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Voiceover: In this video, we are going to talk about somatosensation. If we have someone walking down the street, he is receiving all kinds of information from the world around him. He's receiving different types of sensations, so different types of sensations. He's also receiving information about different intensities of these sensations, so is it really hot outside, is it kind of hot outside? He's receiving information about the intensity of the different sensations. He's also receiving information about the timing. When does he set his foot down on the ground? When does he lift the foot off the ground? If somebody drives by him and throws a hamburger at him, when does the hamburger hit him, when does the hamburger stop hitting him? There's also information about timing that he's receiving from the world around him. Finally, he's also receiving information about where in his body the sense is originating. If a bird comes by and pecks his arm, the brain needs to realize, there's something going on on my arm, so there's also location. Information about location needs to be addressed. What are the different types of senses that he can have, as he's walking down the street? Of course, we need to get information about the temperature, so is it really cold outside, is it really hot outside? The sensation of temperature, is something known as thermoception. Thermoception He also needs to get information about pressure. When does he set his foot down on the ground? If somebody came by, like we said, and threw a hamburger at him, how hard did the hamburger hit him? We need to get information about pressure. This is also known as mechanoception. Mechanoception What else does he need to know? He also needs to get information about pain. If the bird did come down and peck his arm, it would be a little painful, so he needs to get information about pain. Pain is known as nociception. Finally, he needs to get information about where his body is in space, so as he's walking along, is he really close to the sidewalk, is he close to someone else? He needs to get information about his body's location in space. This is known as proprioception. These are the different types of information that he needs to acquire from the world around him, through his body surface. He also needs to know something about the intensity of the various stimuli. If it's really, really hot outside, it would be really nice to know that, versus if it's kind of cold outside. The way intensity is encoded in the body, is by how quickly the neurons fire. There are different neurons that are sensitive to temperature, there are different neurons that are sensitive to pressure, so if it's really cold outside, then the neurons might not fire that much. They might fire a few times. If it's kind of cold outside, it's not that hot, they'll just fire a few times over the course of a minute. If it's really hot outside, they might fire a whole bunch of times. So how quickly they fire, is how intensity is coded. Another thing that we need to acquire from our environment is the timing, so if a bird were to come down, so we've got a little bird coming down, he's going to peck this man. He's coming down, he's flying, he's got a straight shot for the arm, and he pecks him right there. The guy actually needs to know, okay, when does the pecking start? When does the pecking stop? In order for a neuron to encode timing, there are three different ways he can do that. There's a neuron that, the entire time the bird is pecking him, it will consistently fire action potentials. This type of neuron is non-adapting. It's non-adapting because you can look at the action potentials and there is an equal amount of space between each successive action potential. Basically, the entire time some sort of stimulus is being applied, there is no change in the firing rate. Another type of neuron, will start firing really, really quickly, so there's a whole bunch of action potentials, and then over time, it will slow down. The space between the action potentials increases. As you can see here, it starts firing really fast at the beginning of the stimulus, and then it slows down, so the spacing actually increases. This is known as a slow adapting neuron. It's slow adapting because it's really slow to adapt to the change in the stimulus. Another type of neuron is going to fire really quickly, as soon as the stimulus starts, and then it's going to stop firing and then it will fire again when the stimulus stops. This is known as a fast adapting neuron, so fast adapting. Okay, What's the final piece of information that we need to get from the world around us? If this bird is coming in to peck him, he needs to know that it is pecking his arm. There needs to be some sort of way that the brain is able to register that, okay it's my arm that's being pecked, not my leg. In order for the brain to do that, it relies on things called dermatomes. If we were to draw a person here, there is the torso, he's got his arm, he's got his other arm, and he's got his legs. Each little part of the body is innervated by a particular nerve, and that nerve goes up to the brain. If a bird is pecking his arm, we know okay, this arm is being pecked, so this arm will send a nerve to the brain, and the brain is able to figure out, okay it's this arm that's being pecked. All right, so I cleaned that up a bit. Let's imagine that we've got somebody flying in from the sky, and he's headed right at our innocent bystander walking down the street. This man just happens to have a red cape and people call him a super hero, but he's headed right for this guy. As this guy is coming in, he's going to crash into this poor fellow walking down the street. What types of receptors would be activated in this case? Temperature? No, not really. There's no change in temperature. Pressure? Definitely. This guy is just bombarding right into this innocent bystander, so he's going to feel a lot of pressure from the super hero's body. Pain? You betcha. He's going to be in a lot of pain. This guys is coming 100 miles per hour, headed straight for this guy. He's probably going to feel a little pain. He's probably also going to get knocked off his feet, so he's going to feel a change in position. These three different senses will go off. Let's look at intensity. How intense is this interaction? It looks pretty intense. You've got a super hero coming in right at you. His neurons, all these three different types of neurons, are going to fire really, really quickly. Let's look at timing. We're going to definitely have some non-adapting neurons firing, because the entire time this guy, this super hero, is in contact with the innocent bystander, there's going to be different neurons firing steadily just so that it lets his brain know, hey, there's something going on here. There are also going to be fast-adapting neurons that will fire as soon as the super hero hits the innocent bystander, and then as soon as he gets off of him, so there's going to be fast-adapting neurons firing. There will also be slow-adapting neurons firing the entire time the super hero and the innocent bystander are making contact. What about location? The superhero is headed straight for this guy's torso. The entire torso is going to be lit up, maybe some of his arms are going to be lit up. Basically, there are going to be nerves that innervate these three different parts of the body, and they're all going to be sending information to the brain saying, hey we've got a little bit of pain, some pressure, and there's a change in position here.