WEBVTT - Ep56 "We do we care so much about touch?" 0:00:05.519 --> 0:00:09.840 Why does a cold pool feel warmer this second time 0:00:09.920 --> 0:00:12.799 you put your toes in. Why does a person who's 0:00:12.800 --> 0:00:17.320 trying to break into a safe run his fingers over sandpaper? 0:00:17.960 --> 0:00:20.599 Would it be great or not so great if you 0:00:20.680 --> 0:00:25.920 couldn't feel any physical pain? Why does stubbing your toe 0:00:26.400 --> 0:00:32.000 have different sensations through time? Why do Mediterranean cultures touch 0:00:32.040 --> 0:00:36.199 each other more while they're talking than Scandinavian cultures? And 0:00:36.240 --> 0:00:39.080 what does this have to do with cuddle puddles, or 0:00:39.120 --> 0:00:44.559 why NBA players bump chests or why puppies sleep in dogpiles. 0:00:47.560 --> 0:00:51.080 Welcome to Inner Cosmos with me David Eagleman. I'm a 0:00:51.159 --> 0:00:55.080 neuroscientist and author at Stanford and in these episodes we 0:00:55.160 --> 0:00:59.640 sail deeply into our three pound universe to understand why 0:00:59.760 --> 0:01:11.600 and how our lives look the way they do. Today's 0:01:11.600 --> 0:01:16.319 episode is a love story about our sense of touch, 0:01:17.000 --> 0:01:20.399 what it is, how it works, and why it plays 0:01:20.760 --> 0:01:23.760 such a critical role in our lives. So let me 0:01:23.840 --> 0:01:27.120 start with an acquaintance of mine, who many years ago 0:01:27.880 --> 0:01:31.920 worked as a midnight phone operator at AT and T. 0:01:32.440 --> 0:01:35.319 He would get calls from people in the middle of 0:01:35.360 --> 0:01:37.640 the night and he'd say, Hello, this is AT and 0:01:37.680 --> 0:01:40.680 t how can I help you? And they would say hi, 0:01:40.920 --> 0:01:42.959 and he'd say what can I help you with? And 0:01:43.000 --> 0:01:46.640 they'd say, I just need someone to talk with. So 0:01:46.680 --> 0:01:49.640 there's a lot of loneliness in the world now. I 0:01:49.680 --> 0:01:53.320 tend to be suspicious when people talk about problems as 0:01:53.440 --> 0:01:56.480 modern phenomenon, given that most things have a deep history, 0:01:56.880 --> 0:02:01.480 but an increase in loneliness might actually be something new. 0:02:01.560 --> 0:02:02.360 If you look back. 0:02:02.720 --> 0:02:07.280 Even just a century ago, people lived in boarding houses 0:02:07.320 --> 0:02:09.600 with lots of other people, and there was lots of 0:02:09.960 --> 0:02:14.000 intergenerational living where you had at least three generations in 0:02:14.040 --> 0:02:17.600 the same household. Now that was probably crazy making, but 0:02:17.760 --> 0:02:21.480 whatever the case, this situation has changed a lot. 0:02:21.760 --> 0:02:23.320 As the middle class grew. 0:02:23.800 --> 0:02:28.079 The aspiration was for nuclear families to each get their 0:02:28.160 --> 0:02:31.280 own house with their own dishwasher and their own washing 0:02:31.280 --> 0:02:34.560 machine and their own television set. And in most neighborhoods, 0:02:35.000 --> 0:02:38.480 yards became more and more private, and at some point 0:02:38.520 --> 0:02:41.639 people stopped hanging out in their front yards and playing 0:02:41.639 --> 0:02:44.920 on the streets, but instead hung out in their backyards 0:02:44.960 --> 0:02:48.600 behind their fences. And by the year two thousand we 0:02:48.680 --> 0:02:52.640 got books like Bowling Alone, which was written by the 0:02:52.680 --> 0:02:56.360 political scientist Robert Putnam, and this book was about the 0:02:56.440 --> 0:03:01.320 decline of civic engagement since the nineteen sixty people used 0:03:01.320 --> 0:03:04.680 to be in bowling clubs and belong to their church 0:03:04.720 --> 0:03:08.080 or synagogue or mosque and see a community of people 0:03:08.560 --> 0:03:12.040 every day and every weekend. And they were in rotary 0:03:12.080 --> 0:03:16.680 clubs and Shriner's clubs and Kawana's clubs, and people who 0:03:16.760 --> 0:03:20.920 were recent immigrants often had clubs from the towns they 0:03:20.919 --> 0:03:25.280 originally came from. And these clubs lasted for decades until 0:03:25.320 --> 0:03:28.799 the wave of immigrants finally died out and their children 0:03:28.840 --> 0:03:32.000 born in America didn't really need to keep this up anymore. 0:03:32.440 --> 0:03:36.960 So because of lots of factors like suburbanization and the 0:03:37.080 --> 0:03:40.680 rise of television, and the decline of unions and the 0:03:40.760 --> 0:03:45.320 disappearance of clubs, we ended up with this decline of 0:03:45.440 --> 0:03:46.480 social capital. 0:03:46.840 --> 0:03:47.080 Now. 0:03:47.200 --> 0:03:51.200 Of note here is that Putnam's book was published before 0:03:51.320 --> 0:03:55.960 the invention of the smartphone, and while the Internet existed 0:03:55.960 --> 0:03:58.920 and video games existed, they were nothing like what they 0:03:59.000 --> 0:04:03.400 quickly became in the first decade after that book was published. 0:04:03.720 --> 0:04:07.480 And so society has changed much further in the past 0:04:07.560 --> 0:04:11.320 quarter century. And the question is, are we now not 0:04:11.600 --> 0:04:15.640 just bowling alone but not even bowling at all. Has 0:04:15.920 --> 0:04:20.680 society actually gotten lonelier? Now, I'm going to address this 0:04:20.760 --> 0:04:25.240 from both sides, arguing that there's more opportunity for relationship 0:04:25.320 --> 0:04:30.400 and less. Now on the side of more opportunity, one 0:04:30.440 --> 0:04:33.240 of the consequences that has always amazed me about the 0:04:33.279 --> 0:04:37.240 Internet is that now every person has the chance to 0:04:37.360 --> 0:04:41.720 find their tribe. If you are a stamp collector, or 0:04:41.800 --> 0:04:46.320 a mathematical poet, or a corporate mystic or a punk 0:04:46.440 --> 0:04:51.360 rocker historian or whatever, you can find your people. There 0:04:51.400 --> 0:04:54.680 will be no end to the websites and subreddits and 0:04:54.720 --> 0:04:57.400 substacks and whatever else you need to make you feel 0:04:57.800 --> 0:05:00.880 like you have your group where if you grew up 0:05:01.080 --> 0:05:04.520 pre Internet, you would never know that so many others 0:05:04.720 --> 0:05:07.760 like you were out there. And related to this, a 0:05:07.800 --> 0:05:10.800 lot of parents lament that kids are spending all their 0:05:10.880 --> 0:05:13.800 time on video games. But most video games have built 0:05:13.880 --> 0:05:18.000 in social mechanisms so that you can chat with other people. Now, 0:05:18.200 --> 0:05:21.560 chatting is not the same as spending time in real life, 0:05:21.960 --> 0:05:24.600 but with a million games to choose from, you end 0:05:24.680 --> 0:05:28.520 up finding people who share your interest and there's a 0:05:28.600 --> 0:05:32.640 lower barrier to entry from making acquaintances, and sometimes these 0:05:32.680 --> 0:05:36.960 become friends. So with multiplayer games, we have gone from 0:05:37.240 --> 0:05:42.440 bowling alone to bowling virtually virtually but not alone with 0:05:42.720 --> 0:05:45.880 other people's avatars who can chat with us and become 0:05:46.000 --> 0:05:49.000 friends of a sort. And there's another important way in 0:05:49.040 --> 0:05:53.680 which I think technology will have a shot at reducing loneliness, 0:05:53.720 --> 0:05:57.839 and this is with AI. Already we've seen an explosion 0:05:57.880 --> 0:06:02.000 of companies selling AI relationships, and if you're interested in 0:06:02.040 --> 0:06:05.440 the complexities of this, please check out episode thirty nine, 0:06:05.800 --> 0:06:09.360 which is on the future of AI relationships. But beyond 0:06:09.520 --> 0:06:13.160 AI girlfriends or boyfriends, I suspect one of the places 0:06:13.200 --> 0:06:16.680 will see the most immediate impact with AI is in 0:06:16.839 --> 0:06:20.200 areas like mental health therapy. The fact that you can 0:06:20.240 --> 0:06:23.720 have a therapist twenty four to seven, someone you can 0:06:23.839 --> 0:06:25.880 talk to in the middle of the night, someone who 0:06:26.279 --> 0:06:29.880 remembers everything you've said, someone who is not at all 0:06:29.920 --> 0:06:33.240 interested in talking about themselves but just about listening to 0:06:33.279 --> 0:06:36.560 you and giving you good advice and being there and 0:06:36.600 --> 0:06:39.560 truly has your best interests in mind. And so the 0:06:39.680 --> 0:06:43.240 question is does new technology like the Internet and AI 0:06:43.880 --> 0:06:48.800 scratch the itch of loneliness. Because I'm generally a cyber optimist, 0:06:48.839 --> 0:06:53.120 I feel like there are many ways that can be correct. However, 0:06:53.920 --> 0:06:57.200 while this hits on many cylinders, what it misses is 0:06:57.240 --> 0:07:00.960 a big one. Touch And we really care about touch. 0:07:01.200 --> 0:07:03.440 So that's what I'm going to talk about in today's episode, 0:07:03.520 --> 0:07:07.040 what touch is all about? Because our world has become 0:07:07.080 --> 0:07:11.120 a lot of screens and less about holding hands or 0:07:11.200 --> 0:07:14.960 hugging with strangers or friends. And the more I surveyed 0:07:14.960 --> 0:07:17.280 friends of mine on this point, I was surprised to 0:07:17.400 --> 0:07:21.040 realize how much touch is lacking in a lot of 0:07:21.120 --> 0:07:23.960 people's lives. One of my friends goes to Burning Man 0:07:24.000 --> 0:07:26.520 every year, and the thing he loves the most is 0:07:26.520 --> 0:07:29.920 that people hug people. Total strangers will say, give me 0:07:29.960 --> 0:07:33.480 a hug, and it's innocent. People can do this without sexuality, 0:07:33.760 --> 0:07:37.240 and people clearly appreciate it. And there get togethers all 0:07:37.280 --> 0:07:41.239 over the world called cuddle puddles or cuddle parties where 0:07:41.320 --> 0:07:45.280 dozens of people lie on mats and cuddle one another 0:07:45.320 --> 0:07:49.680 and it's not sexual. Instead, it's fulfilling the very primitive 0:07:49.680 --> 0:07:52.880 desire we all have for touch. And by the way, 0:07:52.920 --> 0:07:56.360 the same desire is seen to cross the animal kingdom, 0:07:56.360 --> 0:08:02.240 where you constantly see non sexual physical contact among social animals. 0:08:02.400 --> 0:08:06.040 The way you'll see puppies collect into a big dog 0:08:06.120 --> 0:08:09.160 pile when they're sleeping, And so from the point of 0:08:09.240 --> 0:08:12.360 view of the brain, I've become really interested in what 0:08:12.560 --> 0:08:16.920 we might be losing here. We're in an era with 0:08:17.120 --> 0:08:23.880 infinite hyperlinks between people, but fewer physical links. So for 0:08:23.920 --> 0:08:26.520 today's episode, I want to take us on a journey 0:08:26.960 --> 0:08:30.880 about touch, what it's all about, and how it colors 0:08:31.000 --> 0:08:35.040 our lives. When we look at touch across the animal kingdom, 0:08:35.080 --> 0:08:37.720 we see that it plays a critical role in everything 0:08:37.760 --> 0:08:42.160 from child rearing to finding food to getting mates. But 0:08:42.240 --> 0:08:46.400 not everyone thinks highly of it. Plato thought that vision 0:08:46.559 --> 0:08:50.640 was hot stuff and considered touch to be the most carnal, 0:08:50.720 --> 0:08:56.040 the least noble, of the senses. But his student Aristotle 0:08:56.440 --> 0:09:00.000 developed a different view. He felt, we learn the world 0:09:00.160 --> 0:09:03.000 by going out and touching it. That's how we figure 0:09:03.080 --> 0:09:07.120 things out. Not everything can be achieved purely by looking 0:09:07.160 --> 0:09:11.840 around and reasoning. So Aristotle said, quote, while in respect 0:09:11.960 --> 0:09:16.080 of all other senses we fall below many species of animals, 0:09:16.559 --> 0:09:21.520 in respect of touch, we far excel all other species 0:09:21.760 --> 0:09:26.520 in exactness of discrimination. That is why man is the 0:09:26.559 --> 0:09:32.880 most intelligent of all animals. So why did Aristotle privilege touch. 0:09:33.000 --> 0:09:35.720 What is special about touch and how does it all work? 0:09:36.400 --> 0:09:40.240 So let's start at the beginning. The largest organ in 0:09:40.280 --> 0:09:44.440 your body is actually on your body. It's your skin, 0:09:44.840 --> 0:09:48.600 and it is a miraculous sort of material. Unlike your 0:09:48.600 --> 0:09:52.040 other senses, which are focused at your eyes and your 0:09:52.080 --> 0:09:55.520 ears and so on, touch is spread throughout the body, 0:09:56.080 --> 0:09:58.440 and touch is the only sense that puts you in 0:09:58.920 --> 0:10:02.800 direct con tact with your subject, instead of assessing it 0:10:02.840 --> 0:10:06.240 from a distance, as you do with photons with seeing, 0:10:06.360 --> 0:10:09.920 or air compression waves with hearing. In contrast, you have 0:10:10.000 --> 0:10:13.440 to be right up against something. You can't touch something 0:10:13.520 --> 0:10:17.560 without being touched yourself. So the really amazing part is 0:10:17.640 --> 0:10:21.960 how it works. You have a whole zoo of touch 0:10:22.040 --> 0:10:27.280 receptors in your skin. These are specialized sensors for pressure, 0:10:27.720 --> 0:10:30.240 for itch, for stretch, for temperature. 0:10:30.640 --> 0:10:32.920 Some receptors are specialized for pain. 0:10:33.600 --> 0:10:36.880 Some are for pain caused by chemicals like an acid, 0:10:37.000 --> 0:10:40.120 some are for mechanical pain like a cut, and some 0:10:40.200 --> 0:10:43.800 are for thermal pain like a burn. And all of 0:10:43.840 --> 0:10:49.679 these things work in collaboration to read different sensory aspects 0:10:49.800 --> 0:10:53.240 of the world. So when you pass your finger over 0:10:53.400 --> 0:10:57.080 something like the petal of a flower or the sleeve 0:10:57.160 --> 0:11:00.800 of your jacket or the back of your dog. Different 0:11:00.840 --> 0:11:05.760 receptor types read the object in different ways. And what's 0:11:05.800 --> 0:11:08.839 interesting is that feeling doesn't happen in the top layer 0:11:08.880 --> 0:11:11.040 of your skin, but in the layer underneath. And this 0:11:11.120 --> 0:11:15.800 is why safe crackers sand their fingertips to expose the 0:11:15.920 --> 0:11:18.880 sensitive skin in the layer underneath, so they can be 0:11:18.960 --> 0:11:22.920 really sensitive to the tiny, tiny shifts in the locking 0:11:22.960 --> 0:11:26.920 mechanism of the safe. So collectively, this whole zoo of 0:11:27.000 --> 0:11:29.760 receptors inside your skin allows you to tell a lot 0:11:29.800 --> 0:11:32.439 about what's happening in the outside world in terms of 0:11:32.840 --> 0:11:36.520 texture and temperature and pressure and roughness and hardness, and 0:11:36.559 --> 0:11:39.680 your sense of what's out there is totally dependent on 0:11:39.800 --> 0:11:40.600 these sensors. 0:11:40.920 --> 0:11:44.400 If you block their operation with a local. 0:11:44.240 --> 0:11:47.520 Anesthetic, then you don't sense anything out there at all. 0:11:47.960 --> 0:11:52.880 So these sensors work together to give you exquisite sensitivity. 0:11:53.320 --> 0:11:55.839 This is why you can reach into your pocket and 0:11:55.880 --> 0:11:58.959 you can tell a quarter from a nickel without looking. 0:11:59.440 --> 0:12:05.480 Your sensitive fingertips detect the tiny ridges around the quarter's edge, 0:12:05.960 --> 0:12:08.679 and those are very small, but that's nothing. Your fingers 0:12:08.679 --> 0:12:12.760 can actually detect textures that are way finer. You can 0:12:12.800 --> 0:12:17.800 feel a texture seventy five nanometers high. That's one one 0:12:17.840 --> 0:12:20.880 thousandth of a human hair. If you run your fingers 0:12:20.960 --> 0:12:23.560 over that, you can tell it's there. And you know 0:12:23.600 --> 0:12:26.720 all those ridges and grooves on your fingertips they serve 0:12:26.880 --> 0:12:33.000 as sensation amplifiers. They expand the skin's surface area when 0:12:33.040 --> 0:12:37.880 they encounter pressure. So evolution has sculpted your fingertips into 0:12:37.960 --> 0:12:43.480 incredibly fine instruments of touch. They traffic in information we 0:12:43.640 --> 0:12:58.600 can't otherwise perceive with our other senses. So I'm gonna 0:12:58.600 --> 0:13:00.480 come back to skin in just a second, but first 0:13:00.559 --> 0:13:02.600 I want to make it clear that your system of 0:13:03.040 --> 0:13:07.080 sensation from the body picks up information both from the 0:13:07.080 --> 0:13:10.960 outside but also from the inside, and collectively this is 0:13:11.000 --> 0:13:15.440 grouped into what we call the somatosensory system, which just 0:13:15.559 --> 0:13:19.640 means sensing from the body. As I said a minute ago. 0:13:19.840 --> 0:13:24.760 Unlike vision or hearing, somata sensation doesn't happen in a 0:13:25.400 --> 0:13:29.559 discrete sensory organ like eyes or ears. Instead, the receptors 0:13:29.640 --> 0:13:33.080 are widely distributed in the skin, and the muscles and 0:13:33.120 --> 0:13:37.080 the bones and the joints, in internal organs, in the 0:13:37.120 --> 0:13:40.640 cardiovascular system. So even though we call it one system, 0:13:40.679 --> 0:13:45.680 it's giant, and it's distributed, and every second of your life, 0:13:45.720 --> 0:13:50.440 these millions of receptors are streaming information to your brain, 0:13:51.000 --> 0:13:55.840 where the information then steers your behavior. Now, in today's episode, 0:13:55.840 --> 0:13:58.680 I'm not going to talk too much about the internal monitoring, 0:13:59.160 --> 0:14:01.960 but just for sketching out the landscape, I'll tell you 0:14:02.040 --> 0:14:04.400 two of the main things you're doing on the inside. 0:14:04.440 --> 0:14:10.120 The first is constantly determining where your limbs exist in space, 0:14:10.320 --> 0:14:14.480 your body position, and your limbs movement. This is known 0:14:14.520 --> 0:14:19.920 as proprioception. Now, knowing how your body is positioned this 0:14:20.000 --> 0:14:22.880 is something we just take for granted. But if you 0:14:23.080 --> 0:14:27.200 lose this information, as sometimes happens, for example, from a 0:14:27.320 --> 0:14:31.600 viral infection, you can no longer do things like walk 0:14:32.000 --> 0:14:35.800 because now you need to visually look at your feet 0:14:35.840 --> 0:14:38.120 and your hands to have a sense of where they 0:14:38.160 --> 0:14:42.360 even are. So normally, your brain knows where your limbs 0:14:42.360 --> 0:14:46.840 are by constantly drinking in this fat stream of information. 0:14:47.520 --> 0:14:51.480 But if that goes offline, your only way to estimate 0:14:51.600 --> 0:14:54.600 where your limbs are is to look at them visually. 0:14:54.920 --> 0:14:57.360 And if the lights in the room turn off, you 0:14:57.400 --> 0:15:00.000 are going to fall to the floor. You can't maintain 0:15:00.720 --> 0:15:04.560 posture much less make any kind of fine movements without 0:15:04.720 --> 0:15:08.040 having visual feedback. So even though you may or may 0:15:08.080 --> 0:15:12.120 not have even been aware of this crazy thing called proprioception, 0:15:12.680 --> 0:15:16.800 you cannot function without it. And the other internal monitoring 0:15:16.800 --> 0:15:21.320 that I'll mention is called interoception, which is this amazing 0:15:21.360 --> 0:15:24.720 system by which you monitor the inside of your body, 0:15:24.840 --> 0:15:28.560 like movements of your gut or muscle stretch or changes 0:15:28.600 --> 0:15:32.320 in heart rate or stretch receptors in your lungs that 0:15:32.480 --> 0:15:36.160 modulate your breathing rate, or stretch receptors in your stomach 0:15:36.240 --> 0:15:38.920 and guts that tell you when you're full. You also 0:15:39.080 --> 0:15:43.520 have chemical receptors and the brain which monitor carbon dioxide 0:15:43.600 --> 0:15:46.000 levels to tell you if you're suffocating, and you have 0:15:46.120 --> 0:15:49.520 these in the circulatory system to monitor blood levels and 0:15:49.520 --> 0:15:53.480 trigger thirst. You have receptors in the throat that are 0:15:53.520 --> 0:15:56.280 close cousins of the touch receptors in your skin, and 0:15:56.320 --> 0:15:58.960 these are what detect whether there's an object in there 0:15:59.000 --> 0:16:03.160 to trigger gag. And again, all the stuff we tend 0:16:03.440 --> 0:16:07.360 to take for granted, but every single one of these 0:16:07.480 --> 0:16:11.560 receptors is critical for you to have the experience in 0:16:11.600 --> 0:16:16.120 the world that you do. These underpin your ability to 0:16:16.200 --> 0:16:20.000 be a body moving around in the world. But for 0:16:20.160 --> 0:16:24.440 today we're gonna talk about the other side of somatic sensation, 0:16:24.640 --> 0:16:28.880 not monitoring the inside but touching the outside world. 0:16:29.320 --> 0:16:30.800 So for this your. 0:16:30.840 --> 0:16:34.280 Entire body is covered by that one giant sensory organ, 0:16:34.360 --> 0:16:41.320 the skin. Now, this large, seamless sensory sheet is completely 0:16:41.360 --> 0:16:44.760 packed with receptors that allow you to feel when something 0:16:44.880 --> 0:16:48.920 or someone makes contact with you, when a nearby object 0:16:48.960 --> 0:16:52.000 is vibrating, or when something is hot or cold. Now, 0:16:52.240 --> 0:16:55.680 touch seems pretty simple. You just push your skin up 0:16:55.680 --> 0:16:58.520 against something. You say, oh, that's there, and it's hot 0:16:58.600 --> 0:17:01.720 or it's cold, or it's or it's smooth. But this 0:17:01.920 --> 0:17:07.520 apparent simplicity masks the incredible variety and complexity of how 0:17:07.880 --> 0:17:12.359 your brain and body do this. So let's surface the details. 0:17:13.000 --> 0:17:17.800 So first, to detect pressure and vibration, you have a 0:17:17.960 --> 0:17:24.040 specific flavor of sensory receptors in your skin called mechano receptors, 0:17:24.320 --> 0:17:27.119 and these are just built of special structures that make 0:17:27.200 --> 0:17:32.000 them sensitive to being physically stretched or bent. Now you 0:17:32.080 --> 0:17:35.200 have different types of mechanic receptors that are different structures 0:17:35.200 --> 0:17:38.479 and locations, and that gives you the ability to sense 0:17:38.640 --> 0:17:41.520 and interpret a huge range of things. 0:17:41.560 --> 0:17:42.240 In the world. 0:17:42.480 --> 0:17:44.520 I won't go into too much detail, but i'll mention 0:17:44.600 --> 0:17:48.960 you have some receptors which respond to fine light touch 0:17:49.040 --> 0:17:53.160 if you're interested. These are called Meisner's corpuscles or mercle discs. 0:17:53.520 --> 0:17:56.280 And you have other types of receptors found in the 0:17:56.320 --> 0:18:00.240 deeper layers of the skin that respond to stronger and 0:18:00.400 --> 0:18:04.720 cruder pressure if you're interested. These are called Piscinian corpuscles 0:18:04.840 --> 0:18:09.120 or Ruffini's endings. Now, some of these receptor types respond 0:18:09.240 --> 0:18:11.840 quickly right when something gets touched, but then they stop 0:18:11.880 --> 0:18:15.800 responding if the touch remains there, and others of these 0:18:15.880 --> 0:18:19.760 are slowly adapting, which means they keep firing if the 0:18:19.760 --> 0:18:24.399 touch stays there. So the rapidly adapting ones are well 0:18:24.520 --> 0:18:27.399 suited to respond to something that's going on and off 0:18:27.600 --> 0:18:30.600 like a vibration, because they stop responding right after the 0:18:30.600 --> 0:18:33.720 initial thing, and therefore they're ready to respond again a 0:18:33.760 --> 0:18:37.800 moment later. But others stay firing, so you can know 0:18:37.880 --> 0:18:40.320 when you touch your coffee cup that you're still touching it. 0:18:40.720 --> 0:18:42.800 So the thing I want to illustrate here is that 0:18:42.840 --> 0:18:46.160 you have this interplay of different receptor types that gives 0:18:46.200 --> 0:18:52.240 you a very rich, multilayered sensation of touch, fine and coarse, 0:18:52.480 --> 0:18:56.080 fast and slow. Now it's not just mechanicy receptors that 0:18:56.119 --> 0:18:57.680 you have in your skin, but you've got these other 0:18:57.760 --> 0:19:02.280 things called thermo receptor which tell you about temperature, and 0:19:02.320 --> 0:19:07.600 you have separate thermo receptors for cool and warm. Now, 0:19:07.640 --> 0:19:10.800 if this wasn't something you already knew, it's kind of amazing. Right, 0:19:10.840 --> 0:19:15.000 You're like this sophisticated meat robot with all these sensors 0:19:15.040 --> 0:19:19.760 packed into your skin to detect different things in the world. Now, 0:19:19.800 --> 0:19:23.600 what these thermoreceptors do is they detect changes in temperature 0:19:23.680 --> 0:19:27.760 on the skin surface relative to body temperature. So a 0:19:27.880 --> 0:19:31.320 cold thermo receptor in your finger, it's going to signal 0:19:31.359 --> 0:19:33.600 only a little tiny bit of change. When you test 0:19:33.640 --> 0:19:35.680 the water coming out of the sink. There's going to 0:19:35.720 --> 0:19:38.280 trigger a really strong signal when you dunk your hand 0:19:38.320 --> 0:19:42.440 into a bucket of ice water. Now, speaking of cold water, 0:19:42.520 --> 0:19:46.480 you've probably noticed that a cold lake feels warmer the. 0:19:46.520 --> 0:19:48.119 Second time you dip your foot in. 0:19:48.840 --> 0:19:52.639 Why, obviously the temperature the water hasn't changed, but the 0:19:52.760 --> 0:19:57.280 relative temperature of the water compared to your now chilled skin, 0:19:58.040 --> 0:20:01.080 that difference is decreased, and that changes. 0:20:00.720 --> 0:20:03.080 Your perception of the water temperature. 0:20:03.560 --> 0:20:07.680 They have a separate set of receptors that carry information about. 0:20:07.640 --> 0:20:10.159 Warmth, but these cool and. 0:20:10.040 --> 0:20:13.959 Warm receptors, they track temperature changes only to a point, 0:20:14.080 --> 0:20:17.560 and after that there's signaling drops off and information about 0:20:17.560 --> 0:20:21.680 things that are very cold or very hot that gets 0:20:21.760 --> 0:20:26.200 taken over by separate receptors pain receptors, and I'll return 0:20:26.200 --> 0:20:28.600 to those in a moment, but before we get to pain, 0:20:28.640 --> 0:20:31.400 I just want to mention that these cool and warm 0:20:31.640 --> 0:20:35.280 thermoreceptors can get activated by other things. 0:20:35.000 --> 0:20:36.720 Like certain shapes of molecules. 0:20:36.960 --> 0:20:41.000 For example, you have molecules that happen to bind to 0:20:41.080 --> 0:20:45.679 these cold receptors, like menthol. Menthol is just a shape 0:20:45.720 --> 0:20:50.600 that happens to activate these cool thermoreceptors, giving the pleasant 0:20:50.720 --> 0:20:54.960 illusion of coolness. And because we generally like that, we 0:20:55.119 --> 0:20:58.960 dump that molecule into our toothpastes and our shaving creams 0:20:59.240 --> 0:21:04.919 to activate these cool thermoreceptors. Similarly, the active ingredient in 0:21:05.040 --> 0:21:09.080 chili peppers is called capsaicin, and this is a molecule 0:21:09.119 --> 0:21:13.399 of a certain shape that happens to bind to warm thermoreceptors, 0:21:13.680 --> 0:21:17.679 and therefore it creates the illusion of heat. So capsaicin 0:21:17.800 --> 0:21:20.840 is used in ointments that you rub on your skin, 0:21:21.040 --> 0:21:24.000 like icy hot. Okay, so now we've talked about how 0:21:24.040 --> 0:21:28.159 receptors in your skin detect touch and detect temperature, and 0:21:28.200 --> 0:21:32.200 now let's come back to pain. The sensation of pain 0:21:33.000 --> 0:21:35.960 is one that most of us would probably rather not have, 0:21:36.119 --> 0:21:39.919 but it's actually a gift from nature that's critical for 0:21:40.000 --> 0:21:43.840 our survival. Feeling pain is how we detect damage to 0:21:43.960 --> 0:21:45.480 our cells and our tissues. 0:21:45.800 --> 0:21:47.400 Now, our perception of. 0:21:47.440 --> 0:21:51.919 Pain is mediated by another type of little receptors called 0:21:52.320 --> 0:21:57.440 no susceptors, and what activates these is damage to the tissue. 0:21:57.800 --> 0:22:01.119 And even though pain just seems like pain, you actually 0:22:01.119 --> 0:22:04.760 have very different types of no susceptors. First, you have 0:22:04.840 --> 0:22:09.160 no susceptors that are activated by physical damage to the tissue, 0:22:09.359 --> 0:22:13.880 like caused by pressure, a needlestick, a broken bone. These 0:22:13.920 --> 0:22:19.040 are called mechanical no susceptors. Then you have thermal no susceptors, 0:22:19.040 --> 0:22:22.720 which respond to things that are extremely hot or extremely cold. 0:22:23.040 --> 0:22:26.440 Then you have chemical no susceptors, which are activated by 0:22:26.480 --> 0:22:29.840 things like spider toxins when you get bit, or by 0:22:30.119 --> 0:22:34.520 certain cooking spices, or by poisonous gases like the kind 0:22:34.600 --> 0:22:38.040 used as chemical weapons in World War One. Now again, 0:22:38.400 --> 0:22:41.840 you might think that feeling no pain would be a blessing, 0:22:41.920 --> 0:22:43.720 but how would we know whether that's true or not. 0:22:43.880 --> 0:22:48.720 The answer is this sometimes happens because of genetic mutations, 0:22:49.080 --> 0:22:53.679 and the inability to feel pain is a dangerous curse. 0:22:53.840 --> 0:22:57.159 So take as an example a man who I'll call Paul. 0:22:57.960 --> 0:23:00.720 When Paul was a child, he would push swing and 0:23:00.800 --> 0:23:03.520 let it smash into his face. He didn't mind the 0:23:03.560 --> 0:23:07.520 broken nose and chipped teeth because he has never felt 0:23:07.880 --> 0:23:12.720 physical pain. He has a rare disorder called congenital analgesia 0:23:12.920 --> 0:23:18.280 and inability to register painful sensations. By the way, congenital 0:23:18.480 --> 0:23:22.240 just means you're born with it, and algesia means pain, 0:23:22.480 --> 0:23:28.680 and analgesia means no pain. So congenital analgesia just means 0:23:28.960 --> 0:23:31.399 you can't feel any pain and you were born that way. 0:23:31.640 --> 0:23:35.119 So Paul can feel a knife cutting his finger, but 0:23:35.200 --> 0:23:39.520 only the touch is registered, not the pain of the injury. 0:23:40.080 --> 0:23:43.840 And because his thermoreceptors are intact, he can easily tell 0:23:43.880 --> 0:23:46.879 the difference between warm and cool water, but he doesn't 0:23:46.880 --> 0:23:50.080 feel the pain of things that are really hot or 0:23:50.119 --> 0:23:51.200 really cold. 0:23:51.720 --> 0:23:53.080 So feeling no. 0:23:53.240 --> 0:23:56.560 Pain is a curse for someone like Paul, because pain 0:23:57.040 --> 0:24:00.919 is critical to avoiding bodily damage. It's what tells you 0:24:00.960 --> 0:24:06.080 about harms and threats. When Paul interacts with extreme temperatures, 0:24:06.400 --> 0:24:10.080 he doesn't get any immediate feedback telling him to pull 0:24:10.160 --> 0:24:13.439 his hand away or to avoid the situation. One of 0:24:13.440 --> 0:24:17.280 his most frequent injuries as a child was burning himself. 0:24:17.600 --> 0:24:21.560 He was interested in listening to the sizzling sound that 0:24:21.640 --> 0:24:25.080 his skin made, so his parents were constantly forced to 0:24:25.160 --> 0:24:29.119 take creative and desperate measures to keep their son safe, 0:24:29.160 --> 0:24:32.520 like putting socks over both his hands and goggles over 0:24:32.560 --> 0:24:35.760 his eyes. Often they put a helmet on him to 0:24:35.840 --> 0:24:38.720 protect him from his regular risk of hen injury. They 0:24:39.119 --> 0:24:43.560 constantly checked him for swelling and bruising and burns. Because 0:24:43.600 --> 0:24:47.120 of his lack of sensation, this kind of outside inspection 0:24:47.320 --> 0:24:49.840 was the only way that they or he could tell 0:24:50.200 --> 0:24:54.080 whether tissue had been damaged. So much of Paul's childhood 0:24:54.200 --> 0:24:58.119 was spent in hospitals, whether from jumping off extreme heights 0:24:58.200 --> 0:25:00.760 or banging his head against the wall and his forehead 0:25:00.800 --> 0:25:05.120 swelled up. So Paul is lucky to be alive because 0:25:05.200 --> 0:25:10.080 not everyone with congenital analgesia makes it through childhood. The 0:25:10.240 --> 0:25:14.800 moral of Paul's story is that pain is two faced. 0:25:14.880 --> 0:25:20.120 It hurts, but it also protects, So back to normal 0:25:20.359 --> 0:25:25.479 pain perception. Fascinatingly, you have different types of fibers that 0:25:25.680 --> 0:25:28.760 carry these signals to the brain, and they carry information 0:25:28.880 --> 0:25:30.480 at different speeds. 0:25:31.000 --> 0:25:32.520 So let's say you stub your toe. 0:25:32.560 --> 0:25:35.200 The next time you do this, try to pay attention 0:25:35.400 --> 0:25:39.080 to the way the pain is registered, because thanks to 0:25:39.560 --> 0:25:45.159 some fast fibers called a delta fibers, you'll register a fast, 0:25:45.320 --> 0:25:48.879 sharp pain, and then you have these other much slower 0:25:48.960 --> 0:25:53.080 fibers called sea fibers, and when their signals finally catch up, 0:25:53.119 --> 0:25:57.320 you'll feel the more prolonged but slightly less intense pain. 0:25:57.840 --> 0:25:59.719