 Avalangas are very carefully placed in the rib cages, they are attached to the muscles of the ribs called the intercostal muscles and we have a very important muscle called the diaphragm which is underneath the rib cages. Now these two muscles play a vital role in breathing, we have learnt about it in the mechanism of breathing. Now these muscles they contract and relax and that is how we inhale and exhale. But just like any muscle of our body, these muscles cannot operate or cannot contract without the signals from the brain. Now before I tell you which part of the brain does that, let me ask you something. What do you think breathing is? Is it a voluntary process? That means do you have control over it or is it an involuntary process? What do you think? Okay, let me help you. Think of when you are swimming underwater, you can hold your breath for some time on your own will, right? That means you have control over breathing, that means breathing is voluntary. But then again think of while you are sleeping. Do you put effort to breathe when you sleep? Breathing happens without putting in any conscious effort, right? So that means breathing is involuntary. Okay, did I confuse you? If I did then let me clear it now. The answer to my question is that breathing is both voluntary and involuntary. But in this video we will limit our discussions to only the involuntary regulation of breathing because that is what the syllabus is asking, okay? So let's begin the involuntary control of breathing and let me show you which part of the brain takes care of that. So as you can see on your screen, the highlighted part which is the lower part of the brain which is called pons and medulla oblongata, they have special group of neurons in them that controls the muscles that helps you breathe. The first group of neuron I would like to mention here is the dorsal respiratory group which is present in the medulla oblongata region. And it has nerve connections to the muscles that helps you breathe. That is to the diaphragm and the intercostal muscles. And this DRG has a very special power. It can constantly fire nerve impulses to the muscles of breathing that is only to contract. It do not need the help of any, any other part of the brain to fire nerve impulse to these muscles. And contraction of these muscles means inhalation. But the contraction has to stop somewhere, right? So that the person can exhale. Now these muscles will exhale only if the nerve impulse stops coming into these intercostal muscles. And to switch off these signals there is a special center in the pons called the pneumotexic center. It is again a group of neurons which has the power to only switch off the DRGs. It can only switch it off because it is always switched on all by itself. So there is this pneumotexic center which can only switch it off. And that too for not more than 3 to 5 seconds. And if there is no signal from, from the brain the intercostal muscles will relax and that will lead to exhalation. Now the pneumotexic center switches off the DRGs for at least 12 to 16 times per minute. And that is how exhalation takes place. And again that is how we get a constant rate of breathing. Inhalation and exhalation. Inhalation and exhalation. And again I would like to repeat what I just said that DRGs they only send signal to contract. That means it only sends signal to inspire, okay, only to take in the air. Because when the signal stops the relaxation will take place on its own. It is just like an elastic recoiling and when the signals are switched off expiration will take place on its own, okay. So I hope you got the gist of how your brain or your nervous system has control over your breathing. It regulates your breathing, okay. Now let us consider a different situation. Let's imagine that you are involved in physical exercise. Now while exercising you must have noticed that your breathing pattern changes. You must have noticed that you start breathing really very fast. Now why do you think that happens? Because again you are not doing that on purpose. Your breathing rate increases on its own. It is involuntary. Well it is found that while you are engaged in physical activities your muscles are constantly using a lot of energy and sending out a lot of carbon dioxide into the blood. Now you will say that oh what's big deal in that our muscles or our body tissues anyways does that. It removes the carbon dioxide into the blood. But here you are releasing carbon dioxide faster than you would otherwise do in your resting state. Now this increased carbon dioxide in the blood is actually not good. It decreases the pH of the blood. It makes it very acidic. Because carbon dioxide when it mixes with water it dissociates into hydrogen iron and bicarbonate iron. And the increased level of hydrogen iron is what increases the acidity of the blood. So to tackle the problem of the increased hydrogen iron concentration or let's just say increased carbon dioxide concentration in the blood there are various sensors in our body and one group of sensors are present in the arteries of our heart. So those arteries are carotid artery and it is also present in this arc which is in the aorta which is called the aortic arc. So all these sensors they can sense the increased level of hydrogen iron and carbon dioxide in the blood and they send the signal immediately to this lower part of the brain. Since they can sense the change in the chemistry of the blood they are also known as chemoreceptors. So we found one group of these chemoreceptors in the arteries of the heart and another group of chemoreceptors is found near the medulla oblongata. Let me draw it with some yellow dots okay it's present near the medulla oblongata. And they sense the changed chemistry of the cerebrospinal fluid because there are certain parts of the brain where blood cannot reach. The blood transfers the components which is required for the brain into the fluid which is present in the brain which we call the cerebrospinal fluid. And these receptors the chemoreceptors here they sense the change chemistry in that particular fluid. So we got to see that there are two groups of chemoreceptors one in the medulla and another in the carotid artery and aortic arc. Now after sensing the acidity of the blood what action does it take? The chemoreceptors will send signal to the pneumotexic center and guess what the pneumotexic center does? It comes to the rescue. It starts switching off the DRG's faster than it was otherwise doing. It will switch it off more frequently than it was doing in the resting state. Well what happens then? Well then we will inhale and exhale faster than we were otherwise doing. So that more oxygen can rapidly get into our blood because our body is in need of it, right? So here the pneumotexic center did two things. First it increased the rate of breathing and second it also increased the depth of breathing. Do you see the depth is more in most places? Well it is because the body wants to grasp more oxygen in less time. Okay so this was all about the regulation of breathing. Now let's quickly summarize what we discussed in this video. So in this video we discussed about the involuntary regulation of breathing and we saw that there is both nervous control or neural control and chemical control over breathing. Talking of the neural control we saw that there are two centers in the lower part of the brain, one in the pons and the other in the medulla. The one in the medulla is called the dorsal respiratory group which is always switched on. It always fires nerve impulses to the intercostal muscles causing it to contract and that is how inhalation takes place. And to exhale the pneumotexic center present in the pons switches off the DRG for some time and that is how we exhale, okay? Now coming to the chemical control, the chemical control comes into play when there is an increased level of carbon dioxide or hydrogen ion in the blood or in the cerebrospinal fluid. It basically takes care of the acidity or pH of the blood. So the one group of receptors or sensors which are present near the medulla or blondata region that senses the increased level of acidity in the cerebrospinal fluid is called the central chemoreceptor. And another one which is present in the carotid artery and aortic arc is called the peripheral chemoreceptors and they both report to the pneumotexic center which is present in the pons. This was all about the involuntary regulation of breathing.