1 00:00:00,000 --> 00:00:03,380 When it comes to the nervous system, or just your body in general, let’s face it: 2 00:00:03,380 --> 00:00:05,240 your brain gets all the props. 3 00:00:05,240 --> 00:00:10,160 And it deserves those props! It’s a complicated, and crucial, and sometimes crazy boss of an organ. 4 00:00:10,160 --> 00:00:13,920 But your brain would be pretty useless without a support team that kept it 5 00:00:13,920 --> 00:00:15,780 connected to the outside world. 6 00:00:15,780 --> 00:00:20,000 Because frankly, like any leader, the more isolated your brain gets, the weirder it gets. 7 00:00:20,010 --> 00:00:24,330 Put a person in a watery, pitch-black sensory deprivation tank, and you’ll see the brain 8 00:00:24,330 --> 00:00:29,340 do some really weird stuff. Without a constant flood of external information, the brain starts 9 00:00:29,340 --> 00:00:35,000 to confuse its own thoughts for actual experiences, leading you to hallucinate the taste of cheeseburgers, 10 00:00:35,000 --> 00:00:38,860 or the sound of a choir singing, or the sight of pink stampeding elephants. 11 00:00:38,860 --> 00:00:43,109 It’s your peripheral nervous system that keeps things real, by putting your brain in 12 00:00:43,109 --> 00:00:47,669 touch with the physical environment around you, and allowing it to respond. This network snakes 13 00:00:47,669 --> 00:00:51,069 through just about every part of your body, providing the central nervous system with 14 00:00:51,069 --> 00:00:55,680 information ranging from the temperature, to the touch of a hand on your shoulder, to a twisted ankle. 15 00:00:55,680 --> 00:01:00,080 The peripheral nervous system’s sensory nerve receptors spy on the world for the central 16 00:01:00,089 --> 00:01:03,659 nervous system, and each type responds to different kinds of stimuli. 17 00:01:03,659 --> 00:01:08,570 Thermoreceptors respond to changes in temperature. photoreceptors react to light, chemoreceptors 18 00:01:08,570 --> 00:01:12,950 pay attention to chemicals, and mechanoreceptors respond to pressure, touch, and vibration. 19 00:01:12,950 --> 00:01:16,810 And then we’ve got specialized nerve receptors called nociceptors that, unlike those other 20 00:01:16,810 --> 00:01:21,340 receptors, fire only to indicate pain, which is the main thing I want to talk about today. 21 00:01:21,340 --> 00:01:26,399 Because, as unpleasant as a stick in the eye or tack in the foot may be, pain is actually 22 00:01:26,400 --> 00:01:31,060 a great example of where everything we’ve talked about over the last few weeks all comes together, 23 00:01:31,060 --> 00:01:36,740 as we trace a pain signal through your nervous system, from the first cuss to the Hello Kitty band aid. 24 00:01:36,750 --> 00:01:39,630 By the end of this episode of Crash Course Anatomy & Physiology you’ll never think 25 00:01:39,630 --> 00:01:43,990 of a stubbed toe, pounding headache, or burned tongue the same way again. 26 00:01:53,969 --> 00:01:58,369 Most people go to great lengths to avoid pain, but really, it’s an incredibly useful sensation, 27 00:01:58,369 --> 00:02:01,569 because it helps protect us from ourselves, and from the outside world. 28 00:02:01,569 --> 00:02:06,009 If you’re feeling physical pain, it probably means that your body is under stress, damaged, 29 00:02:06,009 --> 00:02:10,740 or in danger, and your nervous system is sending a cease and desist signal to stop twisting 30 00:02:10,750 --> 00:02:15,380 your arm like that, or to back away from that bonfire, or please seek medical attention, like, RIGHT NOW. 31 00:02:15,380 --> 00:02:18,700 So in that way, pain is actually good for you -- that’s why it exists. I’m not saying 32 00:02:18,700 --> 00:02:22,880 it’s pleasant, but if you’ve ever wished for an X-Men-like power to be impervious to 33 00:02:22,880 --> 00:02:26,480 pain, I’ve gotta say, that is one foolish monkey’s paw of a wish. 34 00:02:26,480 --> 00:02:31,140 Just ask Ashlyn Blocker. She’s got a genetic mutation that’s given her a total insensitivity 35 00:02:31,140 --> 00:02:36,310 to any kind of pain. And as a result, she’s absent-mindedly dunked her hands in pots of 36 00:02:36,310 --> 00:02:41,700 boiling water, run around for days without noticing broken bones, and nearly chewed off her own tongue. 37 00:02:41,700 --> 00:02:45,860 Luckily, such congenital conditions are very rare. The rest of us have a whole nervous 38 00:02:45,870 --> 00:02:51,540 system dedicated to making sure our bodies react with a predictable chain of events at the first sign of damage. 39 00:02:51,540 --> 00:02:54,920 Like say you just wake up and you’re extraordinarily hungry for some reason, so you run downstairs 40 00:02:54,930 --> 00:02:58,170 to grab some clam chowder, but you didn’t put any shoes on and suddenly you’re like, “YOWW!” 41 00:02:58,170 --> 00:03:01,160 There’s a tack, fell out of the wall, and you stepped right on it -- of course. 42 00:03:01,160 --> 00:03:04,160 Your foot immediately lifts off the ground, and then you’re assuring your dog that you’re 43 00:03:04,160 --> 00:03:07,860 not yelling at her, you’re just yelling, and then you limp over to the couch, and sit 44 00:03:07,860 --> 00:03:11,020 down, and you pull up your foot, and remove that spiny devil from your flesh. 45 00:03:11,020 --> 00:03:14,100 You want to talk physiology? So what exactly just happened in your body? 46 00:03:14,100 --> 00:03:18,480 Well, the first step was a change in your environment -- that is, a stimulus that activated 47 00:03:18,480 --> 00:03:19,930 some of your sensory receptors. 48 00:03:19,930 --> 00:03:24,620 In this case, it was a change from the probably completely ignored feeling of bare skin on 49 00:03:24,620 --> 00:03:29,910 a smooth floor to a distinct feeling of discomfort -- the sharp metal tack piercing your skin. 50 00:03:29,910 --> 00:03:34,010 Your peripheral nervous system’s mechano- and nociceptors provided that base sensation, 51 00:03:34,010 --> 00:03:36,190 or awareness that something had changed. 52 00:03:36,190 --> 00:03:39,520 Then it went to your central nervous system -- first to the spinal cord that caused the 53 00:03:39,530 --> 00:03:44,330 immediate reflexive action of pulling up your foot, and then your brain eventually interpreted that 54 00:03:44,330 --> 00:03:50,250 awareness into the perception of pain, and decided to pull the tack out and probably say an expletive or two. 55 00:03:50,250 --> 00:03:56,700 Pain itself is a pretty subjective feeling, but the fact is, we all have the same pain threshold. 56 00:03:56,700 --> 00:04:00,620 That is, the point where a stimulus is intense enough to trigger action potentials in those 57 00:04:00,620 --> 00:04:06,820 nociceptors is the same for everybody. But, you and I might have different tolerances for discomfort. 58 00:04:06,820 --> 00:04:12,500 In general, most doctors think of pain as the perception of pain -- whatever any given brain says pain is. 59 00:04:12,510 --> 00:04:16,870 So, you’ve got the stimulating event -- foot meets tack -- and then the reception of that 60 00:04:16,870 --> 00:04:21,560 signal, as the nociceptors in your foot sense that stimulus, and then the transmission of 61 00:04:21,560 --> 00:04:26,010 that signal through your nerves to your spinal cord and eventually up to the brain. 62 00:04:26,010 --> 00:04:30,640 Now remember back how every neuron in your body has a membrane that keeps positive and negative 63 00:04:30,640 --> 00:04:35,640 charges separated across its boundaries, like a battery sitting around waiting for something to happen? 64 00:04:35,640 --> 00:04:40,140 Well that tack in your flesh is that something. And it snaps those nociceptors to attention. 65 00:04:40,140 --> 00:04:44,700 Some neurons have mechanically-gated receptors that respond to a stretch in their membranes 66 00:04:44,710 --> 00:04:47,530 -- in this case, that happens when the tack punches through them. 67 00:04:47,530 --> 00:04:51,430 Meanwhile, other neurons have ligand-gated receptors that open when the damaged skin 68 00:04:51,430 --> 00:04:55,000 tissue releases chemicals like histamine or potassium ions. 69 00:04:55,000 --> 00:04:59,540 These channels allow sodium ions to flood into the neuron, causing a graded potential, 70 00:04:59,540 --> 00:05:03,720 if that hits the right threshold, it activates the electrical event that sends the signal 71 00:05:03,720 --> 00:05:08,410 all the way up the axon and gets one neuron talking to another -- the action potential. 72 00:05:08,410 --> 00:05:11,980 When that action potential races down the length of its axon to the terminal, the message 73 00:05:11,980 --> 00:05:17,260 hits the synapse that then flings it over that synaptic gap to another neuron that’s in your spinal cord. 74 00:05:17,260 --> 00:05:21,040 Remember, signals travel between neurons either by electrical or chemical synapses. 75 00:05:21,050 --> 00:05:24,870 The electrical ones send an electrical impulse, while the chemical ones -- the ones I’m 76 00:05:24,870 --> 00:05:29,370 talking about now -- first convert that signal from electrical to chemical, by activating 77 00:05:29,370 --> 00:05:33,680 neurotransmitters to bridge the synaptic gap, before the receiving neuron converts that 78 00:05:33,680 --> 00:05:35,980 chemical signal back into an electrical one. 79 00:05:35,980 --> 00:05:41,250 In this case, news of the tack-attack is carried by specific neurotransmitters whose sole job 80 00:05:41,250 --> 00:05:42,980 is to pass along pain messages. 81 00:05:42,980 --> 00:05:46,660 Now, so far, your body’s response to the stimulus has been handled by the sensory, 82 00:05:46,670 --> 00:05:51,090 or afferent, division of your peripheral nervous system. This is the part that’s involved 83 00:05:51,090 --> 00:05:54,660 expressly in collecting data and sending it to the central nervous system. 84 00:05:54,660 --> 00:05:59,000 But at this point, the responsibility changes hands. The torch is passed. 85 00:05:59,000 --> 00:06:04,360 Because the pain signal has just triggered an action potential in a neuron in the spinal 86 00:06:04,360 --> 00:06:09,940 cord, which is part of the central nervous system, and there it reaches an integration center. 87 00:06:09,940 --> 00:06:14,000 From here, the response is taken over by the motor, or efferent division. 88 00:06:14,000 --> 00:06:17,710 Once the integration center interprets the signal, it transmits the message to motor 89 00:06:17,710 --> 00:06:21,900 neurons, which send an action potential back down your leg, where it reaches an effector. 90 00:06:21,900 --> 00:06:26,590 And an effector is just any structure that receives and reacts to a motor neuron’s 91 00:06:26,590 --> 00:06:30,150 signal, like a muscle contracting or a gland secreting a hormone. 92 00:06:30,150 --> 00:06:33,500 From here, the motor neurons complete the whole foot-lifting response until the rest 93 00:06:33,500 --> 00:06:37,050 of your nervous system gets engaged in the complicated tasks of figuring out what the 94 00:06:37,050 --> 00:06:38,740 problem is, and fixing it. 95 00:06:38,740 --> 00:06:43,020 Those are the five steps that your highly specific neural pathways go through to produce 96 00:06:43,020 --> 00:06:44,910 what’s known as a reflex arc. 97 00:06:44,910 --> 00:06:49,340 A lot of your body’s control systems boil down to reflexes just like this -- immediate 98 00:06:49,340 --> 00:06:53,940 reactions that can either be innate or learned, but don’t need much conscious processing in the brain. 99 00:06:53,949 --> 00:06:58,550 Lifting your foot when you step on a tack is an innate, or intrinsic, reflex action 100 00:06:58,550 --> 00:07:01,900 -- a super fast motor response to a startling stimulus. 101 00:07:01,900 --> 00:07:06,510 These reflexes are so invested in your self-preservation that you actually can’t think about them 102 00:07:06,510 --> 00:07:07,730 before you respond. 103 00:07:07,730 --> 00:07:11,430 All this processing happens in the spinal cord, so that the control of muscles can be 104 00:07:11,430 --> 00:07:14,900 initiated before the pain is actually perceived by the brain. 105 00:07:14,900 --> 00:07:18,990 Learned, or acquired reflexes on the other hand, come from experience. Like how you learn 106 00:07:18,990 --> 00:07:23,820 to dodge obstacles while riding a bike or driving a car. That process is also largely 107 00:07:23,820 --> 00:07:29,080 automatic, but you learn those reflexes by spending time behind the wheel, or behind the handlebars. 108 00:07:29,080 --> 00:07:33,140 And reflex arcs stimulate some muscles, while inhibiting others. For example, the tack in 109 00:07:33,150 --> 00:07:37,710 your right foot ended up activating the motor neurons in your right hip flexors and hamstring, 110 00:07:37,710 --> 00:07:40,140 causing that knee to bend and your foot to lift up. 111 00:07:40,150 --> 00:07:44,949 But it also told the quad muscles in your left leg to extend and stand tall, allowing 112 00:07:44,949 --> 00:07:47,150 you to shift your body’s weight off the tack. 113 00:07:47,150 --> 00:07:50,810 Of course not all reflexes come from pain, as you’ve probably experienced when a doctor 114 00:07:50,810 --> 00:07:52,460 tapped your knee and your foot kicked. 115 00:07:52,460 --> 00:07:56,669 Your muscles and tendons are very sensitive to being stretched too far, or too fast, because 116 00:07:56,669 --> 00:07:58,120 that kind of movement can cause injury. 117 00:07:58,120 --> 00:08:03,199 So for this we have receptors called muscle and tendon spindles that specifically sense 118 00:08:03,199 --> 00:08:07,639 stretching. If triggered by an over-stretch, they generate a reflex arc that contracts 119 00:08:07,639 --> 00:08:09,680 the muscle to keep it from stretching further. 120 00:08:09,680 --> 00:08:12,540 So, when does the brain actually get involved in all this? 121 00:08:12,540 --> 00:08:16,830 Well, when your spinal cord sent impulses down the motor neurons, it also sent signals 122 00:08:16,830 --> 00:08:19,300 up your spinal cord toward the brain. 123 00:08:19,300 --> 00:08:23,850 News of the tack arrived first at your thalamus, the information switchboard that then split 124 00:08:23,850 --> 00:08:29,080 the message and sent it to the somatosensory cortex -- which identifies and localizes the 125 00:08:29,080 --> 00:08:34,078 pain, like: “sharp, and foot”; as well as the limbic system, which registers emotional 126 00:08:34,078 --> 00:08:38,479 suffering -- like, “why tack? Why me?!” And it also went to the frontal cortex, which 127 00:08:38,479 --> 00:08:42,909 made sense of it all, assigning meaning to the pain -- like, “oh, I see this tack fell 128 00:08:42,909 --> 00:08:44,959 from the Crash Course poster on the wall here.” 129 00:08:44,959 --> 00:08:48,699 So basically, although your body has been reacting all along, it’s not until those 130 00:08:48,699 --> 00:08:53,670 pain signals hit the brain that you have the conscious thoughts of both “dang, that hurt,” 131 00:08:53,670 --> 00:08:56,930 and “oh, that hurt because I stepped on a specific pointy thing.“ 132 00:08:56,930 --> 00:09:01,309 And this is where I want to point out that we here at Crash Course cannot be held responsible 133 00:09:01,309 --> 00:09:06,169 for any injuries sustained in the process of owning a Crash Course poster. Enjoy them 134 00:09:06,169 --> 00:09:07,480 at your own risk. 135 00:09:07,490 --> 00:09:11,639 Today you got your first look at the peripheral nervous system, by learning how the afferent 136 00:09:11,639 --> 00:09:16,569 and efferent divisions provide information about, and responses to, pain. You learned 137 00:09:16,569 --> 00:09:21,869 about the five steps of the reflex arc, the different kinds of reflexes you have, and 138 00:09:21,869 --> 00:09:25,980 what your brain has to say about all that pain, once the news is finally broken to it. 139 00:09:25,980 --> 00:09:30,339 Crash Course is now on Patreon! Big thanks to all of our supporters on Patreon who make 140 00:09:30,339 --> 00:09:33,930 Crash Course possible for themselves and for the whole rest of the world through their 141 00:09:33,930 --> 00:09:38,339 monthly contributions. If you like Crash Course and you want to help us keep making great 142 00:09:38,339 --> 00:09:42,359 new videos like this one, you can check out Patreon.com/CrashCourse 143 00:09:42,360 --> 00:09:45,820 This episode was written by Kathleen Yale. The script was edited by Blake de Pastino, 144 00:09:45,829 --> 00:09:50,049 and our consultant, is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited 145 00:09:50,049 --> 00:09:53,069 by Nicole Sweeney, and our graphics team is Thought Café.