Lesson video

This lesson is called "Common Defects of 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.

In our lesson today, we're gonna describe some common defects of the human eye and how we treat them.

Now we're gonna come across a good number of keywords in our lesson today, and they're listed up on the screen for you now.

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 going to look at short and long-sightedness and cataracts.

Then we're gonna move on to look at colour blindness, which is only on some specifications.

So are you ready to go? I am.

Let's get started.

So how many people need glasses or contact lenses in the UK do you think? What percentage of the population need them to correct their vision? Would you believe that it is nearly 3/4 of the UK population who need glasses or contact lenses to correct their eyesight? That is astonishing.

So what might go wrong with your eyes? What long-term conditions do you know about that affect vision? Well, maybe you have said: Long and short-sightedness or cataracts.

Maybe you've said about blindness or glaucoma.

Maybe you've mentioned something else, a little less common like macular degeneration or colour blindness.

But whatever you've said, you are aware of a number of conditions that can affect our ability to see clearly and that's what we're looking at today.

So we're gonna start off by looking at long and short-sightedness because these are two very common eye defects and they're relatively easy to treat.

Now they are both the result of the lens in the eye being unable to focus light rays onto the retina.

And because the focal point misses the retina, the picture that people see, if they're long or short-sighted, is blurry.

Now, how those two conditions are caused and how we treat them is different, although there are similarities.

So let's look at them in a bit more detail.

Let's start off with short-sightedness because short-sightedness is relatively common in younger people such as you, and it also goes by another name of myopia.

Now, short-sightedness or myopia is caused by an elongated eyeball.

So if the eyeball is too long, then try as the lens might, it cannot focus light onto the back of the retina because the retina is essentially too far away.

So that's a very common cause of short-sightedness.

However, it might also be caused by the lens being too thick and curved so that it refract more strongly bringing the focal point short of the retina.

But however it is caused, essentially the lens cannot focus light onto the retina.

It comes short of the retina, hence it being called short-sightedness.

Now, short-sightedness means also that we can see things which are close to us, short in distance away, but things which are much further away appear blurred.

So there's a lot of parallels with this name short-sightedness.

Both the focal point is short of the retina and the things that can be seen clearly have to be short, so close to.

So hopefully that will help you to remember that.

Now correcting short-sightedness is relatively straightforward and it requires a concave lens.

So a concave lens is a lens where there are essentially two C's back to back.

If you think about a letter C, put them back to back and you can see that on the screen with the diagram of the concave lens.

The two C's are back to back concave, cuh-cuh, and the C's are back to back.

That's how I remember it anyway.

Now, concave lenses refract light, so they bend light, but they diverge light rays, they push the light rays apart, and you can see that in the diagram that the light rays, after having passed through the concave lens, are now pointing away from each other rather than closer to each other or in a parallel line.

So concave lenses diverge light, and essentially what that means is the lens of the eye then needs to refract that light more to bring it to a focal point.

And by doing that, the focal distance, the focal length is increased, so the focal point lands on the retina instead of short of the retina.

So a concave lens through diverging the rays of light pushes the focal distance further away and onto the retina.

So that's short-sightedness, where objects which are close to a clear but further away are blurry and it's caused because the focal point is short of the retina and is corrected using a concave lens.

So let's just check our understanding.

Which of these statements are true about short-sightedness? A, a concave lens is used to correct vision, B, the corrective lens converges light rays, and C, the eyeball may be elongated.

I'll give you five seconds to decide.

So you should have said that A is correct and C is correct, but B is incorrect because the corrective lens diverges light rays.

Did you get all of those correct? Well done indeed.

Right, let's move on to looking at long-sightedness.

So long-sightedness can occur when the lens stiffens so that it cannot become round enough to focus light of close objects onto the retina.

And this stiffening of the lens generally happens with age, which is why long-sightedness is more common in older people than younger people.

However, that's not the only cause, it's also caused if the eyeball is too short and so the distance between the lens and the retina is not long enough, is too short.

So long-sightedness leads to a focal point which is long of the eyeball.

The lens can't focus light onto the retina because the focal point is beyond the retina.

So this is long-sightedness with a focal point long of the eyeball.

And people who are long-sighted can see things which are far away very clearly, but things which are close up appear blurred.

So long-sightedness is a condition where the focal point is long of the eyeball and things which are a long distance away appear clearly.

Now, how is long-sightedness corrected? Well, long-sightedness is corrected using a convex lens.

So if you think about the word convex, it's got a V in the middle, and if you curve the top of the V, so the points of the V come together, you've got this bulge and that bulge is your convex lens, or at least that's how I remember it.

So a convex lens is essentially helping the lens of the eye in focusing, it's refracting light and converging the light ray slightly further together.

So you can see in the diagram there that the light rays, after having passed through the convex lens, are coming together rather than being in parallel or diverging from each other.

They are converging.

Essentially, the corrective lens is giving the eye lens a helping hand, starting off the correction and the converging of the light rays so that the eye lens has less work to do and the focal point is shortened onto the retina.

So long-sightedness is corrected using a convex lens which converges light rays together.

Now there are lots of ways of adjusting the focal point, and we've discussed using lenses as if we were wearing spectacles, but of course there are other technologies available.

So there are, of course, hard and soft contact lenses which can be worn instead of spectacles, and some people prefer them and obviously they are much less visible on the face as well.

There's also more sophisticated surgery treatment using lasers which are able to change the shape of the cornea.

So laser eye surgery doesn't affect the lens, it affects the cornea on the outer part of the eye, but it is able to change the shape of the cornea and therefore change the amount of refracting the cornea does and therefore affect the way light is focused and hopefully bring it back to the retina instead of past it or short of it.

There's a more substantial surgical option as well.

And this is about replacing the lens itself, but this is not a terribly common procedure, and obviously as a surgical procedure comes with its own risks, but replacing the lens in the eye itself can remove that mis-focusing that is happening with the natural lens by replacing it with an artificial one.

So in addition to wearing spectacles, there are these other options available as well.

So let's just check our understanding.

So which of these statements are true about long-sightedness? A, a convex lens is used to correct vision, B, the lens is stiff, so cannot elongate, and C, the focal point is lengthened by the lens.

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

Okay, let's check our answers.

So you should have said that A is correct, that B is incorrect because the lens is stiff, so cannot become round, and C is incorrect because the focal point is shortened by the lens.

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

So what I'd like you to do now is to summarise our learning so far by firstly completing the table to summarise short and long-sightedness.

And I want you to particularly just briefly summarise the cause of the vision impairment, the problems that are experienced with the vision as a result of that cause, the type of lens that is used to correct vision and the effect of that lens on the focal length.

Once you've completed that, then I would like you to have a look at the two diagrams and I would like you to complete them to show the effect of the condition on the focal point in the eye.

So where is the focal point in the short-sighted eye and where is the focal point in the long-sighted eye? I'd like you to draw both the focal point and also show how light rays are being converged by the eye lens onto that focal point.

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

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

So firstly, I asked you to complete the table.

So for short-sightedness you should have said that the cause of the vision impairment is an elongated eyeball or a too thick lens.

And for long-sightedness, which is also known as hyperopia or hypermetropia, the cause of the vision impairment is a stiff lens or a shortened eyeball.

Now the problems that this causes is for short-sightedness that distant objects appear blurry, whereas for long-sightedness, near objects appear blurry.

So remember that the type of sightedness that you have indicates the objects that can be seen clearly at that length.

So in short-sightedness, objects which are short to us, so nearby, are seen clearly and in long-sightedness, objects that are far away are seen clearly.

So what type of lens can be used to correct these vision impairments? Well, for short-sightedness you should have said a concave lens and for long-sightedness you should have said a convex lens.

And what is the effect of the lens on the focal length? Well, for short-sightedness, the concave lens extends the focal length because it diverges light rays and in long-sightedness it shortens the focal length because it converges light rays.

So just check you've got things around the correct way in your table and then we'll move on.

And what I wanted you to do next was to complete the diagrams to show the effect of the condition on the focal point in the eye.

So for short-sightedness, you should have put the focal point short of the retina, so within the body of the eyeball itself and drawn rays of light with arrowhead on them, which crossover before the retina.

Whereas for long-sightedness, you should have put the focal point beyond the retina and then shown light rays converging at that point, long of the eyeball.

So again, just make sure you've got those around the correct way and well done for both of those tasks.

Okay, let's move on to our next topic, which is about cataracts.

So cataracts occur when the lens becomes cloudy and you can see that in the picture there.

Instead of there being a black pupil in the middle of the green iris, there's a milky, cloudy zone instead and that's where the lens has become cloudy.

And by becoming cloudy it's becoming opaque.

And this happens over time, very slowly and is why it is more common in older people.

So cataracts occur when the lens becomes opaque and ultimately if left untreated it will become worse and worse and can lead to blindness.

Now, cataracts reduce the clarity of vision.

So this means that objects will appear blurry and colours will appear faded, and if the light is really bright, it can glare and dazzle, a bit like if we look at bright lights in a dark room, they can be very intense and quite glaring.

Well, the same sort of thing will happen, but in what we would consider perhaps more normal light conditions.

So the opaqueness of the lens will impact on the clarity of the vision, the colours which are being seen and how bright light is perceived.

Now, correcting cataracts is relatively straightforward and it can be done by removing the lens and replacing the lens with an artificial one.

Now this is a quick piece of surgery, doesn't normally take very long, less than half an hour and is done under local anaesthetic in day surgery.

So it's a really very straightforward process nowadays to correct and you can see in the picture there if you care to look the surgery which is being performed on an eyeball.

So let's just check our understanding.

True or false: Objects appear faded and blurry with cataracts? So you should have said that that is true, but can you justify your answer? Is it because the lens has become stiff and unclear, or is it because the lens has become opaque, which is correct? So you should have justified that by saying that objects appear faded and blurry because the lens has become opaque.

Well done if you got both of those answers correct.

So what I'd like you to do now is to summarise what we've learned by writing a short patient information leaflet about cataracts.

And within that I like you to include what causes cataracts, the impact of cataracts on vision and how cataracts can be treated.

So it doesn't need to be very long, but it does need to be concise and informative.

Once you've completed your patient information leaflet, I would like you to explain to Alex why his granddad who has cataracts can only read in bright light.

So consider what you know about cataracts and explain this scenario to Alex.

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

So for the patient information leaflet about cataracts, you should have included that cataracts occur when the lens becomes cloudy.

And this happens most often in older people as the lens becomes opaque.

Cataracts cause blurry vision, faded colours and glaring bright lights.

So they're the very common symptoms of cataracts.

And if left untreated, this will get worse over time and can lead to blindness.

However, it's relatively easy to treat in day surgery with local anaesthetic.

So what happens in day surgery is that the cloudy lens is removed and it's replaced with a new plastic one.

So just check over your notes, make sure you've included all of those salient points and well done.

Then I asked you to explain to Alex why, because his granddad has cataracts, he can only read in bright light? So you should have said that the cloudy lens reduces the amount of light that is getting through the lens to the retina, and therefore, in dim light, not enough light can get through the lens.

So his granddad will not be able to see well enough to be able to read, which is why he can only read in bright light.

So again, just review your answers and make sure that you've got all of those important points and well done again.

Okay, let's move on to this last section about red-green colour blindness, which is only relevant for some syllabi.

So can you see a number in the picture on the screen? And if so, what is that number? Can you tell? Well, if you can't see the number 97, it might indicate that you are red-green colour blind.

And what this means is that people who are red-green colour blind cannot distinguish between the colours red and green.

Instead they appear grey.

So there are several types of colour blindness including red-green, which is the most common, blue-yellow, blue-only, and monochrome where it's just black and white or grey scale really, I suppose.

So red-green colour blindness is most common, and this affects nearly 1 in 12 men and 1 in 200 women.

So relatively prevalent within the population.

Now red-green colour blindness is a sex-linked genetic disorder.

So what does that mean? Well, the expression of a genetic disorder is determined by the combination of alleles.

So an allele is a variant of a gene.

So the combination of those alleles which are inherited for a certain gene, and that is called the genotype.

Now, the allele for red-green colour blindness is carried on the X chromosome, but only the X chromosome, as you can see in the diagram.

Now, females have two X chromosomes, they are XX.

Therefore, females can have two copies of the red-green colour gene, whereas males only have one X chromosome because their other sex chromosome is a Y chromosome.

Now the Y chromosome is significantly shorter and smaller than the X chromosome as you can see in the picture.

And because of that, it holds fewer genes.

So the gene that allows for red-green colour detection only appears on the X chromosome.

And because males only have one X chromosome, they only have one copy of the gene.

So the gene for red-green colour blindness appears on the X chromosome only and not the Y chromosome.

Now it's a recessive disorder.

That means that it is not dominant and therefore you need the recessive version of the gene in order to have the disorder.

So women who have normal colour vision and are homozygous dominant have two copies of the dominant version of the gene and therefore can be described as having a genotype, big R, big R.

If a woman were to be red-green colour blind, she would need to have two copies of the recessive allele and therefore she would have the genotype little r, little r as you can see in the picture.

But that also means that a female can be what we call a carrier.

That means that she can have the recessive allele in her genotype but not be red-green colour blind in her phenotype.

So that's how it's expressed what it appears to be.

And that means she can have one copy of the dominant allele and one copy of the recessive allele, big R, little r as you can see in the middle picture.

So females can be homozygous dominant, big R, big R, homozygous recessive, and therefore colour blind, little r, little r, or heterozygous big R, little r and therefore carry the allele for colour blindness.

So that's the scenario with females.

Let's look at what happens with males.

So remember that males only have one copy of the X chromosome and therefore they only have one copy of the gene.

So a male with normal colour vision will have one copy of the dominant allele, big R, but if a male is red-green colour blind, they will have one copy of the recessive allele, little r because there's no other X chromosome to carry a second copy.

So this means that males cannot be a carrier.

They either have red-green colour vision or they are red-green colour blind, but they cannot carry the recessive allele and not be red-green colour blind.

And that means that there is a higher probability of males being red-green colour blind because of the way the condition is inherited.

So let's just pause on that and check our learning.

So select the image that shows a male with red-green colour blindness.

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

Okay, so you should have noted that A is with the dominant allele, big R, so that is a normal-vision male.

B is the correct answer, a color-blind male because they've got one copy of the recessive allele, little r.

And C is a female, and C is a female because there are two X chromosomes and she's also colour blind because she has two copies of the recessive allele, little r, little r.

I hope you worked all of those out and got the correct answer as well.

Well done.

Now as we've seen, colour blindness is more common in males than females because of the way it is inherited, and it can be tested using what is called an Ishihara colour test plate, and that's what you can see in that image on the screen.

So you can see that it's a picture made up of lots of different sized coloured circles, and there's a mixture of green and red ones in there, and they're of different hues, different colours of green and red.

So if the person who is looking at the Ishihara colour plate is red-green colour blind, it will be difficult for them to identify which of those dots are red and which of those are green because they will appear grey instead, and therefore they won't be able to read the number present on the plate.

In this case, the number 97.

So let's just check our understanding again.

True or false: Red-green colour blindness is more common in females than males? Okay, so you should have said that that is false, but can you justify your answer? Is it because males only need one recessive allele to be colour blind, or is it because males can be a carrier and be colour blind? Okay, so you should have explained that red-green colour blindness is more common in males because males only need one recessive allele to be colour blind.

Did you get that right? Well done if you did.

So what I'd like you to do now is to summarise this topic on red-green colour blindness by firstly using the chromosome diagrams to label the genotypes and phenotypes found in people with normal colour vision and red-green colour blind vision.

And then I would like you to describe how to test the colour blindness and the impact of the condition on the person with it.

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

Okay, let's just check our work then.

So the first task I asked you to do was to label the genotypes and phenotypes in the diagrams. So for the first one, you should have put a female because there are two X chromosomes.

Remember, the X chromosome is substantially larger than the Y chromosome, and this is a female with normal colour vision because she's got big R, big R alleles.

Then the next one I've put as a carrier, again a female.

So two X chromosomes.

Now she's a carrier because she's got one big R and one little r allele, one dominant and one recessive.

Then the third female version is colour blind, and that's because she's got two recessive alleles, little r, little r.

Then the last two are males.

The first is normal, so we've got X and Y chromosomes and a big R allele.

And then the red-green colour blind male, again XY chromosomes and a little r allele.

So just check that you've got the correct chromosome labelling, X and Y, and the correct allele labelling, Big R or little r, with its correct description, the phenotype of that genotype.

So the genotype is what is written on the purple banners and the phenotype is what is written in the text above the chromosome diagrams. And well done if you got all of those correct.

Then I asked you to describe how to test for colour blindness and the impact of the condition on the person with it.

So you should have said that colour blindness is tested for using an Ishihara colour test plate, and this has a number written in red against a green background, all made of different sized circles, and people who are red-green colour blind cannot read the number on the plate because they cannot distinguish red and green from each other.

They appear grey instead.

What this means for them is that there is no cure for colour blindness because it is sex-linked genetic disorder, and therefore they will have this condition for the whole of their life.

So again, just check over your work and well done indeed for completing that.

So let's just summarise what we've covered in our lesson today.

So we've seen that vision defects are actually incredibly common in the population and short and long-sightedness and cataracts are some of those.

Now short-sightedness, also known as myopia, can be the result of an elongated eyeball and can be corrected using a concave lens, whereas long-sightedness can be the result of the stiffening of the lens and can be corrected using a convex lens.

And in both cases, the lenses that are being worn are changing the focal length and bringing the focal point onto the retina.

Now with cataracts, these occur when the lens becomes cloudy and opaque and cataracts can be treated surgically by replacing the lens.

Then we've also seen that red-green colour blindness is a sex-linked genetic disorder, which cannot be cured.

So we've covered a lot of work in today's lesson.

So thank you very much for working so hard and for joining me in our lesson today, and I hope to see you again soon.

Bye.