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.

Enjoy!

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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  • Thank you for shearing and buy the way you really can’t control the knee
    reflex. I want to ask you why not everyone have the knee jerk reflex?
    Because when I did it to someone in my class mostly the girls have more
    knee jerk the boys. And just let you know when no jeans is on the leg you
    get a better knee jerk and higher kick. And don’t stand in front of anyone
    when doing the might kick you. There are people have more knee jerk then
    others. But thank you and post more if you can. Keep it up!

  • Hi Lee,

    Glad you liked the demonstration πŸ™‚

    I’m not exactly what you mean in terms of the flow chart. What kind of flor chart are you referring to?

  • Wow that was so helpful to me thank you. I learn better from watching rather than reading and this has been the best site i have found, so well done to you (and your wife) and yes it is a lot more interesting which also helps me to remember it too!!

  • That’s awesome Jamie-Marie. Glad to know you find the site valuable. There’s a lot more coming in the future, so stay tuned πŸ™‚

  • such a good teacher i cant believe i understand reflex in 8 MINS… i spend weeks reading and didn’t understand to much.. But now i understand that reflex have interneuron which means that you have more control. no internueuron you have no control over it……

  • @madej1858 That’s Awesome. I’m so glad that I’m able to make it easier to understand for you. Sometimes, it just takes someone breaking it down in a simple way for you to understand it. Stay tuned, because there’s a lot more coming πŸ™‚

  • Great video! Thanks so much. I have a few questions: What would be the evolutionary significance of these two reflexes? Why did these reflexes evolve?

  • I would like to ask something…why don’t we need an interneurone in between the afferent neurone and efferent neurone? doesn’t our brain need to be notify about the knee jerk action we just did?

  • @tieu589 Yes, the brain is notified. However, that’s a totally separate pathway with interneurons involved. The simple reflex arc shown here just gets the muscles contracting. Other pathways go to the brain and the brain interprets those signals.

  • the purpose of the absence of interneurone in knee-jerking(in the movement pathway) is to shorten the time for impulse to be transmitted from receptor to effector right? but what’s the purpose for the reaction to be performed so fast since it doesn’t causes us harm at all? on the other hand, reflex arc which we perform when we accidentally touch anything sharp or hot has interneurone between the afferent and efferent neurone,wouldn’t it take more time for us to react and worsen the injury? ><

  • the purpose of the absence of interneurone in knee-jerking(in the movement pathway) is to shorten the time for impulse to be transmitted from receptor to effector right? but what’s the purpose for the reaction to be performed so fast since it doesn’t causes us harm at all? on the other hand, reflex arc which we perform when we accidentally touch anything sharp or hot has interneurone between the afferent and efferent neurone,wouldn’t it take more time for us to react and worsen the injury? > <

  • @tieu589 That’s a good question, and I’m not sure WHY it is that way. I do know that it gives us info about the health of certain aspects of our nervous system, but not sure what the function is besides that. A reflex involving removing your hand from a hot stove makes complete sense, but this one – I don’t know.

  • That’s a good question, and I’m not sure WHY it is that way. I do know that it gives us info about the health of certain aspects of our nervous system, but not sure what the function is besides that. A reflex involving removing your hand from a hot stove makes complete sense, but this one – I don’t know.

  • That’s a good question, and I’m not sure WHY it is that way. I do know that it gives us info about the health of certain aspects of our nervous system, but not sure what the function is besides that. A reflex involving removing your hand from a hot stove makes complete sense, but this one – I don’t know.

  • @tieu589 That’s a good question, and I’m not sure WHY it is that way. I do
    know that it gives us info about the health of certain aspects of our
    nervous system, but not sure what the function is besides that. A reflex
    involving removing your hand from a hot stove makes complete sense, but
    this one – I don’t know.

  • @tieu589 @InteractiveBiology It is because this reflex performs an important role in keeping balance and posture, e.g. think about jumping – when you land, your thigh muscles will stretch but instead of staying relaxed they (nearly) spontaneously contract to compensate and stop you from falling flat on your face! But this reflex also occurs on a less extreme (yet equally important) scale in such mundane tasks as walking and getting out of bed.

  • @tieu589 @InteractiveBiology It is because this reflex performs an
    important role in keeping balance and posture, e.g. think about jumping –
    when you land, your thigh muscles will stretch but instead of staying
    relaxed they (nearly) spontaneously contract to compensate and stop you
    from falling flat on your face! But this reflex also occurs on a less
    extreme (yet equally important) scale in such mundane tasks as walking and
    getting out of bed.

  • P.S. Awesome channel man; great stuff! (Would appreciate any feedback you could give for some of my attempts at videos of a similar nature, you seem like an experienced teacher!)

  • P.S. Awesome channel man; great stuff! (Would appreciate any feedback you
    could give for some of my attempts at videos of a similar nature, you seem
    like an experienced teacher!)

  • @InteractiveBiology Also, I think you may have mixed up the dorsal/ventral roots and horns; the efferent neuron should enter the dorsal horn and travel along the dorsal root, c.f. the afferent neuron soma should be in the ventral horn and exit via the ventral root. And (not to be picky) but isn’t fine movement control more to do with the ratio of motoneurons:muscle fibres, i.e. size of motor units?

  • @TutorTom10 Not sure what I said about dorsal/ventral roots, but my focus wasn’t the anatomy – just the physiology. Also, you are right about fine motor control. However, I wasn’t dealing with fine motor control in this video. With the eye blink reflex, I was dealing with the ability to stop that reflex from happening. You have more control of the process because there is an interneuron that can be overridden. You have no control over the knee-jerk reflex.

  • Not sure what I said about dorsal/ventral roots, but my focus wasn’t the anatomy – just the physiology. Also, you are right about fine motor control. However, I wasn’t dealing with fine motor control in this video. With the eye blink reflex, I was dealing with the ability to stop that reflex from happening. You have more control of the process because there is an interneuron that can be overridden. You have no control over the knee-jerk reflex.

  • Not sure what I said about dorsal/ventral roots, but my focus wasn’t the anatomy – just the physiology. Also, you are right about fine motor control. However, I wasn’t dealing with fine motor control in this video. With the eye blink reflex, I was dealing with the ability to stop that reflex from happening. You have more control of the process because there is an interneuron that can be overridden. You have no control over the knee-jerk reflex.

  • @InteractiveBiology Yes, you are quite right; I suppose it’s important not to get too carried away with the details but stay more focused on the basics of the concept. Please keep making more videos, these are great!

  • @InteractiveBiology Also, I think you may have mixed up the dorsal/ventral roots and horns; the efferent neuron should enter the dorsal horn and travel along the dorsal root, c.f. the afferent neuron soma should be in the ventral horn and exit via the ventral root. And (not to be picky) but isn’t fine movement control more to do with the ratio of motoneurons:muscle fibres, i.e. size of motor units?

  • That makes perfect sense. Thanks for adding that bit of info. Good stuff!

  • Just checked out your channel and got excited. Great to see someone else doing some similar things. We should connect sometime. Maybe via skype.

  • Not sure what I said about dorsal/ventral roots, but my focus wasn’t the anatomy – just the physiology. Also, you are right about fine motor control. However, I wasn’t dealing with fine motor control in this video. With the eye blink reflex, I was dealing with the ability to stop that reflex from happening. You have more control of the process because there is an interneuron that can be overridden. You have no control over the knee-jerk reflex.

  • @InteractiveBiology Yes, you are quite right; I suppose it’s important not to get too carried away with the details but stay more focused on the basics of the concept. Please keep making more videos, these are great!

  • Whoa. Tutor Tom has it backwards. The dorsal horn is SENSORY and receives AFFERENT impulses. The ventral horn is MOTOR and sends EFFERENT impulses.

  • Whoa. Tutor Tom has it backwards. The dorsal horn is SENSORY and receives AFFERENT impulses. The ventral horn is MOTOR and sends EFFERENT impulses.

  • @tewolde33 All questions are answered in the Interactive Biology forums from now on. Go to the website in the description and then visit the forum. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  • All questions are answered in the Interactive Biology forums from now on. Go to the website in the description and then visit the forum. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  • @hoychris Thank you for watching the video. Unfortunately, Leslie is busy at the moment working a lot on improving the site. He is unavailable to answer any questions. We do have a Facebook fan page where you can interact with other readers and get answers to your questions. In the meantime, stay tuned because we have more Biology videos coming very soon!

  • Thank you for watching the video. Unfortunately, Leslie is busy at the moment working a lot on improving the site. He is unavailable to answer any questions. We do have a Facebook fan page where you can interact with other readers and get answers to your questions. In the meantime, stay tuned because we have more Biology videos coming very soon!

  • Thank you for watching the video. Unfortunately, Leslie is busy at the moment working a lot on improving the site. He is unavailable to answer any questions. We do have a Facebook fan page where you can interact with other readers and get answers to your questions. In the meantime, stay tuned because we have more Biology videos coming very soon!

  • Thank you for watching the video. Unfortunately, Leslie is busy at the moment working a lot on improving the site. He is unavailable to answer any questions. We do have a Facebook fan page where you can interact with other readers and get answers to your questions. In the meantime, stay tuned because we have more Biology videos coming very soon!

  • HEY!!!!!!! I’ve seen you guys! you guys go to school with me at Andrews University!!!!! you dont know me but I have seen you guys before, I didnt know you do this stuff! pretty cool!! Dr. steen and dr. atkins sure are great teachers!!!

  • HEY!!!!!!! I’ve seen you guys! you guys go to school with me at Andrews University!!!!! you dont know me but I have seen you guys before, I didnt know you do this stuff! pretty cool!! Dr. steen and dr. atkins sure are great teachers!!!

  • Well hello there. Very cool. I’m a professor at Andrews and my wife is finishing her Masters there.

  • thank you for explain those things…by the way ur wife is beautifule,i like her nappy style

  • Thx! Really helped me do my homework. What software do you use for the diagrams at the end?

  • Brilliantly explained, and done in a sensitive way. A great teacher, a pleasure to listen. I learnt something!

  • Your video is very educational, I look forward to watching and learning more from your site. Thank you!

  • Hi, I’m confused of why you said there are NO interneurons involved here because, technically when the sensory neurons excite motor neurons, spinal interneurons which inhibits motor neurons to flexor muscles allowing the motor neurons to generate AP’s on extensor muscles causing contraction.

  • You have helped me get the highest grade in my anatomy class! THANK YOU SO MUCH! Big Fan πŸ™‚

  • Thanks for all your fantastic videos. You help me so much in my A&P course. Your wife is stunning!

  • Thanks for all your fantastic videos. You help me so much in my A&P course. Your wife is stunning!

  • Hey guys. Very good video. My brother was formerly an overweight man. He revolutionized his body from 285 lbs of pure fat to 202lbs of pure lean muscle mass. That shit was crazy! I just registered myself as I need to greatly improve my body shape. He used the Muscle Building Bible (Google it)…

  • Hello everyone. Wonderful film.

    My bro had been a fatty. He went from 283lbs of pure fat to 205 lbs of purely natural muscle. Everybody was in shock. I just subscribed myself as I wanna strengthen. He made use of the Muscle Building Bible (Search on Google)…

  • Thank you Leslie!
    Your lecture is very simple and clear.
    It is very easy for me to understand. My English is not good enough to listen to the common speed English. you are very kind and very slow so that I can understand almost all of what you talked. It will be good to my English study! I have to download them to listen to it latter. Thank you very much!

  • Thank you Leslie!
    Your lecture is very simple and clear.
    It is very easy for me to understand. My English is not good enough to listen to the common speed English. you are very kind and very slow so that I can understand almost all of what you talked. It will be good to my English study! I have to download them to listen to it latter. Thank you very much!

  • I love the your video!! they are very clear, fun and extremely helpful. thanks for make them πŸ˜€

  • Thanks a lot Samuel for all your brilliant videos, it helped me a lot. Would you please explain what is stretch reflex and howit is connected to the ballistic stretch?

  • yeah! That was awesome! You should give some sort of diagram first to give a broad overview of what your doing. I stumbled across this during my studies of advanced physiology. Almost went to something else because I wasn’t sure if you’d show me what I wanted see. Thought I’d show some patience and was well and truly rewarded! You guys are cute!

  • Hey guys. Fabulous vid clip. My younger brother had been an obese boy. He revolutionized his body from 285 lbs of fat to 208lbs of massive lean muscle mass. I couldn’t believe it! I just joined personally because I’m attempting to get stronger muscle mass. He made use of the Muscle Building Bible (Look in Google)…

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