Lesson video

This lesson is called the Human Eye 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 today.

In our lesson today, we're going to describe the main structures of the human eye and their functions.

We're going to come across a number of keywords today.

They're listed up on the screen.

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

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

Then we're going to have a look at how the eye focuses light, and then we're going to have a look at how the iris reacts to light.

So are you ready to go? I certainly am.

Let's get started.

So the human eye is a sense organ.

It has receptors which are sensitive to visible light intensity, that's about its brightness, and visible light wavelength, which is to do with its colour.

And it transmits this information to the brain for processing.

Now the eye has many key features to enable this to happen.

It has a cornea, it has an iris and a lens.

Then attached to the lens are suspensory ligaments and ciliary muscles.

And then at the back of the eye there's the sclera, the retina, and the optic nerve.

So take a note of those, make sure you understand which part is which.

So let's quickly check our understanding.

Which label indicates the lens? A, B, C, D, E, or F.

Did you choose C? Excellent, well done.

What about the retina? Which label is pointing to the retina? So have you chosen E this time? Well done if you did.

Right, now we know where the bits are, what do they do? So the cornea, which is on the front of the eye, refracts light, and it also protects the eye.

The iris is the coloured part of the eye and that controls how much light enters the eyeball itself.

The lens focuses light and allows us to see a clear picture and that focusing happens on the retina.

We'll come to that in a moment.

The suspensory ligaments connect the ciliary muscles to the lens and the ciliary muscles contract and relax in order to change the shape of the lens to allow us to focus on items which are close to us or far away from us.

So they're the parts of the front of the eye.

What about the other parts then? The sclera is the white outer layer that protects the eye.

The retina is where light is focused onto, and that is where the sensory receptors are located.

And then the optic nerve transmits electrical information from the sensory receptors to the brain for processing.

So these are sensory neurons.

Have you got all of those? Make sure you understand what various different parts of the eye do, and you match those functions to the correct part of the eye.

Now, sensory receptors in the retina are really highly specialised and there's a fascinating picture of them here on the screen.

There are two types of sensory receptors, cones, which are kind of conical in shape.

You can see from their shape.

Cones allow us to see colour, C-C, cones for colour.

Now they need bright light in order to work properly, which is why in dim light our vision is really quite black and white.

The other type of receptor are called rods.

And rods are, well, you can see they look a bit like rods, don't they? And they work at much lower light intensities.

So this is what we are using when it's quite dim and dark.

We are not using the cones 'cause they require bright light.

The rods are what allows us to see in darker conditions.

They don't perceive colour, but they are sensitive to movement.

And so when you see something out the corner of your eye moving, that's being picked up by the rods.

So cones are for colour perception and rods are for movement.

So let's quickly check our understanding.

Which part or parts of the eye change size in bright light? I'll give you five seconds to think about it.

So did you choose B, which is pointing to the iris? Well done if you did.

How about this, which parts of the eye contain the sensory receptors? I'll give you five seconds again.

So have you chosen E? That's pointing to the retina.

Again, well done if you did.

So what I'd like you to do is to summarise that information.

I'd like you to label the image of the eye with the key parts and then I'd like you to complete the table to describe the function of these key parts.

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

Okay, let's see how you got on with that.

So firstly, I asked you to label the image of the eye with the key parts.

So you should have labelled the cornea, the iris, the lens, the ciliary muscles, connected to suspensory ligaments, which connect to the lens.

Also identified the sclera, the retina, and the optic nerve.

Just check that you have labelled your diagram correctly and make any amendments if you've got any mistakes.

Well done.

Then I asked you to complete the table with the different parts of the eye in their function.

So for the cornea, you should have said that this refracts light and protects the eye.

That the iris controls how much light enters the eye.

That the lens focuses light onto the retina.

That the ciliary muscles change the shape of the lens, and that suspensory ligaments attach the ciliary muscles to the lens.

That the retina contains the sensory receptors and the optic nerve transmits information to the brain.

Then finally, the sclera is the outer layer that protects the eye.

So again, just check your work over, make sure you've got the correct functions against the parts of the eye.

Make any changes if necessary and well done again.

Okay, let's move on to having a look at how the eye focuses light.

Now, the cornea and the lens are both refractive.

That means that they can bend light and they converge light rays in order to bring them to a focal point to bring them into focus on the retina.

So you can see there in the picture that this lens in the way of the light beam is bringing the light rays together to a focal point.

And that's because the lens is refractive and it's focusing those light rays to a focal point.

And that's pretty much what is happening in both the cornea and the lens.

Now the cornea in lens work together to help focus light and this enables a clear image to be created onto the retina and then interpreted by the brain.

So light comes from the source and it passes through the cornea, which refracts it.

That means bending it and the cornea converges those light rays slightly, but not enough.

So the light rays then pass through the lens which converges those light rays further so that they come to a focal point on the retina.

So you can see that both the cornea and the lens are both converging the light rays through refraction to ultimately bring those light rays together to the focal point.

And that focal point is on the retina.

So a nice, clear image can be created.

Now, let's just clarify a few things.

Firstly, the cornea does not change shape regardless of how near or far the object is.

It is a fixed shape covering the outer part of the eye.

And actually if you look at the side of your eye, you'll see the cornea because that's that kind of clear blob sticking out from the eyeball when you look sideways into the eye.

And you can see that in the cut through diagram there.

So the cornea does not change shape.

However, the lens does.

So the lens is attached to suspensory ligaments, which are attached to ciliary muscles.

And the ciliary muscles can contract or relax to change the shape of the lens.

And by doing so, it is enabling clear focus to be brought onto the retina.

And that contraction or relaxation of the muscles is called the accommodation reflex response.

Now let's look at that in a bit more detail because there's some precision to add to this description.

So to focus on an object nearby the ciliary muscles contract and the suspensory ligaments loosen, and this makes the lens thicker, which refracts the light rays more strongly.

So you can see the lens is quite round and fat in nature, and this is because the ciliary muscles have contracted and the suspensory ligaments have loosened.

When we are looking at an object near to us.

When we're focusing on an object much further away, the ciliary muscles relax and the suspensory ligaments are pulled tight.

And what this does is make the lens thinner so that light rays are only refracted slightly.

And you can see there in the diagram the lens is really skinny and quite tall, and that's because the ciliary muscles have relaxed and the sensory ligaments are pulled tight.

So let's just quickly check our understanding.

Which one of these correctly describes your eye if you are observing an object close up? Is it A, the ciliary muscles contract and the suspensory ligaments loosen to make the cornea round? Is it B, the ciliary muscles contract and the suspensory ligaments loosen to make the lens round? Is it C, the ciliary muscles relax and the suspensory ligaments become tight to make the cornea thinner? Or is it D, the ciliary muscles relax and the suspensory ligaments become tight to make the lens thinner? I'll give you five seconds to decide.

Okay, so you should have first of all, discounted the items which had the cornea stated because it is not the cornea which changes shape.

So A and C can first of all be scratched off the list.

Then when we're observing something which is near to us, the lens becomes round.

And that's because ciliary muscles contract and the suspensory ligaments loosen and therefore statement B is correct.

Did you get that correct with the reasoning behind it? Well done if you did.

So what I'd like you to do now is to complete the flow chart.

We're starting with light entering the eye, then light is refracted by what? Focused by something else, and light is sensed by something at the end of this flow chart.

So complete the parts.

Then I'd like you to complete the diagram by drawing two rays of light from the source and show how they are brought to focus on the retina.

Then label the diagram with the various different parts of the eye which are shown in the diagram.

Then finally, I would like you to describe how the lens is shaped and what happens to the ciliary muscles and suspensory ligaments when you are viewing objects that are near and objects that are far away.

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

Okay, let's check our work.

So the first thing I asked you to do was to complete the flow chart.

So light enters the eye, then light is refracted by the cornea, then light is focused by the lens, and then light is sensed by the retina.

Did you get all of those parts right? Just check your work over, make sure that you did.

Well done.

Then I asked you to complete the diagram by drawing two rays of light from the source and show how they are brought to focus on the retina and then labelled diagram parts.

So you first of all should have had two light rays spreading outwards from the source to the cornea, both of which having an arrow head on them to show the direction the light rays are travelling in.

Then the light rays should have gone parallel to the lens before being drawn to a point at the focal point on the retina.

And again, those lines should also have arrow heads on them pointing in the direction that the light rays are travelling in.

Then you should have added labels to the diagram to show the cornea, the lens and the retina.

So check your work over.

Have you got the arrows on the light rays? That's probably the most likely thing you've missed, if you've missed anything.

And have you labelled up the parts of the eye, the cornea, the lens, and the retina properly as well? Well done indeed.

Then next, I asked you to describe how the lens is shaped when we are viewing objects that are nearby and far away.

So you should have written that when viewing objects that are near to us, the ciliary muscles contract and the suspensory ligaments loosen to make the lens thicker so that it refracts strongly.

Also, you should have said that when we're viewing objects far away, the ciliary muscles relax and the suspensory ligaments are pulled tight to make the lens thinner and refract only slightly.

So again, check that you've got what is happening to the ciliary muscles and the suspensory ligaments round the right way, and what's happening to the size of the lens and the refraction of light listed correctly as well.

And well done again.

Okay, let's move on to our final part of the lesson where we're going to look at how the iris reacts to light.

So the iris is the coloured part of the eye that surrounds the black pupil.

And you can see that in the diagram there, the iris is the green part and the pupil is that black hole in the centre.

And light enters the eye through the pupil so that it can be focused onto the retina at the back of the eye.

Now the iris is made up of a set of antagonistic muscles.

That means a set of muscles that work in opposite directions from each other.

Now these antagonistic muscles are called radial muscles and these span from the centre to the outside edge in lines and circular muscles, which are around in a circle in the centre of the iris, essentially circling the pupil.

So we've got radial muscles which are radiating out from the centre and circular muscles, which are forming a circle in the centre of the iris.

Now in bright light, the circular muscles in the iris contract and the radial muscles relax.

So the circular muscles in the centre of the eye contract and the radial muscles around the outer part of the iris relax.

And what this causes to happen, as you can see in the diagram there is for the size of the pupil to reduce and in very bright light, the size of the pupil can become almost nothing can be so pinprick small.

Now what this does is let less light into the eye and therefore protects it from damage because too much light can harm our retina and could cause us blindness.

Now in dim light, the circular muscles in the iris relax and the radial muscles contract.

So the circular muscles which are around the outside of the inner part of the iris, essentially edging the pupil, these relax and the radial muscles which radiate out from the centre of the iris to the outside, these contract.

And what they do is pull the edge of the iris, the circular muscles out towards the edge of the iris, and that as you can see in the diagram there on the screen, makes the pupil much, much larger and in really dim conditions, the pupil can be really quite significant and the iris can be almost imperceptible.

And what this does is maximise the amount of light that can enter the eye, and this means that we can harvest as much light as possible, which enables us to see better in low light conditions.

Now this changing of size of the iris is a reflex response and we can observe this reflex response in humans quite easily.

So we're going to investigate that reflex response of the iris to different light conditions, and we're going to do that by facing each other and closing our eyes maybe for about 30 seconds and let the teacher turn off the lights in the room.

Let your eyes have time to adjust and to customise themselves to the lower light conditions.

Then wait about 30 seconds, turn the lights back on, open your eyes together at the same time as your partner and observe the change in the pupil size.

They should go from large to small.

Before we do that, let's just check our understanding.

So in dim light, the pupil size decreases.

True or false? Okay, so you should have said that that is false, but why? Can you justify your answer with one of these two statements? Either that pupil size increases to allow more light in so we can see better, or pupil size increases to allow less light in so we can see better.

Which one is the correct justification? Again, I'll give you five seconds.

So you should have justified that answer with statement A.

That pupil size increases to allow more light in so we can see better.

Well done if you got that right.

So what I'd like you to do is to first of all follow the method so that you can observe your partner's change in pupil size as the brightness changes in the room.

Then I'd like you to describe the changes in pupil size as the light conditions change, and state why this is an important reflex response.

Then I'd like you to consider this scenario that some patients are given eye drops to relax iris muscles and expand pupil size when they visit the optician.

In fact, you might well have had this treatment done to yourself or know somebody who has.

And what that enables the optician to get is a better view into the eye and the inner part of the eye so they can see more easily what's going on in the retina and so on, and if there's any problems. But what I'd like you to do is to explain why it is important that the patient after this procedure, wears sunglasses and avoids bright light for a couple of hours after this treatment whilst it wears off.

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

Okay, I hope you enjoyed doing that little practical and that you did actually notice the change of pupil size in your partner's eyes.

Let's have a look though at what you wrote for the tasks.

So firstly, I asked you to describe the changes in pupil size as the light conditions change and say why this is an important reflex response.

So you should have said that in bright light, the pupil size decreases, the circular muscles contract and the radial muscles relax.

And what this does is reduce the amount of light entering the eye and reduces the likelihood that our eyes will be damaged.

Conversely, you should have said in low light levels, pupil size increases.

This is because the circular muscles relax and the radial muscles contract and this allows more light into the eye so we can see better in darker conditions.

So just check your work over, make sure you've got all of those salient points and well done for that response.

Then I asked you to consider this scenario of people visiting the optician and being given eye drops that relax the iris muscles and expand the pupil size and why it's important that they wear sunglasses and avoid bright light for a couple of hours after this treatment whilst it wears off.

So you should have said that pupils are dilated by the eyedrops, which means that more light is allowed into the eye.

So bright lights will appear brighter because the pupil cannot adjust in size to reduce the light entering the eye.

And in bright light, this could cause damage to the eye because the amount of light entering the eye isn't being restricted and therefore wearing sunglasses will reduce the amount of light entering the eye, offer protection, and help to prevent damage from occurring to the eye.

So again, just check your response over, have you got those important points there and well done for composing that response.

Okay, so that's the end of our lesson today, and that was quite a lot to get through.

But in our lesson today, we've seen that the human eye is a sense organ that detects light so that we can see.

There are many parts to the eye and we've looked particularly at the cornea, the iris, lens, ciliary muscles, retina, and the optic nerve.

We've seen how the cornea and the lens both refract light because they are convex lenses and they bring the light to focus onto the retina.

And that the lens does that specifically because its shape can be changed by the ciliary muscles in what is known as the accommodation reflex.

We've also seen how the iris expands and contracts to change the size of the pupil, and this adjusts the amount of light that is led into the eye according to the light levels.

And this is a reflex response and we can observe this reflex response of the iris as brightness levels change within the room.

So I hope you've enjoyed our lesson today.

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

Bye.