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  • Consider your heart for a moment.

  • For the average person -- at rest, like you probably are, sitting there watching me -- the

  • heart beats at around 60 beats per minute.

  • Once a second. Nice and easy.

  • But if you were to somehow disconnect your heart from your autonomic nervous system,

  • things, as you might imagine, would change.

  • But, your heart would not stop. Actually, it would be the opposite.

  • It would speed up.

  • It would start beating at around 100 beats per minute -- and that’s just at rest.

  • With your heart beating two-thirds faster than normal, before you even broke a sweat,

  • your cardiac muscle would experience a lot of extra wear and tear. The surrounding blood

  • vessels would be under enormous pressure. And your body would suddenly require -- and

  • waste -- a lot of energy.

  • Basically, you’d be out of balance.

  • Part of what keeps your heart under control is your parasympathetic nervous system.

  • It’s often described as the calming side of your autonomic system -- a kind of antidote

  • to the effects of stress created by the sympathetic system.

  • But it’s really much more than that.

  • Unlike your sympathetic division, which lets you deal with the crisis of the now, the parasympathetic

  • system allows your body to handleeverything else.

  • It not only calms you down after being stressed out, it’s what allows you to digest food,

  • to reproduce, to excrete waste, to fight off infections.

  • Basically, it lets you do the business of living.

  • But our bodies can only do that when they are in balance, somewhere between excitement

  • and inhibition, both aroused enough and calm enough to keep things working.

  • So the parasympathetic system is why our hearts don’t pump so hard that they explode, sure.

  • But it also explains a lot of other stuff about our bodies.

  • Oh, but just one thing?

  • Learning about the parasympathetic system is going to involve a lot of memorizing.

  • Hope that doesn’t stress you out.

  • You will recall that our sympathetic & parasympathetic systems not only have different functions

  • -- more or less engaging the same organs to opposite effects -- they also have different structures.

  • Their ganglia, for example, are located in different places: The sympathetic ganglia

  • are located near the spinal cord, while on the parasympathetic side, theyre close to the effectors.

  • And likewise, the use of neurotransmitters in the two systems is similar, but not quite the same.

  • In both systems, neurons release acetylcholine, or ACh, in their preganglionic synapses.

  • But in your parasympathetic system, the postganglionic neurons release ACh at their synapses with

  • the effector organs, too...

  • ...as opposed to in the sympathetic system, where effectors get a dose of norepinephrine instead.

  • But the biggest anatomical difference between these two systems has to do with the physical

  • networks that they form as they reach throughout your body.

  • While the sympathetic nerves all spring from the thoracolumbar area of your spinal cord,

  • right around your midsection, the nerves of the parasympathetic division are craniosacral.

  • And with the exception of a couple of sacral nerves near the tailbone that run to the bladder

  • and genitals and rectum, most of these nerves never go through the spinal cord.

  • Instead, they run right from the brain almost all the way to their effectors.

  • There are 12 of these cranial nerves, and they vary in terms of what kinds of neurons they contain.

  • I mean, were talking about the autonomic system here, but they are not all autonomic motor fibers.

  • Some of your cranial nerves also carry motor fibers that control voluntary functions, like

  • moving your eyeballs around.

  • And others carry only sensory fibers, which relay data to and from your sensory organs.

  • And, you know, just to keep things interesting, some of your cranial nerves carry both motor

  • and sensory neurons.

  • So, which ones are where? And what exactly does each one of these 12 nerves do?

  • As anatomists, we have to keep track of the human wiring-diagram that are cranial nerves,

  • because you don’t want to end up like some sidekick in a ‘90s action movie who has

  • to defuse a bomb all by himself.

  • SHOULD I CUT THE RED WIRE OR THE BLACK WIRE?

  • Honestly, though, if you find yourself inside of somebody’s brain stem you probably shouldn’t

  • be cutting anything.

  • Since all 12 of these cranial nerves are important, youre gonna have to come up with some kind

  • of mnemonics to help you keep track of both their names and their functions.

  • Youll need to know what each one is called, whether it’s a sensory nerve, a motor nerve, or both.

  • And the map that we follow of the cranial nerves is based on a ventral view of the brain

  • -- looking at its underside, with the anterior portion at the top, and posterior on the bottom.

  • First, let’s tackle the names.

  • Starting at the top, the first cranial nerve you encounter is the olfactory nerve, which

  • takes scent information gathered by the nose and sends it to the brain.

  • Followed by the optic nerve, which does the same, but with visual data.

  • Then there’s oculomotor, which controls four of the six muscles that control the movements of your eyes.

  • The next nerve, near the center of the brain’s ventral side, is the trochlear nerve, which

  • controls a single muscle in the eye, and it lets you do this.

  • Just below that is the trigeminal nerve, the largest of the cranial nerves, which branches

  • into three main strands -- hence thetri’ -- and innervates the face and jaw muscles.

  • After that there’s the abducens, which stimulates the muscles that let your eyes do this -- from

  • side to side, followed by the facial nerve, which operates the muscles that make most

  • facial expressions possible.

  • Then there’s the auditory nerve. You can probably guess what that’s for.

  • You might notice that, up until the auditory nerve, the cranial nerves mostly control organs

  • in the front of the cranium -- mainly the eyes and facial muscles.

  • But as you work your way down, the nerves tend to innervate the lower and more posterior

  • portions of the head.

  • Like the glossopharyngeal nerve, which leads to your tongue and your pharynx.

  • That’s followed by your vagus nerve -- you should definitely remember that one -- and

  • then the spinal accessory nerve -- which has to do with moving your head and shoulders,

  • and not whether your belt matches your shoes.

  • Lastly there’s the hypoglossal, the nerve that allows you to swallow and talk, among

  • other things that you do with your mouth and tongue.

  • That was a lot of information and probably new words, so how are you gonna remember it all?

  • Well, by finding a way to remember the first letter of each name, in order. Which is:

  • O-O-O .. T-T … A-F-A … G-V-S-H.

  • That doesn’t spell anything useful at all. There is a mnemonic that youll probably

  • hear in school that goes like this:

  • On old Olympustowering top, a Fin and German viewed some hops.

  • That’s pretty weird sounding -- not terribly easy to remember. I mean, Olympus? Fin? Hops?

  • There’s gotta be something more relevant to us 21st century science lovers. Like, the

  • Lord of the Rings fans out there might prefer something along the lines of:

  • Onward old orcs! Toward the Argonath for a Great Villain! Slay Hobbits!

  • I’m just trying to help.

  • Whatever device you use to remember the names of the cranial nerves, you also have to keep

  • track of their functions -- that is, whether theyre sensory, motor, or both. So, again

  • from top to bottom, a lot of teachers use this sequence of S’s, M’s and B’s to remember:

  • Some say marry money, but my brother says big brains matter more.

  • That one’s not so bad. But I don’t know, maybe youll have better luck with something like this:

  • Sorry, Sherlock -- Mean Moriarty Beat Me, But Some Bobbies Busted Moriarty Masterfully!

  • You are, of course, invited to think up your own. And feel free to share them in the comments,

  • hopefully there will be some good ones down there -- anything would be better than Fins and Hops

  • But if youre going to commit one cranial nerve type to memory, it should be 10, the vagus nerve.

  • This long and extensive nerve stretches from near the brainstem down to most of your visceral

  • organs, including your heart, lungs, and stomach.

  • The vagus nerves work as a two-way street, ferrying incoming sensory information from

  • the peripheral system to the brain, and transmitting outgoing motor instructions from the brain

  • to the rest of the body.

  • So it’s a “B” nerve, because it hasbothsensory and motor functions. And

  • usually you don’t notice this nerve at work, because its functions are mostly automatic.

  • Say youve had a really stressful day, so your sympathetic system is charged up. You

  • come home, crash on the couch, mow down a half a pizza.

  • Your stomach sends signals to your brain through the sensory nerve axons in your vagus nerve,

  • telling you that your belly is full of starch and protein and fat.

  • Your brain sees that your stomach is churning away, which is a usual parasympathetic activity,

  • so it sends signals back down through the vagus nerve, triggering other parasympathetic

  • responses -- like slowing down your heart rate, putting some glucose back into storage, and

  • reducing all that norepinephrine that your sympathetic system was pumping out all day.

  • Soon, you start feeling more relaxed. Which is just one reason why, for some people, eating

  • is a way of reducing stress and anxiety. In fact, it can feel so good that even though

  • your stomach is full, you might continue eating.

  • So, like I mentioned before, it can be easy to think of the two divisions of your autonomic

  • system as opposites or even rivals, but that’s a little off the mark.

  • Looking at your body as a whole, you should picture them as two sides of a scale -- sometimes

  • it’s balanced in the middle, and sometimes it leans to the left or right, depending on what's happening.

  • That balance is the essence of homeostasis, and as youll recall, homeostasis is the key to life.

  • Here’s something else that’s important for life: sex.

  • It mostly falls within the parasympathetic domain ofnecessary but not an emergency.”

  • But in order to effectively do it, you need help from both systems.

  • First, the parasympathetic system has to make sure youre calm enough to even think about

  • sex, and then funnel extra blood away from your muscles and down to your genitals -- which

  • is why too much stress and anxiety can lead to sexual dysfunction.

  • But you also need a burst of that sympathetic system to excite you, and keep you excited.

  • So like two sides of the scale, the balance depends on having the right amount of both.

  • The rate of action potentials travelling through each division is known as yoursympathetic

  • toneand yourparasympathetic tone.”

  • And, most of the time, our parasympathetic tone is actually dominant, keeping down the

  • caged animal that is your sympathetic response.

  • That’s why you need your parasympathetic system to keep your heart from racing like

  • a rabbit’s. And why, most of the time, our bodies can do the eating, and sex-having,

  • and all of the other fun tasks that make up the business of living.

  • Today on Crash Course: Anatomy & Physiology we looked at the parasympathetic nervous system,

  • its structural differences from the sympathetic system, and the 12 cranial nerves. We also

  • looked at the vagus nerve as a way of understanding how parasympathetic responses work, and how

  • they make our lives possible.

  • Big shout out and thank you to our Headmaster of Learning, Thomas Frank whose generous contribution

  • on Patreon keeps 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 co-sponsored by Jennifer K. Koons, Tim Wisard, and Mich Acosta.

  • This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio.

  • We got a plaque. I wanted to show you the plaque before we put it up on the wall.

  • It 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, our sound designer is Michael Aranda, and the graphics team is Thought Café

Consider your heart for a moment.

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Parasympathetic Nervous System: Crash Course A&P #15

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