 Let's explore reflex actions. Reflex actions are involuntary response to a stimulus. Let's see what that means with an example, couple of examples actually. Here I have my hand is coming towards my flame and see what happens. Whoa, what did you see? See, when my hand came close to the flame, there was a heat that was produced over here. That heat is the stimulus. And what happened because of that? I responded. My hand moved away. That's the response, but what's important is that it was an involuntary response. Meaning, I didn't think about it. I didn't think that, oh, my hand is getting hot and therefore I have to move it. No, it just happened automatically. That's what a reflex action is. Let's take another example. This time, let's shine light into our eyes and you will see something. You will see that this pupil, ooh, did you see that? It became smaller. Another example. So let's see, here the stimulus was the light and the response was that the pupil became smaller and that was an involuntary response. It happened automatically. So you see what reflex actions are? Involuntary response to a stimulus. But how does it happen? How does it all become automatic? Let's see that in the next slide. So here's the same example. When I move my hand close to the flame, what really happens? Well, there are a few neurons that are involved. The first neuron is what we call the sensory neuron. It's a long neuron that starts from our fingers and goes all the way into our backbone. This is our spinal cord. Yes, neurons can be that long. Neurons are one of the longest cells in our bodies. So what the sensory neuron does, as the name suggests, it senses that there is some heat and it sense that signal all the way to your backbone. Then that signal goes to the brain, just like all the signals are supposed to go to the brain, but the beauty over here is it doesn't wait for the brain to process and then tell. No, no, no, no, it doesn't wait for the brain. What now happens to that signal is that that signal goes to another neuron which is called the relay neuron. Relay means it takes that signal and gives it to someone else. Who does it give it to? It gives it to the third neuron which is called the motor neuron. That signal immediately goes to the motor neuron which goes to your muscle and immediately it contracts that muscle and makes your hand move away. And this is how it became automatic. It became automatic because although the signal went to your brain, your brain understood it that your hand is getting hot, it didn't wait for your brain to respond. It happened automatically. And so this automatic response, this whole pathway which consists of sensory neuron, the relay neuron and the motor neuron, this entire pathway is what we call the reflex arc. Reflex arcs are amazing because without them we wouldn't have survived. They are a beautiful survival mechanism. And guess what? Reflex arcs are believed to have evolved before our intelligence evolved. And that makes sense because you need reflex arcs for survival.