That Neuroscience Guy - Neuroscience Bites-Why the Doctor Hits Your Knee with a Hammer

Episode Date: June 19, 2023

In today's neuroscience bite, we discuss the neuroscience behind the patellar tendon tap, a common test used by doctors. 

Hi, my name is Olof Kregolsen and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to another Neuroscience Bite. Have you ever thought about why the doctor taps your knee? You know, if you go in for a physical, they get you to sit on the table and they take out that funny little thing that's got the little triangle shaped piece of rubber on the end and it's usually got a metal handle. And then they tap your patellar tendon. They tap just below your kneecap. And what they're looking to see is whether your knee pops out or not. Well, first of all, why do doctors do that? You know, why do they tap your knee? Well, they're actually testing how well your nervous system works. If you had ALS, for instance, and they tapped your knee, your knee wouldn't pop

out as quickly or as far as it does when you do have ALS. And it's not just ALS. There's a whole bunch of other neurodegenerative diseases that impact how well your nervous system works. So what they actually do is they're tapping your knee just to see it pop out. And if it pops out, it's a very quick assessment of how well your peripheral nervous system is working. Now, if your knee doesn't pop out, they do other tests, of course, that's not the full diagnostic. The most common thing they do is they actually measure the speed with which nerves conduct electricity. And there's ways to do that. You can literally put an electrical signal into the bottom of your arm and then you measure how long it takes to get to the

top of your arm. And then you repeat that with the leg and they compute what's called the nerve conduction velocity. And if that velocity is too slow, then they do even more tests to figure out why it's slow. But how does it work? This is also kind of cool as well. How does this simple reflex work? Well, the way it works is pretty simple. When the doctor taps the patellar tendon, there's a sensory neuron that receives that information. And that information is conveyed to the spinal cord. It's conveyed by what's called a 1A sensory neuron. And it's basically, it fires because it's being tapped. All right. It's a stretch reflex. And you could do it with your finger if you really wanted to. The reason that

they use the little rubber handled thing is because they want to do it very quickly and sharply. And that sensory, that neuron fires. So you tap the tendon, sensory neuron fires, and it goes to the spinal cord. Now what's cool is the reflex doesn't go to the brain. It just happens right at the spinal cord. That 1A sensory neuron is connected to an alpha motor neuron. And what that alpha motor neuron does is it fires the leg. That's the reflex response. So when that tendon is stretched suddenly and unexpectedly, the 1A sensory neuron fires, it goes to the spinal cord, and that fires an alpha motor neuron that causes the leg to extend. And that's the patellar tendon reflex. It's a very simple reflex system. It involves literally two neurons. Now,

there's all sorts of other connections. The information does go up the spinal cord to the brain. It does go to other muscles and things, but that's the reason they do it is to fire that little reflex system and make sure that it's working. And that's how it works. Literally a pathway of two neurons. Now, the other thing to think about is why do they do it when you're sitting? Well, when you're standing, if you tried to do it, it doesn't work. And why doesn't it work? It's because your nervous system adjusts. It says, well, hang on, I've got weight on this foot. So the last thing I want to do is kick my leg out.

So your brain literally dials down the reflex response as soon as you put weight on it, which is why they have to do it when you're sitting down. Anyway, I just thought that was an interesting tidbit. Hopefully you did too. That's why the doctor taps your knee in the neuroscience behind it. Remember, check out the website, thatneuroscienceguy.com. DM me on Twitter at thatneuroscienceguy if you've got ideas for merch or for future episodes. And of course, thank you for subscribing to the podcast and listening.

My name's Olive Craig Olson, and I'm that neuroscience guy, and that's another Neuroscience Byte.