Levers of power: how the brain controls breathing
Do you like to have everything under control? So you are the same as me. Welcome to the club! It is difficult to assign something to someone, delegate it, let it out of control-all this is difficult. But I will tell you one secret: with all the craving for control, we can only have a very limited influence on most of the processes occurring in our body. Don’t you believe it? Then try to consciously narrow the pupil. Is it not working? You see! When it comes to the main functions of the body, everything happens without our participation: heart rate, activity of the intestines and bladder, blood pressure, gland secretion, pupil diameter, blood supply to the skin, temperature regulation. You were deprived of your powers on the sly, without offering anything in return, without any explanations and negotiations. Someone else has the right, and in this case it is a part of our nervous system, which we call autonomous or vegetative, because it does not lend itself to arbitrary control. And this is good, because the conscious regulation of these processes would eventually exceed the capabilities of even the most perfect control center. Thus, you should only welcome such a division of labor. Leave the issues of personality optimization to your ego and free will, and let everyday routine tasks be solved without your participation. If the words “autonomous nervous system” are associated with hooligans throwing stones, car arsonists and provocateurs at peaceful demonstrations attacking special forces, then you can calm down. Your internal “autonomists” are reliable, correct, accurate, punctual, hardy, loyal and under strict control. In addition, they work around the clock, in three shifts, seven days a week and do not require additional payment. What more could you want?
In the system of separation of powers of our nervous system, there is only one exception — breathing. Here the party of “autonomists” had to make a serious compromise. And there is a reason for this. Breathing performs not only a prosaic function of obtaining energy — it is an important part of social interaction. Without it, speech, singing, shouting, whispering, love courtship, playing musical instruments are impossible. And in all these cases, breathing needs conscious control. Of course, they are not vital, but in the long term they ensure the successful reproduction and preservation of the species. Even the autonomous nervous system has to admit this. That is why there are special, compromise rules for breathing. Both Contracting Parties have published a joint press statement on this issue: “Free will and an autonomous nervous system establish stable and effective control over breathing, which benefits the body in which we peacefully coexist.” Or maybe everything was different? It doesn’t matter. The main thing is that breathing remains the only vital process that can be controlled both autonomously and arbitrarily. And it happens surprisingly harmoniously. And if there are rare failures, then the Little Mermaid from the cartoon is to blame for this. In what way? Patience, you’ll find out now.
Death came in a dream. The patient, who was taken to the neurological department of a local hospital with signs of a stroke, initially seemed to be recovering well. The blood circulation of the medulla oblongata significantly improved after several days of treatment. Everyone agreed that they were dealing with a routine situation. And then there was a dramatic turn: during sleep, the patient stopped breathing and fell into a coma. Immediately, measures were taken for artificial ventilation of the lungs, and the patient was brought to consciousness. But, although the condition stabilized during the day, at night after falling asleep everything started all over again: lethargy, respiratory arrest, coma. And again, equipment was connected to the patient, with the help of which his condition was quickly brought back to normal. The next day, the same thing happened again: while the patient was conscious, he had regular deep breathing, and as soon as he fell asleep, it stopped, as if someone had pulled the cord out of the socket. But the patient did not suffer the next night episode and the next coma. Against the background of oxygen starvation, he had a heart attack, and soon he died. He was struck by the “curse of the Undine” (so we came to the Little Mermaid). American doctors first described this rare and mysterious case in 1962. Soon the phenomenon was repeated in three patients who underwent brain surgery: in the waking state, they all breathed completely normally, but as soon as night fell and they fell asleep, their breathing stopped. If they were not woken up in time, there was a serious lack of oxygen, which threatened life. Such an involuntary loss of the body’s functions reminded doctors of the legend of the mermaid Undine: to ensure the fidelity of a loved one who lived on land, she enchanted him in such a way that in case of treason he lost control of vegetative vital functions. Therefore, doctors who described cases of involuntary nocturnal respiratory arrest called this disease “the curse of Undine”. The character of Undine inspired Hans Christian Andersen to write the fairy tale “The Little Mermaid”, and Walt Disney made a cartoon based on it. Of course, there were no deaths, no stops of breathing, and everyone lived happily until the end of time!
What lies behind this “curse”? The brain controls both voluntary and autonomous breathing, but different parts of it are responsible for this. The main respiratory center of a person is located deep in the brain near its transition to the spinal cord. This is the so-called medulla oblongata, or rather, its part, called the “bridge”. Here are nerve cells that, like a metronome, regularly send impulses that activate breathing, which ensures a calm, even breathing rhythm of 10 to 15 breaths per minute, including during sleep. The command center in the medulla oblongata is connected by nerve fibers of the spinal cord with the respiratory muscles. These fibers at the level of the third cervical vertebra depart from the spinal canal, forming the right and left diaphragmatic nerves that descend through the thoracic cavity to the diaphragm. Therefore, injuries to the cervical spine always pose a threat to life. Unlike a transverse lesion of the spinal cord in the thoracic or cervical region, in which paralysis occurs, here we are talking about a complete failure of breathing. Uncontrolled excitation of these nerves is expressed in such an unpleasant phenomenon as hiccups. It is caused by sudden twitching of the diaphragm under the influence of random nerve impulses.
But the respiratory center is not only a transmitting, but also a receiving station, which is also important for regulating breathing. In particular, it should respond to changes in the body’s need for air depending on physical activity and regulate the respiratory rate accordingly. The respiratory center primarily receives this information from the so — called chemoreceptors-sensors that are located on the walls of the aorta and in the medulla oblongata itself and that respond to changes in the content of carbon dioxide and oxygen in the blood. In addition, there are stretch sensors in large muscle groups that transmit signals to the brain about increased muscle activity so that it increases the respiratory rate. There is a kind of closed circuit of automatic regulation. With increased muscle activity, oxygen consumption increases to replenish energy, and due to this, the production of carbon dioxide increases. Together with other waste products in the muscles, such as lactic acid compounds, carbon dioxide causes an increase in the acidity of the blood. Both factors — a high content of carbon dioxide and a change in the pH index-activate sensors in the aorta and brain, and that, in turn, increases the frequency of respiratory impulses. The diaphragm makes deeper and more frequent movements, as a result of which more carbon dioxide is excreted from the body, and more oxygen enters it. The pH level is normalized. The regulating circuit closes, and the respiratory rate decreases again. Oddly enough, the respiratory center is literally obsessed with carbon dioxide. No matter how important oxygen is for energy production and maintaining the vital functions of organs, all the sensors of the respiratory center take care exclusively of waste disposal, react only to changes in the concentration of carbon dioxide and blood acidity indicators. Fluctuations in the oxygen content do not bother them at all, and there is a good reason for this: almost all metabolic processes in the body occur only at certain pH values. So maintaining their stability is the main task of the medulla oblongata.
In addition, the respiratory center receives nerve impulses from other areas of the brain, in particular from the hypothalamus. This leads to the fact that the nature of breathing involuntarily changes under the influence of emotions such as sadness, joy, excitement, anger, aggression, love. Arbitrary control of breathing is carried out in the cerebral cortex. It is able to make changes to the basic rhythm set by the medulla oblongata when breathing is required for other processes, usually for speech. But if the cerebral cortex is resting (for example, in a dream), then the medulla oblongata automatically takes command. Sometimes, even in one particular cycle of breathing, there is a division of labor: an autonomous inhalation and an arbitrary exhalation. After all, unlike inhalation, which is carried out by actively contracting the diaphragm and expanding the chest, exhalation is almost always a purely passive process: the lungs, chest and diaphragm simply return to their original state, like a stretched spring from which the load has been removed. The brain takes a certain amount of time for this return. In healthy people, the exhalation lasts about twice as long as the inhale. If the process is delayed (for example, due to a decrease in the elasticity of the lungs due to a disease), the brain turns on the mode of active efforts for exhalation in order to stay “on schedule”. The same thing happens with a high respiratory rate, when the body is working under load — in this situation, the duration of a normal passive exhalation would be too long. However, with all the harmony, one thing remains indisputable: the autonomous component of breath control plays a dominant role. Try to hold your breath yourself, as much as possible. In the end, the medulla oblongata will still win.
Violations in the work of the respiratory center are always the most serious diseases. In patients with the “Undine curse”, the structures of the medulla oblongata that control involuntary breathing during sleep are completely or partially destroyed, for example, as a result of a stroke. Injuries, neoplasms and infections can also affect. There is also an innate form of”curse”. If the cerebral cortex regularly performs its functions, then in the waking state it replaces the medulla oblongata. In order not to deprive patients of sleep, at night they have to be connected to an artificial lung ventilation device or an electric stimulator of the diaphragm is installed.
The smooth operation of the medulla oblongata is also important because it not only controls the autonomic functions in sleep, but also controls them. As soon as there are emergency situations (a decrease in blood pressure, pain impulses from various parts of the body, a change in the content of carbon dioxide in the blood), he immediately raises the alarm cortex of the brain, and the person instantly wakes up. Unfortunately, this clever system of” checks and balances ” does not always work perfectly. Like all complex control processes of the central nervous system, it needs development and training. A particularly tragic example of a system failure is the sudden infant death syndrome. In babies, for some unknown reason, the alarm system is broken in case of respiratory failure. Pauses in breathing, which occur regularly in infants and are a sign of the” training ” of the respiratory center, suddenly stop giving the brain a wake-up signal, and the child dies in his sleep for no apparent reason. This is a nightmare for any parent.
The opposite situation can also happen, when the cerebral cortex fails, and the medulla oblongata retains its functions. This happens, for example, with a severe traumatic brain injury or a brain infection. In this state of apallic syndrome, which is also called a waking coma, consciousness completely disappears and patients lose the ability to freely breathe. However, the medulla oblongata continues to work, so there is no need for artificial ventilation of the lungs. The control of respiratory processes by medical personnel is of great importance for patients in a coma. Prolonged absence of respiratory activity indicates irreversible damage to the medulla oblongata. Since this part of the brain usually dies last in severe injuries, the termination of its functions (along with other criteria) allows us to conclude about the final death of the brain and, consequently, to state the death of the patient.
Another, much more frequent, but mostly harmless violation of the control functions of breathing is called hyperventilation syndrome. Emotional or mental arousal, caused, for example, by fear or panic, leads to excessive stimulation of the respiratory center in the medulla oblongata. Deep accelerated breathing reduces the level of carbon dioxide in the blood, and the pH increases, creating an alkaline reaction. The result is convulsions, dizziness and confusion of consciousness. These symptoms further increase the feeling of fear in the hypothalamus, and a vicious circle occurs. If the patient cannot calm down on his own, then re-inhalation of exhaled carbon dioxide helps to normalize his condition (it is enough to put a plastic bag to his mouth and breathe from it). The symptoms disappear, and the emotional excitement subsides. Thus, if your boss starts “hyperventilating” again, treat it leniently — perhaps it’s all about the hypothalamus. In such cases, an energetic exclamation is enough: “Hold your breath!” Such an order should not be perceived as inappropriate audacity, but as a valuable medical recommendation that replaces the use of a plastic bag: thanks to this, carbon dioxide temporarily ceases to be removed from the body, its content in the blood is normalized, and the condition returns to normal. The cerebral cortex intervenes in the process, breaking the chain between the hypothalamus and the medulla oblongata. This example demonstrates that when it comes to the vegetative consequences of an emotional outburst, one should not watch what is happening indifferently. You can take over the command functions of the nervous system and influence the situation. The close connection of emotions, the autonomous nervous system and the arbitrary control of breathing opens up wide opportunities. At least one of the elements of this chain is under your personal control! It is only necessary to master the techniques with which you can influence your own well-being, to calm the autonomic nervous system with a conscious effort. But more on this later.
Let’s go back to the lungs. Are they really not involved in the management of breathing at all? Does this mean that the lungs are only an executive organ — an amorphous sponge, which, under the influence of uniform movements of the diaphragm, repeatedly expands and contracts in a rhythm set by the medulla oblongata? I must agree that at first glance, the lungs are not characterized by special nervousness, they are insensitive to pain and touch. If, for example,you feel pain in the chest, then it hurts the rib pleura, bones or chest muscles, but not the lungs. However, the lungs are by no means a cold and indifferent organ, but, on the contrary, very sensitive. However, all sentiments and manifestations of feelings occur without the participation of the public — in the subconscious.
The intertwining of the pulmonary nerves is part of the autonomic nervous system. The nerves of the lungs, like the nerves of the diaphragm, originate in the spinal cord of the cervical spine. From there, they descend on both sides of the spine into the thoracic cavity to the left and right gates of the lungs, through which the main bronchi, as well as the pulmonary arteries and veins pass. Nerve fibers branch together with the bronchi and blood vessels, creating a thick interweaving. Signals from the brain to the lungs are distributed along one part of the nerve fibers, and information from the lungs to the brain is distributed along the other. In the first case, the nerves perform command functions, and in the second — sensitive ones. The command nerves control the work of the ring muscles located on the bronchi and mucous glands. Some of them activate the muscles, and some of them slow down. These two subgroups are called sympathetic and parasympathetic nerve fibers, and they are the main antipodes of the autonomous nervous system-like yin and yang, plus and minus, North and South poles. Sympathetic nerves represent the gas pedal, and parasympathetic nerves represent the brake pedal. Sympathetic nerves are activated during typical anxiety reactions (such as “fight or flight”) and guarantee maximum physical return in dangerous situations. At the same time, there is an acceleration of the pulse, an increase in blood pressure, maximum burning of glucose in the muscles, increased breathing — pure adrenaline! When the sympathetic nerves speak, the parasympathetic ones are silent, at least temporarily. Parasympathetic nerves regulate the body’s functions that are important for regeneration and growth. For them, it is important to be calm, completely calm. The heart rate, blood pressure and blood supply to the muscles slow down, but the digestive processes and the secretion of glands in the gastrointestinal tract are activated. The duration of the parasympathetic system is night, rest in a dream. If we do not take into account the extreme states — gas and braking, the sympathetic and parasympathetic systems are in harmonious equilibrium with respect to each other. Each of them acts half-heartedly, creating a comfortable speed of movement as a result.
How do sympathetic and parasympathetic nerves interact in the respiratory tract? Activation of sympathetic nerve fibers causes relaxation of the bronchial muscles, so that they expand to allow the maximum amount of air to pass. At the same time, the secretion of the mucous glands slows down. Stimulation of the parasympathetic nerves causes the opposite effect: the bronchi narrow due to the tension of the annular muscles, and the glands of the mucous membrane are involved in active work. Relaxation of the bronchial muscles can be carried out not only through the endings of the sympathetic nerves, but also through the adrenaline contained in the blood. With an alarming reaction, the stress hormone adrenaline is released into the blood from the adrenal glands in large quantities, which puts the entire body in a state of “combat readiness”. However, the effect of bronchial expansion through the sympathetic system in healthy people is almost imperceptible, since their bronchial diameter is already optimal at rest and therefore can hardly be at least somewhat noticeably increased. Then why all this? The system of sympathetic activation plays the role of just a “safety cushion”, a kind of insurance against hasty reactions of the parasympathetic system. The parasympathetic nerve, in addition to providing a general soporific effect, controls the protective reflexes of the bronchi, which should prevent harmful substances from entering the lower respiratory tract, for example, makes the bronchi narrow and wash out foreign bodies with mucus. This method is unsafe, because if the parasympathetic nervous system uses this reflex too zealously, in certain circumstances it can cause more harm than the extraneous body itself. In an extreme case, the result will be a spasmodic overlap of all the bronchi and death from suffocation. The oppositely directed action of the sympathetic system prevents excessive narrowing of the bronchi and restores the original balance. Peace and friendship! Wherever you look, the principle of balance, homeostasis reigns everywhere. The balance and harmony of biological functions guarantee the long-term maintenance of the body’s health.
But what if everything goes wrong? If there is a disorder in the relations of the sympathetic and parasympathetic systems? Then people will begin to suffer from narrowing of the airways, as is the case with asthma and COPD. One of the main causes of both diseases is the dominance of the parasympathetic nervous system, which causes narrowing of the bronchi and increased secretion of the mucous glands. And since this dominance increases at night, many patients suffer from coughing and characteristic wheezing all night. But the German poet Gelderlin already knew: where there is danger, there is salvation! Doctors apply their knowledge of the chemical processes used to transmit signals from the sympathetic system to the parasympathetic, and try to restore the disturbed balance with their help. The effect of one of the oldest drugs for asthma — datura and belladonna-is based on the pharmacological suppression of signals of the parasympathetic nervous system: the endings of the parasympathetic nerves are simply turned off, and the reflex spasm is weakened. As an alternative treatment, an oppositely directed action of the sympathetic system can be used. Already at the beginning of the XX century, doctors began to use chemicals that resemble adrenaline to expand the bronchi. Today, almost all therapy aimed at expanding the bronchi is based on these two principles: suppression of parasympathetic nerves (with the help of parasympatholytics) and activation of sympathetic nerves (with the help of sympathomimetics).
This is all about the command functions of the nervous system. And what about the nerve endings in the lungs that collect information? It’s even more interesting here, much is still unclear. One thing is for sure: there is a “Reply” button in the e-mail of the lungs, and with its help a huge amount of information is sent to the brain. Apparently, this feedback is extremely important for our computing center: 20 percent of the fibers of the powerful vagal nerve, which accumulates information from all internal organs for its subsequent transmission to the brain, come from the lungs. Thus, a fifth of all data comes from an organ that does not feel anything, does not experience pain, does not feel pressure.
What do the lungs transmit through their channels? Empty gossip? Or are we dealing with unused excess capacity? By no means, no less information comes from the lungs than from the senses, but all this information is processed by the brain in the subconscious. However, there is an exception: irritation leading to a reflex cough, or lack of air are perceived directly, as well as signals from the sensory organs. But the information processed by the subconscious mind affects other autonomous functions of the body, for example, blood pressure, heart rate, digestion, sweating, the manifestation of emotions… And also on mental processes.
What kind of information do the lungs send, if we are not talking about optical and acoustic signals, pain or tactile sensations? Almost all of these signals are of a chemical or physical nature. Although the breathing process looks monotonous, none of the 15 breaths we take per minute is like another, because every liter of inhaled air is special. The lungs treat the air not as a consumer, but as a subtle connoisseur. Like a sommelier who finds in a tiny sip of wine the flavors of an oak barrel, earth, apricot, peach, cigar and wet skin, the lungs in the inhaled air emit parameters such as temperature, humidity, salt content, pH, and gas composition. In addition, the air may contain irritating and harmful substances, foreign particles, allergens. In the lungs, as well as on the tongue and in the nose, there are taste buds and smell receptors. They can detect the products of bacterial metabolism and determine the taste of many poisons. They have the same receptors that perceive, for example, the refreshing aroma of vegetable essential oils in the nose and mouth. But since the processing of signals from these receptors in the lungs takes place without the participation of consciousness, we can only guess what effect the stimuli have on the respiratory tract and the autonomous nervous system. What is indisputable is that very sensitive nerves are needed to recognize, distinguish and measure all these components. And there are enough of them in the lungs.
The sensitive nerve fibers of the lungs begin where it is possible to collect maximum information: in the bronchial muscles, glands, alveoli and, above all, in the epithelium. The main events take place here. Why lay transmission lines from connective tissue cells, in which nothing happens, if life is raging nearby? The epithelium of the respiratory tract offers the best and most diverse program. There are regular troubles and scandals that ensure the highest audience rating! Not all fibers transmit sensations from the epithelium, some have to be content with the boring but important work of tissue stretching sensors. Their signals are of great importance, because they literally protect the lungs from ruptures. When the lungs reach a certain degree of stretching under the influence of the diaphragm, the sensors send a stop signal to the respiratory center of the medulla oblongata. The brain, in turn, stops the contraction of the diaphragm and gives a signal for the beginning of exhalation. The main thing is not to tear anything. Athletes know how important stretching is as a means of preventing injuries. The lungs also perform spontaneous stretching from time to time — during yawning. If breathing is calm and shallow for a long time, then the stretching sensors begin to get bored and cause a yawning reflex. Just like we arrange a draft at home to quickly ventilate the rooms.
But let’s return to the nerve endings of the epithelium of the respiratory tract. There is a dense network of receptors that react to chemical and physical stimuli, which can be dust particles, substances dissolved in water vapor, bacterial waste products, hydrochloric acid, capsaicin, responsible for the burning taste of chili pepper, mucus, as well as signaling substances of the immune system released during inflammation, and even cold and heat. The nerves report everything that affects them. In a healthy state, their sensitive endings are protected by the epithelium of the respiratory tract, but if it is damaged, the endings lose their protection, protrude above the surface and begin to react to irritations. The most common causes of epithelial damage are cold viruses and inflammation that occur, for example, as a result of allergies, infections or contact with harmful substances. In this case, the sensitive nerve endings send alarm signals to the brain, which responds to them with protective reflexes of the bronchi, eliminating the cause of irritation or preventing its spread to deeper parts of the respiratory tract. Such reflex reactions include coughing, mucus production and spasm of the bronchial muscles.
Sensitive nerve endings of the epithelium are especially interesting in terms of studying chronic respiratory diseases. According to their functions in the bronchi, they surprisingly resemble the receptors that fix skin damage — nociceptors. The task of the latter is to warn the brain about the impending damage to the skin as a result of external influence. By creating a painful sensation, they provoke an immediate reaction, for example, pulling your hand away from a hot kitchen stove. In the respiratory tract in such cases, instead of pain, a cough occurs. Like the pain receptors of the skin, sensitive nerve endings in the lungs can be subjected to constant irritation. If in the first case there are chronic pains, then in the second case there is a chronic endless cough that can last for several months. It is still unclear how the disturbed cough reflex can be normalized. But the fact that there is a fundamental possibility to manipulate the degree of excitability of nerve endings is proved by smokers. The initial reflex cough disappears over time, otherwise everything would have ended already on the first cigarette. Another aspect that attracts attention is observed in novice smokers: despite the strong cough that occurs when smoking the first cigarette, there is practically no spasmodic narrowing of the bronchi. Thus, all three reflex reactions may not necessarily appear, sometimes they share responsibilities among themselves. This is also confirmed by the daily observations of practicing doctors: only a few asthmatics, along with narrowing of the bronchi, suffer from a strong cough. With bronchitis, some patients have a dry cough, and others have excessive mucus formation. Why this happens, we do not yet know.
But there is more and more new knowledge in this area. For example, countless chemical compounds have an irritating effect on the nerves of the lungs. There is not enough accurate information yet, whether it is positive or negative, but one thing can be said with certainty: an increasingly close study of brain activity will help to understand the complex relationships between the lungs and the respiratory center. Perhaps it is the nine-tenths of the iceberg that we have not yet seen that will help logically link together all the factors of health and lung diseases. The lungs, as a sensory organ, undoubtedly need further study. There is still a long way to go before we learn how to correct violations in their work with the help of appropriate ” glasses “or”hearing aids”.