So you’re 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 you’re, 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 you’re 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 you’re 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 you’re
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 bodies’ stress 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 you’re 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 as “stress”? 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 you’re 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 say … stressful! All right, we’re 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,
you’re 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 they’re 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 they’re 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 they’re 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 you’ll 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 you’re 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, we’re 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 we’re 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.