Test the respiratory system. respiratory center

The respiratory center not only provides a rhythmic alternation of inhalation and exhalation, but is also able to change the depth and frequency of respiratory movements, thereby adapting pulmonary ventilation to the current needs of the body. Environmental factors, such as the composition and pressure of atmospheric air, ambient temperature, and changes in the state of the body, for example, during muscle work, emotional arousal, etc., affecting the intensity of metabolism, and, consequently, oxygen consumption and carbon dioxide release, affect the functional state of the respiratory center. As a result, the volume of pulmonary ventilation changes.

Like all other processes of automatic regulation of physiological functions, the regulation of respiration is carried out in the body on the basis of the feedback principle. This means that the activity of the respiratory center, which regulates the supply of oxygen to the body and the removal of carbon dioxide formed in it, is determined by the state of the process regulated by it. The accumulation of carbon dioxide in the blood, as well as the lack of oxygen, are factors that cause excitation of the respiratory center.

The value of blood gas composition in the regulation of respiration was shown by Frederick by experiment with cross-circulation. To do this, in two dogs under anesthesia, their carotid arteries and separately jugular veins were cut and cross-connected (Figure 2). the head of the second dog is from the body of the first.

If one of these dogs clamps the trachea and thus suffocates the body, then after a while it stops breathing (apnea), while the second dog develops severe shortness of breath (dyspnea). This is explained by the fact that the clamping of the trachea in the first dog causes the accumulation of CO 2 in the blood of its trunk (hypercapnia) and a decrease in the oxygen content (hypoxemia). Blood from the body of the first dog enters the head of the second dog and stimulates its respiratory center. As a result, increased breathing occurs - hyperventilation - in the second dog, which leads to a decrease in CO 2 tension and an increase in O 2 tension in the blood vessels of the body of the second dog. The oxygen-rich, carbon-dioxide-poor blood from the torso of this dog enters the head first and causes apnea.

Figure 2 - Scheme of Frederick's experiment with cross-circulation

Frederick's experience shows that the activity of the respiratory center changes with a change in the CO 2 and O 2 tension in the blood. Let us consider the influence on respiration of each of these gases separately.

Importance of carbon dioxide tension in the blood in the regulation of respiration. An increase in carbon dioxide tension in the blood causes excitation of the respiratory center, leading to an increase in lung ventilation, and a decrease in carbon dioxide tension in the blood inhibits the activity of the respiratory center, which leads to a decrease in lung ventilation. The role of carbon dioxide in the regulation of respiration was proved by Holden in experiments in which a person was in a closed space of a small volume. As the inhaled air decreases in oxygen and increases in carbon dioxide, dyspnea begins to develop. If the released carbon dioxide is absorbed by soda lime, the oxygen content in the inhaled air can decrease to 12%, and there is no noticeable increase in pulmonary ventilation. Thus, the increase in lung ventilation in this experiment was due to an increase in the content of carbon dioxide in the inhaled air.

In another series of experiments, Holden determined the volume of ventilation of the lungs and the content of carbon dioxide in the alveolar air when breathing a gas mixture with different carbon dioxide content. The results obtained are shown in table 1.

breathing muscle gas blood

Table 1 - The volume of ventilation of the lungs and the content of carbon dioxide in the alveolar air

The data given in Table 1 show that, simultaneously with an increase in the content of carbon dioxide in the inhaled air, its content in the alveolar air, and hence in the arterial blood, also increases. In this case, there is an increase in ventilation of the lungs.

The results of the experiments gave convincing evidence that the state of the respiratory center depends on the content of carbon dioxide in the alveolar air. It was found that an increase in the content of CO 2 in the alveoli by 0.2% causes an increase in lung ventilation by 100%.

A decrease in the content of carbon dioxide in the alveolar air (and, consequently, a decrease in its tension in the blood) lowers the activity of the respiratory center. This occurs, for example, as a result of artificial hyperventilation, i.e., increased deep and frequent breathing, which leads to a decrease in the partial pressure of CO 2 in the alveolar air and CO 2 tension in the blood. As a result, respiratory arrest occurs. Using this method, i.e., by making a preliminary hyperventilation, you can significantly increase the time of arbitrary breath holding. This is what divers do when they need to spend 2-3 minutes underwater (the usual duration of an arbitrary breath-hold is 40-60 seconds).

The direct stimulating effect of carbon dioxide on the respiratory center has been proven by various experiments. Injection of 0.01 ml of a solution containing carbon dioxide or its salt into a certain area of the medulla oblongata causes an increase in respiratory movements. Euler exposed the isolated medulla oblongata of a cat to the action of carbon dioxide and observed that this causes an increase in the frequency of electrical discharges (action potentials), indicating the excitation of the respiratory center.

The respiratory center is affected increase in the concentration of hydrogen ions. Winterstein in 1911 expressed the point of view that the excitation of the respiratory center is caused not by carbonic acid itself, but by an increase in the concentration of hydrogen ions due to an increase in its content in the cells of the respiratory center. This opinion is based on the fact that an increase in respiratory movements is observed when not only carbonic acid is injected into the arteries that feed the brain, but also other acids, such as lactic. The hyperventilation that occurs with an increase in the concentration of hydrogen ions in the blood and tissues promotes the release of part of the carbon dioxide contained in the blood from the body and thereby leads to a decrease in the concentration of hydrogen ions. According to these experiments, the respiratory center is a regulator of the constancy of not only the tension of carbon dioxide in the blood, but also the concentration of hydrogen ions.

The facts established by Winterstein were confirmed in experimental studies. At the same time, a number of physiologists insisted that carbonic acid is a specific irritant of the respiratory center and has a stronger stimulating effect on it than other acids. The reason for this turned out to be that carbon dioxide penetrates more easily than the H + ion through the blood-brain barrier that separates blood from the cerebrospinal fluid, which is the immediate environment that bathes the nerve cells, and more easily passes through the membrane of the nerve cells themselves. When CO 2 enters the cell, H 2 CO 3 is formed, which dissociates with the release of H + ions. The latter are the causative agents of the cells of the respiratory center.

Another reason for the stronger action of H 2 CO 3 in comparison with other acids is, according to a number of researchers, that it specifically affects certain biochemical processes in the cell.

The stimulating effect of carbon dioxide on the respiratory center is the basis of one intervention that has found application in clinical practice. With the weakening of the function of the respiratory center and the resulting insufficient supply of oxygen to the body, the patient is forced to breathe through a mask with a mixture of oxygen with 6% carbon dioxide. This gas mixture is called carbogen.

The mechanism of action of increased CO voltage 2 and increased concentration of H+-ions in the blood for respiration. For a long time it was believed that an increase in carbon dioxide tension and an increase in the concentration of H+ ions in the blood and cerebrospinal fluid (CSF) directly affect the inspiratory neurons of the respiratory center. It has now been established that changes in CO 2 voltage and H+ ion concentration affect respiration by stimulating chemoreceptors located near the respiratory center, which are sensitive to the above changes. These chemoreceptors are located in bodies about 2 mm in diameter, located symmetrically on both sides of the medulla oblongata on its ventrolateral surface near the exit site of the hypoglossal nerve.

The importance of chemoreceptors in the medulla oblongata can be seen from the following facts. When these chemoreceptors are exposed to carbon dioxide or solutions with an increased concentration of H+ ions, respiration is stimulated. Cooling of one of the chemoreceptor bodies of the medulla oblongata entails, according to the experiments of Leshke, the cessation of respiratory movements on the opposite side of the body. If the chemoreceptor bodies are destroyed or poisoned by novocaine, breathing stops.

Along With chemoreceptors in the medulla oblongata in the regulation of respiration, an important role belongs to the chemoreceptors located in the carotid and aortic bodies. This was proved by Heimans in methodically complex experiments in which the vessels of two animals were connected in such a way that the carotid sinus and carotid body or the aortic arch and aortic body of one animal were supplied with the blood of another animal. It turned out that an increase in the concentration of H + -ions in the blood and an increase in CO 2 tension cause excitation of carotid and aortic chemoreceptors and a reflex increase in respiratory movements.

There is evidence that 35% of the effect caused by the inhalation of air With high content of carbon dioxide, due to the effect on chemoreceptors of an increased concentration of H + -ions in the blood, and 65% are the result of an increase in CO 2 tension. The action of CO 2 is explained by the rapid diffusion of carbon dioxide through the chemoreceptor membrane and the shift in the concentration of H + -ions inside the cell.

Consider effect of lack of oxygen on respiration. Excitation of the inspiratory neurons of the respiratory center occurs not only with an increase in the carbon dioxide tension in the blood, but also with a decrease in the oxygen tension.

Reduced oxygen tension in the blood causes a reflex increase in respiratory movements, acting on the chemoreceptors of the vascular reflexogenic zones. Direct evidence that a decrease in oxygen tension in the blood excites the chemoreceptors of the carotid body was obtained by Geimans, Neil and other physiologists by recording bioelectric potentials in the carotid sinus nerve. Perfusion of the carotid sinus with blood with low oxygen tension leads to an increase in action potentials in this nerve (Figure 3) and to an increase in respiration. After the destruction of chemoreceptors, a decrease in oxygen tension in the blood does not cause changes in respiration.

Figure 3 - Electrical activity of the sinus nerve (according to Nile) A- when breathing atmospheric air; B- when breathing a gas mixture containing 10% oxygen and 90% nitrogen. 1 - recording the electrical activity of the nerve; 2 - record of two pulse fluctuations of arterial pressure. Calibration lines correspond to pressure values of 100 and 150 mm Hg. Art.

Recording electrical potentials B shows a continuous frequent impulse that occurs when the chemoreceptors are stimulated by a lack of oxygen. High-amplitude potentials during periods of pulsed increases in blood pressure are due to the impulsation of pressoreceptors in the carotid sinus.

The fact that the stimulus of chemoreceptors is a decrease in the tension of oxygen in the blood plasma, and not a decrease in its total content in the blood, is proved by the following observations of L. L. Shik. With a decrease in the amount of hemoglobin or when it is bound by carbon monoxide, the oxygen content in the blood is sharply reduced, but the dissolution of O 2 in the blood plasma is not disturbed and its tension in the plasma remains normal. In this case, excitation of chemoreceptors does not occur and respiration does not change, although oxygen transport is sharply impaired and the tissues experience a state of oxygen starvation, since insufficient oxygen is delivered to them by hemoglobin. With a decrease in atmospheric pressure, when the tension of oxygen in the blood decreases, there is an excitation of chemoreceptors and an increase in respiration.

The nature of the change in respiration with an excess of carbon dioxide and a decrease in oxygen tension in the blood is different. With a slight decrease in the tension of oxygen in the blood, a reflex increase in the rhythm of breathing is observed, and with a slight increase in the tension of carbon dioxide in the blood, a reflex deepening of the respiratory movements occurs.

Thus, the activity of the respiratory center is regulated by the effect of an increased concentration of H+ ions and an increased tension of CO 2 on the chemoreceptors of the medulla oblongata and on the chemoreceptors of the carotid and aortic bodies, as well as by the effect on the chemoreceptors of these vascular reflexogenic zones of a decrease in oxygen tension in the arterial blood.

Causes of the first breath of a newborn are explained by the fact that in the womb the fetal gas exchange occurs through the umbilical vessels, which are in close contact with the mother's blood in the placenta. The termination of this connection with the mother at birth leads to a decrease in oxygen tension and the accumulation of carbon dioxide in the blood of the fetus. This, according to Barcroft, irritates the respiratory center and leads to inhalation.

For the onset of the first breath, it is important that the cessation of embryonic breathing occurs suddenly: when the umbilical cord is slowly clamped, the respiratory center is not excited and the fetus dies without taking a single breath.

It should also be taken into account that the transition to new conditions causes irritation of a number of receptors in the newborn and the flow of impulses through the afferent nerves that increase the excitability of the central nervous system, including the respiratory center (I. A. Arshavsky).

The value of mechanoreceptors in the regulation of respiration. The respiratory center receives afferent impulses not only from chemoreceptors, but also from the pressoreceptors of the vascular reflexogenic zones, as well as from the mechanoreceptors of the lungs, airways, and respiratory muscles.

The influence of pressoreceptors of vascular reflexogenic zones is found in the fact that an increase in pressure in an isolated carotid sinus, connected with the body only by nerve fibers, leads to inhibition of respiratory movements. This also happens in the body when blood pressure rises. On the contrary, with a decrease in blood pressure, breathing quickens and deepens.

Important in the regulation of respiration are impulses coming to the respiratory center along the vagus nerves from the receptors of the lungs. The depth of inhalation and exhalation largely depends on them. The presence of reflex influences from the lungs was described in 1868 by Hering and Breuer and formed the basis for the idea of reflex self-regulation of breathing. It manifests itself in the fact that when inhaling, impulses appear in the receptors located in the walls of the alveoli, reflexively inhibiting inhalation and stimulating exhalation, and with a very sharp exhalation, with an extreme degree of decrease in lung volume, impulses appear that enter the respiratory center and reflexively stimulate inhalation. . The following facts testify to the presence of such reflex regulation:

In the lung tissue in the walls of the alveoli, i.e., in the most extensible part of the lung, there are interoreceptors, which are the endings of the afferent fibers of the vagus nerve that perceive irritation;

After transection of the vagus nerves, breathing becomes sharply slow and deep;

When the lung is inflated with an indifferent gas, such as nitrogen, with the obligatory condition of the integrity of the vagus nerves, the muscles of the diaphragm and intercostal spaces suddenly cease to contract, the breath stops before reaching the usual depth; on the contrary, with artificial suction of air from the lung, a contraction of the diaphragm occurs.

Based on all these facts, the authors came to the conclusion that the stretching of the pulmonary alveoli during inspiration causes irritation of the receptors of the lungs, as a result of which the impulses coming to the respiratory center along the pulmonary branches of the vagus nerves become more frequent, and this reflex excites the expiratory neurons of the respiratory center, and, therefore, causes exhalation. Thus, as Hering and Breuer wrote, "each breath, as it stretches the lungs, prepares its own end."

If you connect the peripheral ends of the cut vagus nerves to an oscilloscope, you can register the action potentials that arise in the receptors of the lungs and go along the vagus nerves to the central nervous system not only when the lungs are inflated, but also when air is artificially sucked out of them. In natural respiration, frequent currents of action in the vagus nerve are found only during inspiration; during natural exhalation, they are not observed (Figure 4).

Figure 4 - Currents of action in the vagus nerve during stretching of the lung tissue during inspiration (according to Adrian) From top to bottom: 1 - afferent impulses in the vagus nerve: 2 - breath recording (inhale - up, exhale - down); 3 - timestamp

Consequently, the collapse of the lungs causes reflex irritation of the respiratory center only with such a strong compression, which does not happen during a normal, ordinary exhalation. This is observed only with a very deep exhalation or a sudden bilateral pneumothorax, to which the diaphragm reacts reflexively with a contraction. During natural breathing, the vagus nerve receptors are irritated only when the lungs are stretched and reflexively stimulate exhalation.

In addition to the mechanoreceptors of the lungs, the mechanoreceptors of the intercostal muscles and the diaphragm take part in the regulation of respiration. They are excited by stretching during exhalation and reflexively stimulate inhalation (S. I. Franshtein).

Correlation between inspiratory and expiratory neurons of the respiratory center. There are complex reciprocal (conjugated) relationships between inspiratory and expiratory neurons. This means that excitation of inspiratory neurons inhibits expiratory neurons, and excitation of expiratory neurons inhibits inspiratory neurons. Such phenomena are partly due to the presence of direct connections that exist between the neurons of the respiratory center, but they mainly depend on reflex influences and on the functioning of the pneumotaxis center.

The interaction between the neurons of the respiratory center is currently represented as follows. Due to the reflex (through chemoreceptors) action of carbon dioxide on the respiratory center, excitation of inspiratory neurons occurs, which is transmitted to motor neurons that innervate the respiratory muscles, causing the act of inspiration. At the same time, impulses from inspiratory neurons arrive at the pneumotaxis center located in the pons, and from it, along the processes of its neurons, impulses arrive at the expiratory neurons of the respiratory center of the medulla oblongata, causing excitation of these neurons, cessation of inhalation and stimulation of exhalation. In addition, the excitation of expiratory neurons during inspiration is also carried out reflexively through the Hering-Breuer reflex. After transection of the vagus nerves, the influx of impulses from the mechanoreceptors of the lungs stops and expiratory neurons can only be excited by impulses coming from the center of pneumotaxis. The impulse that excites the expiratory center is significantly reduced and its excitation is somewhat delayed. Therefore, after transection of the vagus nerves, inhalation lasts much longer and is replaced by exhalation later than before the transection of the nerves. Breathing becomes rare and deep.

Similar changes in breathing with intact vagus nerves occur after transection of the brainstem at the level of the pons, which separates the center of pneumotaxis from the medulla oblongata (see Figure 1, Figure 5). After such a transection, the flow of impulses that excite the expiratory center also decreases, and breathing becomes rare and deep. The excitation of the expiratory center in this case is carried out only by impulses coming to it through the vagus nerves. If in such an animal the vagus nerves are also cut or the propagation of impulses along these nerves is interrupted by cooling them, then exhalation of the exhalation center does not occur and breathing stops in the phase of maximum inspiration. If after that the conduction of the vagus nerves is restored by warming them, then excitation of the exhalation center periodically occurs again and rhythmic breathing is restored (Figure 6).

Figure 5 - Scheme of nerve connections of the respiratory center 1 - inspiratory center; 2 - pneumotaxis center; 3 - expiratory center; 4 - lung mechanoreceptors. After crossing along the lines / and // separately, the rhythmic activity of the respiratory center is preserved. With simultaneous transection, breathing stops in the inspiratory phase.

Thus, the vital function of breathing, which is possible only with the rhythmic alternation of inhalation and exhalation, is regulated by a complex nervous mechanism. When studying it, attention is drawn to the multiple ensuring the operation of this mechanism. The excitation of the inspiratory center occurs both under the influence of an increase in the concentration of hydrogen ions (an increase in CO 2 tension) in the blood, which causes excitation of the chemoreceptors of the medulla oblongata and chemoreceptors of the vascular reflexogenic zones, and as a result of the effect of a reduced oxygen tension on the aortic and carotid chemoreceptors. The excitation of the expiratory center is due to both reflex impulses coming to it along the afferent fibers of the vagus nerves and the influence of the inhalation center through the center of pneumotaxis.

The excitability of the respiratory center changes under the action of nerve impulses coming through the cervical sympathetic nerve. Irritation of this nerve increases the excitability of the respiratory center, which intensifies and speeds up breathing.

The influence of the sympathetic nerves on the respiratory center partly explains the changes in breathing during emotions.

Figure 6 - The effect of turning off the vagus nerves on breathing after cutting the brain at the level between the lines I and II(See Figure 5) (by Stella) A- breath recording; b- a mark of nerve cooling

Respiratory system. Breath.

A) does not change B) shrinks C) expands

2. The number of cell layers in the wall of the pulmonary vesicle:
A) 1 B) 2 C) 3 D) 4

3. The shape of the diaphragm during contraction:
A) flat B) domed C) elongated D) concave

4. The respiratory center is located in:
A) medulla oblongata B) cerebellum C) diencephalon D) cerebral cortex

5. A substance that causes the activity of the respiratory center:
A) oxygen B) carbon dioxide C) glucose D) hemoglobin

6. Portion of the tracheal wall without cartilage:
A) front wall B) side walls C) back wall

7. The epiglottis closes the entrance to the larynx:
A) during a conversation B) when inhaling C) when exhaling D) when swallowing

8. How much oxygen is in exhaled air?
A) 10% B) 14% C) 16% D) 21%

9. An organ that is not involved in the formation of the wall of the chest cavity:
A) ribs B) sternum C) diaphragm D) pericardial sac

10. An organ that does not line the pleura:
A) trachea B) lung C) sternum D) diaphragm E) ribs

11. Eustachian tube opens at:
A) nasal cavity B) nasopharynx C) pharynx D) larynx

12. The pressure in the lungs is greater than the pressure in the pleural cavity:
A) when inhaling B) when exhaling C) in any phase D) when holding the breath while inhaling

14. The walls of the larynx are formed:
A) cartilage B) bones C) ligaments D) smooth muscles

15. How much oxygen is in the air of the pulmonary vesicles?
A) 10% B) 14% C) 16% D) 21%

16. The amount of air that enters the lungs during a quiet breath:
A) 100-200 cm 3 B) 300-900 cm 3 C) 1000-1100 cm 3 D) 1200-1300 cm 3

17. The sheath that covers each lung from the outside:
A) fascia B) pleura C) capsule D) basement membrane

18. During swallowing occurs:
A) inhale B) exhale C) inhale and exhale D) hold the breath

19 . The amount of carbon dioxide in the atmospheric air:
A) 0.03% B) 1% C) 4% D) 6%

20. Sound is generated by:

A) inhale B) exhale C) hold the breath while inhaling D) hold the breath while exhaling

21. Does not take part in the formation of speech sounds:
A) trachea B) nasopharynx C) pharynx D) mouth E) nose

22. The wall of the pulmonary vesicles is formed by tissue:
A) connective B) epithelial C) smooth muscle D) striated muscle

23. Relaxed diaphragm shape:
A) flat B) elongated C) domed D) concave into the abdominal cavity

24. The amount of carbon dioxide in the exhaled air:
A) 0.03% B) 1% C) 4% D) 6%

25. Airway epithelial cells contain:
A) flagella B) villi C) pseudopods D) cilia

26 . The amount of carbon dioxide in the air of the pulmonary vesicles:
A) 0.03% B) 1% C) 4% D) 6%

28. With an increase in chest volume, pressure in the alveoli:
A) does not change B) decreases C) increases

29 . The amount of nitrogen in the atmospheric air:
A) 54% B) 68% C) 79% D) 87%

30. Outside the chest is located (s):
A) trachea B) esophagus C) heart D) thymus (thymus gland) E) stomach

31. The most frequent respiratory movements are characteristic of:
A) newborns B) children 2-3 years old C) teenagers D) adults

32. Oxygen moves from the alveoli into the blood plasma when:

A) pinocytosis B) diffusion C) respiration D) ventilation

33 . Number of breaths per minute:
A) 10-12 B) 16-18 C) 2022 D) 24-26

34 . A diver develops gas bubbles in the blood (a cause of decompression sickness) when:
A) slow ascent from depth to surface B) slow descent to depth

C) rapid ascent from depth to surface D) rapid descent to depth

35. Which cartilage of the larynx protrudes forward in men?
A) epiglottis B) arytenoid C) cricoid D) thyroid

36. The causative agent of tuberculosis refers to:
A) bacteria B) fungi C) viruses D) protozoa

37. The total surface of the pulmonary vesicles:
A) 1 m 2 B) 10 m 2 C) 100 m 2 D) 1000 m 2

38. The concentration of carbon dioxide at which a person begins to poison:

39 . The diaphragm first appeared in:
A) amphibians B) reptiles C) mammals D) primates E) humans

40. The concentration of carbon dioxide at which a person loses consciousness and death:

A) 1% B) 2-3% C) 4-5% D) 10-12%

41. Cellular respiration occurs in:
A) nucleus B) endoplasmic reticulum C) ribosome D) mitochondria

42. The amount of air for an untrained person during a deep breath:
A) 800-900 cm 3 B) 1500-2000 cm 3 C) 3000-4000 cm 3 D) 6000 cm 3

43. The phase when the pressure of the lungs is above atmospheric:
A) inhale B) exhale C) hold the breath D) hold the breath

44. The pressure that begins to change during breathing earlier:
A) in the alveoli B) in the pleural cavity C) in the nasal cavity D) in the bronchi

45. A process that requires the participation of oxygen:
A) glycolysis B) protein synthesis C) fat hydrolysis D) cellular respiration

46. The composition of the airways does not include the organ:
A) nasopharynx B) larynx C) bronchi D) trachea E) lungs

47 . The lower respiratory tract does not include:

A) larynx B) nasopharynx C) bronchi D) trachea

48. The causative agent of diphtheria is classified as:
A) bacteria B) viruses C) protozoa D) fungi

49. Which component of the exhaled air is present in the largest quantity?

A) carbon dioxide B) oxygen C) ammonia D) nitrogen E) water vapor

50. The bone in which the maxillary sinus is located?
A) frontal B) temporal C) maxillary D) nasal

Answers: 1b, 2a, 3a, 4a, 5b, 6c, 7d, 8c, 9d, 10a, 11b, 12c, 13c, 14a, 15b, 16b, 17b, 18d, 19a, 20b, 21a, 22b, 23c, 24c, 25d, 26d, 27c, 28b, 29c, 30d, 31a, 32b, 33b, 34c, 35d, 36a, 37c, 38c, 39c, 40d, 41d, 42c, 43b, 44a, 45d, 46e, 47b, 48a, 4 9g, 50v

The main function of the respiratory system is to ensure the exchange of oxygen and carbon dioxide between the environment and the body in accordance with its metabolic needs. In general, this function is regulated by a network of numerous CNS neurons that are associated with the respiratory center of the medulla oblongata.

Under respiratory center understand the totality of neurons located in different parts of the central nervous system, providing coordinated muscle activity and adaptation of breathing to the conditions of the external and internal environment. In 1825, P. Flurans singled out a “vital knot” in the central nervous system, N.A. Mislavsky (1885) discovered the inspiratory and expiratory parts, and later F.V. Ovsyannikov described the respiratory center.

The respiratory center is a paired formation, consisting of an inhalation center (inspiratory) and an exhalation center (expiratory). Each center regulates the breathing of the side of the same name: when the respiratory center is destroyed on one side, the respiratory movements stop on that side.

expiratory department - part of the respiratory center that regulates the process of exhalation (its neurons are located in the ventral nucleus of the medulla oblongata).

Inspiratory department- part of the respiratory center that regulates the process of inhalation (located mainly in the dorsal part of the medulla oblongata).

The neurons of the upper part of the bridge that regulate the act of breathing were named pneumotaxic center. On fig. 1 shows the location of the neurons of the respiratory center in various parts of the CNS. The inspiratory center has automatism and is in good shape. The expiratory center is regulated from the inspiratory center through the pneumotaxic center.

Pneumatic complex- part of the respiratory center, located in the region of the pons and regulating inhalation and exhalation (during inhalation causes excitation of the expiratory center).

Rice. 1. Localization of the respiratory centers in the lower part of the brain stem (posterior view):

PN - pneumotaxic center; INSP - inspiratory; ZKSP - expiratory. The centers are double-sided, but to simplify the diagram, only one is shown on each side. Transection along line 1 does not affect breathing, along line 2 the pneumotaxic center is separated, below line 3 respiratory arrest occurs

In the structures of the bridge, two respiratory centers are also distinguished. One of them - pneumotaxic - promotes the change of inhalation to exhalation (by switching excitation from the center of inhalation to the center of exhalation); the second center exerts a tonic effect on the respiratory center of the medulla oblongata.

The expiratory and inspiratory centers are in reciprocal relations. Under the influence of spontaneous activity of the neurons of the inspiratory center, an act of inhalation occurs, during which, when the lungs are stretched, mechanoreceptors are excited. Impulses from mechanoreceptors through the afferent neurons of the excitatory nerve enter the inspiratory center and cause excitation of the expiratory and inhibition of the inspiratory center. This provides a change from inhalation to exhalation.

In the change of inhalation to exhalation, the pneumotaxic center plays an important role, which exerts its influence through the neurons of the expiratory center (Fig. 2).

Rice. 2. Scheme of nerve connections of the respiratory center:

1 - inspiratory center; 2 - pneumotaxic center; 3 - expiratory center; 4 - mechanoreceptors of the lung

At the moment of excitation of the inspiratory center of the medulla oblongata, excitation simultaneously occurs in the inspiratory department of the pneumotaxic center. From the latter, along the processes of its neurons, impulses come to the expiratory center of the medulla oblongata, causing its excitation and, by induction, inhibition of the inspiratory center, which leads to a change from inhalation to exhalation.

Thus, the regulation of respiration (Fig. 3) is carried out due to the coordinated activity of all departments of the central nervous system, united by the concept of the respiratory center. The degree of activity and interaction of the departments of the respiratory center is influenced by various humoral and reflex factors.

Respiratory center vehicles

The ability of the respiratory center to automaticity was first discovered by I.M. Sechenov (1882) in experiments on frogs under conditions of complete deafferentation of animals. In these experiments, despite the fact that no afferent impulses were delivered to the CNS, potential fluctuations were recorded in the respiratory center of the medulla oblongata.

The automaticity of the respiratory center is evidenced by Heimans' experiment with an isolated dog's head. Her brain was cut at the level of the bridge and deprived of various afferent influences (the glossopharyngeal, lingual and trigeminal nerves were cut). Under these conditions, the respiratory center did not receive impulses not only from the lungs and respiratory muscles (due to the preliminary separation of the head), but also from the upper respiratory tract (due to the transection of these nerves). Nevertheless, the animal retained the rhythmic movements of the larynx. This fact can only be explained by the presence of rhythmic activity of the neurons of the respiratory center.

The automation of the respiratory center is maintained and changed under the influence of impulses from the respiratory muscles, vascular reflexogenic zones, various intero- and exteroreceptors, as well as under the influence of many humoral factors (blood pH, carbon dioxide and oxygen content in the blood, etc.).

The effect of carbon dioxide on the state of the respiratory center

The influence of carbon dioxide on the activity of the respiratory center is especially clearly demonstrated in Frederick's experiment with cross-circulation. In two dogs, the carotid arteries and jugular veins are cut and connected crosswise: the peripheral end of the carotid artery is connected to the central end of the same vessel of the second dog. The jugular veins are also cross-connected: the central end of the jugular vein of the first dog is connected to the peripheral end of the jugular vein of the second dog. As a result, blood from the body of the first dog goes to the head of the second dog, and blood from the body of the second dog goes to the head of the first dog. All other vessels are ligated.

After such an operation, the first dog was subjected to tracheal clamping (suffocation). This led to the fact that after some time an increase in the depth and frequency of breathing in the second dog (hyperpnea) was observed, while the first dog stopped breathing (apnea). This is explained by the fact that in the first dog, as a result of clamping the trachea, gas exchange was not carried out, and the content of carbon dioxide in the blood increased (hypercapnia occurred) and the oxygen content decreased. This blood flowed to the head of the second dog and affected the cells of the respiratory center, resulting in hyperpnea. But in the process of increased ventilation of the lungs in the blood of the second dog, the content of carbon dioxide (hypocapnia) decreased and the content of oxygen increased. Blood with a reduced content of carbon dioxide entered the cells of the respiratory center of the first dog, and the irritation of the latter decreased, which led to apnea.

Thus, an increase in the content of carbon dioxide in the blood leads to an increase in the depth and frequency of respiration, and a decrease in the content of carbon dioxide and an increase in oxygen leads to its decrease up to respiratory arrest. In those observations, when the first dog was allowed to breathe various gas mixtures, the greatest change in respiration was observed with an increase in the content of carbon dioxide in the blood.

Dependence of the activity of the respiratory center on the gas composition of the blood

The activity of the respiratory center, which determines the frequency and depth of breathing, depends primarily on the tension of the gases dissolved in the blood and the concentration of hydrogen ions in it. The leading role in determining the amount of ventilation of the lungs is the tension of carbon dioxide in the arterial blood: it, as it were, creates a request for the desired amount of ventilation of the alveoli.

The terms "hypercapnia", "normocapnia" and "hypocapnia" are used to designate increased, normal and reduced carbon dioxide tension in the blood, respectively. Normal oxygen content is called normoxia, lack of oxygen in the body and tissues - hypoxia in blood - hypoxemia. There is an increase in oxygen tension hyperxia. The condition in which hypercapnia and hypoxia exist at the same time is called asphyxia.

Normal breathing at rest is called epnea. Hypercapnia, as well as a decrease in blood pH (acidosis) are accompanied by an involuntary increase in lung ventilation - hyperpnea aimed at removing excess carbon dioxide from the body. Lung ventilation increases mainly due to the depth of breathing (increase in tidal volume), but at the same time, the respiratory rate also increases.

Hypocapnia and an increase in the pH level of the blood lead to a decrease in ventilation, and then to respiratory arrest - apnea.

The development of hypoxia initially causes moderate hyperpnea (mainly as a result of an increase in the respiratory rate), which, with an increase in the degree of hypoxia, is replaced by a weakening of breathing and its stop. Apnea due to hypoxia is deadly. Its cause is the weakening of oxidative processes in the brain, including in the neurons of the respiratory center. Hypoxic apnea is preceded by loss of consciousness.

Hyperkainia can be caused by inhalation of gas mixtures with an increased content of carbon dioxide up to 6%. The activity of the human respiratory center is under arbitrary control. Arbitrary holding of breath for 30-60 seconds causes asphyxic changes in the gas composition of the blood, after the cessation of the delay, hyperpnea is observed. Hypocapnia is easily induced by voluntary increased breathing, as well as by excessive artificial ventilation of the lungs (hyperventilation). In an awake person, even after significant hyperventilation, respiratory arrest usually does not occur due to the control of breathing by the anterior brain regions. Hypocapnia is compensated gradually, within a few minutes.

Hypoxia is observed when climbing to a height due to a decrease in atmospheric pressure, during extremely hard physical work, as well as in violation of breathing, blood circulation and blood composition.

During severe asphyxia, breathing becomes as deep as possible, auxiliary respiratory muscles take part in it, and there is an unpleasant feeling of suffocation. This breathing is called dyspnea.

In general, maintaining a normal blood gas composition is based on the principle of negative feedback. So, hypercapnia causes an increase in the activity of the respiratory center and an increase in lung ventilation, and hypocapnia - a weakening of the activity of the respiratory center and a decrease in ventilation.

Reflex effects on breathing from vascular reflex zones

Breathing reacts especially quickly to various stimuli. It changes rapidly under the influence of impulses coming from the extero- and interoreceptors to the cells of the respiratory center.

The irritant of the receptors can be chemical, mechanical, temperature and other influences. The most pronounced mechanism of self-regulation is the change in respiration under the influence of chemical and mechanical stimulation of vascular reflexogenic zones, mechanical stimulation of the receptors of the lungs and respiratory muscles.

The sinocarotid vascular reflexogenic zone contains receptors that are sensitive to the content of carbon dioxide, oxygen and hydrogen ions in the blood. This is clearly shown in Heimans' experiments with an isolated carotid sinus, which was separated from the carotid artery and supplied with blood from another animal. The carotid sinus was connected to the CNS only by a nervous route - Hering's nerve was preserved. With an increase in the content of carbon dioxide in the blood surrounding the carotid body, excitation of the chemoreceptors of this zone occurs, as a result of which the number of impulses going to the respiratory center (to the center of inhalation) increases, and a reflex increase in the depth of breathing occurs.

Rice. 3. Regulation of breathing

K - bark; Ht - hypothalamus; Pvc - pneumotaxic center; Apts - the center of respiration (expiratory and inspiratory); Xin - carotid sinus; Bn - vagus nerve; Cm - spinal cord; C 3 -C 5 - cervical segments of the spinal cord; Dfn - phrenic nerve; EM - expiratory muscles; MI — inspiratory muscles; Mnr - intercostal nerves; L - lungs; Df - aperture; Th 1 - Th 6 - thoracic segments of the spinal cord

An increase in the depth of breathing also occurs when carbon dioxide acts on the chemoreceptors of the aortic reflexogenic zone.

The same changes in respiration occur when the chemoreceptors of these reflexogenic zones of the blood are stimulated with an increased concentration of hydrogen ions.

In those cases, when the oxygen content in the blood increases, the irritation of the chemoreceptors of the reflexogenic zones decreases, as a result of which the flow of impulses to the respiratory center weakens and a reflex decrease in the frequency of breathing occurs.

The reflex causative agent of the respiratory center and the factor influencing respiration is the change in blood pressure in the vascular reflexogenic zones. With an increase in blood pressure, the mechanoreceptors of the vascular reflexogenic zones are irritated, as a result of which reflex respiratory depression occurs. A decrease in blood pressure leads to an increase in the depth and frequency of breathing.

Reflex effects on respiration from the mechanoreceptors of the lungs and respiratory muscles. An essential factor causing the change of inhalation and exhalation is the influence from the mechanoreceptors of the lungs, which was first discovered by Hering and Breuer (1868). They showed that each breath stimulates the exhalation. During inhalation, when the lungs are stretched, mechanoreceptors located in the alveoli and respiratory muscles are irritated. The impulses that have arisen in them along the afferent fibers of the vagus and intercostal nerves come to the respiratory center and cause excitation of expiratory neurons and inhibition of inspiratory neurons, causing a change from inhalation to exhalation. This is one of the mechanisms of self-regulation of breathing.

Like the Hering-Breuer reflex, there are reflex influences on the respiratory center from the receptors of the diaphragm. During inhalation in the diaphragm, when its muscle fibers contract, the endings of the nerve fibers are irritated, the impulses arising in them enter the respiratory center and cause the inhalation to stop and the exhalation to occur. This mechanism is of particular importance during increased breathing.

Reflex influences on breathing from various receptors of the body. The considered reflex influences on breathing are permanent. But there are various short-term effects from almost all receptors in our body that affect breathing.

So, under the action of mechanical and temperature stimuli on the exteroreceptors of the skin, breath holding occurs. Under the action of cold or hot water on a large surface of the skin, breathing stops on inspiration. Painful irritation of the skin causes a sharp breath (shriek) with the simultaneous closure of the vocal cord.

Some changes in the act of breathing that occur when the mucous membranes of the respiratory tract are irritated are called protective respiratory reflexes: coughing, sneezing, holding the breath, which occurs under the action of pungent odors, etc.

Respiratory center and its connections

Respiratory center called a set of neural structures located in various parts of the central nervous system that regulate rhythmic coordinated contractions of the respiratory muscles and adapt breathing to changing environmental conditions and the needs of the body. Among these structures, vital sections of the respiratory center are distinguished, without the functioning of which breathing stops. These include departments located in the medulla oblongata and spinal cord. In the spinal cord, the structures of the respiratory center include motor neurons, which form the phrenic nerves with their axons (in the 3-5th cervical segments), and motor neurons, which form the intercostal nerves (in the 2-10th thoracic segments, while the respiratory neurons are concentrated in the 2- 6th, and expiratory - in the 8th-10th segments).

A special role in the regulation of respiration is played by the respiratory center, represented by departments localized in the brain stem. Part of the neuronal groups of the respiratory center is located in the right and left halves of the medulla oblongata in the region of the bottom of the IV ventricle. There is a dorsal group of neurons that activate the inspiratory muscles - the inspiratory section and a ventral group of neurons that control predominantly exhalation - the expiratory section.

In each of these departments there are neurons with different properties. Among the neurons of the inspiratory section, there are: 1) early inspiratory - their activity increases 0.1-0.2 s before the start of contraction of the inspiratory muscles and lasts during inspiration; 2) full inspiratory - active during inspiration; 3) late inspiratory - activity increases in the middle of inhalation and ends at the beginning of exhalation; 4) neurons of an intermediate type. Part of the neurons of the inspiratory region has the ability to spontaneously rhythmically excite. Neurons similar in properties are described in the expiratory section of the respiratory center. The interaction between these neural pools ensures the formation of the frequency and depth of breathing.

An important role in determining the nature of the rhythmic activity of the neurons of the respiratory center and respiration belongs to signals coming to the center along afferent fibers from receptors, as well as from the cerebral cortex, limbic system, and hypothalamus. A simplified diagram of the nerve connections of the respiratory center is shown in fig. 4.

The neurons of the inspiratory department receive information about the tension of gases in the arterial blood, the pH of the blood from the chemoreceptors of the vessels, and the pH of the cerebrospinal fluid from the central chemoreceptors located on the ventral surface of the medulla oblongata.

The respiratory center also receives nerve impulses from receptors that control the stretching of the lungs and the condition of the respiratory and other muscles, from thermoreceptors, pain and sensory receptors.

The signals coming to the neurons of the dorsal part of the respiratory center modulate their own rhythmic activity and influence the formation of efferent nerve impulse flows transmitted to the spinal cord and further to the diaphragm and external intercostal muscles.

Rice. 4. Respiratory center and its connections: IC - inspiratory center; PC - insvmotaksnchsskny center; EC - expiratory center; 1,2 - impulses from stretch receptors of the respiratory tract, lungs and chest

Thus, the respiratory cycle is triggered by inspiratory neurons, which are activated due to automation, and its duration, frequency, and depth of breathing depend on the influence of receptor signals on the neuronal structures of the respiratory center that are sensitive to the level of p0 2 , pCO 2 and pH, as well as other factors. intero- and exteroreceptors.

Efferent nerve impulses from inspiratory neurons are transmitted along descending fibers in the ventral and anterior part of the lateral funiculus of the white matter of the spinal cord to a-motoneurons that form the phrenic and intercostal nerves. All fibers following to the motor neurons innervating the expiratory muscles are crossed, and 90% of the fibers following to the motor neurons innervating the inspiratory muscles are crossed.

Motor neurons, activated by the flow of nerve impulses from the inspiratory neurons of the respiratory center, send efferent impulses to the neuromuscular synapses of the inspiratory muscles, which provide an increase in the volume of the chest. Following the chest, the volume of the lungs increases and inhalation occurs.

During inhalation, stretch receptors in the airways and lungs are activated. The flow of nerve impulses from these receptors along the afferent fibers of the vagus nerve enters the medulla oblongata and activates expiratory neurons that trigger exhalation. Thus, one circuit of the mechanism of respiration regulation is closed.

The second regulatory circuit also starts from the inspiratory neurons and conducts impulses to the neurons of the pneumotaxic department of the respiratory center located in the pons of the brainstem. This department coordinates the interaction between the inspiratory and expiratory neurons of the medulla oblongata. The pneumotaxic department processes the information received from the inspiratory center and sends a stream of impulses that excite the neurons of the expiratory center. Streams of impulses coming from the neurons of the pneumotaxic section and from the stretch receptors of the lungs converge on the expiratory neurons, excite them, the expiratory neurons inhibit (but on the principle of reciprocal inhibition) the activity of the inspiratory neurons. Sending nerve impulses to the inspiratory muscles stops and they relax. This is enough for a calm exhalation to occur. With increased exhalation, efferent impulses are sent from expiratory neurons, causing contraction of the internal intercostal muscles and abdominal muscles.

The described scheme of neural connections reflects only the most general principle of the regulation of the respiratory cycle. In reality, afferent signal flows from numerous receptors of the respiratory tract, blood vessels, muscles, skin, etc. come to all structures of the respiratory center. They have an excitatory effect on some groups of neurons, and an inhibitory effect on others. The processing and analysis of this information in the respiratory center of the brain stem is controlled and corrected by the higher parts of the brain. For example, the hypothalamus plays a leading role in changes in respiration associated with reactions to pain stimuli, physical activity, and also ensures the involvement of the respiratory system in thermoregulatory reactions. Limbic structures influence breathing during emotional reactions.

The cerebral cortex ensures the inclusion of the respiratory system in behavioral reactions, speech function, and the penis. The presence of the influence of the cerebral cortex on the sections of the respiratory center in the medulla oblongata and spinal cord is evidenced by the possibility of arbitrary changes in the frequency, depth and breath holding by a person. The influence of the cerebral cortex on the bulbar respiratory center is achieved both through the cortico-bulbar pathways and through subcortical structures (stropallidarium, limbic, reticular formation).

Oxygen, carbon dioxide and pH receptors

Oxygen receptors are already active at a normal pO 2 level and continuously send streams of signals (tonic impulses) that activate inspiratory neurons.

Oxygen receptors are concentrated in the carotid bodies (the bifurcation area of the common carotid artery). They are represented by type 1 glomus cells, which are surrounded by supporting cells and have synaptic connections with the endings of the afferent fibers of the glossopharyngeal nerve.

Glomus cells of the 1st type respond to a decrease in pO 2 in arterial blood by increasing the release of the mediator dopamine. Dopamine causes the generation of nerve impulses at the endings of the afferent fibers of the tongue of the pharyngeal nerve, which are conducted to the neurons of the inspiratory section of the respiratory center and to the neurons of the pressor section of the vasomotor center. Thus, a decrease in oxygen tension in arterial blood leads to an increase in the frequency of sending afferent nerve impulses and an increase in the activity of inspiratory neurons. The latter increase ventilation of the lungs, mainly due to increased respiration.

Receptors sensitive to carbon dioxide are found in carotid bodies, aortic bodies of the aortic arch, and also directly in the medulla oblongata - central chemoreceptors. The latter are located on the ventral surface of the medulla oblongata in the area between the exit of the hypoglossal and vagus nerves. Carbon dioxide receptors also perceive changes in the concentration of H + ions. Receptors of arterial vessels respond to changes in pCO 2 and blood plasma pH, while the supply of afferent signals to inspiratory neurons from them increases with an increase in pCO 2 and (or) a decrease in arterial blood plasma pH. In response to the receipt of more signals from them in the respiratory center, the ventilation of the lungs reflexively increases due to the deepening of breathing.

Central chemoreceptors respond to changes in pH and pCO 2 , cerebrospinal fluid and intercellular fluid of the medulla oblongata. It is believed that the central chemoreceptors predominantly respond to changes in the concentration of hydrogen protons (pH) in the interstitial fluid. In this case, a change in pH is achieved due to the easy penetration of carbon dioxide from the blood and cerebrospinal fluid through the structures of the blood-brain barrier into the brain, where, as a result of its interaction with H 2 0, carbon dioxide is formed, which dissociates with the release of hydrogen runs.

Signals from the central chemoreceptors are also conducted to the inspiratory neurons of the respiratory center. The neurons of the respiratory center themselves have some sensitivity to a shift in the pH of the interstitial fluid. The decrease in pH and the accumulation of carbon dioxide in the CSF is accompanied by the activation of inspiratory neurons and an increase in lung ventilation.

Thus, the regulation of pCO 0 and pH are closely related both at the level of effector systems that affect the content of hydrogen ions and carbonates in the body, and at the level of central nervous mechanisms.

With the rapid development of hypercapnia, an increase in lung ventilation of only approximately 25% is caused by stimulation of peripheral chemoreceptors of carbon dioxide and pH. The remaining 75% are associated with the activation of the central chemoreceptors of the medulla oblongata by hydrogen protons and carbon dioxide. This is due to the high permeability of the blood-brain barrier to carbon dioxide. Since the cerebrospinal fluid and the intercellular fluid of the brain have a much lower capacity of buffer systems than the blood, an increase in pCO 2 similar to blood in magnitude creates a more acidic environment in the cerebrospinal fluid than in the blood:

With prolonged hypercapnia, the pH of the cerebrospinal fluid returns to normal due to a gradual increase in the permeability of the blood-brain barrier for HCO 3 anions and their accumulation in the cerebrospinal fluid. This leads to a decrease in ventilation that has developed in response to hypercapnia.

An excessive increase in the activity of pCO 0 and pH receptors contributes to the emergence of subjectively painful, painful sensations of suffocation, lack of air. This is easy to verify if you hold your breath for a long time. At the same time, with a lack of oxygen and a decrease in p0 2 in the arterial blood, when pCO 2 and blood pH are maintained normal, a person does not experience discomfort. This may result in a number of hazards that arise in everyday life or in the conditions of human breathing with gas mixtures from closed systems. Most often they occur during carbon monoxide poisoning (death in the garage, other household poisoning), when a person, due to the lack of obvious sensations of suffocation, does not take protective actions.

Respiratory system. Breath.

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