This lesson is the structure and function of a reflex arc and is from the unit coordination and control: the human nervous system.

Hi there, my name's Mrs. McCready and I'm here to guide you through today's lesson.

So thank you very much for joining me.

In our lesson today, we're going to describe what a reflex response is and the path a nerve impulse takes through a reflex arc in the nervous system in order to cause a reflex response.

Now we'll come across a few keywords in our lesson today and they listed up on the screen now for you.

You may wish to pause the video to make a note of them, but I will introduce them to you as we come across them.

So in our lesson today, we're gonna first of all look at reflexes before we look at the reflex arc in detail.

So are you ready to go? I am.

Let's get started.

So have a think about this.

What is similar about your reactions to a loud sudden noise, a sneeze and touching a very hot object? Just have a think about those.

How do you react? Well, you should have said that all of your reactions are super fast and that also involuntary.

You have the reaction before you realise you've had the reaction.

And that's because all of your reactions are what are called reflex responses.

They are fast and they are involuntary.

They happen without you really having very much control over them, and certainly before you are consciously aware of your actions.

So which of these could generate a reflex response? Touching something sharp, a firework explosion, music increasing to a loud volume and someone touching your arm unexpectedly.

Which could generate a reflex response? I'll give you five seconds to decide.

So will touching something sharp generator reflex response? Yes, it will.

So might a firework explosion and so might someone touching your arm unexpectedly.

But music that's already playing and is just building to a loud volume is very unlikely to generate a reflex response.

So reflex responses are an important survival mechanism and they've developed over evolutionary time in order to enable us to survive.

So they enable us to survive because they allow us to react really quickly and without thinking to situations which may well cause us harm.

And because we can react super fast and unconsciously to these situations, so things flying at us or loud explosions happening near to us, hot things, sharp things, that sort of thing, then we can get outta the way before those things are able to cause us any more harm.

And that enables us to be more likely to survive in our environment because we are hopefully able to get outta the way of danger before it really does cause us harm.

So think about that in the modern world because of course these reflex responses have developed over evolutionary time before all of our mod-cons.

So just consider that you are cooking dinner and you accidentally touch a hot saucepan with your arm.

Your reflex response moves your arm rapidly and automatically, involuntarily, away from the heat and out of danger.

And could you stop that? Well, it would be very difficult in order to do so because it happens before you were aware of it.

What would happen if you didn't respond with reflex response? And therefore why is that reflex response beneficial? Well, hopefully you've realised that if you didn't respond with a reflex, then you would've touched the saucepan for longer and therefore been exposed to the heat of the source pan for longer.

And this will have likely caused you a deeper and more serious burn than you might have got if you moved outta the way really quickly.

And a more serious burn is more damage.

So a reflex response has minimised the damage and helped to protect you, to keep you safe and enable you to survive.

And that is why it is a survival instinct.

Now all animals exhibit reflex responses.

Let's have a look at some examples.

So woodlice curl up into a ball if disturbed.

I dunno if you've ever disturbed them underneath a brick or a rock or something else while they'll curl up really quickly into a ball to protect themselves.

And that's a reflex response.

Worms, if they are prodded or touched, will twist and turn really quickly.

So I dunno if you've ever tried to pick up a worm or if you've seen birds picking up worms, you'll see the worm thrashing around really quickly and quite violently really in order to try and escape and stop themselves from being picked up and therefore eaten and damaged.

Tortoises have a reflex response.

If they get alarmed by something, they'll retract their arms and legs and their head into their shell where they're protected by that outer hard casing.

And again, that's a reflex response.

And tadpoles, if they're touched on their side, they will flex and swim away really quickly, again to avoid danger, to get outta the way of something that might well cause them harm.

So animals exhibit reflex responses and we are an animal and therefore we exhibit reflex responses as well.

Now, some reflex responses can be overridden.

So let's imagine now we're back in the kitchen, but this time we're carrying a really hot tray with oven gloves, but the heat is starting to really come through the oven gloves and really cause us harm.

Would we drop the tray? Probably not, and that's because some reflexes can be overridden, but we might compensate by perhaps walking to the table faster or putting it down sooner than we might otherwise have done in order to help minimise any damage to us.

So some reflexes can be overridden, but not all of them and not terribly easily either.

So let's just consider Andeep, Sam and Izzy are describing a reflex response, but who has correctly described a reflex response? So Andeep says, "All animals exhibit reflex responses: worms curl up into a ball when touched." Sam says, "Reflex responses help us to survive.

Tortoises hide in their shells to protect themselves." And Izzy says, "If something scary is about to happen in a film, I hide behind a cushion and put my fingers in my ears." But who has correctly described a reflex response? I'll give you five seconds to think about it.

Well, hopefully you've realised that it's Sam who has correctly described a reflex response because Andeep should have cited a woodlouse rather than a worm.

And Izzy's response is a choice rather than a reflex response.

So what I'd like you to do is to use the keywords to write three sentences to describe reflex responses.

Then I would like you to with permission, clap near to your volunteer's face and note their reflex response and describe it briefly.

But only do that if you have permission from the person that you are clapping near to.

Don't just clap randomly in their face without their permission.

And then I'd like you to consider this.

This is really interesting actually.

There's a really very rare condition called Congenital Insensitivity to Pain, CIP.

What it means is that the person can't feel pain.

You might think, "Oh crikey, that's a really good thing, that'd be great." But actually it's a really bad thing because they can't feel pain.

So what I'd like you to do is have a think about what the ramifications of that? What are the consequences of not being able to feel pain? And explain why this is therefore dangerous to the person who has this congenital deficiency.

So pause the video and come back to me when you're ready.

Okay, let's see how we got on them.

So firstly, I asked you to use the keywords to write three sentences to describe reflex responses.

So you might have written that reflex responses are fast and involuntary.

That they are a survival mechanism and minimise damage to our body.

And some reflex responses can be overridden.

So just check over, have you written words to the effect of that? Have you used those words correctly? Well done if you have.

Then I asked you to have a go at clapping with permission near to your volunteers face and see what their reflex response is and then describe it.

So assuming you had permission, you should have noticed that their eyes blink and their head twitches quickly away from the sound and the movement.

Did you notice that? And then I asked you to consider this condition called Congenital Insensitivity to Pain and explain why this could be very dangerous for a person with this condition.

So you might have said that they will not be able to respond to injuries including those which would normally trigger a reflex response.

And this means that they would have to be very careful to avoid injuring themselves.

And if they think they've injured themselves, or even if they haven't, they would have to thoroughly check themselves for any signs of injury to avoid anything sort of sitting there and getting worse because they wouldn't know whether they'd broken something, cut themselves, become bruised, burnt, that sort of thing.

They wouldn't notice that.

And so they'd have to be really careful to just make sure that their body is still okay at the end of an activity or the day.

So now we're gonna move on to having a look at how this reflex response works by examining the reflex arc.

So we know that reflex responses occur rapidly and involuntarily.

And that is because they bypass the brain in their initial response.

So we react with a reflex response automatically and before we realise what has happened.

And that happens through what is known as the reflex arc.

Now the reflex arc involves three neurons, the sensory neuron, which is sitting within the peripheral nervous system, the relay neuron, which is within the central nervous system and in the example we are going to have a look at within the spinal cord.

And the motor neuron, which is back in the peripheral nervous system.

So we've got these three neurons involved within a reflex arc, and they are fixed in a pathway which does not initially involve the brain.

So those neurons are connected one to the next to the next in this fixed pathway, which means a really quick response can occur.

Now each neuron is connected via synapse as normal.

So there's nothing different about these neurons or the way they are connected to each other, just that this pathway is embedded within our body.

So the reflex pathway is a short neural pathway, and let's see what that really means.

So the stimulus, in this case a sharp pin, is detected by the receptor and that's same as usual.

And that detection is then sent down the sensory neuron as an nerve impulse.

It then reaches the relay neuron within the spinal cord, and that relay neuron transfers the message from the sensory neuron immediately to the relevant motor neuron, which is in the fixed pathway.

And then the motor neuron triggers the effector, usually a muscle, to contract and produce the response that we're after.

So in this case, it would be to rapidly contract and move the hand outta the way of the sharp object.

Meanwhile, however, the relay neuron also sends the message to the brain for processing.

But essentially by the time it's got to the brain and is being processed, the reflex response has already happened, which is why we have the action before we recognise consciously that we've had a response to the scenario.

So this is the reflex arc, this fixed pathway through the nervous system, and the reflex response is fast because this pathway is really short.

It involves three neurons and that's it.

And it's really not a very long distance to travel either.

And it's involuntary because that pathway is fixed and in the first instance does not involve the brain.

So there's no cognitive processing that needs to happen.

The brain doesn't have to um and ah about what it is that's going to be best to do.

The response has already happened before the brain acknowledges that something has occurred.

Now, we've said that sometimes the reflex response can be modified, and that's because sometimes the brain can override the reflex response.

And it does that by sending a signal down the motor neuron via reflex neurons.

So the brain communicates with that motor neuron and essentially prevents it from making the muscle contract, the effector muscle contract.

And that overrides our response.

So it would mean that we would keep holding the hot pan, for instance, and stop us from dropping it.

But this doesn't happen all of the time, and it's only for certain reflexes that we can do this.

So consider this then please.

A reflex response is fast because it bypasses the relay neurons.

True or false? So this is false, but can you justify your answer with one of these two responses? Either that the reflex response is fast because it bypasses the brain in the initial response.

Or the reflex response is fast because the impulse never reaches the brain.

Which is correct? So you should have justified that by saying that the reflex response is fast because it bypasses the brain in the initial response, but the brain is kept in the loop.

That's really important as well.

Can you also then explain why reflex responses are fast and involuntary? So which of these statements help do that? Is it A, the brain is not involved in the initial response, B, neurons are in a fixed pathway C, synapses connect neurons, or D there are three neurons involved in the reflex arc.

Which explain why reflex responses are fast and involuntary? I'll give you five seconds to decide.

Okay, so you should have said that they are fast and involuntary because the brain is not involved in the initial response and the neurons are in a fixed pathway.

Whilst the other two statements are also correct, they do not explain why reflex responses are fast and involuntary.

Well done if you chose the correct two.

So what I'd like you to do now is to summarise our learning so far.

I would like you to firstly label the diagram to show the neurons involved in the reflex arc.

Then I would like you to explain the features of the reflex arc that lead to a fast, involuntary response.

And then I'd like you to consider what Andeep is saying.

So he says, "I can carry hot pans without dropping them because I have no reflex response to hot things." Now, CIP, Congenital Insensitivity to Pain is extremely rare.

So let's assume that he doesn't have that and therefore explain how Andeep's brain is overriding his reflex response to prevent him from dropping the hot pan.

So pause the video and come back to me when you are ready.

Okay, let's see how you got on then.

So I asked you firstly to label the diagram to show the neurons involved in the reflex arc.

So you should have identified the sensory neuron, the relay neuron, and the motor neuron.

You might have added some other labels such as the effector muscle and the fact that this is a vertebrate in the spinal cord.

Then I asked you to explain the features of the reflex arc that lead to a fast, involuntary response.

So you should have said that the reflex response is fast because the pathway is short and the response is involuntary because the neurons are connected via a fixed pathway.

Also that the brain is not involved in the initial pathway.

So no decision has to be made on what action to take.

And these things lead to a fast and involuntary response via the reflex arc.

Then I asked you to explain how Andeep's brain is overriding his reflex response to prevent him from dropping the hot pan.

So you might have said that some reflexes can be overridden, and this happens if the brain sends an overriding message down the motor neuron to the effector, and it's that that prevents Andeep from dropping the hot pan.

So just check over your answer and have you got the essence of that correct? Well done if you have.

So we've seen in our lesson today that reflex responses are fast and involuntary and they therefore form an important survival mechanism for organisms. And because of that, most organisms have reflex responses.

Now, the reflex arc initially bypasses the brain to generate a quick response through a fixed neuron pathway, and the relay neuron is connected directly to a motor neuron from the sensory neuron.

It also connects up to the brain and the brain can override some reflex responses by triggering the motor neuron in the reflex arc instead of letting it respond through the fixed pathway of the reflex arc.

So I hope you found that lesson interesting and enjoyable.

Thank you very much for joining me today, and I hope to see you again soon, bye.