That Neuroscience Guy - The Neuroscience of Somatosensation

Episode Date: July 19, 2025

In today's episode of That Neuroscience Guy, we discuss the neuroscience behind somatosensation, or our sense of touch. 

Hi, my name is Olav Krogolson and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to the podcast. Well, hi everybody. A bit of an unexpected break for us. Matt was away at a conference, I was away at a conference, sadly both different conferences at the same time, and we fell behind. But we're back and we're going to keep on going until we stop going. So when we left off, I had done, we'd promised five

episodes on sensory feedback or sensation. So here's number two, the neuroscience of somatosensation. So I've always found sensory feedback in terms of somatosensation to be fascinating because you have to think of your entire body as a detector, right? As you're sitting there right now or standing or whatever you're doing, the sensory receptors in your body are listening and they're listening for the state of you and they're listening for the state of the world. Now they're not actually listening. We're going to talk about hearing in an episode or two, but what I mean by that is they're waiting to detect something.

So if you zoom in underneath the skin, you would see a myriad of receptors. You would see touch receptors, and not just any kind of touch. You have what are called light touch receptors, which are closer to the surface, and you have strong pressure receptors, and you have strong pressure receptors that are deeper. All right, and the reason for these two types of receptors is that you want something that's sensitive to what's going on on the surface, and then you want something that's sensitive to something that's deep, and you want everything in between.

So there are a array of these sensory receptors at various levels of depth throughout the skin. Alright these things that are close to the surface and they're deep in fact technically they're under the skin. Now that's just for touch, all right? At the same time, intermixed in there, there are receptors for cold and for heat. And one way to think about that is if all your heat receptors are firing, it's very hot. If only some of them are firing, it's not as hot. And the same is true for cold. You have pain receptors that fire when the body's experiencing pain or what's perceived to be pain.

All those hairs on your body, they're receptors, all right? At the base of the hair follicle, it's attached to the dendrite of a neuron and when the hair moves, whether it's through wind or something brushes it, it fires. So the purpose of all of these detectors are to fire when stimulated. So your body, you're sitting there right now, just imagine this. Just close your eyes and just feel all those receptors.

You know me, I'm sitting on a chair. It's not the most comfortable chair. So I've got receptors on the back of my front that are firing, because there's pressure on my rear end. All right, it's a little bit chilly, I can feel a bit of a breeze, so those hair receptors are firing. Now, that's good if you're stationary,

but what happens when you move? Well, you have somatosensory receptors in the muscles of your body as well. So each muscle is comprised of muscle spindles. And those spindles, I always think of them as mini muscles. It's not really what they are, but a whole bunch of muscle spindles add up to a muscle. And within each spindle, there are receptors that tell you if that spindle is contracting or if it's stretching. So if the muscle is being stretched or if it's being contracted

and they fire. And the purpose of those receptors firing, and again these receptors are wrapped around say a muscle spindle and then they effectively are dendrites at that stage, and when they fire it means the muscles being stretched or it's being contracted. And you also have receptors in the joints, alright, things like the Golgi tendon organ, alright, that fire if the joint is being moved. So what this means, all these receptors under the skin, in all of these receptors in your muscles and in the joints, it tells the brain the state of the body. What's going on? You know, what the body is experiencing and what the body is doing in terms of

movement, if moving at all. Now all of this information goes to the spinal cord and then it goes up the spinal cord to the brain. So that sensory information fires. All right, those receptors fire and the information goes to your brain. So I want you to try something right now. We'll call this an at-home experiment. All right? What I want you to do is to just sit quietly and close your eyes. sit quietly and close your eyes. And all I want you to do is contract your left biceps muscle. Now you're doing this with your eyes closed, but you do have the sensation of movement. Now why do you have that sensation of movement? Well guess what? Because those receptors in the biceps muscle are firing and they're

saying, hey I'm being contracted and that message is being sent to the primary sensory cortex in the brain and it's detecting that. So when the neuron fires in the muscle there's a corresponding neuron that fires in the brain. Alright, and we'll get to that in a little bit. I've talked about it before, but the key thing to think is all of this sensory information, this massive pattern of neural information that's going up to your brain constantly. It doesn't happen just once. It's continually going up there, so your brain knows the state of the body. I'll give you a second to try at home example of this.

If you close your eyes, alright, pick either your right index finger or your left index finger and quickly touch your nose. Now how are you able to do that? Think about it. You know, it's actually pretty impressive because if I had have asked you to touch out, to reach out and touch something else, something that's not attached to you, you probably wouldn't be able to do it that well. But

with touching your nose, you're incredibly quick and you can try your ear and try your knee, whatever you want. Well, how are you able to do this complex thing? Well, basically all those receptors we've just talked about, they're sending information to the brain constantly, like I said, about the state thing? Well basically all those receptors we've just talked about, they're sending information to the brain constantly, like I said, about the state of the body. So your brain has an accurate representation of where your nose is in space. It knows where your finger is because of all that information that's firing. It knows where your nose is so it's able to solve that mathematical problem and create a path for your finger to move to touch your

nose. But it's only because of all that sensory information that's coming in. And as you're moving, it's able to adjust your limb if it needs to, to make sure that you accurately touch your nose. This is actually not too hard to disrupt. Maybe it would be hard to do at home, but one way we do this in the lab is through vibration. If you get something that creates a vibration, you might have one of these things. I've got a home massage device that I use on my calves.

If you actually put some small amount of vibration onto the muscle that you're trying to move, you'll find you won't be able to move it as well. And the reason being is that vibration disrupts the sensory information you're not used to the vibration. And as a result, the brain doesn't have as an accurate representation of where the limb is in space. So you might want to try that yourself and see if you can do it. Okay, so muscles have detectors, joints have detectors, there's all these detectors under the skin and attached to hair follicles

and all of that information goes up the spinal cord and it actually goes to a place called the thalamus. First, we've talked about the thalamus. You know the story hasn't changed. The thalamus is basically a relay station. Information from the thalamus goes to other parts of the brain but everything comes in through the thalamus. Alright and from the thalamus in the case of the somatosensation, it goes to the primary sensory cortex. Now the primary sensory cortex is the most anterior part of the parietal lobe, alright, some people call it the somatosensory cortex, and basically for all of these receptors there's a

target site. So for a cold receptor there is a corresponding neuron in the primary sensory cortex and if you feel a bit of cold on your left biceps, well then guess what the left biceps cold receptor in the primary sensory cortex fires. Now think of that for every single hair on your body, for every single one of these receptors, whether they're stretch receptors, contraction receptors, whether they're hair receptors or heat receptors, there is a one-for-one representation in the primary sensory cortex.

And this is how your brain interprets what's going on in the world. It builds a map of the body from this information. So if you go onto Google Images right now and you type in primary sensory cortex, I encourage you to do so. You'll probably see a representation of the body, we call it the homunculus, alright, and it's stretched over the primary sensory cortex.

And if you actually look at that map, you'll see some regions have greater representation than others. We have a ton of receptors in the face of our body because we have to be able to control our face for all the facial expressions we can do. But we don't have as many receptors, say, on your shoulder because the shoulder basically needs to do a few simple things and it's basically gross or large movements. So not as many receptors there. And in fact, when I talked about this before,

you can try this at home, alright? You can actually get a pin. Now be very careful. And you need a friend to do this, someone you really trust. And if you went to an area where you have lots of sensory receptors, say the cheek, and you gently just tap the skin, and you want to do it say two millimeters apart and the person who's being tapped should be able to feel that difference. Now as you move those taps closer together because you tap back and

forth little breaks in between at some point people probably less than half a millimeter people won't be able to tell the difference anymore because that's the size of the sensory receptor. There's a receptor responsible for that region of space and if you go past that then all of a sudden you can't tell the difference. On the shoulder if you do this you might find it as much as two millimeters. So you can actually, the areas on that sensory homunculus with large representation, the area of sensitivity will be a lot more precise than the larger areas. But the point of that sensory homunculus is to basically represent the body in space, all right?

Your body takes that pattern of firing, and we've talked about this a lot over the years, but remember, it's just a pattern of neural firing, and the pattern of neural firing for walking is different than the pattern of neural firing for running, which is different from standing, which is different from sitting, which is different from lying down.

And your body learns, that Lily learns from the time you're born as you move through life, how to interpret these patterns, right? So if you're lying down listening to this, there's a pattern of neural activity that would be different than if you were standing up. And your brain can tell the difference between the two.

So that is somatosensation in a nutshell. The key to understanding somatosensation are two key ideas, and I'll just review them quickly. Receptors everywhere. All right under the skin, in the joints, in the muscles. Receptors everywhere. All right. And they're detecting heat, touch, pain, cold, movement, stretching, contraction,

joints moving. All of that information is stretching, contraction, joints moving. All of that information is transmitted through the nervous system up to the primary sensory cortex. Now, we'll split the second half of this summary into two pieces. What happens in the primary sensory cortex? Number one, there's a one-for-one representation of the body in the outside world. All right, so all of those receptors have their their partner, if you will, there. And then what the somatosensory cortex does is it uses that pattern of information to make a statement or to

decide about the position of the body in space, whether it's moving or stationary, whether it's standing or sitting. So that is somatosensation in a nutshell and That's all I have for today's podcast. Hopefully you found that interesting Don't forget you can check out the website that neuroscience guy calm we have new merch There's links to our Etsy store and we have lots of new merch check it all out. We have patreon Thank you to those people that support us there store, Etsy store, and we have lots of new merch, check it all out. We have Patreon. Thank you to those people that support us there. I know it sounds silly, but if you pledge a dollar a week or a dollar a month or five dollars a month, it all goes to graduate

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