 Hey everybody, Dr. O here. In this video I'm going to introduce the two main divisions of the autonomic nervous system. So remember the autonomic nervous system is a visceral motor system so it controls the organs of the thoracic and abdominal pelvic cavity and that can be further broken down into the sympathetic division which is known as the fight or flight system and the parasympathetic division which is rest and digest. We're going to go into much more detail later but I just want to kind of compare and contrast the two here together before we do separate videos on each of those divisions. So a really big picture, the autonomic nervous system is responsible for controlling the organs of the thorax and the abdominal pelvic cavity. So your cardiovascular centers, respiratory centers, reproductive functions, urinary, all of these are under the influence of the autonomic nervous system. The integrative center, so where all this information comes and goes, is going to be in the hypothalamus which is very important because that is the hypothalamus is in the diencephalon, meaning it's below the cerebrum, meaning it's subconscious or autonomic, automatic. This is not under voluntary control at all. So the two key divisions you see here on the right are the sympathetic nervous system which is the one we call the fight or flight system but really it should be called fight, flight or freeze because freezing is a response. I see that with students when they have test anxiety. So the sympathetic nervous system is the fight, flight or freeze system. The parasympathetic division is the rest and digest system. So a really big picture, your sympathetic nervous system is designed to mobilize energy so you can fight or you can run. So it's going to be releasing energy stores, it's going to be increasing your heart rate and blood pressure, it's going to be sending blood to your muscles and to your skin and your heart and your lungs. It's going to be opening your airways. It's all about preparing for physical activity. I'm going to fight this bear and try to make it my supper or I'm going to run from this bear so I don't become its supper. That's the sympathetic nervous system, your fight or flight response system. The parasympathetic nervous system is all about rest and digest, relaxation, regeneration. So it's going to be all about storing energy, decreasing blood flow to the heart and lungs in those areas, increasing blood flow to your gastrointestinal system. So for digestion, urination, defecation, these are all going to be parasympathetic activities. All right, and again, we'll cover those in more details later. But most parts of the body do have what's called dual innervation. They're under control of both. And in most cases, they have opposing effects. You can see some examples here. The sympathetic nervous system dilates the pupils. Parasympathetic system does the opposite, constrict it. Sympathetic nervous system increases heart rate. Parasympathetic system decreases heart rate. So most parts of the body are controlled by both and they have opposing effects. But that's not always the case. The two systems can work independently. So there are some parts of the body that are only under control of the sympathetic nervous system. That your blood vessels would be the key example there. And then there's one part of your body that's only under control of the parasympathetic nervous system. And that would be your lacrimal glands. So they can either, some parts of your body are only controlled by one or the other. Or the two systems do work together, but they work together in a way when they're controlling a complex process where they need each other. By far the best example or the textbook example there would be how achieving an erection is a parasympathetic response, whereas ejaculation is a sympathetic response. So a man could not reproduce without the ability to use both halves of the autonomic nervous system. So I do have to mention in passing here there is a third division to the autonomic nervous system. And that's called the enteric nervous system. It's a huge extensive network of neurons. We don't know how many, but somewhere between 200 and 600 million neurons that are in the walls of the digestive tract. So a lot of the subconscious control of digestion is under the purview here of the enteric nervous system. So it has as many, it has more neurons than your spinal cord. It's a big part of your nervous system, but we're still kind of teasing, teasing out everything that it does. All right. A few more of the effects here. So we taught, if you look, if you look at the effects of these two, the two halves of the ANS, sympathetic system is all about preparing for physical activity. Your pupils are dilating. So, so you can, you can see more, get more light. Your lenses are looking off in the distance. Where's that predator coming from? These types of things. Saliva is going to get real dry. Like if you get stressed, your mouth gets dry. It's not about, you're not, it is, you're still producing saliva, but it's a real sticky saliva. You don't want to be losing a lot of fluid in case you get bitten, things like that. Your heart and lungs preparing for physical activity, but you can't have all the blood go everywhere. So at the same time, your GI tract and kidneys are shutting down. You're not, you're not worried about digesting food right now. You're worried about not becoming food, right? You're not worried about urination at this point. And then we already talked about a few of the other ones there. The parasympathetic division is all about resting and regenerating. So digestion, relaxation, lowering your heart rate, lowering your blood pressure, lowering your respiratory rate. All these are all under the, under the effects of the parasympathetic system. So we talked about how dual innervation, most areas are controlled by both. We already hit that. But in areas where it's not, so here's how I think of it. Like I think of it like the gas and the brakes, if there's dual innervation. If one thing excites an area, the other turns it down. So gas versus brakes. So if you're under sympathetic stimulation, your heart rate goes up, that's the gas. Parasympathetic stimulation, heart rate goes down, that's the brakes. But some areas aren't controlled by both. So you see here like your blood vessels. Your arteries are only under control of sympathetic system. So I think of that would be like gas, no gas. So like a bumper car that only has a gas or like the old golf carts that only had a gas. And how did you stop that golf cart? When you took your foot off the gas, it stopped it. So that's kind of how I think about here. So we need the ability for our blood vessels to constrict and to dilate. So but when you put your foot on the gas, the sympathetic system constricts the blood vessels. When you take your foot off the gas, they'll dilate again. So you still do get control. It only takes the one system, the sympathetic system to do so. Just a couple more things here. Here you see a good example of the opposing effects. So you see the sympathetic nervous system. And we won't go through all the complex processes yet. But the sympathetic nervous system is going to dilate the pupils. So you can see more and see off in the distance. The parasympathetic system is going to constrict your pupils. There's no predators coming. You don't have to worry about them when you're relaxing or as far as you know. Right. All right. Last thing is I just want to show you this, the yin-yang or some say yin-yang. I guess it's important because the key with the auto nervous system is balance. We need both. We need stress. Stress motivates us. Stress drives us. Exercise is an example of a stressor that makes us healthier. As a student, deadlines, due dates, these things drive you to perform, to do what you need to do. So the automatic nervous system is, or the sympathetic system is important. Life can't just be relaxation. We need balance. So here you see that, that balance between physical activity and stress and things that drive and motivate us with regeneration. We have to take our foot off the gas sometimes too. So are you getting enough sleep? Are you doing things that bring you joy? Are you relaxing as much as you're stressing and worrying? I mean, I know from talking to a lot of you that's probably not the case. But it's important to note that these two systems, there's not a good one and a bad one. They're both needed. The key is balance between the two. We're having so much problem with stress, anxiety, depression because there's an imbalance. Our foot's always on the gas and it's never on the brake anymore. So just keep that in mind. Okay, that's just a quick overview of the two halves of this system. We'll cover some more of the anatomical details here coming up. I hope this helps. Have a wonderful day. Be blessed.