 Check out these four videos. In the second one look at that black pupil at the at the center see what happens to it as a shine light Okay, and over here my leg is coming up automatically. I'm not doing this on purpose the moment. I hit it the leg comes up automatically So the question is what did you find common in all the four videos? Well, besides the fact that I'm torturing myself at least mildly One thing common you can find is that in all four cases there is a stimulus What's a stimulus you ask? stimulus is some kind of a change in the environment of a body for example over here the temperature of the finger changes That's a stimulus over here the light the amount of light entering my eye changes stimulus Here before I sneezed I sniffed something so the environment inside my nose changed stimulus and over here I'm hitting myself Somewhere at the knee so the pressure changed again a stimulus. Okay, what else is common? Well, I am responding to that stimulus in all the four cases over here I move my hand away here my pupils become smaller over here. I sneeze trying to get that You know get something out and over here my leg raises So there's a response to the stimulus great. There's one more thing that's common in all the four. What's that? The thing is these responses are all involuntary meaning I am not consciously deciding to do that. It's happening Automatically right so it's an involuntary response to a stimulus that my dear friend is what we call a reflex action So, let's write that down. What's a reflex action? reflex action What is that? Well, it's it's an involuntary response in voluntary response to a stimulus Now what's so interesting about reflex actions is that it's different than other involuntary processes like your heart beating and your Breathing or maybe digestion those are also involuntary processes But over here it requires a stimulus only then the reflex action happens. That's what's different over here So I guess the question is how does this work? I mean, how can you respond to something without being conscious about it without even thinking about it? I mean look at these cases. I'm not even thinking about it and it's just happening. So how does it work? Well, it turns out some of the reflex actions are more complicated than the others So we will only study the one that is most commonly found in our body Okay, and so it turns out that the mechanism of this one the mechanism of this Reflex action is the most common. So let's look at this one in detail So here is my arm. I have shown the bicep muscle over here because that will be important for us So in such reflex action, it turns out that there are only three neurons involved only three cells are involved and It involves our spinal cord. So let me just draw that as well So this is a section of this spinal cord if I were to show you a little bit more of my spinal cord Here it is. This is the down part so you can kind of see right what this what I mean by a section and These are the nerves that are coming out the bundle of nerves that are coming out from the spinal cord Okay, so let's see what happens So the moment the temperature of my finger increases that is detected by a neuron which is present over here That neuron converts that heat into electrical signal and that electrical is signal is sent to the spinal cord So this is a neuron and if you're wondering why am I looping it over here? I'll tell you in a second But anyways this single neuron and if you're wondering are Can a cell be this long? The answer is yes neural neurons are the longest cells in our body neurons can be really really long Okay, so that single neuron Takes that electrical signal and sends it to the spinal cord. What happens after that? Well, then that signal is taken by another neuron. So this signal goes to another neuron over here Another neuron over here and Then that signal goes to the brain the neuron sends that signal to the brain. So it's going like this Goes this way and then from here that signal goes to the brain Okay, and this is what happens all the time. So there's nothing different over here But what's different in reflex action? What happens next is that? It will not wait for the brain to give command Okay, usually once the signal is sent to the brain The brain is the one that processes that signal then decides what to do But in reflex action that in this reflex action at least that's not what happens What happens is it does not wait for the brain to give the command the signal from this neuron is Immediately sent to yet another neuron and that's what's that's what's different over here. It's sent to yet another neuron Which sends that signal Directly to my bicep muscle in this case. It sends it directly to that bicep muscle So the electricity gets directly sent to the bicep the bicep contracts pooling my you know my hand and as a result my hand moves away from the fire and So the speciality of the reflex action is that it does not wait For your brain to process that information and then to give you command it immediately redirects it to the muscle and The reflex action is carried out and this is why even without thinking about it Even if you're not thinking about it, even if this happens accidentally automatically the hand moves away That's the speciality of reflex action Okay, so the obvious question might be why is this happening? Why are we not waiting for the brain to process the information and then give the command? Can you think about this? I want you to pause the video and think about why? The information is directly sent to the muscle. Why not? Why aren't we waiting for the brain? Can you answer that pause the video and think about it? Well, I'm pretty sure you guessed it if we had to wait for the brain to think about it and then decide What to do in that time the damage would have already been done, right? Because this is a dangerous situation for our survival We need immediate action and in such cases it does not wait for your brain That's the speciality of this reflex action And so you see since reflex actions are super important for our survival We still have them even though we have a complex brain In fact, would you be surprised if I told you that reflex actions were first evolved Before a complex brain, right? Before complex intelligence evolved, reflex actions evolved first Because they are the ones that will ensure your survival So the last thing to do is just go ahead and label this diagram And I'll also tell you a couple of things to remember while drawing this So this big neuron that senses the stimulus We call it, no surprise, the sensory neuron So this is called the sensory neuron And this neuron that acts on the muscle and makes it contract and makes your hand move This one is called the motor neuron Motor neuron It's called so because it's causing motion It's because of this neuron, motion is happening, that's why it's called motor neuron And the neuron that transfers the information from the sensory neuron to the motor neuron So in the spinal cord this one is called This one is called the relay neuron Relay means transfer of information over here So this is called the relay neuron And of course this is our spinal cord This is our spinal cord And lastly this entire pathway Which consists of sensory neuron, the relay neuron and the motor neuron The entire pathway is also given a name It's called the reflex arc Reflex arc So in our example the reflex arc consists of one sensory neuron One relay neuron and one motor neuron Of course some other reflex arcs might have more neurons in them Some reflex arcs can have fewer neurons in them, that's right For example that knee reflex that we saw, it turns out it only has sensory and motor It does not contain relay So some reflex arcs can have only two neurons But the most common ones will have three neurons in them Okay now a couple of things to remember while drawing this First of all, why do I draw a loop over here in the sensory neuron Well that's because most of the time when we see a neuron This is the picture that comes to our head, right But neurons don't have to look like this Neurons can have a variety of shapes And one of the shapes that neurons can have is like this Neurons can also look like this And sensory neurons do look like this All sensory neurons look like this And so this thing that I've drawn, this loop I've drawn Is actually the body of the neuron over here Alright, that's why sensory neurons are drawn that way So keep that in mind So let me get rid of that And the second thing you may want to remember Is that sensory neurons always go to the back side of our spinal cord So this is the back side And the motor neurons will always come out of the front side Of the spinal cord Just something to remember while drawing the diagram That's it Time to now recall Recall what we just learned So can you define what reflex action is? What is a reflex arc? Can you tell what sensory neurons relay neurons Which are also called interneurons by the way I forgot to mention that And motor neurons, what do they do? And finally, can you try drawing the entire reflex arc drawing? See, if you try to recall this now You will be able to remember this much longer Okay, so please try and recall And of course, if you are stuck anywhere You can always go back and re-watch the video