Protective airway reflexes: sneezing and coughing (briefly). Reflex regulation of breathing Protective respiratory reflexes include

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The nervous system usually sets such alveolar ventilation rate, which almost exactly matches the body's needs, so the tension of oxygen (Po2) and carbon dioxide (Pco2) in arterial blood changes little even during severe physical activity and during most other cases of respiratory stress. This article outlines neurogenic system function regulation of breathing.

Anatomy of the respiratory center.

Respiratory center consists of several groups of neurons located in the brain stem on either side of the medulla oblongata and the pons. They are divided into three large groups of neurons:

dorsal group of respiratory neurons, located in the dorsal part of the medulla oblongata, which mainly causes inspiration;
ventral group of respiratory neurons, which is located in the ventrolateral part of the medulla oblongata and mainly causes exhalation;
pneumotaxic center, which is located dorsally at the top of the pons and controls mainly the rate and depth of breathing. The dorsal group of neurons plays the most important role in the control of breathing, so we will consider its functions first.

Dorsal group respiratory neurons extend over most of the length of the medulla oblongata. Most of these neurons are located in the nucleus of the solitary tract, although additional neurons located in the nearby reticular formation of the medulla oblongata are also important for the regulation of breathing.

The nucleus of the solitary tract is the sensory nucleus For wandering And glossopharyngeal nerves, which transmit sensory signals to the respiratory center from:

peripheral chemoreceptors;
baroreceptors;
different types of lung receptors.

Generation of respiratory impulses. Breathing rhythm.

Rhythmic inspiratory discharges from the dorsal group of neurons.

Basic breathing rhythm generated mainly by the dorsal group of respiratory neurons. Even after all peripheral nerves entering the medulla and the brainstem below and above the medulla have been cut, this group of neurons continues to generate repeated bursts of action potentials from inspiratory neurons. The underlying cause of these volleys is unknown.

After some time, the activation pattern is repeated, and this continues throughout the life of the animal, so most physiologists involved in respiratory physiology believe that humans also have a similar network of neurons located within the medulla oblongata; it is possible that it includes not only the dorsal group of neurons, but also the adjacent parts of the medulla oblongata, and that this network of neurons is responsible for the basic rhythm of breathing.

Increasing inspiratory signal.

Signal from neurons that is transmitted to inspiratory muscles, mainly the diaphragm, is not an instantaneous burst of action potentials. During normal breathing it gradually increases for about 2 seconds. After that he declines sharply for about 3 seconds, which stops the excitation of the diaphragm and allows the elastic traction of the lungs and chest wall to exhale. Then the inspiratory signal begins again, and the cycle repeats again, and in the interval between them there is an exhalation. Thus, the inspiratory signal is a rising signal. Apparently, this increase in signal ensures a gradual increase in lung volume during inspiration instead of sudden inspiration.

Two moments of the rising signal are monitored.

The rate of increase of the rising signal, so during difficult breathing the signal grows quickly and causes rapid filling of the lungs.
A limiting point at which the signal suddenly disappears. This is a common way of controlling the rate of breathing; The sooner the increasing signal stops, the shorter the duration of inspiration. At the same time, the duration of exhalation is reduced, as a result, breathing becomes more frequent.

Reflex regulation of breathing.

Reflex regulation of breathing is carried out due to the fact that the neurons of the respiratory center have connections with numerous mechanoreceptors of the respiratory tract and alveoli of the lungs and receptors of vascular reflexogenic zones. The following types of mechanoreceptors are found in the human lungs:

irritant, or rapidly adapting, receptors of the mucous membrane of the respiratory tract;
Stretch receptors of smooth muscles of the respiratory tract;
J-receptors.

Reflexes from the mucous membrane of the nasal cavity.

Irritation of irritant receptors of the nasal mucosa, for example, tobacco smoke, inert dust particles, gaseous substances, water causes narrowing of the bronchi, glottis, bradycardia, decreased cardiac output, narrowing of the lumen of blood vessels in the skin and muscles. The protective reflex occurs in newborns when briefly immersed in water. They experience respiratory arrest, preventing water from entering the upper respiratory tract.

Reflexes from the pharynx.

Mechanical irritation of the receptors of the mucous membrane of the posterior part of the nasal cavity causes a strong contraction of the diaphragm, external intercostal muscles, and, consequently, inhalation, which opens the airway through the nasal passages (aspiration reflex). This reflex is expressed in newborns.

Reflexes from the larynx and trachea.

Numerous nerve endings are located between the epithelial cells of the mucous membrane of the larynx and main bronchi. These receptors are irritated by inhaled particles, irritating gases, bronchial secretions, and foreign bodies. All this causes cough reflex, manifested in a sharp exhalation against the background of a narrowing of the larynx and contraction of the smooth muscles of the bronchi, which persists for a long time after the reflex.
The cough reflex is the main pulmonary reflex of the vagus nerve.

Reflexes from bronchiole receptors.

Numerous myelinated receptors are located in the epithelium of the intrapulmonary bronchi and bronchioles. Irritation of these receptors causes hyperpnea, bronchoconstriction, laryngeal contraction, and hypersecretion of mucus, but is never accompanied by a cough. Receptors are most sensitive to three types of stimuli:

tobacco smoke, numerous inert and irritating chemicals;
damage and mechanical stretching of the airways during deep breathing, as well as pneumothorax, atelectasis, and the action of bronchoconstrictors;
pulmonary embolism, pulmonary capillary hypertension and pulmonary anaphylactic phenomena.

Reflexes from J-receptors.

In the alveolar septa are in contact with capillaries special J receptors. These receptors are especially sensitive to interstitial edema, pulmonary venous hypertension, microembolism, irritant gases and inhaled narcotic substances, phenyl diguanide (with intravenous administration of this substance).

Stimulation of J receptors initially causes apnea, then superficial tachypnea, hypotension and bradycardia.

Hering-Breuer reflex.

Inflation of the lungs in an anesthetized animal reflexively inhibits inhalation and causes exhalation. Transection of the vagus nerves eliminates the reflex. Nerve endings located in the bronchial muscles play the role of lung stretch receptors. They are classified as slowly adapting stretch receptors of the lungs, which are innervated by myelinated fibers of the vagus nerve

The Hering-Breuer reflex controls the depth and frequency of breathing. In humans, it has physiological significance for tidal volumes greater than 1 L (e.g. during physical activity). In an awake adult, short-term bilateral vagus nerve blockade using local anesthesia does not affect the depth or rate of breathing.
In newborns, the Hering-Breuer reflex clearly manifests itself only in the first 3-4 days after birth.

Proprioceptive control of breathing.

Receptors in the joints of the chest send impulses to the cerebral cortex and are the only source of information about chest movements and tidal volumes.

The intercostal muscles, and to a lesser extent the diaphragm, contain a large number of muscle spindles. The activity of these receptors is manifested during passive muscle stretching, isometric contraction and isolated contraction of intrafusal muscle fibers. Receptors send signals to the corresponding segments of the spinal cord. Insufficient shortening of the inspiratory or expiratory muscles increases impulses from muscle spindles, which dose muscle force through motor neurons.

Chemoreflexes of breathing.

Partial pressure of oxygen and carbon dioxide(Po2 and Pco2) in the arterial blood of humans and animals is maintained at a fairly stable level, despite significant changes in O2 consumption and CO2 release. Hypoxia and decreased blood pH ( acidosis) cause increased ventilation(hyperventilation), and hyperoxia and increased blood pH ( alkalosis) - decreased ventilation(hypoventilation) or apnea. Control over the normal content of O2, CO2 and pH in the internal environment of the body is carried out by peripheral and central chemoreceptors.

An adequate stimulus for peripheral chemoreceptors is decrease in arterial blood Po2, to a lesser extent, an increase in Pco2 and pH, and for central chemoreceptors - an increase in the concentration of H+ in the extracellular fluid of the brain.

Arterial (peripheral) chemoreceptors.

Peripheral chemoreceptors found in the carotid and aortic bodies. Signals from arterial chemoreceptors along the sinocarotid and aortic nerves initially arrive at the neurons of the nucleus of the solitary fasciculus of the medulla oblongata, and then switch to the neurons of the respiratory center. The response of peripheral chemoreceptors to a decrease in Pao2 is very rapid, but nonlinear. With Pao2 in the range of 80-60 mm Hg. (10.6-8.0 kPa) there is a slight increase in ventilation, and when Pao2 is below 50 mm Hg. (6.7 kPa) severe hyperventilation occurs.

Paco2 and blood pH only potentiate the effect of hypoxia on arterial chemoreceptors and are not adequate stimuli for this type of respiratory chemoreceptors.
Response of arterial chemoreceptors and respiration to hypoxia. Lack of O2 in arterial blood is the main irritant of peripheral chemoreceptors. Impulse activity in the afferent fibers of the sinocarotid nerve stops when Pao2 is above 400 mmHg. (53.2 kPa). In normoxia, the frequency of discharges of the sinocarotid nerve is 10% of their maximum reaction, which is observed at Pao2 of about 50 mm Hg. and below. The hypoxic respiratory reaction is practically absent in the indigenous inhabitants of the highlands and disappears approximately 5 years later in the inhabitants of the plains after the beginning of their adaptation to the highlands (3500 m and above).

Central chemoreceptors.

The location of the central chemoreceptors has not been definitively established. Researchers believe that such chemoreceptors are located in the rostral parts of the medulla oblongata near its ventral surface, as well as in various areas of the dorsal respiratory nucleus.
The presence of central chemoreceptors is proven quite simply: after transection of the sinocarotid and aortic nerves in experimental animals, the sensitivity of the respiratory center to hypoxia disappears, but the respiratory response to hypercapnia and acidosis is completely preserved. Transection of the brainstem immediately above the medulla oblongata does not affect the nature of this reaction.

An adequate stimulus for central chemoreceptors is change in H* concentration in the extracellular fluid of the brain. The function of the regulator of threshold pH shifts in the area of central chemoreceptors is performed by the structures of the blood-brain barrier, which separates the blood from the extracellular fluid of the brain. Through this barrier, O2, CO2 and H+ are transported between the blood and the extracellular fluid of the brain. The transport of CO2 and H+ from the internal environment of the brain into the blood plasma through the structures of the blood-brain barrier is regulated with the participation of the enzyme carbonic anhydrase.
Respiration response to CO2. Hypercapnia and acidosis stimulate, and hypocapnia and alkalosis inhibit central chemoreceptors.

The airways are divided into upper and lower. The upper ones include the nasal passages, nasopharynx, the lower ones include the larynx, trachea, and bronchi. The trachea, bronchi and bronchioles are the conducting zone of the lungs. The terminal bronchioles are called the transition zone. They have a small number of alveoli, which make a small contribution to gas exchange. The alveolar ducts and alveolar sacs belong to the exchange zone.

Nasal breathing is physiological. When inhaling cold air, a reflex dilation of the vessels of the nasal mucosa and a narrowing of the nasal passages occur. This promotes better air heating. Its hydration occurs due to moisture secreted by the glandular cells of the mucous membrane, as well as tear moisture and water filtered through the capillary wall. Air purification in the nasal passages occurs due to the settling of dust particles on the mucous membrane.

Protective breathing reflexes occur in the airways. When inhaling air containing irritating substances, a reflex slowdown occurs and a decrease in the depth of breathing. At the same time, the glottis narrows and the smooth muscles of the bronchi contract. When the irritant receptors of the epithelium of the mucous membrane of the larynx, trachea, and bronchi are irritated, impulses from them arrive along the afferent fibers of the upper laryngeal, trigeminal and vagus nerves to the inspiratory neurons of the respiratory center. A deep breath takes place. Then the muscles of the larynx contract and the glottis closes. Expiratory neurons are activated and exhalation begins. And since the glottis is closed, the pressure in the lungs increases. At a certain moment, the glottis opens and air leaves the lungs at high speed. A cough occurs. All these processes are coordinated by the cough center of the medulla oblongata. When dust particles and irritating substances affect the sensitive endings of the trigeminal nerve, which are located in the nasal mucosa, sneezing occurs. When sneezing, the inhalation center is also initially activated. Then a forced exhalation occurs through the nose.

There are anatomical, functional and alveolar dead space. Anatomical is the volume of the airways - the nasopharynx, larynx, trachea, bronchi, bronchioles. No gas exchange occurs in it. Alveolar dead space refers to the volume of alveoli that are not ventilated or there is no blood flow in their capillaries. Therefore, they also do not participate in gas exchange. Functional dead space is the sum of anatomical and alveolar. In a healthy person, the volume of alveolar dead space is very small. Therefore, the size of the anatomical and functional spaces is almost the same and is about 30% of the respiratory volume. On average 140 ml. In violation of ventilation and blood supply to the lungs, the volume of functional dead space is much larger than the anatomical one. However, the anatomical dead space plays an important role in the processes of respiration. The air in it is warmed, humidified, and cleaned of dust and microorganisms. Here respiratory protective reflexes are formed - coughing, sneezing. It is where smells are perceived and sounds are produced.

Sneezing- this is an unconditioned reflex, with the help of which dust, foreign particles, mucus, vapors of caustic chemicals, etc. are removed from the nasal cavity. Due to this, the body prevents them from entering other respiratory tracts. The receptors for this reflex are located in the nasal cavity, and its center is in the medulla oblongata. Sneezing can also be a symptom of an infectious disease accompanied by a runny nose. With a stream of air from the nose, when chi-hani, a lot of viruses and bacteria are thrown out. This frees the body from infectious agents, but contributes to the spread of infection. That's why, When you sneeze, be sure to cover your nose with a tissue.

Cough- it is also a protective unconditioned reflex, aimed at removing dust, foreign particles through the oral cavity, if they got into the larynx, pharynx, trachea or bronchi, sputum, which is formed during inflammation of the respiratory tract. Sensitive cough receptors are found in the mucous membrane of the respiratory tract. Its center is in the medulla oblongata. Material from the site

In smokers, the protective cough reflex is first strengthened through irritation of its receptors by tobacco smoke. That's why they cough constantly. However, after some time, these receptors die along with the ciliary and secretory cells. The cough disappears, and the mucus that smokers continuously produce is retained in the airways, which are deprived of protection. This leads to severe inflammatory lesions of the entire respiratory system. Smoker's chronic bronchitis occurs. A person who smokes snores loudly during sleep due to the accumulation of mucus in the bronchi.

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Questions about this material:

It has now been established that irritation of any visceral or somatic nerves can affect breathing and that many afferent pathways are involved in respiratory reflexes. There are at least nine respiratory reflexes arising from the chest organs, and five of them are fairly well appreciated and deserve special mention.

Bloat reflex(Hering-Breuer). Hering and Breuer showed in 1868 that while maintaining the lungs in an inflated state reduces the respiratory rate in anesthetized animals, maintaining the lungs in a collapsed state has the opposite effect. Vagotomy prevents the development of these reactions, which proves their reflex origin; Adrian in 1933 showed that this reflex is carried out through stretch receptors in the lung, which are not encapsulated and are believed to be smooth muscle endings, usually located in the walls of the bronchi and bronchioles. The swell reflex is present in newborns, but weakens over the years. Its importance faded into the background when the role of chemical regulation of respiration was established. Currently, it is considered only as one of the many chemical and neural mechanisms that regulate breathing. Apparently, it affects the tone of the bronchial muscles.

Fall reflex. Collapse of the lungs stimulates respiration by activating a group of receptors believed to be located in or distal to the respiratory bronchioles. The exact role of the collapse reflex is difficult to determine, since collapse of the lungs also changes breathing through many other mechanisms. Although the extent of the influence of the collapse reflex in normal breathing is unclear, it is likely to be important in forced collapse of the lung and in atelectasis, the frequency and force of inspiration being increased by its action in these circumstances. Vagotomy usually relieves the collapse reflex in animals.

Paradoxical reflex. Head in 1889 showed that inflation of the lungs in rabbits with partial blockade of the vagus nerve (during the recovery period after freezing) does not give an inflation reflex, but, on the contrary, leads to a prolonged and powerful contraction of the diaphragm. The reflex is relieved by crossing the vagus and, since its action is the opposite of that of the normal inflation reflex, it is called “paradoxical.” Two observations support a possible physiological role for the paradoxical reflex. Occasional deep breaths, which punctuate normal quiet breathing and appear to prevent microatelectasis that might otherwise occur, disappear after vagotomy and are thought to be associated with the paradoxical reflex. Cross et al. observed convulsive sighs when the lungs of newborns were inflated in the first 5 days. They suggested that the mechanism in this case is similar to the paradoxical reflex and may provide aeration of the newborn lung.

Irritation reflexes. The cough reflex is associated with subepithelial receptors in the trachea and bronchi. Clusters of these receptors are usually present on the posterior wall of the trachea and bronchial bifurcations (up to the proximal end of the respiratory bronchioles) and are most numerous in carina. In order to perform a good bronchoscopy under local anesthesia, sufficient anesthesia of the tracheal bifurcation is essential.

Inhalation of mechanical or chemical irritants leads to reflex closure of the glottis and bronchospasm. There is likely a peripheral internal reflex arc in the bronchial wall with a central component acting through the vagus nerve.

Pulmonary vascular reflex. An increase in pressure in the vessels of the lungs of cats and dogs leads to the appearance of accelerated shallow breathing in combination with hypotension. This effect can be prevented by vagotomy and it manifests itself more when the venous rather than the arterial bed is stretched. The exact location of the receptors has not yet been established, although recent evidence suggests that they are located in the pulmonary veins or capillaries.

With multiple pulmonary embolism in animals and humans, prolonged, rapid, shallow breathing occurs. In animals, this effect is reversed by vagotomy. As well as this respiratory reflex, embolism causes many other changes that affect breathing. These include a drop in blood pressure and increased heart rate, generalized pulmonary vasospasm and possible edema, decreased lung compliance and increased resistance to air flow. Since the administration of 5-hydroxytryptamine closely resembles the action of embolism, it is believed that this substance is released during the formation of vascular thrombi, probably from platelets. That this is not a complete explanation is supported by the fact that anti-5-hydroxytryptamine drugs are only partially effective in reversing embolic events.

Reflexes in the upper respiratory tract. They are primarily protective. Sneezing and coughing are pronounced reflex efforts. Sneezing is a reaction to irritation in the nose, but can also occur when a bright light suddenly falls on the retina. Coughing is a reaction to irritation of the parts located downward from the pharynx. The closure (gag) reflex prevents unwanted substances from entering the esophagus, but at the same time the glottis also closes. There are reports that bronchoconstrictor inhibitory cardiac activity and vasomotor reflexes occur as a result of irritation of the nose or pharynx.

Other breathing reflexes. Reflexes from the respiratory muscles, tendons and joints, from the heart and systemic circulation, from the digestive tract, from pain and temperature receptors, as well as some postural reflexes, can all affect breathing. A well-known example is the gasping for air after sudden exposure to cold on the skin.

For a detailed description of respiratory reflexes, we refer the reader to the Widdicombe review.