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  • So youre sound asleep, when your smoke alarm goes off. Before you even know what’s

  • going on, you start to feel it. Those smoke alarms are loud -- for a good reason. Your

  • heart starts to race, your breathing picks up, you become sweaty all over your body.

  • You are stressed. And I’m not talking about the my-iPhone-just-died kind of stress. I’m

  • talking about the I’m-afraid-I-might-die kind of stress.

  • Even though it’s often seen as a dirty word, stress, like pain, isn’t all bad -- it’s

  • actually very useful if youre, y’know, trying to get out of a burning building.

  • Your sympathetic nervous system is the part of your nervous system that responds to stress,

  • and it does its job exceedingly well by focusing on what your body needs to do right now.

  • Like, when youre facing a life-or-death ordeal, you don’t need to be digesting that

  • cashew cluster in your intestines, or producing reproductive cells, or fighting off an infection.

  • That’s all stuff that you can deal with later, when youre out of harm’s way.

  • So your sympathetic nervous system sweeps these suddenly trivial functions aside to

  • blast all of your energy to your brain and heart and muscles to deal with the threat at hand.

  • So, this is where I tell you that youre lucky to have a sympathetic nervous system.

  • And that it keeps you alive. And that you would probably die in X Period of Time if you didn’t have one.

  • All of which is true. But here’s the thing: the problem is, nowadays our bodiesstress

  • responses are triggered all the time, practically every day, even when we are not in mortal danger.

  • I mean, worrying about paying your wireless bill or being late for an important meeting

  • -- those things are terrible, but they will not kill you.

  • But, good luck explaining that to your nervous system.

  • Because your physiological responses to non-immediate stresses are largely the

  • same as when youre fighting for survival.

  • So, if stress is, like, ruining your life, that’s why. And that’s part of the reason

  • that should get to know how it works. Because by learning about your sympathetic

  • nervous system, you come to understand one of the key players in the physiology of stress.

  • You may recall from our tour of the anatomy of your autonomic nervous system, that in

  • both your sympathetic and parasympathetic divisions, almost every signal has to cross two synapses.

  • Each neuron travels from its root in the spinal cord to a ganglion, where it synapses -- and

  • yes, that is a verb as well -- with another nerve fiber. And that one, in turn, leads

  • to an effector organ, where it synapses again to create whatever response was signaled -- like

  • sending more blood to your skeletal muscles, or making your heart pump faster.

  • But you gotta wonder -- or at least I gotta wonder: how do these neurons and effectors

  • actually communicate with each other? And how do all of those signals result in the

  • high-octane sensations that we know asstress”?

  • By and large, the stress response includes two kinds of chemicals, both of which I’m sure you've heard of.

  • The first, of course, are neurotransmitters. These are made and released from neurons themselves,

  • and like we talked about in our lesson about synapses, they are what neurons use to communicate

  • with each other -- or their effector organs -- across a synapse.

  • The other chemicals involved in stress are hormones, which are secreted by your glands.

  • There are at least 50 different hormones at work in your body right now, and they do everything

  • from regulating your sleep cycles to making your body retain water so youre not dying

  • of dehydration all over the place.

  • I’m telling you all of this now, up front, because hormones and neurotransmitters are

  • 100% necessary for understanding how your sympathetic division ultimately works.

  • BUT! When you trace a single sympathetic signal, from the initial stimulus to the final response,

  • those chemicals can be kind of hard to keep track of.

  • That’s because the very same substance can have different effects -- actually, sometimes,

  • totally opposite effects -- depending on where it’s received in your body.

  • And to make things even more fun, even though neurotransmitters are part of your nervous

  • system, and hormones are products of your endocrine system, a compound can be considered

  • either a neurotransmitter or a hormone -- even though it hasn’t changed one iota -- depending

  • on where it happens to be operating in your body.

  • So all of this can make understanding your stress responses pretty confusing! You might

  • even saystressful!

  • All right, were going in.

  • The smoke alarm wakes you up. You smell smoke. It is time to move muscles. Fast.

  • Your brain sends action potentials down your spinal cord and preganglionic neuronal axons.

  • Those signals flow all the way to their ganglia.

  • When the signals reach the synapses inside the ganglia, the nerve fibers then release

  • a neurotransmitter -- called acetylcholine, known to its friends as ACh.

  • If you haven’t heard of acetylcholine yet, youre gonna wanna remember that name.

  • In addition to working in sympathetic ganglia like this, it’s also what the rest of your

  • peripheral nervous system and lots of your central nervous system uses to communicate.

  • So when it comes to nervous communication, ACh is really the coin of the realm. The premium currency.

  • So, that acetylcholine crosses the synapse and, if there’s enough of it, it can stimulate

  • action potentials in several neurons on the other end -- in the postganglionic fibers.

  • That’s all it does, but it’s important. It’s basically a signal booster.

  • Those postganglionic neurons then carry the action potential to the effector organs -- in

  • this case, let’s say your leg muscles, which are going to need an influx of blood

  • if theyre going to hustle you out of that house.

  • And at the end of that second, postganglionic neuron, the fiber releases a different neurotransmitter.

  • This one’s called norepinephrine. And it is always norepinephrine that’s released

  • from postganglionic fibers in the sympathetic nervous system.

  • It’s what crosses that final synapse and creates a response in the effector, like opening

  • up blood vessels that lead to the leg muscles.

  • So, the preganglionic fiber releases ACh, and the postganglionic releases norepinephrine.

  • Boom. Congrats. Your life is on its way to being saved.

  • But, your body has more than one mechanism for responding to things, especially things

  • like a burning house.

  • There’s another alternative for getting the message out.

  • I mentioned those hormones, remember?

  • In addition to nerve fibers that lead to ganglia and then your effectors, there’s also a

  • set leaving the spinal cord that goes directly to your adrenal glands.

  • Like all preganglionic fibers, these release acetylcholine, too. But here, the signal doesn’t

  • end up in another neuron that triggers blood vessels to open or whatever. Instead, it triggers

  • your adrenal medulla to release a flood of epinephrine and norepinephrine -- hormones

  • that rush through your bloodstream toward your heart, lungs, and other organs.

  • Now, hold up! Did you notice what I just said?

  • Yeah, I said the adrenal glands release norepinephrine as a hormone.

  • Whereas in that first scenario I said that norepinephrine was a neurotransmitter that

  • sent the final signal to control blood flow to the leg muscle.

  • Now, how can I say both of those things?

  • Because theyre both true.

  • Norepinephrine is BOTH a neurotransmitter and a hormone, and which one it is depends

  • on how it’s being used. If it’s being released from a neuron and travelling across a synapse,

  • we refer to a messenger chemical -- no matter what it is -- as a neurotransmitter. If it’s

  • being secreted by a gland into the bloodstream for more widespread distribution, it’s a hormone.

  • Even if it’s the same chemical. And to an effector, hormonal norepinephrine is just

  • as good as neurotransmitter norepinephrine. But as scientists, we describe them differently,

  • because theyre functioning differently.

  • Now, the ways in which a neurotransmitter-slash- hormone like norepinephrine works, is a good example

  • of another confusing aspect of your sympathetic nervous system. Because it works by both stimulating

  • and inhibiting the same systems in your body at the same time!

  • So, in our house-burning scenario, the norepinephrine your system releases causes an increase of

  • blood flow in some parts of your body -- like your leg muscles -- while restricting blood

  • flow in other places where it’s not urgently needed -- like your guts.

  • How can the same chemical cause opposite responses? Well, it all depends on the particular kind

  • of receptors that an effector has for receiving that chemical.

  • In the case of norepinephrine, its effector is smooth muscle -- the muscle that controls

  • all of your involuntary functions of hollow organs, like the stomach, and bladder,

  • and also your blood vessels.

  • On the smooth muscle cells controlling some blood vessels, there are receptors called

  • alpha receptors -- when norepinephrine, or epinephrine, bind to those receptors, they

  • make those smooth muscle cells contract, thereby restricting blood flow.

  • But on smooth muscle cells that control other blood vessels, there are lots of beta receptors

  • for epinephrine and norepinephrine, and when they are activated, they make the muscles

  • relax, letting more blood flow through.

  • So it makes sense that the smooth muscle around your blood vessels, which feed your skeletal

  • muscles -- which youll need to get out of that smoky house -- are covered in beta

  • receptors. Because you want those blood vessels to relax, and provide plenty of oxygen to

  • the muscles in your arms and legs.

  • And since running away is more important than digesting your dinner, the blood vessels leading

  • to your stomach and intestines have lots of alpha receptors, which reduce blood flow to

  • those areas, because that burrito can wait until youre out of the house.

  • So, there’s a lot going on in your sympathetic responses. And much of it can seem complicated,

  • or even contradictory.

  • But the thing is, all of these functions work together to create a full-body response, which

  • is exactly what you need in an emergency.

  • After all, it wouldn’t do you much good to speed up your heart without sending that

  • blood to your muscles, where it’s needed. It’s up to those neurotransmitters and hormones,

  • and the receptors on the corresponding effectors, to make sure that everyone is on the same page.

  • So, the system works well. Really well. Sometimes, too well.

  • Remember when I said at the beginning, how your body doesn’t know life-threatening

  • stress from life-annoying stress?

  • Since your body’s reaction tends to be a full-body response either way, it can become

  • pretty taxing over time.

  • I mean, were talking about throwing parts of your body into overdrive, while depriving

  • others of blood and oxygen.

  • That’s not something you want happening every morning.

  • So the irony here? The real kick in the head? It’s that non-life-threatening stressors can

  • actually end up endangering your life in the long run, because your body’s stress response is so effective.

  • The frequent activation of your sympathetic nervous system, and the triggering the other

  • part of your stress response -- the part that’s driven by hormones -- can have nasty consequences,

  • like high blood pressure, digestive problems, and even the suppression of your immune system.

  • So what your body needs to do is figure out how to relax. Rest and digest. Feed and breed.

  • That is where your sympathetic system’s more mellow half-brother, the parasympathetic system comes in.

  • And yeah, that’s what were gonna be talking about next time.

  • For now you learned that your sympathetic nervous system controls your body’s stress

  • response and how the signals in your sympathetic nervous system travel to an effector, using

  • the neurotransmitters acetylcholine in the ganglion and norepinephrine at the effector.

  • And you learned that other signals can go right to the adrenal glands, where norepinephrine

  • and epinephrine are secreted as hormones.

  • And you also learned that the same messenger chemical can evoke different responses depending

  • on the receptors, with alpha receptors causing smooth muscles to constrict, for example,

  • while beta receptors cause smooth muscle to relax.

  • A big shoutout and thank you to our Headmaster of Learning, Thomas Frank, 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, check out patreon.com/crashcourse

  • This episode of Crash Course was co-sponsored by Harry Brisson, David Thompson, Jason Constam,

  • and Tuseroni.

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

  • was written by Kathleen Yale, edited by Blake de Pastino, and our consultant, is Dr. Brandon

  • Jackson. Our director and script supervisor is Nicholas Jenkins, the editor is Nicole

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

So youre sound asleep, when your smoke alarm goes off. Before you even know what’s

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Sympathetic Nervous System: Crash Course A&P #14

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