January 25, 2011

023 How Reflexes Work (Knee Jerk and Eye Blink)

In this video, Leslie’s wife helps to demonstrate both the knee jerk and the eye blink reflexes.

Watch as Leslie explains how both of these reflexes work.


Transcript of Today’s Episode

Hello and welcome to another episode of Interactive Biology TV. My name is Leslie Samuel. In this episode, Episode 23, we’re going to be talking about reflexes and how they work. We’re going to talk about the knee jerk reflex and we’re going to talk about the eye blink reflex. I have a very special guest here today. This is my wife, Marguerite.

Marguerite: Hello!

Alright, so this is my wife, and she’s going to help me to illustrate these 2 reflexes, and then we’re going to talk about how it happens.

Let’s first look at the knee jerk reflex. Sorry, I don’t have a tendon hammer here. I have a remote control, but the principle is still the same. What I’m going to do, here we have the patella bone, and right beneath that, we have the patellar tendon. What I’m going to do is try to hit her exactly on that patellar tendon, and we’re going to see what happens. So here we go. Ooh! One more time. Okay, so I hit, and what happens is, you know this already, but her leg kicks forward. There’s a muscle contraction happening, and we’re going to look at that process in a little while.

Now, what I want you to do is I want you to try not to kick. Can you do that?

Marguerite: Yup!

Okay, let’s do that. She’s trying not to kick, she’s tensing up or whatever she needs to do. We’re going to try it again. It still happened. You could still see that reflex. Even though she’s trying not to do it, once I stimulate it, something is going to happen and she’s going to do that reflex.

Now, we’re going to look at the eye blink reflex. What I’m going to do is I’m just going to take my hand and I’m going to do it towards her eyes. And you see that she blinked. I’m going to do it again, and you see that she blinked.

This is what I need you to do, honey. I need you to think about not blinking, and try hard not to blink. Can you do that?

Marguerite: I can try.

Okay, so she’s going to try. Alright, don’t blink. Try really hard. Okay, she didn’t blink. I’m going to try it again, and she didn’t blink. So she had more control of this reflex than she did with the knee jerk reflex. So now we’re going to talk about how this process actually works.

Let’s look at the mechanism behind the knee jerk reflex. Here, we have someone’s leg, and here’s the knee, and right here we have the patellar tendon. So let’s say I took a tendon hammer and I applied pressure to this tendon by just striking it there. What that’s going to do is it’s going to stimulate sensory neurons.

We spoke about the different types of neurons in Episode 2. We spoke about sensory neurons that send signals to the central nervous system, which is the brain and the spinal cord. We spoke about motor neurons that send signals away from the central nervous system and to muscles and organs and glands. And then we spoke about interneurons that are fully contained within the central nervous system.

Right here, what happens is when you strike the patellar tendon, that sends a signal via this sensory neuron. Let me show that in a different color. Via this sensory neuron to the spinal cord. So this is looking at the spinal cord as a cross-section. And what you’ll see is that makes a synaptic connection to a motor neuron, and that’s this other neuron that’s coming here. That neuron goes and stimulates the muscles to contract, causing the leg to be extended.

The key thing here is that when we look at this connection that’s happening inside the spinal cord, so if we look right here, we’ll see that it’s just a sensory neuron connected to a motor neuron. There are no interneurons whatsoever in this process. So I’m going to write here “no interneuron.” So because we don’t have any interneurons in this what we call a simple reflex arc, let me write that here, there’s hardly any processing happening here.

Once a signal comes in, a signal will go out, the muscles will contract, and the knee jerk reflex will happen, the leg will extend. The key here is that there is no interneuron, and because there’s no interneuron, there won’t be any significant processing happening. Once a signal comes in, a signal will go out.

Now, let’s look at the eye blink reflex. The eye blink reflex is a little different. I’m going to attempt to draw an eye here, looking at it from the side. I’m just going to simplify it. This is my eye, and here we have the pupil. This is a green-eyed person. So when I took my hand and I swung my hand towards my wife’s eye, she blinked.

What you have here is we have sensory neurons, once again, coming into the central nervous system, and I’m going to show this very simply. And then we have 1 interneuron, so I’m going to draw that interneuron here. And then, we have 1 motor neuron. This is my soma, and then we have the motor neuron sending signals to the muscles surrounding the eye. When this happens, it stimulates the muscles to contract, and then you blink your eye.

However, because there is 1 interneuron here, there is more control that you can have, because you can block the signal that goes to the motor neuron. So by putting that 1 interneuron here, we’re giving some more control. And you can control that eye blink reflex. It’s a little harder to control than other movements, but you can still control it.

Now, if you are to look at other movements that you make, like moving your hands and moving your legs and so on, there are many interneurons that are involved in that process. And the more interneurons you have, the more control you can have. If you look at something as complex as speech where you have muscles in the mouth and you have the vocal cords that you’re causing to vibrate, and there’s a lot of details that’s happening when you want to speak. In order for that to be the case, there needs to be many interneurons. The more interneurons you have in the process, the more control you’re going to have over the muscle contractions and over the movements.

So that’s how the reflexes work. That is why the knee jerk reflex happens whether you want it to happen or not, and that is why the eye blink reflex happens but you do have some control over it. These are tests that doctors use to see if the nervous system is functioning correctly, and it gives them specific details about specific aspects of the nervous system functioning.

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