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Hank takes you on a tour of your two-part autonomic nervous system. This episode explains how your sympathetic nervous system and parasympathetic nervous system work together as foils, balancing each other out. Their key anatomical differences - where nerve fibers originate and where their ganglia are located - drive their distinct anatomical functions, making your sympathetic nervous system the "fight or flight" while your parasympathetic nervous system is for "resting and digesting."

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Table of Contents
The Basic Two-Part System of the Autonomic Nervous System 0:48
Sympathetic Nervous System 2:33
Parasympathetic Nervous System 2:54
Their Nerve Fibers Originate in Different Parts of the Body 3:22
Sympathetic Ganglia Are Close to the Spinal Cord 4:36
Parasympathetic Ganglia Are Close to Their Effectors 4:59


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No matter what you’re a fan of, you can probably think of two characters who fit the description. Holmes and Watson. Brennan and Booth. Kirk and Spock. Ron and Hermione. They’re close. They understand each other in ways that others don’t, and you really can’t picture one without the other. But when stuff happens, they usually react in different ways. Like, if you’re snooping around Hogwarts at night, and you come across some beastie, the smart money’s on Hermione to come up with a spell that saves the day, while Ron probably standin’ there with his mouth open. And when Sherlock starts to go crazy because there hasn’t been a fiendishly clever jewel heist or whatever for him to figure out, you can count on Watson to calm him down. Mostly. My brother John, who is a writer of books, will tell you that these characters are foils -- they’re not opposites, exactly. They’re contrasts, and the more you see them together, the more they seem to complement or round one another out. And that is basically your autonomic nervous system in a nutshell. Your autonomic system is the branch of your peripheral nervous system that regulates the functions of your internal organs, like your heart and stomach, and also controls your smooth and cardiac muscles, and your glands. All things that you you do not consciously control, so, yes, you could say it has a lot of power over you. But the funny -- or maybe confusing -- thing about this system is that its effects on your organs and muscles and glands are by no means consistent. Not at all. At any given moment, whether you happen to be totally relaxed or completely flipping your wig, your autonomic system is constantly making involuntary, fine-tuned adjustments to your body, based on what signals your central nervous system is picking up. This could mean changing your body temperature, sending extra blood to a particular area, slowing your heart beat, or tweaking your stomach secretions. Its effects change, depending on the situation you’re in -- and also which part of your autonomic system is in charge at that moment. Because this weird little corner of your nervous system that keeps you alive is actually run by two competing interests. Two divisions that serve the same organs, but they create opposite effects in them, battling it out back and forth, to either excite your body’s functions or subdue them. One of them is dedicated to amping you up and preparing you for activity -- that’s your sympathetic nervous system. And the other one talks you down and effectively undoes what its foil did. And that is the parasympathetic nervous system. Together, they’re what make your body experience stress, fear, relaxation, and defiance. Courage and cowardice. Panic and peace. If there’s an epic novel going on in your body right now it is probably being written by these two. Let’s talk about names for a minute. One of the two divisions of your autonomic nervous system is called the sympathetic system. That sounds kinda nice, doesn’t it? It’s like understanding, and calming, and telling you that it’s not so bad after all. WRONG! Contrary to its comforting name, the sympathetic system is what sounds your internal alarm bells. It’s the hardware behind the famous “fight or flight” response. It is synonymous with stress. If the sympathetic is for “fight or flight,” the parasympathetic is for “resting and digesting” -- it’s responsible for maintaining your body and conserving energy for later. I recognize that this is confusing. But, when you explore the anatomy of these two systems, like we’re gonna do today, they start to make a little more sense. Because, even though their basic components are essentially the same, their physical structures turn out to be different in a few really important ways. And those differences can help explain why they act like the foils that they are, and why sometimes you feel more like Sherlock than Watson, or the other way around. First big difference: the nerves of these two divisions originate at different sites in your body. Your sympathetic fibers are thoraco-lumbar -- meaning that they originate from between your thoracic vertebrae where your ribs attach, and the lumbar vertebrae just inferior to your ribs. Early anatomists saw how a network of nerves radiating from the middle of the spine like this could quickly coordinate the functions of many major organs at once. So it was called the sympathetic system, from the Greek words for “feeling together.” But the nerve fibers of your parasympathetic system begin both above and below where the sympathetic ones do. They’re craniosacral, meaning they sprout from the base of your brain and also from your sacral spinal cord, just superior to the tailbone. And because the roots of these nerve fibers basically frame the starting points of the sympathetic nerves, they were called parasympathetic -- literally “beside the sympathetic.” Another difference between these two foils? Their ganglia. Unlike your sensory or motor neurons, where a single axon can reach all the way from your spinal cord to whatever muscle or touch receptor it works with, both parts of your autonomic system require two neurons in order to work. And those two neurons meet in ganglia -- clusters of neuron cell bodies that house millions of synapses. But where these ganglia appear relate to their function, and which division of the autonomic system they’re serving. Sympathetic ganglia are found closer to the spinal cord, because in those fight-or-flight moments of high excitement or activity, they need to be able to send a single message far and wide, like the Bat Signal. This way, excitatory signals traveling into a ganglion near the spine -- ganglion being the singular of ganglia -- can trigger action potentials in a whole bunch of other neurons that lead to many different effectors, like the heart, lungs, stomach, and adrenal glands. By contrast, most parasympathetic ganglia are found way out from the spine -- near, or even inside of their effector organs. Because this system is responsible for taking care of particular functions only when you have the time and energy to do it -- like digesting food or excreting waste -- it uses more specific, strategic signals. It’s more like Commissioner Gordon calling Batman on the Batphone, one on one just to talk about how things are going and you know whether Alfred’s doing OK after his meningitis. It’s a private conversation -- not everybody needs to be involved. Anyway, because the ganglia of these two divisions appear in different places in your body, it also makes sense that their neurons themselves have slightly different forms, namely the length of their axons. Now, ganglia can be kind of complex -- it actually comes from the Greek word for “a knot in a string” -- so when dealing with neurons around these structures, we look at the fibers before they run into the ganglion, as well as after they come out of it. Understandably enough, the axon lengths of the neurons before the ganglion are called the preganglionic fibers, and the ones coming out are postganglionic. The key here is that, in the sympathetic system, the preganglionic fibers are much shorter than the postganglionic ones. Which makes sense, when you think about it, because sympathetic ganglia are really close to the spinal cord, and the axons don’t have, or need, very far to go from the central nervous system. But they do have a lot of distance to cover, on the other side of the ganglion, in order to reach their effectors. So naturally the fibers leading out of the ganglia are a lot longer. And, foils being what they are, the reverse is of course true for the parasympathetic system. Since parasympathetic ganglia are so close to, or even inside of, their effector organs, the preganglionic fibers are a lot longer. They extend from the cranium and sacrum where they start, out to the lungs or liver or bladder -- wherever their effector is -- where they reach their ganglion. From there, the postganglionic fibers are super short -- just long enough to communicate with their effector. So, once again -- it’s anatomy and physiology -- the structure of each of these systems is related to its function. The sympathetic nervous system is set up in such a way that even a small stress signal sent down one path could trigger a response in many effectors at once. Which is one reason why your reaction to a sudden, stressful event can feel so all-encompassing. By the same token, the resting and digesting that’s overseen by the parasympathetic system doesn’t require urgent, all-hands-on deck communication. If you need to process a burrito or take a nap or maybe a trip to the bathroom, it can communicate with the organs involved, one on one. But still none of this tells us how these systems do what they do -- how these nerves communicate with your organs, and muscles, and glands in times of either stress, or relaxation. We’ll start that next week, with a white-knuckle ride through your sympathetic nervous system. So, between now and then, rest up. For now, you learned the basic two-part system of your autonomic nervous system. Mainly that it consists of two primary, complementary divisions, the sympathetic nervous system, which arouses your body, and the parasympathetic nervous system, which is charge of resting, digesting, and repair. We also talked through the three main anatomical differences between the two systems. Their nerve fibers originate in different parts of the body, the ganglia of the sympathetic system are located close to the spinal cord, while the ganglia of the parasympathetic system are close to their effectors. And finally, the nerve fibers themselves have different structures. Announcing Thomas Frank, our new "Headmaster of Learning," whose generous contribution on Patreon helps keep Crash Course alive and well for everyone. Thank you, Thomas. If you want to help us keep making great videos like this one, you can check out Patreon.com/CrashCourse This episode of Crash Course was cosponsored by Link, Kelly Naylor, Tim Webster, and Steven Meekel. This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. Our director is Nicholas Jenkins, the script supervisor and editor is Nicole Sweeney, the sound designer is Michael Aranda, and our graphics team is Thought Café.