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Raja Narayan 创建




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 the basic idea behind it is that 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 kind of pokes us right here and of course it doesn't this poke doesn't penetrate into our skin but it causes a force that's transmitted deep into the different layers what'll happen then is that there's a Meccano receptor 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 f ferrant nerve fibers notice there's an a right here and what a ferret 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 F ferent nerve fiber is it's taking this stimulus and telling the central nervous system something's going on and this is the contrast 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 ooh 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 will focus only on afferent fibers and there are two main afferent fibers we talked 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 talked about for all of 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 we 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 my sinners with two S's Meister'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 it could be anything else of that variety and it transmits a force deep into the skin we'll have mice nurse 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 mice nerves corpuscle and they're even these nuclei that you can see right here see there's one nucleus there's another nucleus there's another nucleus right there there's a whole bunch of nuclei and they're all within these cells now I'll just draw one like right there and let's say there's another disk right there and even another right there and I'll 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 deep down right here and it kind of nudges this disc it's also known as an epithelial or a laminar disc 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 disc to nudge over this way it actually shifts across from where this other epithelial disc was and so this disc 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 appeared nudges that way and when this disc moves away that actually allows sodium and other ions to kind of leak into the disc 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 the cells until they get to the bottom of my snares corpuscle which actually ends in an afferent nerve fiber and as you know if a bunch of sodium is present within a nerve it'll 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 if it then is that this mice nurse 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 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 meissner's corpuscle only works when you're putting this smooth cotton t-shirt on it's not firing after the t-shirts 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 Meisner scorp us Kohl 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 is 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'll eventually lead into this afferent nerve fiber and so keep that in mind we'll mention two other mechanoreceptors that have the same criterion here and to 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 my snares corpuscle sits in the papillary dermis so the papillary dermis this is the top layer of the dermis right below the epidermis and so good you'll notice this trick is going to work when we talk about all the other mechanoreceptors below