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Video transcript
- [Voiceover] And then down to the next mechanoreceptor. This is another corpuscle right here. We're familiar with corpuscles by now, and it's named after another scientist. This is Ruffini's corpuscle. You may have actually seen this by another term. Ruffini's Ending is another phrase they use for this mechanoreceptor. The idea is if we have some external stimulus again, this guy poking us all over again, generating this force that goes deep within our skin, Ruffini's corpuscle, which looks like this guy right here, will perceive it. It's very interesting because there are no disks or rings like we had with the other corpuscles. Instead, what you can kind of clearly see right here, and this is so beautiful, there's an actual nerve fiber right here, and I'll outline it. So that thing right here that's kind of being cast this way, that's the nerve fiber. This is our afferent nerve fiber right here. Do you remember what type it is? I think I heard you say a beta, so you're absolutely right. So this is our afferent nerve fiber, and actually this guy sort of branches into the corpuscle. So Ruffini's corpuscle right here has a whole bunch of afferent nerve fiber branches in it. But I didn't highlight the whole thing here, what else do we have hanging out up here? Well the other thing that we've got sort of coating this is collagen. Collagen you might remember is a structural protein. So I'll draw it just all around there and I'll label it right here. So there's collagen right here, collagen like what you might remember from the dermis. So what will happen is that if we stretch this skin up here with this external stimulus and we stretch it hard and we generate this force that goes deep into our skin and kind of hits Ruffini's Ending right here, it'll cause the collagen here to shift and be perturbed. As you might notice, the collagen is very intimately connected to the nerve fiber branches here and as the collagen shifts it opens up ion channels on these nerve branches that allow sodium that's kind of hanging out in the extra cellular matrix to sneak in. It will sneak in and then kind of go along this afferent nerve fiber to generate an action potential and move on to the central nervous system. It's really interesting because as long as this stretcher, this stimulus, is being applied to our skin, we're going to have the collagen being stretched and pushed out of the way to allow sodium or other ions to enter into our afferent nerve fiber. So you might have guessed this is going to be one of those receptors that respond to sustained touch, but the trick here is because this depends heavily on collagen you've got to think, what part of our skin has the most amount of collagen? Well we're going to have to go pretty deep for that because the part of our skin that has collagen is the dermis. So it'll be the dermis, but the dermis has two layers and the part of the dermis that has the most collagen is the reticular dermis. So that's how you can reason that out. It's sustained touch that causes the collagen to move and allow sodium to keep on entering. So where do we have the most collagen? The reticular dermis, and that's pretty deep. Awesome. Now let's move on to the last mechanoreceptor. The last mechanoreceptor we're going to talk about is called a hair follicle receptor. I'm hoping there are some neurons firing in your brain right now about this because we talked a little earlier about a non-hairy receptor, and now we're talking specifically about hair. So this should be a callback to the hairy mechanoreceptor that'll close that story that we talked about at the beginning. Likewise, I'm going to draw this stimulus a little differently. This perturbation right here will be something like this and I'll draw the nail if this is going to be like a finger, and it's kind of initiating a force in that direction. If this is over a part of skin that's relatively hairy and there was hair here, the hair will be deflected. So that is deflected hair, very beautiful shaft of hair right here that goes deep into the skin because you remember that hair protrudes from a follicle. So there's the hair shaft that goes at the bottom here. I'll draw a little follicle that kind of nests it right here. So it kind of goes like this right here, and that goes in the back, and that comes out at the front. The interesting thing about the hair follicle is that there is a nerve fiber that is actually wrapped around the part of the hair that sits in the follicle, and sure enough this is an afferent nerve fiber, and a beta nerve fiber. What you'll notice is that when the hair is deflected that allows for ions to leak into this afferent nerve kind of where the hair and the nerve are interfacing, and when it leaks in it kind of enters and goes along the length of this nerve and goes down here. That generates an action potential which can then convey a signal to our central nervous system. This all started from hair deflection. Hair deflection was the impetus for this signal to occur. So this is going to be used to perceive light touch because we're not even touching the skin right here. We're actually just touching the hair, so it's light touch on hairy skin. So light touch on hairy skin because our hairs are being deflected as we perceive the stimulus. While the hair kind of runs many layers of skin, remember that the follicle itself, this guy sits anchored in the reticular dermis. It sits in the reticular dermis and so that's the location we subscribe the hair follicle receptor to. Lastly, as long as we're kind of pushing along right here, what'll happen is that the sodium will leak in so long as there is an active space that's present, but because the reticular dermis is a place of such thick, dense connective tissue if we push the hair shaft and make an opening right here, remember that there's a ton of collagen kind of sitting around that can come and fill the gap and just block it so sodium could not enter. So what does that mean? This implies then that we need constantly changing stimuli in order to have a signal be generated. So this is the third of three mechanoreceptors that requires constantly changing stimuli so that the hair shaft can constantly be opening up a gap for sodium to enter. It has to do so around so much collagen that will plug the opening if we just have the hair held, deflected. That's the whole point of why you don't notice a hairband that you might be wearing over a long period of time because that's definitely deflecting your hair, but over time it's as noticeable as a smooth cotton t-shirt. It's there, but you're not constantly detecting it. It's important to emphasize that this is a very essential distinction from the other type of light touch we mentioned earlier with Merkel's disk.