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Autonomic nervous system

The autonomic nervous system, a part of our overall nervous system, regulates smooth muscle cells, cardiac muscle, and gland cells autonomously. It consists of two subsystems: the sympathetic nervous system for fight or flight responses, and the parasympathetic nervous system for rest and digestion. These systems influence our bodies in various circumstances. Created by Matthew Barry Jensen.

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Video transcript

Voiceover: In this video, I want to introduce the autonomic nervous system, autonomic nervous system, which is part of the overall nervous system, and this is a functional division of the nervous system, not a structural division, like the central nervous system and the peripheral nervous system. The autonomic nervous system consists of efferent neurons in the peripheral nervous system that do specific jobs. So, these are efferent neurons, and these neurons control three different types of cells. The first are smooth muscle cells, which are in all sorts of structures all over our body, like around our blood vessels, and they control cardiac muscle, the muscle that makes up our heart tissue, so, cardiac muscle, and these muscle tissue types are different than the skeletal muscle, the muscle that's all over, attached to our skeleton that moves us around, because those are controlled by different efferent neurons of the peripheral nervous system. Those are controlled by lower motor neurons, not autonomic neurons. The last thing that autonomic neurons control are gland cells. Some gland cells are controlled by the autonomic nervous system. Now, the autonomic nervous system is called this because it tends to control all these things without conscious involvement. It doesn't require the involvement of consciousness to control these things. So, it's kind of autonomous. It kind of does this stuff on its own, without our conscious selves having to be involved, for the most part, and we divide the autonomic nervous system into two big subsystems. So, let me write two big arrows here. And this part we call the sympathetic nervous system, the sympathetic nervous system, which is the first big part of the autonomic nervous system. So, I'll just write SNS for short, for sympathetic nervous system, and this other big part we call the parasympathetic nervous system. Parasympathetic, so I'll just write PNS for short, for parasympathetic nervous system, and there are a number of big differences between these two parts of the autonomic nervous system that we can talk about in this kind of introductory talk. The first big difference is kind of where they start in the central nervous system. The sympathetic nervous system starts in the middle of the spinal cord, and at the middle part of the spinal cord, let me draw a bunch of somas here, and I'll just take one of these here, and I'll draw a little short axon on the first neuron that's coming out of the central nervous system, and then it's going to synapse with the second neuron in a ganglia close to where the first neuron is, and then the second neuron is going to send a longer axon to reach its target cell. So, let me just draw a big T, to represent some kind of target cell that it's going to synapse on, and this target cell will be a smooth muscle cell, a cardiac muscle cell, or a gland cell. And here's an illustration of kind of the entire autonomic nervous system, and here they're showing kind of the middle part of the spinal cord that all these first neurons in the sympathetic nervous system are starting, and then there's a short axon until they synapse in a ganglia that's pretty close to the spine. Here's a set of ganglia, and here are a few other ganglia, but they all tend to be pretty close to the spine. This set of ganglia are actually often linked together in kind of a chain, which we actually call the sympathetic chain, and here's just a different illustration of the same thing. So, here it's showing in the middle part of the spinal cord that first axon's coming out, synapsing at a ganglia close to the spine, with a lot of these ganglia linked together in a chain, and then the second neuron sending a longer axon out to synapse on the target cell in whatever tissue you're talking about that contains smooth muscle cells, cardiac muscle cells, or gland cells. Now, the parasympathetic nervous system has its first neurons start in a different place in the central nervous system. They start either up here in the brain stem, or they start way down here at the bottom of the spinal cord, and then their first neuron tends to send a long axon out to synapse with the second neuron in a ganglion at a distance from the first neuron, and then that second neuron usually sends out a short axon to synapse on its target cell. I'll just write a big T here for target cell. And here this illustration is showing this as well, where it's showing the first neurons of the parasympathetic nervous system either up here in the brain stem or down here at the bottom of the spinal cord, and then it's showing these long axons on the first neuron, until it reaches a ganglia at a distance from the first neuron's soma and then a shorter axon on that second neuron, until it reaches its target cell. And here's another illustration, just showing the same thing. So, here's these first axons coming out of either the brain stem up high or the bottom part of the spinal cord down low, and then these first long axons go all the way until they meet a ganglion at a distance from the first neuron soma, and then the second neuron sends a shorter axon to the target cells. So, the similarities in the structure of the different parts of the autonomic nervous system are that they both usually consist of a chain of two neurons connecting the central nervous system to the target cell, but the differences are where those first neurons start and whether there's a short first axon and a long second axon or a long first axon and a short second axon. But, more importantly than these structural differences between the different parts of the autonomic nervous system, are the functional differences, and these neurons do so many different things in so many tissues of the body, that it's a little hard to talk about them in general, but there are these great phrases that can kind of help think through lots of the changes that these different parts of the autonomic nervous system do, and for the sympathetic nervous system, the phrase is fight or flight, fight or flight, that the sympathetic nervous system, when it's activated, will cause lots of changes in the body that'll prepare to either fight or run away, which can kind of help you deal with threatening or dangerous situations. So, I'll write that in red here for the sympathetic nervous system, whereas the parasympathetic nervous system I'll write in a nice cool green here, because its phrase is rest and digest, rest and digest. So, when it's active, it often causes lots of changes in the body that are more important for homeostasis and just maintenance of the body in nonthreatening situations. So, let's take a few examples of a few tissues where these different responses happen, to get a feel for what this means. So, first let's look down here at the gastrointestinal system, the intestines or the gut, and both the sympathetic and the parasympathetic nervous system play a role in a lot of activities of the gastrointestinal system, but one is blood flow to the intestines, because the amount of blood flowing through the intestines plays a big role in how much digestion the intestines can do. Blood flow to intestines, and it also plays a big role in how much blood is available for other parts of the body. So, when the sympathetic nervous system is activated in some kind of fight or flight situation, blood flow to the intestines decreases, and that blood is actually diverted away from the intestines, often to skeletal muscle. So, all our muscles all over our body that can help us move to deal with dangerous situations, the blood is going to leave the intestines and go to that, because during a dangerous situation is not the time to be digesting food. It's the time to be moving, so the blood's flow decreases to the intestines and is diverted to skeletal muscle, whereas most of the time, when you're in a nonthreatening situation and it's time to rest and digest, the peripheral nervous system is activated, and that increases blood flow to the intestines. That'll divert blood away from skeletal muscle, because now you're not in a fight or flight situation, and you want to rest and digest. So, it's going to bring the blood flow back to the intestines, to increase your ability to digest food. If we look at the heart, both the sympathetic and parasympathetic nervous systems innervate the heart, and we look at the heart output, kind of how much blood the heart is pumping out over any particular unit of time. Heart output of blood. When the sympathetic nervous system is activated, the heart output increases. The heart pumps harder and pumps faster and pushes more blood out, so that things like skeletal muscle can get more blood flow. In addition to diverting blood flow from the intestines to skeletal muscle, the heart's just going to push more out, so there's more available for the skeletal muscle. When the parasympathetic nervous system is activated, the heart output goes down. The heart is pumping less hard, and it's beating less often. It's just working less, because you don't need as much blood flow to the muscles for movement, so you go to kind of a baseline level that's sufficient for activities that involve resting and digesting. So, these examples of blood flow involve the activity of smooth muscle, because smooth muscle is around our blood vessels and determines where the blood is going to flow to, and cardiac muscle, because the cardiac muscle makes up the heart, and then if we think about gland cells, there's a bunch of different glands that the autonomic nervous system controls, and they tend to be activated kind of differently at different times. So, one type of gland that's activated during fight or flight situations, when the sympathetic nervous system is active, are sweat glands out here in the skin, and the sweat glands are activated to secrete sweat, which helps cool us down, which increases our ability to move faster and farther, if we're able to stay cool, whereas some glands that are activated by the parasympathetic nervous system include things like the salivary glands that produce saliva in our mouth, because saliva is very useful for digestion, and it's part of a number of activities that happen that help us digest food. So, I find these phrases helpful when I'm thinking about what effects the autonomic nervous system will have on different tissues of the body during different situations, because like in all of these examples, most of the things that the sympathetic nervous system does when it's activated increase the body's ability to turn stored energy into movement, to deal with dangerous situations, like moving blood from the intestines to skeletal muscle and increasing the amount of blood being pumped around from the heart and increasing sweat production from sweat glands to keep us cool while we're moving to deal with a dangerous situation, whereas all of these things that the parasympathetic nervous system is doing, make sense in nonthreatening situations, where we're actually trying to conserve and store energy, like diverting blood flow away from skeletal muscle to the intestines, to increase digestion, decreasing heart/cardiac output to conserve energy, and increasing saliva production from the salivary glands to help with digestion as well. But the autonomic nervous system affects many more structures and has many more functions than I can cover in this little introductory video. For instance, autonomic neurons play a role in changing the size of your pupils in your eyes, in sexual responses, and in secretion from a whole bunch of other glands, and because it does so many different things, I find it best to actually not cover it all in one sitting, but instead to cover these things as you're studying each individual organ system. Because almost any organ system you're going to cover is going to have autonomic neurons coming in and affecting how that organ system functions.