What is the reflex principle of the nervous system responsible for? Abstract: The reflex nature of the activity of the human nervous system.

The reflex mechanism is the main one in the activity of the nervous system. A reflex is the body’s response to external irritation, carried out with the participation of the nervous system.

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

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

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

HOMEOSTASIS

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

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

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

2.5. THE OCCASION OF EXCITATION AND ITS CARRYING OUT 2.5.1. MEMBRANE POTENTIALS

The cell membrane consists of a double layer of lipid molecules, with their “heads” facing outward and their “tails” facing each other. Lumps of protein molecules float freely between them. Some of them penetrate the membrane right through. Some of these proteins contain special pores or ion channels through which ions involved in the formation of membrane potentials can pass (Fig. I-A).

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

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

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

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

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

and a resting closed sodium channel (3).

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

membranes.

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

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

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

This entire process is extremely short-lived. It only takes about

1-2 ms, after which the sodium channel gate closes. At this point, the permeability for potassium ions, which slowly increases during excitation, reaches a large value. Potassium ions leaving the cell cause a rapid decrease in the action potential. However, the final restoration of the original charge continues for some time. In this regard, in the action potential, a short-term high-voltage part is distinguished - the peak (or spike) and long-term small fluctuations - trace potentials. Motor neuron action potentials have a peak amplitude of about

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

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

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

CONDUCTING EXCITATION

Action potentials (excitation impulses) have the ability to propagate along nerve and muscle fibers.

In a nerve fiber, the action potential is a very strong stimulus to adjacent sections of the fiber. The amplitude of the action potential is usually 5-6 times the depolarization threshold. This ensures high speed and reliability.

Between the excitation zone (which has a negative charge on the surface of the fiber and a positive charge on the inner side of the membrane) and the adjacent non-excited section of the nerve fiber membrane (with an inverse charge ratio), electric currents- so-called local currents. As a result, depolarization of the neighboring area develops, an increase in its ionic permeability and the appearance of an action potential. In the original excitation zone, the resting potential is restored. Then the excitation covers the next section of the membrane, etc. Thus, with the help of local currents, excitation spreads to neighboring sections of the nerve fiber, i.e. conduction of a nerve impulse. As it is carried out, the amplitude of the action potential does not decrease, i.e., the excitation does not die out even with long length nerve.

In the process of evolution, with the transition from non-pulp nerve fibers to pulpal ones, there was a significant increase in the speed of nerve impulse conduction. The soft fibers are characterized by continuous conduction of excitation, which sequentially covers each adjacent section of the nerve. The pulpal nerves are almost completely covered with an insulating myelin sheath. Ionic currents in them can pass only in exposed areas of the membrane - nodes of Ranvier, devoid of this membrane. During the conduction of a nerve impulse, excitation jumps from one interception to another and can even cover several interceptions. This type of exercise is called saltatory (lat. saltus-jump). This increases not only the speed, but also the cost-effectiveness of the process. Excitation does not capture the entire surface of the fiber membrane, but only a small part of it. Consequently, less energy is spent on active transport of ions across the membrane during excitation and during recovery.

The conduction speed in different fibers is different. Thicker nerve fibers conduct excitation at a higher speed: they have larger distances between nodes of Ranvier and longer jumps. Motor and proprioceptive afferent nerve fibers have the highest conduction speed - up to 100. In thin sympathetic nerve fibers (especially in unmyelinated fibers), the conduction velocity is low - on the order of 0.5 - 15.

During the development of an action potential, the membrane completely loses excitability. This state is called complete inexcitability or absolute refractoriness. It is followed by relative refractoriness, when the action potential can occur only with very strong stimulation. Gradually, excitability is restored to its original level.

NERVOUS SYSTEM

The nervous system is divided into peripheral (nerve fibers and nodes) and central. The central nervous system (CNS) includes the spinal cord and brain.

BASIC FUNCTIONS OF THE CNS

All the most important human behavioral reactions are carried out with the help of the central nervous system.

The main functions of the central nervous system are:

Uniting all parts of the body into a single whole and their regulation;

Controlling the state and behavior of the body in accordance with environmental conditions and its needs.

In higher animals and humans, the leading part of the central nervous system is the cerebral cortex. It controls the most complex functions in human life - mental processes (consciousness, thinking, speech, memory, etc.).

The main methods for studying the functions of the central nervous system are methods of removal and irritation (in the clinic and on animals), recording electrical phenomena, and the method of conditioned reflexes.

New methods for studying the central nervous system continue to be developed: with the help of so-called computed tomography, one can see morphofunctional changes in the brain at different depths; photography in infrared rays(thermal imaging) allow you to detect the “hottest” spots in the brain; New data on the functioning of the brain is provided by the study of its magnetic oscillations.

Related information.

The nervous system performs two main functions:

1. Ensuring adequate reactions of the body to constantly changing environmental conditions.

2. Regulation and coordination of the work of internal organs.

The concept of nervous regulation of functions is based on the doctrine of reflex. Reflex is defined as the body's response to irritation, carried out with the participation of the nervous system. However, not every response of the body is a reflex. For example, a bruise in response to mechanical irritation occurs due to rupture of skin vessels and blood clotting; however, the nervous system does not take part in this, and the appearance of a bruise cannot be called a reflex. In order to provide a response, the NS must first receive information about the current situation from the senses. Based on this information, as well as signals from memory centers, needs, motivations and some other things, the nervous system “makes a decision” about which response will be the most optimal. After this, the NS sends control impulses to the executive organs (muscles or glands), which carry out the appropriate reaction.

It is clear that for the reflex to occur, it is first necessary for the nervous excitation that arises in the central nervous system in response to any stimulation to reach the executive organ. The structural basis for this process is reflex arc.

Reflex arc- the path along which the nerve impulse passes during the implementation of the reflex. It consists of five sections: 1) receptor; 2) a sensitive neuron that transmits an impulse to the central nervous system; 3) nerve center; 4) motor neuron; 5) a working organ that reacts to the received irritation.

Receptor- a sensitive formation that transforms the energy of a stimulus into a nervous process (usually electrical excitation). The receptor is followed by a sensory neuron located in the peripheral nervous system. The peripheral processes (dendrites) of such neurons form a sensory nerve and go to the receptors, and the central ones (axons) enter the central nervous system and form synapses on its interneurons. In some cases (skin sensitivity, sense of smell), the receptors are the endings of the peripheral processes of sensory neurons. In this case, the first two sections of the reflex arc are formed by the same neuron. The interneuron of the central nervous system (or, more precisely, neurons, since there are usually several of them) are the nerve center of each specific reflex. The axons of interneurons form synapses on motor neurons, along the axons of which the nerve impulse in turn reaches the executive organ, causing corresponding activity. The axons of motor neurons form motor nerves.

Thus, the arcs of even simple reflexes usually include about 5-10 sequentially located neurons. In the very simple case The reflex arc includes only two neurons - sensory and motor. Examples of such reflexes include the knee reflex, which occurs in response to a blow to the quadriceps tendon, or the Achilles reflex, which occurs in response to a blow to the gastrocnemius tendon (see Fig. 18).

For a more adequate understanding of the regulation of the body’s functioning, it is necessary to examine in more detail the concept of “nerve center”. Nerve center- a group of neurons necessary to carry out a specific reflex or more complex shapes behavior. The nerve center processes information that comes to it from the senses or from other nerve centers and, in turn, sends commands to the peripheral organs (muscles and glands) or other nerve centers.

In invertebrate animals, the nerve center may consist of only a few neurons. Thus, in the marine mollusk Aplysia, only four neurons control the heart function. In vertebrates, nerve centers are part of the central nervous system and can include thousands or even millions of neurons.

Each nerve center is located in a specific place in the nervous system. For example, the respiratory center, which regulates the functioning of the respiratory muscles, is located in the medulla oblongata. When this center is destroyed, breathing stops. But in fact, many other neurons are involved in breathing. Thus, nerve fibers from the respiratory center in the medulla oblongata go to groups of motor neurons in the spinal cord that directly control the respiratory muscles. The pons contains a nerve center that regulates the correct alternation of inhalation and exhalation. The highest center of the brain - the cerebral cortex - also takes part in breathing, thanks to which breathing can be voluntarily regulated. The same can be said about most other functions of the body (movement in space, eye movements, reactions to pain, etc.). Therefore, in the broad sense of the word, the nerve center is all structures that coordinately influence the performance of a particular function.

It is thanks to the reflex principle that the nervous system ensures the processes self-regulation. If any physiological parameter decreases excessively, then mechanisms are automatically (reflexively) activated to ensure its increase. And vice versa, if any parameter increases, mechanisms for its reduction are activated. For example, when body temperature rises, the VNS ensures dilation of skin blood vessels and sweating, thereby removing excess heat. This operating principle is also called a negative feedback mechanism.

In some physiological systems, a positive feedback mechanism has also been discovered, thanks to which the process, having arisen, strengthens and maintains itself for some time.

To explain the mechanisms of self-regulation, Russian physiologist Academician P.K. Anokhin proposed the concept “ functional system».

Functional system- a temporary or permanent association of various elements of the nervous system (from receptors to executive organs), which arose or exists to perform a specific physiological task. Very important in this concept is the idea that when performing any action, information about its results enters the central nervous system (in the form of impulses from the corresponding receptors). This link of the functional system closes the reflex arc into a ring. If the result of actions partially or completely does not correspond to what was expected, then the central nervous system, through a feedback mechanism, directs the course of reactions in the required direction. Thus, behavior is built on the principle of continuous circular interaction between the organism and the environment, constant assessment of the results of activity - the principle of a reflex ring. This principle significantly complements the idea of ​​the reflex arc, known since the time of R. Descartes.

The reflex nature of the activity of the human nervous system

3. Reflex principle of building the nervous system Feedback principle

From point of view modern science The nervous system is a collection of neurons connected by synapses into cellular chains that operate on the principle of reflection, i.e. reflexively. Reflex (from Latin reflexus - “turned back”, “reflected”) is the body’s reaction to irritation, carried out using the nervous system. The first ideas about the reflected activity of the brain were expressed in 1649 by the French scientist and philosopher Rene Descartes (1590-- 1650). He viewed reflexes as the simplest movements. However, over time the concept has expanded.

In 1863, the founder of the Russian school of physiologists, Ivan Mikhailovich Sechenov, uttered a phrase that went down in the history of medicine: “All acts of conscious and unconscious activity, according to their method of origin, are reflexes.” Three years later, he substantiated his statement in the classic work “Reflexes of the Brain.” Another Russian scientist I.P. Pavlov built on the statement of his brilliant compatriot the doctrine of higher nervous activity. Pavlov divided the reflexes that underlie it into unconditioned, with which a person is born, and conditioned, acquired throughout life.

The structural basis of any reflex is the reflex arc. The shortest one consists of three neurons and functions within the body. It turns on when receptors are irritated (from the Latin recipio - “receive”); they are sensitive nerve endings or special cells that convert this or that influence (light, sound, etc.) into biopotentials (from the Greek “bios” - “life” plat. potentia - “power”).

Through centripetal - afferent (from the Latin affero - “I bring”) fibers, signals arrive to the so-called first (sensitive) neuron located in the spinal ganglion. It is he who passes through the initial information, which the brain converts into familiar sensations in a split second: touch, injection, warmth... Along the axon of the sensitive nerve cell, impulses follow to the second neuron - the intermediate (intercalary) one. It is located in the posterior sections, or, as experts say, the posterior horns, of the spinal cord; a horizontal section of the spinal cord really looks like the head of a strange beast with four horns.

From here the signals have a direct path to the anterior horns: to the third - motor - neuron. The axon of the motor cell extends beyond the spinal cord along with other efferent (from the Latin effero - “I carry out”) fibers as part of the nerve roots and nerves. They transmit commands from the central nervous system to the working organs: a muscle, for example, is ordered to contract, a gland is ordered to secrete juice, blood vessels are ordered to expand, etc.

However, the activity of the nervous system is not limited to the “highest decrees”. She not only gives orders, but also strictly monitors their execution - she analyzes signals from receptors located in organs that work on her instructions. Thanks to this, the amount of work is adjusted depending on the condition of the “subordinates”. In fact, the body is a self-regulating system: it carries out life activities according to the principle of closed cycles, with feedback about the achieved result. Academician Pyotr Kuzmich Anokhin (1898-1974) came to this conclusion back in 1934, who combined the doctrine of reflexes with biological cybernetics.

Sensory and motor neurons are the alpha and omega of a simple reflex arc: it begins with one, and ends with the other. In complex reflex arcs, ascending and descending cellular chains are formed, connected by a cascade of interneurons. This is how extensive bilateral connections are made between the brain and the spinal cord.

The formation of a conditioned reflex connection requires a number of conditions:

1. Multiple coincidence in time of the action of the unconditioned and conditioned stimuli (more precisely, with some precedence of the action of the conditioned stimulus). Sometimes a connection is formed even with a single coincidence of the action of stimuli.

2. Absence of extraneous irritants. The action of an extraneous stimulus during the development of a conditioned reflex leads to inhibition (or even cessation) of the conditioned reflex reaction.

3. Greater physiological strength (factor of biological significance) of the unconditioned stimulus compared to the conditioned stimulus.

4. Active state of the cerebral cortex.

According to modern ideas, nerve impulses are transmitted during reflexes along reflex rings. Reflex ring includes at least 5 links.

It should be noted that the latest research data from scientists (P.K. Anokhin and others) confirm precisely this ring-shaped reflex pattern, and not the reflex arc pattern, which does not fully reveal this difficult process. The body needs to receive information about the results perfect action, information about each stage of the ongoing action. Without it, the brain cannot organize purposeful activity, cannot correct the action when any random (interfering) factors interfere with the reaction, cannot stop activities in necessary moment, upon achieving the result. This led to the need to move from the idea of ​​an open reflex arc to the idea of ​​a cyclic innervation structure in which there is feedback - from the effector and object of activity through receptors to the central nervous structures.

This connection (reverse flow of information from the object of activity) is a mandatory element. Without it, the organism would be cut off from the environment in which it lives and to change which its activity is aimed, including human activity associated with the use of tools of production. .

theory reflex nerve system

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Nervous system regulates the activity of all organs and systems, determining their functional unity, and ensures the connection of the body as a whole with the external environment.

The structural unit of the nervous system is a nerve cell with processes - neuron. The entire nervous system is a collection of neurons that contact each other using special devices - synapses. Based on structure and function, there are three types of neurons:

• receptor, or sensitive;
• insertion, closing (conductor);
• effector, motor neurons, from which the impulse is sent to the working organs (muscles, glands).

The nervous system is conventionally divided into two large sections - somatic, or animal, nervous system and vegetative, or autonomic nervous system. The somatic nervous system primarily carries out the functions of connecting the body with the external environment, providing sensitivity and movement causing contraction skeletal muscles. Since the functions of movement and feeling are characteristic of animals and distinguish them from plants, this part of the nervous system is called animal (animal).

The autonomic nervous system influences the processes of so-called plant life, common to animals and plants (metabolism, respiration, excretion, etc.), which is where its name comes from (vegetative - plant). Both systems are closely related to each other, but the autonomic nervous system has a certain degree of independence and does not depend on our will, as a result of which it is also called the autonomic nervous system. It is divided into two parts sympathetic And parasympathetic.

In the nervous system there are central part - brain and spinal cord - central nervous system and peripheral, represented by nerves extending from the brain and spinal cord, is the peripheral nervous system. A cross-section of the brain shows that it consists of gray and white matter.

Gray matter formed in clusters nerve cells(with the initial sections of processes extending from their bodies). Individual limited accumulations of gray matter are called cores.

White matter form nerve fibers covered with a myelin sheath (the processes of nerve cells that form the gray matter). Nerve fibers in the brain and spinal cord form pathways.

Peripheral nerves, depending on what fibers (sensory or motor) they consist of, are divided into sensitive, motor And mixed. The bodies of neurons, the processes of which make up the sensory nerves, lie in the nerve ganglia outside the brain. The cell bodies of motor neurons lie in the anterior horns of the spinal cord or motor nuclei of the brain.

I.P. Pavlov showed that the central nervous system can have three types of effects on organs:

• 1) launcher causing or stopping the function of an organ (muscle contraction, gland secretion);
• 2) vasomotor, changing the width of the lumen of blood vessels and thereby regulating the flow of blood to the organ;
• 3) trophic, increasing or decreasing and therefore the consumption of nutrients and oxygen. Thanks to this, it is constantly consistent functional state organ and its need for nutrients and oxygen. When impulses are sent to a working skeletal muscle through motor fibers, causing its contraction, then at the same time impulses are sent through the autonomic nerve fibers, dilating blood vessels and strengthening them. This ensures the energetic ability to perform muscle work.

The central nervous system perceives afferent(sensitive) information that arises when specific receptors are stimulated and, in response to this, forms appropriate efferent impulses that cause changes in the activity of certain organs and systems of the body.

"...if you turn off all the receptors, then a person should fall asleep
dead sleep and never wake up."
THEM. Sechenov

Reflex- the main form of nervous activity. The body's response to stimulation from the external or internal environment, carried out with the participation of the central nervous system, is called reflex.

The path along which a nerve impulse travels from the receptor to the effector (acting organ) is called reflex arc.

There are five links in the reflex arc:

• receptor;
• sensitive fiber conducting excitation to the centers;
• the nerve center where the switching of excitation from sensory cells to motor cells occurs;
• motor fiber carrying nerve impulses to the periphery;
• the acting organ is a muscle or gland.

Any irritation - mechanical, light, sound, chemical, temperature, perceived by the receptor, is transformed (converted) or, as is now commonly said, encoded by the receptor into a nerve impulse and in this form is sent along sensory fibers to the central nervous system.

In the central nervous system, this information is processed, selected and transmitted to motor nerve cells, which send nerve impulses to the working organs - muscles, glands and cause one or another adaptive act - movement or secretion.

The reflex, as an adaptive reaction of the body, ensures a subtle, precise and perfect balancing of the body with the environment, as well as control and regulation of functions within the body. This is his biological significance. A reflex is a functional unit of nervous activity.

All nervous activity, no matter how complex it is, consists of reflexes of varying degrees of complexity, i.e. it is reflected, caused by an external reason, an external push.
From clinical practice: in the clinic of S.P. Botkin observed a patient in whom, of all the body’s receptors, one eye and one ear were functioning. As soon as the patient's eyes were closed and his ear plugged, he fell asleep.

In the experiments of V.S. Galkin's dogs, whose visual, auditory and olfactory receptors were simultaneously turned off by surgery, slept 20-23 hours a day. They awoke only under the influence of internal needs or energetic influence on skin receptors. Consequently, the central nervous system works on the principle of reflex, reflection, and on the stimulus-response principle.

The reflex principle of nervous activity was discovered by the great French philosopher, physicist and mathematician Rene Descartes more than 300 years ago.
The reflex theory was developed in the fundamental works of Russian scientists I.M. Sechenov and I.P. Pavlova.

The time that elapses from the moment the stimulus is applied to the response to it is called the reflex time. It is composed of the time necessary to excite receptors, conduct excitation along sensory fibers, through the central nervous system, through motor fibers, and, finally, the latent (hidden) period of excitation of the working organ. Most of the time is spent on conducting excitation through the nerve centers - central reflex time.

The reflex time depends on the strength of stimulation and the excitability of the central nervous system. With strong irritation it is shorter; with a decrease in excitability, caused, for example, by fatigue, the reflex time increases; with an increase in excitability it decreases significantly.

Each reflex can only be evoked from a specific receptive field. For example, the sucking reflex occurs when the baby's lips are irritated; pupil constriction reflex - in bright light (illumination of the retina), etc.

Each reflex has its own localization(location) in the central nervous system, i.e. that part of it that is necessary for its implementation. For example, the center of pupil dilation is in the upper thoracic segment of the spinal cord. When the corresponding section is destroyed, the reflex is absent.

Only with the integrity of the central nervous system is the perfection of nervous activity preserved. The nerve center is a collection of nerve cells located in various parts of the central nervous system, necessary for the implementation of the reflex and sufficient for its regulation.

Braking

It would seem that the excitation that arises in the central nervous system can spread unhindered in all directions and cover all nerve centers. In reality, this does not happen. In the central nervous system, in addition to the process of excitation, a process of inhibition simultaneously occurs, turning off those nerve centers that could interfere or impede the implementation of any type of activity of the body, for example, bending a leg.

Excitement called a nervous process that either causes the activity of an organ or enhances an existing one.

Under braking understand a nervous process that weakens or stops activity or prevents its occurrence. The interaction of these two active processes underlies neural activity.

The process of inhibition in the central nervous system was discovered in 1862 by I.M. Sechenov. In experiments on frogs, he made transverse sections of the brain into various levels and irritated the nerve centers by placing a crystal of table salt on the cut. It was discovered that when the diencephalon is irritated, spinal reflexes are depressed or completely inhibited: the frog's leg, immersed in a weak solution of sulfuric acid, did not withdraw.

Much later, the English physiologist Sherrington discovered that the processes of excitation and inhibition are involved in any reflex act. When a muscle group contracts, the antagonist muscle centers are inhibited. When bending an arm or leg, the centers of the extensor muscles are inhibited. A reflex act is possible only with coupled, so-called reciprocal inhibition of antagonist muscles. When walking, bending the leg is accompanied by relaxation of the extensors and, conversely, when extending, the flexor muscles are inhibited. If this did not happen, then a mechanical struggle of the muscles, convulsions, would arise, and not adaptive motor acts.

When a sensory nerve is irritated,

Dominant

In the central nervous system, under the influence of certain reasons, a focus of increased excitability may arise, which has the property of attracting excitations from other reflex arcs and thereby increasing its activity and inhibiting other nerve centers. This phenomenon is called dominant.

The dominant is one of the main patterns in the activity of the central nervous system. It can arise under the influence of various reasons: hunger, thirst, self-preservation instinct, reproduction. The state of food dominance is well formulated in the Russian proverb: “A hungry godfather has bread on his mind.” In a person, the cause of dominance can be passion for work, love, or parental instinct. If a student is busy preparing for an exam or reading an exciting book, then extraneous noise do not interfere with him, but even deepen his concentration and attention.

A very important factor in the coordination of reflexes is the presence in the central nervous system of a certain functional subordination, that is, a certain subordination between its departments that arises in the process of long evolution. Nerve centers and receptors of the head as an “avant-garde” part of the body, paving the way for the body in environment, develop faster. The higher parts of the central nervous system acquire the ability to change the activity and direction of activity of the lower parts.

It is important to note: the higher the level of the animal, the stronger the power of the highest departments of the central nervous system, “the more the highest department is the manager and distributor of the body’s activity” (I.P. Pavlov).

In humans, such a “manager and distributor” is the cerebral cortex. There are no functions in the body that are not subject to the decisive regulatory influence of the cortex.

Scheme 1. Propagation (direction shown by arrows) of nerve impulses along a simple reflex arc

1 - sensitive (afferent) neuron; 2 - intercalary (conductor) neuron; 3 - motor (efferent) neuron; 4 - nerve fibers of the thin and wedge-shaped bundles; 5 - fibers of the corticospinal tract.

Reflex. Reflex arc A reflex is the body's response to external or internal stimuli, carried out with the participation of the central nervous system. Receptors are highly sensitive to stimuli specific to them and convert their energy into the process of nervous excitation. The neural pathway along which nerve impulses are carried out during a reflex is called a reflex arc.

Reflex. Reflex arc The simplest reflex arcs are formed by just two neurons. The processes of sensory nerve cells form contacts directly on the executive neurons, which send their long processes to the muscles or glands. An example of the simplest reflexes is the knee reflex, which is usually caused by a doctor examining a patient. To do this, the patient is asked to cross his legs and hit the tendon ligament just below the kneecap with a rubber mallet.

Reflex. Reflex Arc The reflex arc of this reflex consists of only two neurons. The executive neuron is located in the spinal cord. The vast majority of reflex arcs have a more complex structure. A reflex arc is the path along which a nerve impulse travels during a reflex. There are 5 elements in the reflex arc: 1 – receptors, 2 – sensory neuron, 3 – nerve center, 4 – motor neuron, 5 – executive organ.

Reflex. Reflex arc They are formed by a chain of sensitive, one or more intercalary and executive neurons. Touching a hot object with your hand creates painful sensation and causes the hand to be withdrawn. Pain signals from receptors enter the spinal cord and are transmitted to interneurons. These, in turn, excite executive neurons that send commands to the arm muscles. The muscles contract and the arm bends.

Reflex. Reflex arc Part of the reflex arc of any reflex is always located in a certain area of ​​the central nervous system and consists of intercalary and executive neurons. This is the nerve center of this reflex. In other words, the nerve center is a combination of neurons designed to participate in the performance of a specific reflex act.

Reflex. Reflex arc The knee and flexion reflexes described above are classified as innate. To carry out an innate reflex, the body has ready-made reflex arcs. Therefore, their implementation does not require any special additional conditions, which is why they are called unconditioned reflexes.

Repetition: **Test 1. Correct judgments: 1. Reflex is the body’s response to external or internal irritation. 2. Reflex is the body’s response to irritation, carried out with the participation of the nervous system. 3. The movement of the amoeba towards food is a reflex. 4.The movement of the hydra towards food is a reflex. **Test 2. K without conditioned reflexes include: 1. Knee reflex. 2. Withdrawal of the hand when touching a hot object. 3. The dog salivates when food enters the mouth. 4. The dog salivates at the sight of food. **Test 3. Correct judgments: 1. Conditioned reflexes have ready-made reflex arcs already at birth. 2. The doctrine of conditioned reflexes was created by I.M. Sechenov. 3. The basis of learning is the formation of conditioned reflexes. 4. The basis of learning is the formation of unconditioned reflexes.

Repetition: **Test 4. Conditioned reflexes include: 1. The dog’s reaction to the word “Face”. 2. Withdrawal of the hand when touching a hot object. 3. The dog salivates when food enters the mouth. 4. The dog salivates at the sight of food. Test 5. The reflex arc consists of: 1. Of receptors and a sensitive neuron that transmits excitation to the nerve center. 2. From receptors, sensitive neurons, nerve centers that analyze information. 3. From receptors, sensory neuron, nerve center, motor neuron and working organ. 4. From receptors, sensory neuron, nerve center, motor neuron, transmitting excitation to the organ and feedback, with the help of which the nerve center controls the reflex.

Repetition: Test 6. A simple reflex arc consists of: 1. A sensitive neuron that transmits excitation to the nerve center. 2. From a sensory neuron and a motor neuron. 3.From sensitive, insertion and motor neurons. 4. From sensory, intercalary, motor neurons and feedback connections, with the help of which the nerve center controls the reflex. Test 7. A complex reflex arc consists of: 1. A sensitive neuron that transmits excitation to the nerve center. 2. From a sensory neuron and a motor neuron. 3. From sensory, intercalary and motor neurons. 4. From sensory, intercalary, motor neurons and feedback connections, with the help of which the nerve center controls the reflex.

Repetition: Test 8. The nerve center of the reflex consists of: 1. A sensitive neuron with receptors. 2. From a sensory neuron and a motor neuron. 3. From intercalary and executive neurons. 4. From sensory, intercalary, motor neurons and feedback connections, with the help of which the nerve center controls the reflex.