Okay. So in this video, we're going to be talking about adaptation. Now, adaptation is a reduction in receptor sensitivity when it is in the presence of a constant stimulus. This is basically what's going to happen if we have some kind of stimulus in our environment that is constant, maintained, or unchanging. Our receptors will basically begin to stop responding to it after a while. That is adaptation. And this is very important for neural health. So we don't want our neurons to have to be firing constantly if they don't need to be. By adapting, we can kind of save energy and save metabolic resources and give our neurons a little bit of a break. Now in the peripheral nervous system, we see two types of adaptation and we basically have these two types of receptors. First, we have phasic receptors, and these are also called fast-adapting receptors. As you probably guessed, these phasic receptors will adapt very quickly when we have a constant stimulus. So basically, when that stimulus is first applied, they will respond to it, they'll send up action potentials, but then as long as that stimulus is maintained and unchanging, they'll kind of just stop and they'll slow down their firing rate. What phasic receptors really do is report changes in the environment. Next, we have tonic receptors, and these are also called slow-adapting receptors. These receptors actually exhibit very little adaptation; what they do is they provide a sustained response. So basically, even though that stimulus is constant and unchanging, they are going to keep on firing. They are not going to adapt to it. Even though we're talking about these two types of receptors, I don't want you to think about these as like these black and white boxes and we're going to be organizing sense receptors neatly into either box. It is much more nuanced than that. I would encourage you to think of adaptation more like a continuum where some receptors tend to act more phasic, some tend to act more tonic, and some of them will have both phasic and tonic properties and kind of fall somewhere in the middle there. To give you an example of a phasic receptor, a nice example here is a thermoreceptor because, remember, what do thermoreceptors do? They report changes in temperature, right? That's their entire job. So these tend to act phasic, so when you enter a room with a different temperature, they will respond to that very quickly. And then, as long as that temperature is constant and maintained, they will kind of slow down their firing rate. Now, I do want to be very clear that thermoreceptors are not purely phasic in their behavior; they do have tonic qualities. They just exhibit this one very classic phasic property, and that's why we're using them as an example. Over here on the tonic end, a classic example of a tonic receptor is a nociceptor or a pain receptor. These give you a very sustained response. They take a long, long time to adapt, and that's very important. Evolutionarily speaking, if we have pain somewhere in our body, we need to be aware of that pain, and we have to adjust our behavior accordingly. Right? So it's unfortunate, but very good for us that nociceptors are tonic in their behavior. Another great example of a tonic receptor is a proprioceptor, so those receptors that give us information about the location of our muscles and joints and information about how our body is moving through space. Right? And that information is constantly getting sent up to our brain, and that is super important information to help us be able to successfully navigate the world, to be able to walk across the room, even just sit upright in my chair. My brain has to be getting constant signals from the muscles in my back, in my abs, in my legs, and all of that positioning to be able to keep me upright. So those are nociceptors and proprioceptors tend to be very very tonic in their behavior. And an example of one that can kind of exhibit, sometimes they'll have phasic properties, sometimes they'll have tonic properties, are mechanoreceptors. So some types of mechanoreceptors, these are of course the ones that respond to things like pressure and vibration, some types will be very tonic, they'll be very, slow adapting, and they'll give you this very sustained information about whatever pressure you're encountering, for example. And then other types of mechanoreceptors really focus more on reporting changes in the environment and they adapt very quickly to those changes, again as long as that stimulus is constant. Alright. So that is adaptation in a nutshell and I will see you guys in our next video. Bye bye.