. Then we act as observers without violating the animal, and, accordingly, we get more objective results.” What exactly the author means by “violence” of an animal and what the “more objective” results are, the author does not specify.

Reflex activity ensures the body’s connection with the environment, allows it to adequately respond to external and internal changes and quickly protect itself from external harmful air and respond to internal changes. Eating – finding prey. Maintain constant parameters of the internal environment and regulate these parameters.

Reflex arc and reflex act.

The material substrate of the reflex is the reflex arc, which is formed by a chain of neurons connected by synaptic connections. Along the reflex arc, nerve impulses from excited sensory receptors travel through the central nervous system to the cells of executive tissues and organs.

The reflex arc consists of the following elements:

1. Sensitive receptor– highly specialized formations that perceive and transform the energy of an external stimulus and transmit nerve impulses to the central structures along the sensory nerves

2. Sensory neuron– an afferent neuron, the cat conducts a nerve impulse to the central nervous system and a set of sensory neurons is located outside the central nervous system

3. Interneurons/association/interneurons– are located in the central nervous system, receive information from the sensory neuron and transmit it to the efferent neuron - motor neuron/executive

4. Efferent neuron/motoneuron– receives information from the interneuron and transmits it to the effector/executive organ. The bodies of motor neurons are located in the central nervous system, and axons belong to the peripheral nervous system

5. End-effector/effector-muscles and glands. Therefore, all reflex responses can be reduced either to mc reduction, or to secretion.

Excitation along the reflex arc due to synapses goes in 1 direction: from sensitive receptors through the central nervous system to the effector. A set of sensitive receptors, the irritation of which causes a certain reflex, is called receptive field of the reflex.

Reflex time- time from the moment of action of the stimulus on sensitive receptors until the response from the effector.

Depending on the number of synapses included in the reflex arc, they are distinguished:

1. Polysynaptic reflex arcs – consisting of 3 or more neurons

2. Monosynaptic consisting of 1 synapse, when information from the senses is transmitted to the motor one. In humans, only tendon reflexes are monosynaptic - the knee, plantar, and Achilles reflexes.

Reflex is a complex nervous process, there are 4 functional units:

1- Irritation of receptors and conduction of impulses along the afferent pathways of n.impulses in the central nervous system

2- Deployment of the nervous process in the central nervous system, i.e. in structures called nerve centers and central departments analyzers.

3- Conduction of nerve impulses along efferent/descending pathways, which causes or regulates organ function

Any reflex act should be assessed depending on the achievement of the desired result (Have the muscles contracted sufficiently to ensure flexion of the arm elbow joint?) Such an assessment is carried out on the basis feedback: the effector contains sensitive receptors, information from which enters the central nervous system (in skeletal muscles these are proprioceptors)

4- Conduction of afferent impulses from the own sensitive receptors of a functioning organ to the central nervous system - reverse information. Such a connection allows the organs to be corrected, as it makes it possible to regulate the intensity and nature of the organ’s activity. Therefore, with reflex circuits, it is more correct to talk about a reflex ring, taking into account feedback. The reflex ring includes: a reflex arc and ways to receive feedback.

If the result of the reflector is not achieved, excitation switching to new afferent pathways.

Therefore, the number of afferent and efferent neurons correlates as 5 to 1. That is, the same reflex response can be observed to a variety of stimuli. Can use 1 and the same final path. That is, certain motor neurons mc groups, and the afferent links of these reflexes differ.

Charles Sherrington formulated this pattern as the principle of a common final path.

In the absence of the 4th link of the reflex act/feedback, normal functional activity of the organ becomes impossible, because without feedback mechanisms, without signals that ensure the result of the performed action, it is impossible to correct the body’s reactions, which means adaptation to the environment

Particular physiology of the NS

Physiology of the spinal cord