Lesson video

This lesson is called neurons and synapses, and it is from the unit coordination and control, the human nervous system.

Hi, there.

Welcome to today's lesson.

My name's Mrs. McCready, and I'm going to guide you through today's lesson, so I hope you're looking forward to it.

In our lesson today, we're going to describe the structures and functions of neurons and the synapses that exist between neurons.

So we're gonna come across a number of keywords in our lesson today.

They're on the screen at the moment for you.

You may wish to make a note of them and pause the video if you wish to do so, but I will introduce them to you as we go.

Now in our lesson today, we're going to first of all look at the structure of the neuron.

Then we're gonna have a look at a part of the neuron called the myelin sheath.

And then we're gonna have a look at synapses and see how they work.

So you ready to go? I am.

Let's get started.

So nerve cells are also known as neurons.

And neurons are specialised cells which are responsible for transmitting electrical impulses between different parts of the body.

Now, nerve cells, neurons, we're gonna call them neurons because that's their scientific term, neurons are really complex cells.

They are very large and often very long, and they are incredibly detailed and you can get an idea of that from the picture on the screen there.

And because of that, if you damage a nerve cell, it makes it very difficult to repair them, which is why nerve damage is often and most usually a long-term problem that you then have to live with.

Now, there are three types of neurons.

There are sensory neurons which transmit electrical impulses from a sensory receptor, so that's a receptor that can sense a change in the environment, and they send that message from the sensory receptor towards the central nervous system.

There are also motor neurons which transmit electrical impulses from the central nervous system to an effector.

So that's something which will complete a job based on that instruction.

So that's usually a muscle, but it can also be a gland.

And then there are also other neurons called relay neurons, and these transmit electrical impulses between sensory and motor neurons and between other relay neurons as well.

And relay neurons exist only within the central nervous system, and sensory and motor neurons only exist within the peripheral nervous system.

So let's just check our understanding.

What I'd like you to do is match the type of neuron on the left to the pathway that it is involved in on the right.

I'll give you five seconds to think about it.

Okay, so hopefully you've had a good think about this and you've matched the sensory neuron to the statement which says, from receptor to central nervous system.

You've matched the relay neuron with the statement which says within the central nervous system.

And you've matched the motor neuron to the statement which says from the central nervous system to the effector.

So A to E, B to D, and C to F.

Did you get all of those right? Well done if you did.

Now, when we're looking at a neuron, despite the differences between sensory, motor, and relay neurons, they all have the same key parts.

So they all have a cell body, which is where the nucleus is found, and this is a large bulbous part of the neuron, and it might be at one end or it might be in the middle.

They also all have dendrons, which are the endings of a nerve which receive the signal.

So these are only found at one end of the neuron because that's where the signal is coming in the cell from.

And they all have an axon.

And an axon is a long extension part to it, which is what carries the signal along from one end of the neuron to the other.

So these are really important parts to the neuron, the cell body, dendrons, and the axon.

So let's just check our understanding of that.

Which letter correctly points to the axon on the diagram? A, B, or C? I'll give you five seconds to think about it.

Okay, so hopefully you have decided that part C labels the axon of this neuron.

Well done if you got that right.

Okay, so what I'd like you to do now is to summarise that information by firstly labelling the diagram with the cell body, dendrons, and the axon, and state the function of those cell parts.

Then I would like you to complete the table to list the three main types of neurons and their functions.

And then finally, I'd like you to use the parts of the neuron and write a sentence to describe how the electrical impulse is transmitted through the neuron from one end to the other.

So you need to list the parts of the neuron cell in the order through which the signal will move.

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

Okay, let's review our work then.

So the first thing I asked you to do was to label the cell body, the dendrons, and the axon on the diagram, and state their function.

So you should have added the cell body in the bulbous part, the dendrons on the left, and the axon is the long skinny bit, and you should have added that the cell body includes the nucleus, the dendrons receive the signal, and the axon carries the signal.

So just make sure you've labelled all of those parts up correctly and that you've described their function properly too.

Then I asked you to complete the table to list the three different categories of neurons and their functions.

So against the sensory neuron, you should have listed that these transmit electrical impulses from the receptor to the central nervous system.

For the relay neurons, you should have said that these transmit electrical impulses between neurons within the central nervous system.

And for the motor neuron, you should have said that these transmit electrical impulses from the central nervous system to the effector.

And you might have added that that might be a muscle or a gland.

So just check your work over, make sure that it's correct and well done for both of those two tasks.

Then finally, I asked you to use the parts of the neuron essentially from the diagram and write a sentence to describe how the electrical impulse is transmitted through the neuron from one end to the other, essentially naming the parts.

So you should have included something along the lines that the signal is received by the dendrons and travels through the neuron, past the cell body, via the axon, to the other end of the neuron.

So you should have written something along those lines.

So just check your work over and make sure that you did and well done indeed.

Okay, let's move on to the next part, which is looking at a very specific part of the cell called the myelin sheath.

So some of those neurons, the relay, sensory, motor neurons, some of them have another feature called the myelin sheath.

And you can see that clearly in this diagram, those light yellow parts wrapped around the axon.

So the myelin sheath is a fatty layer and it wraps itself around the axon and insulates it.

So you might think about this a bit like the plastic that is wrapped around electrical cables and insulates them, well, the myelin sheath is doing a similar thing.

Now, insulating the neurons is really important because it speeds up the signal.

And in some cases, this can speed up the signal by 100 times, make it go 100 times faster.

So instead of going at one metre a second, it can go at 100 metres per second.

So if you think about that, that's the same as running the 100 metre race in one second.

That's really quick.

Insulating the axon is also important because it stops the nerve impulse from being interrupted by other things that are going on inside the body, either other nerve impulses which are running down adjacent neurons or other things that are happening inside the body outside of the nervous system.

And if the nerve isn't insulated, then those messages, those electrical signals can be interrupted just like they can be within the electrical cables that power your computer, your smartphone charger or whatever.

They can be interrupted if the insulation is not good around the wires that are charging those devices.

The same thing can happen in the nervous system.

So insulating the axon with the myelin sheath is a really important feature for many neurons.

So what I'd like you to think about is this, neurons without a myelin sheath will transmit the nerve impulse faster than one with a myelin sheath.

Is that true or false? I'll give you a few seconds to think about that.

Okay, so you should have said that that is false, but can you justify your answer? So can you either justify it with the fact that the myelin sheath insulates the nerve signal using the fatty layers, or the myelin sheath conducts signals from one end of the neuron to the other? So which is the correct justification for this statement? I'll give you a few more seconds to think about it.

Okay, so you should have justified your decision, your true or false statement with the fact that the myelin sheath insulates the nerve signal using fatty layers.

So did you get both of those right? Well done if you did.

So what I'd like you to do now is to review this idea of the myelin sheath, but in the context of motor neuron disease.

So motor neuron disease is a condition which is caused by the gradual breakdown of the myelin sheath on motor neurons.

Now, those motor neurons control skeletal muscle, so that's muscle which makes us move, makes our body move.

And you might have heard motor neuron disease being mentioned in respect to Stephen Hawking or the rugby player Rob Burrows.

So what I would like you to do is to write a short patient information leaflet about motor neuron disease.

And what I'd like you to do is include the role of neurons and their structure, the role of the myelin sheath, the likely symptoms that you might get if motor neurons are affected by myelin sheath breakdown.

So just think about what the motor neurons are triggering to move.

So they're connected to skeletal muscle.

So just think about what the impact, if those signals don't get through to skeletal muscle, what that might mean for the body, how it might move instead.

And you also need to include a labelled diagram of a neuron.

So take your time with this, pause the video and come back to me when you're ready.

Okay, so let's see what you've put together for your patient information leaflet.

So you might have included that neurons are part of the nervous system and they transmit messages from one neuron to another.

They have a cell body, dendrons, and an axon, and some axons are wrapped with a myelin sheath.

Now, the myelin sheath is important because it insulates the axon and this means that the transmission of electrical impulses is sped up.

And this also prevents the messages from being disrupted by other nearby neurons.

So they're the functions of the myelin sheath.

You might also have included that if the myelin sheath breaks down, the messages will be slower and might be disrupted.

Now, this is likely to lead to movement that is slow or inconsistent, interrupted, unexpected, or difficult to control.

You might have mentioned some of those things or maybe even all of them.

You might also have phrased them in a slightly different way, but that's the kind of idea that you're getting at hopefully.

And you should have also noted that only skeletal muscle will be affected.

So heart, muscle, cardiac muscle, and smooth muscle in the digestive system, for instance, will not be affected.

So the heart will continue to pump in the same way and the digestive system will function normally as well.

It's just skeletal muscle that is affected by motor neuron disease.

And then you should have also included a labelled diagram, which includes the cell body, the dendrons, the myelin sheath, importantly, and the axon.

So just review your work, make sure you've included all those important points and you've got your correctly labelled diagram as well.

Well done, that was quite a tricky task and quite a lot of information to pull together in one place.

So good job, well done.

Now let's look at this last section about the synapses.

So we know that nerve signals are transmitted from one neuron to another.

So from a sensory neuron to a relay neuron, relay neurons to relay neurons, and relay neurons to motor neurons.

But if we zoom in on that junction between different neurons, we will see that there is a really tiny gap about 20 nanometers wide, and that gap is called the synapse.

So that is a very specific technical term that is essentially to do with the gap between one neuron and another.

It's not a very big gap, but it is a gap nevertheless.

Now, at the synapse, electrical signals which are running along the axon of the nerve are converted into a chemical signal and then converted back again into an electrical signal.

So in this diagram, we can see at the top of the diagram is the first neuron, and that's the end of the axon.

Then we've got this gap, the synaptic gap between the first neuron and the second neuron.

And at the bottom of the picture, there is the second neuron, which is the dendron of the receiving neuron, the second neuron.

So in that first neuron, there's an electrical signal coming along the axon to the end, and that is converted into a chemical signal.

And you can see that those little red dots moving between the first neuron and the second neuron.

So that's a chemical signal.

And then once that signal reaches the second neuron, it's converted back into an electrical signal.

So that's the first thing to note about the synapse, that we've got electrical to chemical to electrical signal exchange going on.

So let's look at that in more detail.

So here's the synapse again, the first thing that happens is that the nerve signal arrives at the end of the axon and it triggers vesicles, which are packets of neurotransmitters.

So vesicles are these little circular balls and they are packed full of neurotransmitters.

And a neurotransmitter is the chemical, those little red dots in this diagram.

So the vesicle is triggered by the arrival of the nerve signal to the end of the axon, and those neurotransmitters are then released into the synaptic gap.

That's the gap between the first neuron and the second neuron.

Now, those neurotransmitters diffuse across the gap and they bind to a receptor on the far side, on the second neuron, and that's on the dendron of the second neuron.

And once they arrive there, they can then pass that signal on and the signal carries on in an electrical way.

So this is the process which is happening at the synapse between, at the junction between two neurons.

So what I'd like you to do is to put the stages in the correct order to describe this process happening at the synapse.

So have a read over.

Statement A is about the neurotransmitter diffuses across the synaptic gap.

Statement B says the neurotransmitters bind to a receptor on the second neuron.

Statement C is the nerve signal arrives at the end of the first neuron, and statement D is the vesicles release neurotransmitters into the synaptic gap.

But which order do they go in? I'll give you five seconds to think about it.

Okay, so let's see.

So you should have, first of all, put statement C.

The nerve signal arrives at the end of the first neuron.

Then you should have added statement D, the vesicles release neurotransmitters into the synaptic gap.

Then statement A, the neurotransmitters diffuse across the synaptic gap.

And then finish with statement B, the neurotransmitters bind to a receptor on the second neuron.

Did you get them in the correct order? Well done if you did.

Now, neurotransmitters are chemicals and you've probably heard of quite a few of them because they include serotonin, dopamine, acetylcholine, endorphins, and adrenaline.

Have you heard of any of them? I expect you probably have.

Now, they're chemicals and here's a picture of dopamine.

You don't need to know what it looks like as such, but I think it's quite interesting to see that these are quite small chemicals.

They're not complicated particularly, and they're not particularly large either.

So these neurotransmitters are stored in the vesicles and the vesicles are only found in the end of the first neuron.

Vesicles are not found at the dendron of the second neuron.

They're only found in the end of the axon of the first neuron.

Also receptors, which are what the neurotransmitter binds to, these are only found on these dendrons of the second neuron.

So we only have neurotransmitters stored within vesicles at the end of the first neuron, and we only have receptors to which the neurotransmitters will bind in the dendrons of the second neuron.

And because the vesicles are only in one place and the receptors are only in the second place, the message between the neurons can only pass in one direction from the first neuron to the second neuron, but they cannot pass in the other direction.

They can't go backwards, they can only go forwards.

And this is a really important point of the synapse because it controls the direction that the nerve signal passes in.

So let's summarise that then.

Izzy, Jacob, and Alex have been trying to describe a synapse, but whose description is correct? So Izzy says, "A synapse uses vesicles to diffuse and bind to neurotransmitters on the second neuron." Jacob says, "At a synapse, neurotransmitters diffuse across the gap and bind to receptors on the dendrons." And Alex says, "Electrical signals cause synaptic vesicles to bind to receptors then diffuse to the dendrons on the axon." But whose description is correct? I'll give you five seconds to think about it.

Okay, did you decide that Jacob is correct? If you did, well done for spotting that.

So what I'd like you to do now is to consolidate your understanding about the synapse by firstly completing the sentences to describe how the synapse transmits signals from one neuron to another.

And then I'd like you to explain two features of a synapse that ensure that the signal travels in one direction only.

Once you've done those two tasks, I'd like you to consider this scenario that Botox treatment uses the botulinum toxin produced by Clostridium botulinum bacteria, and it is sometimes used to relax the muscles in the forehead when clinically treating extreme migraines.

Now, how it works is that it prevents the release of the neurotransmitter acetylcholine from the axon terminals of motor neurons that are connecting the motor neuron to the muscles in the forehead.

So what I'd like you to do is take that information and then explain how preventing the release of neurotransmitter will cause the muscles to relax.

Have a real think about that 'cause that's quite a challenging question to answer.

Now pause the video, take your time, and come back to me when you're ready.

Okay, let's have a look at what you might have written for those questions.

So the first task, I asked you to complete the sentences to describe how the synapse works.

So you should have said that the nerve impulse arrives at the first neuron.

Vesicles containing neurotransmitter fuse with the cell membrane.

Neurotransmitter diffuses across the synaptic gap.

Neurotransmitter binds to receptors on the second neuron, and this triggers an onward nerve impulse.

Did you get all of those statements correct? Well done if you did.

Just amend your answers if you got some of them wrong.

Then I asked you to explain two features of a synapse that ensure that the signal travels in one direction only.

So you should have said that the vesicles containing neurotransmitters are only found in the first neuron, and that the receptors that the neurotransmitters bind to are only found on the second neuron.

And it is this that means that the signal can only travel in one direction.

Then I gave you scenario about how preventing the release of neurotransmitter using the botulinum toxin will cause muscles in the forehead to relax when being treated for extreme migraine.

So you should have said that nerve signals are passed on from one neuron to another using neurotransmitters.

And neurotransmitters cross the synaptic gap and bind with receptors on the second neuron.

This triggers the onward signal.

And if you block that process, if you prevent the neurotransmitter from being released, no onward signal will be received and therefore the muscle cells will not contract and therefore they will relax.

So review your answer and check that you've got all those important points and well done if you had a go at that 'cause that's actually a really tricky scenario to think your way through.

So well done if you've even had a go at that, good work.

So in our lesson today, we've seen that neurons are specialised cells that transmit nerve impulses, and that they all have a cell body, an axon, and dendrons, but some neurons also have a myelin sheath and this insulates the electrical signals and speeds up transmission.

Now, nerve impulses pass from one neuron to another via a really small gap called synapse, and the message is transmitted between nerves over the synapse using neurotransmitters, which are released from vesicles in the first neuron, and they diffuse across the synaptic gap.

When they get to the other side, they fuse with receptors on the second neuron and trigger an onward signal in the second neuron.

So that's what we've covered in our lesson today.

I hope you've enjoyed it and found it interesting.

Thanks very much for joining me, and I hope to see you again soon.

Bye.