[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:03.38,Default,,0000,0000,0000,,When it comes to the nervous system, or just\Nyour body in general, let’s face it:
Dialogue: 0,0:00:03.38,0:00:05.24,Default,,0000,0000,0000,,your brain gets all the props.
Dialogue: 0,0:00:05.24,0:00:10.16,Default,,0000,0000,0000,,And it deserves those props! It’s a complicated,\Nand crucial, and sometimes crazy boss of an organ.
Dialogue: 0,0:00:10.16,0:00:13.92,Default,,0000,0000,0000,,But your brain would be pretty useless without\Na support team that kept it
Dialogue: 0,0:00:13.92,0:00:15.78,Default,,0000,0000,0000,,connected to the outside world.
Dialogue: 0,0:00:15.78,0:00:20.00,Default,,0000,0000,0000,,Because frankly, like any leader, the more\Nisolated your brain gets, the weirder it gets.
Dialogue: 0,0:00:20.01,0:00:24.33,Default,,0000,0000,0000,,Put a person in a watery, pitch-black sensory\Ndeprivation tank, and you’ll see the brain
Dialogue: 0,0:00:24.33,0:00:29.34,Default,,0000,0000,0000,,do some really weird stuff. Without a constant\Nflood of external information, the brain starts
Dialogue: 0,0:00:29.34,0:00:35.00,Default,,0000,0000,0000,,to confuse its own thoughts for actual experiences,\Nleading you to hallucinate the taste of cheeseburgers,
Dialogue: 0,0:00:35.00,0:00:38.86,Default,,0000,0000,0000,,or the sound of a choir singing, or the sight\Nof pink stampeding elephants.
Dialogue: 0,0:00:38.86,0:00:43.11,Default,,0000,0000,0000,,It’s your peripheral nervous system that\Nkeeps things real, by putting your brain in
Dialogue: 0,0:00:43.11,0:00:47.67,Default,,0000,0000,0000,,touch with the physical environment around you,\Nand allowing it to respond. This network snakes
Dialogue: 0,0:00:47.67,0:00:51.07,Default,,0000,0000,0000,,through just about every part of your body,\Nproviding the central nervous system with
Dialogue: 0,0:00:51.07,0:00:55.68,Default,,0000,0000,0000,,information ranging from the temperature, to the touch\Nof a hand on your shoulder, to a twisted ankle.
Dialogue: 0,0:00:55.68,0:01:00.08,Default,,0000,0000,0000,,The peripheral nervous system’s sensory\Nnerve receptors spy on the world for the central
Dialogue: 0,0:01:00.09,0:01:03.66,Default,,0000,0000,0000,,nervous system, and each type responds to\Ndifferent kinds of stimuli.
Dialogue: 0,0:01:03.66,0:01:08.57,Default,,0000,0000,0000,,Thermoreceptors respond to changes in temperature.\Nphotoreceptors react to light, chemoreceptors
Dialogue: 0,0:01:08.57,0:01:12.95,Default,,0000,0000,0000,,pay attention to chemicals, and mechanoreceptors\Nrespond to pressure, touch, and vibration.
Dialogue: 0,0:01:12.95,0:01:16.81,Default,,0000,0000,0000,,And then we’ve got specialized nerve receptors\Ncalled nociceptors that, unlike those other
Dialogue: 0,0:01:16.81,0:01:21.34,Default,,0000,0000,0000,,receptors, fire only to indicate pain, which\Nis the main thing I want to talk about today.
Dialogue: 0,0:01:21.34,0:01:26.40,Default,,0000,0000,0000,,Because, as unpleasant as a stick in the eye\Nor tack in the foot may be, pain is actually
Dialogue: 0,0:01:26.40,0:01:31.06,Default,,0000,0000,0000,,a great example of where everything we’ve talked\Nabout over the last few weeks all comes together,
Dialogue: 0,0:01:31.06,0:01:36.74,Default,,0000,0000,0000,,as we trace a pain signal through your nervous\Nsystem, from the first cuss to the Hello Kitty band aid.
Dialogue: 0,0:01:36.75,0:01:39.63,Default,,0000,0000,0000,,By the end of this episode of Crash Course\NAnatomy & Physiology you’ll never think
Dialogue: 0,0:01:39.63,0:01:43.99,Default,,0000,0000,0000,,of a stubbed toe, pounding headache, or burned\Ntongue the same way again.
Dialogue: 0,0:01:53.97,0:01:58.37,Default,,0000,0000,0000,,Most people go to great lengths to avoid pain,\Nbut really, it’s an incredibly useful sensation,
Dialogue: 0,0:01:58.37,0:02:01.57,Default,,0000,0000,0000,,because it helps protect us from ourselves,\Nand from the outside world.
Dialogue: 0,0:02:01.57,0:02:06.01,Default,,0000,0000,0000,,If you’re feeling physical pain, it probably\Nmeans that your body is under stress, damaged,
Dialogue: 0,0:02:06.01,0:02:10.74,Default,,0000,0000,0000,,or in danger, and your nervous system is sending\Na cease and desist signal to stop twisting
Dialogue: 0,0:02:10.75,0:02:15.38,Default,,0000,0000,0000,,your arm like that, or to back away from that bonfire,\Nor please seek medical attention, like, RIGHT NOW.
Dialogue: 0,0:02:15.38,0:02:18.70,Default,,0000,0000,0000,,So in that way, pain is actually good for\Nyou -- that’s why it exists. I’m not saying
Dialogue: 0,0:02:18.70,0:02:22.88,Default,,0000,0000,0000,,it’s pleasant, but if you’ve ever wished\Nfor an X-Men-like power to be impervious to
Dialogue: 0,0:02:22.88,0:02:26.48,Default,,0000,0000,0000,,pain, I’ve gotta say, that is one foolish\Nmonkey’s paw of a wish.
Dialogue: 0,0:02:26.48,0:02:31.14,Default,,0000,0000,0000,,Just ask Ashlyn Blocker. She’s got a genetic\Nmutation that’s given her a total insensitivity
Dialogue: 0,0:02:31.14,0:02:36.31,Default,,0000,0000,0000,,to any kind of pain. And as a result, she’s\Nabsent-mindedly dunked her hands in pots of
Dialogue: 0,0:02:36.31,0:02:41.70,Default,,0000,0000,0000,,boiling water, run around for days without noticing\Nbroken bones, and nearly chewed off her own tongue.
Dialogue: 0,0:02:41.70,0:02:45.86,Default,,0000,0000,0000,,Luckily, such congenital conditions are very\Nrare. The rest of us have a whole nervous
Dialogue: 0,0:02:45.87,0:02:51.54,Default,,0000,0000,0000,,system dedicated to making sure our bodies react with\Na predictable chain of events at the first sign of damage.
Dialogue: 0,0:02:51.54,0:02:54.92,Default,,0000,0000,0000,,Like say you just wake up and you’re extraordinarily\Nhungry for some reason, so you run downstairs
Dialogue: 0,0:02:54.93,0:02:58.17,Default,,0000,0000,0000,,to grab some clam chowder, but you didn’t put\Nany shoes on and suddenly you’re like, “YOWW!”
Dialogue: 0,0:02:58.17,0:03:01.16,Default,,0000,0000,0000,,There’s a tack, fell out of the wall, and\Nyou stepped right on it -- of course.
Dialogue: 0,0:03:01.16,0:03:04.16,Default,,0000,0000,0000,,Your foot immediately lifts off the ground,\Nand then you’re assuring your dog that you’re
Dialogue: 0,0:03:04.16,0:03:07.86,Default,,0000,0000,0000,,not yelling at her, you’re just yelling,\Nand then you limp over to the couch, and sit
Dialogue: 0,0:03:07.86,0:03:11.02,Default,,0000,0000,0000,,down, and you pull up your foot, and remove\Nthat spiny devil from your flesh.
Dialogue: 0,0:03:11.02,0:03:14.10,Default,,0000,0000,0000,,You want to talk physiology? So what exactly\Njust happened in your body?
Dialogue: 0,0:03:14.10,0:03:18.48,Default,,0000,0000,0000,,Well, the first step was a change in your\Nenvironment -- that is, a stimulus that activated
Dialogue: 0,0:03:18.48,0:03:19.93,Default,,0000,0000,0000,,some of your sensory receptors.
Dialogue: 0,0:03:19.93,0:03:24.62,Default,,0000,0000,0000,,In this case, it was a change from the probably\Ncompletely ignored feeling of bare skin on
Dialogue: 0,0:03:24.62,0:03:29.91,Default,,0000,0000,0000,,a smooth floor to a distinct feeling of discomfort\N-- the sharp metal tack piercing your skin.
Dialogue: 0,0:03:29.91,0:03:34.01,Default,,0000,0000,0000,,Your peripheral nervous system’s mechano-\Nand nociceptors provided that base sensation,
Dialogue: 0,0:03:34.01,0:03:36.19,Default,,0000,0000,0000,,or awareness that something had changed.
Dialogue: 0,0:03:36.19,0:03:39.52,Default,,0000,0000,0000,,Then it went to your central nervous system\N-- first to the spinal cord that caused the
Dialogue: 0,0:03:39.53,0:03:44.33,Default,,0000,0000,0000,,immediate reflexive action of pulling up your foot,\Nand then your brain eventually interpreted that
Dialogue: 0,0:03:44.33,0:03:50.25,Default,,0000,0000,0000,,awareness into the perception of pain, and decided to\Npull the tack out and probably say an expletive or two.
Dialogue: 0,0:03:50.25,0:03:56.70,Default,,0000,0000,0000,,Pain itself is a pretty subjective feeling, but the\Nfact is, we all have the same pain threshold.
Dialogue: 0,0:03:56.70,0:04:00.62,Default,,0000,0000,0000,,That is, the point where a stimulus is intense\Nenough to trigger action potentials in those
Dialogue: 0,0:04:00.62,0:04:06.82,Default,,0000,0000,0000,,nociceptors is the same for everybody. But, you and\NI might have different tolerances for discomfort.
Dialogue: 0,0:04:06.82,0:04:12.50,Default,,0000,0000,0000,,In general, most doctors think of pain as the perception\Nof pain -- whatever any given brain says pain is.
Dialogue: 0,0:04:12.51,0:04:16.87,Default,,0000,0000,0000,,So, you’ve got the stimulating event -- foot\Nmeets tack -- and then the reception of that
Dialogue: 0,0:04:16.87,0:04:21.56,Default,,0000,0000,0000,,signal, as the nociceptors in your foot sense\Nthat stimulus, and then the transmission of
Dialogue: 0,0:04:21.56,0:04:26.01,Default,,0000,0000,0000,,that signal through your nerves to your spinal\Ncord and eventually up to the brain.
Dialogue: 0,0:04:26.01,0:04:30.64,Default,,0000,0000,0000,,Now remember back how every neuron in your body\Nhas a membrane that keeps positive and negative
Dialogue: 0,0:04:30.64,0:04:35.64,Default,,0000,0000,0000,,charges separated across its boundaries, like a battery\Nsitting around waiting for something to happen?
Dialogue: 0,0:04:35.64,0:04:40.14,Default,,0000,0000,0000,,Well that tack in your flesh is that something.\NAnd it snaps those nociceptors to attention.
Dialogue: 0,0:04:40.14,0:04:44.70,Default,,0000,0000,0000,,Some neurons have mechanically-gated receptors\Nthat respond to a stretch in their membranes
Dialogue: 0,0:04:44.71,0:04:47.53,Default,,0000,0000,0000,,-- in this case, that happens when the tack\Npunches through them.
Dialogue: 0,0:04:47.53,0:04:51.43,Default,,0000,0000,0000,,Meanwhile, other neurons have ligand-gated\Nreceptors that open when the damaged skin
Dialogue: 0,0:04:51.43,0:04:55.00,Default,,0000,0000,0000,,tissue releases chemicals like histamine or\Npotassium ions.
Dialogue: 0,0:04:55.00,0:04:59.54,Default,,0000,0000,0000,,These channels allow sodium ions to flood\Ninto the neuron, causing a graded potential,
Dialogue: 0,0:04:59.54,0:05:03.72,Default,,0000,0000,0000,,if that hits the right threshold, it activates\Nthe electrical event that sends the signal
Dialogue: 0,0:05:03.72,0:05:08.41,Default,,0000,0000,0000,,all the way up the axon and gets one neuron\Ntalking to another -- the action potential.
Dialogue: 0,0:05:08.41,0:05:11.98,Default,,0000,0000,0000,,When that action potential races down the\Nlength of its axon to the terminal, the message
Dialogue: 0,0:05:11.98,0:05:17.26,Default,,0000,0000,0000,,hits the synapse that then flings it over that synaptic\Ngap to another neuron that’s in your spinal cord.
Dialogue: 0,0:05:17.26,0:05:21.04,Default,,0000,0000,0000,,Remember, signals travel between neurons either\Nby electrical or chemical synapses.
Dialogue: 0,0:05:21.05,0:05:24.87,Default,,0000,0000,0000,,The electrical ones send an electrical impulse,\Nwhile the chemical ones -- the ones I’m
Dialogue: 0,0:05:24.87,0:05:29.37,Default,,0000,0000,0000,,talking about now -- first convert that signal\Nfrom electrical to chemical, by activating
Dialogue: 0,0:05:29.37,0:05:33.68,Default,,0000,0000,0000,,neurotransmitters to bridge the synaptic gap,\Nbefore the receiving neuron converts that
Dialogue: 0,0:05:33.68,0:05:35.98,Default,,0000,0000,0000,,chemical signal back into an electrical one.
Dialogue: 0,0:05:35.98,0:05:41.25,Default,,0000,0000,0000,,In this case, news of the tack-attack is carried\Nby specific neurotransmitters whose sole job
Dialogue: 0,0:05:41.25,0:05:42.98,Default,,0000,0000,0000,,is to pass along pain messages.
Dialogue: 0,0:05:42.98,0:05:46.66,Default,,0000,0000,0000,,Now, so far, your body’s response to the\Nstimulus has been handled by the sensory,
Dialogue: 0,0:05:46.67,0:05:51.09,Default,,0000,0000,0000,,or afferent, division of your peripheral nervous\Nsystem. This is the part that’s involved
Dialogue: 0,0:05:51.09,0:05:54.66,Default,,0000,0000,0000,,expressly in collecting data and sending it\Nto the central nervous system.
Dialogue: 0,0:05:54.66,0:05:59.00,Default,,0000,0000,0000,,But at this point, the responsibility changes\Nhands. The torch is passed.
Dialogue: 0,0:05:59.00,0:06:04.36,Default,,0000,0000,0000,,Because the pain signal has just triggered\Nan action potential in a neuron in the spinal
Dialogue: 0,0:06:04.36,0:06:09.94,Default,,0000,0000,0000,,cord, which is part of the central nervous system,\Nand there it reaches an integration center.
Dialogue: 0,0:06:09.94,0:06:14.00,Default,,0000,0000,0000,,From here, the response is taken over by the\Nmotor, or efferent division.
Dialogue: 0,0:06:14.00,0:06:17.71,Default,,0000,0000,0000,,Once the integration center interprets the\Nsignal, it transmits the message to motor
Dialogue: 0,0:06:17.71,0:06:21.90,Default,,0000,0000,0000,,neurons, which send an action potential back\Ndown your leg, where it reaches an effector.
Dialogue: 0,0:06:21.90,0:06:26.59,Default,,0000,0000,0000,,And an effector is just any structure that\Nreceives and reacts to a motor neuron’s
Dialogue: 0,0:06:26.59,0:06:30.15,Default,,0000,0000,0000,,signal, like a muscle contracting or a gland\Nsecreting a hormone.
Dialogue: 0,0:06:30.15,0:06:33.50,Default,,0000,0000,0000,,From here, the motor neurons complete the\Nwhole foot-lifting response until the rest
Dialogue: 0,0:06:33.50,0:06:37.05,Default,,0000,0000,0000,,of your nervous system gets engaged in the\Ncomplicated tasks of figuring out what the
Dialogue: 0,0:06:37.05,0:06:38.74,Default,,0000,0000,0000,,problem is, and fixing it.
Dialogue: 0,0:06:38.74,0:06:43.02,Default,,0000,0000,0000,,Those are the five steps that your highly\Nspecific neural pathways go through to produce
Dialogue: 0,0:06:43.02,0:06:44.91,Default,,0000,0000,0000,,what’s known as a reflex arc.
Dialogue: 0,0:06:44.91,0:06:49.34,Default,,0000,0000,0000,,A lot of your body’s control systems boil\Ndown to reflexes just like this -- immediate
Dialogue: 0,0:06:49.34,0:06:53.94,Default,,0000,0000,0000,,reactions that can either be innate or learned, but\Ndon’t need much conscious processing in the brain.
Dialogue: 0,0:06:53.95,0:06:58.55,Default,,0000,0000,0000,,Lifting your foot when you step on a tack\Nis an innate, or intrinsic, reflex action
Dialogue: 0,0:06:58.55,0:07:01.90,Default,,0000,0000,0000,,-- a super fast motor response to a startling\Nstimulus.
Dialogue: 0,0:07:01.90,0:07:06.51,Default,,0000,0000,0000,,These reflexes are so invested in your self-preservation\Nthat you actually can’t think about them
Dialogue: 0,0:07:06.51,0:07:07.73,Default,,0000,0000,0000,,before you respond.
Dialogue: 0,0:07:07.73,0:07:11.43,Default,,0000,0000,0000,,All this processing happens in the spinal\Ncord, so that the control of muscles can be
Dialogue: 0,0:07:11.43,0:07:14.90,Default,,0000,0000,0000,,initiated before the pain is actually perceived\Nby the brain.
Dialogue: 0,0:07:14.90,0:07:18.99,Default,,0000,0000,0000,,Learned, or acquired reflexes on the other\Nhand, come from experience. Like how you learn
Dialogue: 0,0:07:18.99,0:07:23.82,Default,,0000,0000,0000,,to dodge obstacles while riding a bike or\Ndriving a car. That process is also largely
Dialogue: 0,0:07:23.82,0:07:29.08,Default,,0000,0000,0000,,automatic, but you learn those reflexes by spending\Ntime behind the wheel, or behind the handlebars.
Dialogue: 0,0:07:29.08,0:07:33.14,Default,,0000,0000,0000,,And reflex arcs stimulate some muscles, while\Ninhibiting others. For example, the tack in
Dialogue: 0,0:07:33.15,0:07:37.71,Default,,0000,0000,0000,,your right foot ended up activating the motor\Nneurons in your right hip flexors and hamstring,
Dialogue: 0,0:07:37.71,0:07:40.14,Default,,0000,0000,0000,,causing that knee to bend and your foot to\Nlift up.
Dialogue: 0,0:07:40.15,0:07:44.95,Default,,0000,0000,0000,,But it also told the quad muscles in your\Nleft leg to extend and stand tall, allowing
Dialogue: 0,0:07:44.95,0:07:47.15,Default,,0000,0000,0000,,you to shift your body’s weight off the\Ntack.
Dialogue: 0,0:07:47.15,0:07:50.81,Default,,0000,0000,0000,,Of course not all reflexes come from pain,\Nas you’ve probably experienced when a doctor
Dialogue: 0,0:07:50.81,0:07:52.46,Default,,0000,0000,0000,,tapped your knee and your foot kicked.
Dialogue: 0,0:07:52.46,0:07:56.67,Default,,0000,0000,0000,,Your muscles and tendons are very sensitive\Nto being stretched too far, or too fast, because
Dialogue: 0,0:07:56.67,0:07:58.12,Default,,0000,0000,0000,,that kind of movement can cause injury.
Dialogue: 0,0:07:58.12,0:08:03.20,Default,,0000,0000,0000,,So for this we have receptors called muscle\Nand tendon spindles that specifically sense
Dialogue: 0,0:08:03.20,0:08:07.64,Default,,0000,0000,0000,,stretching. If triggered by an over-stretch,\Nthey generate a reflex arc that contracts
Dialogue: 0,0:08:07.64,0:08:09.68,Default,,0000,0000,0000,,the muscle to keep it from stretching further.
Dialogue: 0,0:08:09.68,0:08:12.54,Default,,0000,0000,0000,,So, when does the brain actually get involved\Nin all this?
Dialogue: 0,0:08:12.54,0:08:16.83,Default,,0000,0000,0000,,Well, when your spinal cord sent impulses\Ndown the motor neurons, it also sent signals
Dialogue: 0,0:08:16.83,0:08:19.30,Default,,0000,0000,0000,,up your spinal cord toward the brain.
Dialogue: 0,0:08:19.30,0:08:23.85,Default,,0000,0000,0000,,News of the tack arrived first at your thalamus,\Nthe information switchboard that then split
Dialogue: 0,0:08:23.85,0:08:29.08,Default,,0000,0000,0000,,the message and sent it to the somatosensory\Ncortex -- which identifies and localizes the
Dialogue: 0,0:08:29.08,0:08:34.08,Default,,0000,0000,0000,,pain, like: “sharp, and foot”; as well\Nas the limbic system, which registers emotional
Dialogue: 0,0:08:34.08,0:08:38.48,Default,,0000,0000,0000,,suffering -- like, “why tack? Why me?!”\NAnd it also went to the frontal cortex, which
Dialogue: 0,0:08:38.48,0:08:42.91,Default,,0000,0000,0000,,made sense of it all, assigning meaning to\Nthe pain -- like, “oh, I see this tack fell
Dialogue: 0,0:08:42.91,0:08:44.96,Default,,0000,0000,0000,,from the Crash Course poster on the wall here.”
Dialogue: 0,0:08:44.96,0:08:48.70,Default,,0000,0000,0000,,So basically, although your body has been\Nreacting all along, it’s not until those
Dialogue: 0,0:08:48.70,0:08:53.67,Default,,0000,0000,0000,,pain signals hit the brain that you have the\Nconscious thoughts of both “dang, that hurt,”
Dialogue: 0,0:08:53.67,0:08:56.93,Default,,0000,0000,0000,,and “oh, that hurt because I stepped on\Na specific pointy thing.“
Dialogue: 0,0:08:56.93,0:09:01.31,Default,,0000,0000,0000,,And this is where I want to point out that\Nwe here at Crash Course cannot be held responsible
Dialogue: 0,0:09:01.31,0:09:06.17,Default,,0000,0000,0000,,for any injuries sustained in the process\Nof owning a Crash Course poster. Enjoy them
Dialogue: 0,0:09:06.17,0:09:07.48,Default,,0000,0000,0000,,at your own risk.
Dialogue: 0,0:09:07.49,0:09:11.64,Default,,0000,0000,0000,,Today you got your first look at the peripheral\Nnervous system, by learning how the afferent
Dialogue: 0,0:09:11.64,0:09:16.57,Default,,0000,0000,0000,,and efferent divisions provide information\Nabout, and responses to, pain. You learned
Dialogue: 0,0:09:16.57,0:09:21.87,Default,,0000,0000,0000,,about the five steps of the reflex arc, the\Ndifferent kinds of reflexes you have, and
Dialogue: 0,0:09:21.87,0:09:25.98,Default,,0000,0000,0000,,what your brain has to say about all that\Npain, once the news is finally broken to it.
Dialogue: 0,0:09:25.98,0:09:30.34,Default,,0000,0000,0000,,Crash Course is now on Patreon! Big thanks\Nto all of our supporters on Patreon who make
Dialogue: 0,0:09:30.34,0:09:33.93,Default,,0000,0000,0000,,Crash Course possible for themselves and for\Nthe whole rest of the world through their
Dialogue: 0,0:09:33.93,0:09:38.34,Default,,0000,0000,0000,,monthly contributions. If you like Crash Course\Nand you want to help us keep making great
Dialogue: 0,0:09:38.34,0:09:42.36,Default,,0000,0000,0000,,new videos like this one, you can check out\NPatreon.com/CrashCourse
Dialogue: 0,0:09:42.36,0:09:45.82,Default,,0000,0000,0000,,This episode was written by Kathleen Yale.\NThe script was edited by Blake de Pastino,
Dialogue: 0,0:09:45.83,0:09:50.05,Default,,0000,0000,0000,,and our consultant, is Dr. Brandon Jackson.\NIt was directed by Nicholas Jenkins, edited
Dialogue: 0,0:09:50.05,0:09:53.07,Default,,0000,0000,0000,,by Nicole Sweeney, and our graphics team is\NThought Café.