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- [Voiceover] So now that we understand the structure of skin, let's talk about how our skin helps us perceive our environment. How does it enable us to sense what's going on around us? And the basic idea behind it is when we have our skin, let's say this is just a schematic of skin right here and there are all the layers that are going on here, and there's some type of perturbation. Let's say someone just kinds of pokes us right here. And of course this poke doesn't penetrate into our skin but it causes a force that is transmitted deep into the different layers. What'll happen then is that there's a mechanoreceptor that sits about here that will perceive this force and then from it will generate an Action Potential, or a signal that it can then transmit to a neuron, or this nerve fiber that sits here that'll carry this message on to our Central Nervous System. And that's the basic layout of how these mechanoreceptors, and I'll write it out right here, how these mechanoreceptors help us perceive our external environment. And the other thing I guess I should mention at this point when we're talking about these nerve fibers is that they're called Afferent Nerve Fibers. So Afferent Nerve Fibers. And notice there's an a right here. What Afferent Nerve Fibers do is that they'll take some stimulus from our external environment right here and then create a signal that will then be transmitted to our central nervous system. And so that's what this Afferent Nerve Fiber is it's taking this stimulus and telling the Central Nervous System something is going on. And this is to contrast of what happens on the flip side. So that would be an Efferent Nerve Fiber. Notice that there's an e right there. And that does the exact opposite, where the Central Nervous System is going to respond to some stimulus. So this would be a nerve that goes to a muscle fiber that says, "Oh there's something that's really itching us "right here let's move our hand away." So that's what an Efferent Nerve Fiber would do. But because in this video we're talking about sensation we'll focus only on afferent fibers. And there are two main afferent fibers we talk about. There are A beta fibers and A delta fibers. A delta fibers are used to perceive pain and temperature. Pain and temperature, I'll just write temp right here. The A beta fibers perceive everything else so these are going to be the afferent fibers we talk about for all the mechanoreceptors in this video. There's a separate video that talks about the A delta nerve fiber. And the main thing to keep in mind as go and talk about the mechanism through which each of these mechanoreceptors work or their structure is that their structure is very important to understand because the structure of the mechanoreceptor actually determines their function. So structure determines function. So if we understand the structure we'll understand or be able to figure out what the function is. But furthermore, once we understand what the function of the mechanoreceptor is, from there the function will actually help us determine the location of the mechanoreceptor. So the function will help us figure out the location. So let's test that out below. The first mechanoreceptor we're going to talk about is called Meissner's, with two s's, Meissner's Corpuscle. And you'll see this word often, "corpuscle," and all this means is just body. So let's run our scenario if there's something that perturbs the skin or applies a force to it, it could be a finger or anything else of that variety and it transmits a force deep into the skin. We'll have Meissner's Corpuscle right here, that's what it looks like actually under a microscope. It'll perceive this force, so what happens then? Well one thing that you can kind of see here is that there are different layers of disks in Meissner's Corpuscle and there're even these nuclei that you can see right here. See there's one nucleus, there's another nucleus, and there's anther nucleus right there. There's a whole bunch of nuclei and they're all within these cells. Now I'll just draw one right there and let's say there's another disk right there and even another right there. And I'll draw one right here just kind of stacked right on top of this one right here. And so let's say this force is transmitted deep down right here and it kind of nudges this disk. It's also known as an epithelial or a laminar disk or a laminar disk. They're just specialized epithelial cells. So this initial nudge that we have from our external environment, our stimulus right here will then in turn cause this epithelial disk to nudge over this way. It actually shifts across from where this other epithelial disk was. And so this disk right here will stay put. So I'm just drawing these dots right here that kind of show the center. So these aren't going to be nudging but this guy up here nudges that way. And when this disk moves away that actually allows sodium and other ions to kind of leak into the disk below it. And these disks are actually connected and so when they shift across you'll have ions enter and they'll kind of propagate through all of the cells until they get to the bottom of Meissner's Corpuscle which actually ends in an Afferent Nerve Fiber. And as you know if a bunch of sodium is present within a nerve it will fire an Action Potential. And that's exactly what happens, an Action Potential is generated and we send a signal on to the Central Nervous System. And so notice here, this all started from a nudge. And this entire mechanism initiated. And so if this is a nudge, the way I think of it then is that this Meissner's Corpuscle can't sit too deep in our skin if it's perceiving a nudge. And so the function of this Corpuscle then is to perceive light touch. Light touch, and specifically in non-hairy skin. Non hairy skin. And we'll talk about what works in hairy skin. But another term for non-hairy skin is Glabrous skin. Glabrous skin. So it perceives light touch in Glabrous skin. And an example of light touch would be something like putting on a smooth cotton t-shirt. So putting on some type of smooth cotton t-shirt. And I want to emphasize here that Meisnner's Corpuscle only works when you're putting this smooth cotton t-shirt on. It's not firing after the t-shirt's already on you. You don't feel that you're wearing clothes every second of the day, that would be too much stimuli. And so a very important thing to keep in mind here is that Meissner's Corpuscle, in order to fire, requires constantly, not constant, but constantly changing stimuli. Constantly changing stimuli. And this kind of makes sense when we take a look at the mechanism involved here. Because when this disk moves across, that's when sodium can enter into the disk below. When it's moved across already and it's just kind of sitting adjacent here, the sodium's not going to be able to enter. It's the movement that allows the sodium to kind of tumble across or between these two disks to enter into a gap that will eventually lead into this Afferent Nerve Fiber. And so keep that in mind. We'll mention two other mechanoreceptors that have this same criterion here. And then continue with this thing that we were talking about that a nudge started all of this happening here and it's light touch that we're perceiving. The location can't be somewhere too deep in our skin. And so sure enough Meissner's Corpuscle sits in the papillary dermis. So the papillary dermis, this is the top layer of the dermis, right below the epidermis. So good you'll notice this trick is going to work when we talk about all the other mechanoreceptors below.