That Neuroscience Guy - The Neuroscience of Walking

Episode Date: May 1, 2022

A lot of us can't go a day without needing to walk somewhere. Most of the time, we don't even have to think about it. But what's actually going on in the nervous system when you're walking? In today's... episode of That Neuroscience Guy, we discuss the neuroscience behind walking. 

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Starting point is 00:00:00 Hi, my name is Olof Kreg Olsson, and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to the podcast. Have you ever wondered about how you actually walk? You know, back in season one, we talked about grabbing an apple, a very conscious decision to use the motor system to achieve something. But what about walking? You know, is it really your brain sending down messages going left, right, left, right, left, right? Well, no, as it turns out, that's not what happens whatsoever. In fact, it's quite fascinating and involves some complex neural circuitry within your spinal cord. On today's podcast, the neuroscience of walking.
Starting point is 00:00:53 Now, before we tackle walking, we have to tackle reflexes. Have you been to the doctor's office and you sat on the bench and they take out that funny little device? It's typically a metal thing with a little rubber triangle. And what does the doctor do? He taps your knee and if things go well, your foot goes forward a little bit. Have you ever wondered why the doctor actually does that? Well, the doctor's examining your reflex response. Basically, when the doctor taps the tendon, you get a tendon tap reflex response.
Starting point is 00:01:27 The leg goes forward. And the reason the doctor does this is if that doesn't happen, it's a big problem. Now, we'll talk a little bit more about that in a bit, but the doctor is literally looking for your leg to pop forward. Now, how does that actually work? Why is your leg popping forwards? Well, it's got to do with the neural wiring of what's happening. Within the muscle, there are a whole bunch of things called stretch receptors. And essentially, they're looking for stretch. The same is true within a tendon. And what happens is from the tendon in the muscle, you have what are called 1A sensory neurons. These neurons project to the spinal cord. And these are kind of cool neurons because there's not a whole series
Starting point is 00:02:11 of them. Like most people think of neurons being really, really small. But in this case, the cell body of the neuron is in the spinal cord and the dendrite literally projects all the way from the tendon or the muscle to the spinal cord. So as opposed to being measured in micrometers, the dendrites of these neurons are measured in centimeters or feet. So when the tendon is tapped, the 1A sensory neuron fires. It basically goes, whoa, hang on, I've been stretched. And when that signal reaches the spinal cord, some of that information is transmitted up the spinal cord to the brain, so the brain's aware of what's going on.
Starting point is 00:02:52 And if you remember, we talked about the primary sensory cortex and how it's getting all of these input signals, so you have awareness of the body. But something simpler is happening here. Basically, there's a very, very simple loop. And that 1A sensory neuron is connected directly to an alpha motor neuron. And what that does is it fires a muscle. So when all of a sudden the tendon is tapped,
Starting point is 00:03:15 your leg goes forward because the alpha motor neuron fires. Now, of course, there's going to be multiple neurons involved in this process. But this very simple loop, sensory neuron connecting directly to the alpha motor neuron. So the sensory neuron fires the alpha motor neuron, which fires a muscle and your leg goes forward. Now, the reason the doctor is tapping your tendon the way he is, is that if this doesn't happen, your leg doesn't pop forward. It basically means there's damage to the sensory neuron and or the alpha motor neuron. And this can be quite serious. For instance, people with ALS, when you tap the knee, it doesn't go forward. And that's because the myelin and the alpha motor
Starting point is 00:03:57 neuron is being degraded and the muscle can't be contracted anymore. So the doctor is doing a very, very, very quick test just to see how well your peripheral nervous system is working. Now, don't worry, if you fail the tendon tap, they don't just give you a diagnosis. There's some really cool technology that actually allows them to measure what's called nerve conduction speed or velocity. It's how quickly the electrical signal is moving through the motor neuron or the sensory neuron. And that's what they actually use when they're trying to figure out, you know, if there's something wrong with you. Now, that's what we call a stage one or a type one reflex. Now we have to talk about stage two or type two reflexes.
Starting point is 00:04:38 So, you know, I used to play this game when I was younger. I was a teenager and I was a bit silly. But, you know, all of a sudden you shove someone from behind unexpectedly. And what typically happens when you do this is the person throws their leg forward. It's a learned reflex. So this one isn't hardwired like the type one reflex. This is one that you learn. And basically what happens is it's kind of similar. The sensory system detects the sudden movement forwards, and as a reflex, it's learned to throw that leg out to try to stay on balance. And it's a detection of the unexpected movement. But what's so cool about this is this processing is going on in the
Starting point is 00:05:19 spinal cord. Now, again, these sensory signals are sent up to the brain for processing, but it's not like the sensory system is going, hey, all of a sudden we're moving forward and it goes up to the premotor cortex and the primary motor cortex and goes, all right, now I must throw my leg out. of this is a withdrawal reflex. You reach out towards a hot element on a stove or you stand on a tack, for instance. The reason your limb comes back so quickly is it's spinal level processing. You've learned on the stimulus, do this. Now, there's other examples of this as well. For example, if you've ever been on a bus and it starts to accelerate, you might notice that you lean forward. You're not even thinking about this. Again, these are learned reflex responses. This is your brain processing, hey, the bus is moving and you need to do something about it. If you're a bit younger, you use what's called an ankle strategy. You basically just lean forward with movement about the ankle. As you get older, you use a hip strategy where there's actually movement about the hips. The reason you do that is it's posturally more stable.
Starting point is 00:06:30 But again, these are learned reflex responses. And the idea is processing within the spinal cord. Now that finally gets us to walking. At the outset, I sort of said, you know, how does walking actually work? If you think about grabbing an apple, we talked about visual processing within the posterior parietal cortex. We talked about movement planning with the cerebellum and the premotor cortex. And we talked about sending signals to the primary motor cortex, which finally allow you to move. Well, is that what happens with walking?
Starting point is 00:07:01 You know, is the brain literally sending down a continual motor signal, left leg forward, right leg forward, left leg forward, right leg forward? Well, no. The brain does get involved, but what's really happening is again within your spinal cord. So what's happening when you walk in the current working theory is you're activating what's called a central pattern generator. Now, what is a central pattern generator? It's basically just a loop of circuits. It's kind of like that original alpha motor neuron, 1A sensory neuron example, but the idea is one neuron. Now, it's, again, I'm going to use a small number of neurons, which we like to do just to make the example clear. I hope you all realize that there's actually lots of neurons involved. But imagine a little circuit where the left leg activates an intermediate neuron, and that neuron activates the right leg. And then the right leg activates
Starting point is 00:07:54 an intermediate neuron, and that intermediate neuron activates a left leg. Think of a little pattern going in a square. Neuron A fires intermediate neuron, which fires neuron B, which fires another intermediate neuron, which fires neuron A. And that's the left-right, left-right locomotion. It would be the same with swinging your arms. So any repetitive back and forth movement is controlled by these central pattern generators. It's a pattern of neural activity where the left leg activation literally activates an intermediary, which then activates the right leg. And that activates another intermediary neuron, which goes back to the left leg. So how do we know this is true? Well, this has actually been seen in cats. If you find cats
Starting point is 00:08:39 that have been deafferented, in other words, they don't have a connection below the top of the spinal cord. If you put them on a treadmill, they begin walking. And the reason they begin walking is because the central pattern generator has been turned on. And once it's turned on, the cat keeps walking. This is also seen in babies. Babies have been shown to have these sort of repetitive back and forth movements. And these back and forth movements are hardwired. And what's happening in the case of babies, the reason they don't walk is that the muscles have to gain strength and they have to learn to control it. But the actual neural circuitry of the central pattern generator is hardwired. Now, I did mention that there is some top stuff happening there. And what I mean by that is the cerebral cortex plays a role.
Starting point is 00:09:27 We call this top-down control. In terms of the reflexes, your reflexes are adjustable. If you go back to the start and you think of that tendon tap reflex, it doesn't work when you're standing up. And the reason it doesn't work is your neural system can detect that there is a load on the feet. And if you throw your leg out, you'd be off balance. So that actual load by you standing up prevents that tendon tap reflex from firing. So it only works when you're sitting down because the leg is not supporting any weight, if you will.
Starting point is 00:10:00 This is also true of walking. In terms of top-down control, your walking pattern is controlled by the brain. You can imagine that the brain is saying, all right, let's walk now, so it sends a signal down to the central pattern generators, which then initiate you walking. And it can adapt that. If you're walking on sand versus walking on gravel versus walking on grass versus walking on gravel, versus walking on grass, versus walking on a sidewalk, it can control how fast the central pattern generator is firing, and the number of central pattern generators that are involved, and the amount of force that's being produced. So in that way, there is this top-down control.
Starting point is 00:10:41 Your brain is allowing you to do these things. It's turning it on, but once it's on, you just keep walking. The central pattern generator just keeps firing over and over and over again, and off you go walking. This is one of the reasons when people have major injuries to, say, their legs and they have trouble walking, they have to relearn to walk. And that's because, you know, obviously there's the musculature damage possibly or damage to tendons or joints, but this all throws the central pattern generators off. They're not able to produce the rhythmic movement the way they're used to, so these generators have to relearn the new movement pattern. And as you heal, you sort of come back in line with this and you adapt and you're able
Starting point is 00:11:22 to walk again. Now, again, I just want to emphasize the thing I find so cool about this is all of this processing is in your spinal cord. So while you think that neuroscience is just in the brain, never forget the peripheral nervous system. There's some pretty cool stuff happening within your own spinal cord. Now, you can always email us show ideas.
Starting point is 00:11:44 This one was inspired by someone that asked us about reflexes. So, thatneuroscienceguy at gmail.com. Or of course, follow me on Twitter at thatneuroscienceguy. You can DM me and I'll take your ideas and we'll process them and see if we can turn them into episodes. Our website is up, thatneuroscienceguy.com. We have links to our merch, to our podcast, towards our YouTube channel, and of course to Patreon. Please think about supporting us. We already have a couple of people and it's great. All of the money from Patreon and the merch, to be fair, is going to graduate students in the Craig Olson Lab. I've talked a little bit about this, but these people don't
Starting point is 00:12:20 live off much and everything you can do to help them helps us do some more cool neuroscience. And finally, of course, listen to the podcast and subscribe. Thank you so much for listening. My name is Olive Kurgolson and I'm that neuroscience guy. I'll see you on Wednesday next week for another neuroscience bite.

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