1. The Structure and Function of the Eye
A2 Biology. Unit 4.

2. Structure and Function of the Eye
Learning Objectives
Be able to name the structures of the eye.
Be able to explain the function of the parts of the eye.
Be able to explain how rods and cones function.
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Structure and Function of the Eye

3. Structure and Function of the Eye
Windows to the World
Eyes are photosensitive organs designed to detect light and pass on the information as electrical signal to the brain.
Vision such as ours and animals are know as ‘camera’ like vision.
Camera like vision has evolved at least seven separate times through out natures history.
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Structure and Function of the Eye

4. The External Structure of the Eye
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Structure and Function of the Eye

5. Structure and Function of the Eye
Structure of the Eye
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Structure and Function of the Eye

6. Structure and Function of the Eye
Sclera
The sclera, commonly known as "the white of the eye," is the tough, opaque tissue that serves as the eye's protective outer coat.
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Structure and Function of the Eye

7. Structure and Function of the Eye
The Iris
The coloured part of the eye is called the iris.  It controls light levels inside the eye similar to the aperture on a camera. 
The round opening in the centre of the iris is called the pupil. 
The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size. 
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Structure and Function of the Eye

8. Structure and Function of the Eye
The Cornea
The cornea is the transparent, dome-shaped window covering the front of the eye. It is a powerful refracting surface, providing 2/3 of the eye's focusing power. Like the crystal on a watch, it gives us a clear window to look through
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Structure and Function of the Eye

9. Structure and Function of the Eye
Conjunctiva
The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye. 
It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids. 
It is nourished by tiny blood vessels that are nearly invisible to the naked eye.
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Structure and Function of the Eye

10. Structure and Function of the Eye
Lens
The purpose of the lens is to focus light onto the back of the eye. 
The nucleus, the innermost part of the lens is surrounded by softer material called the cortex. 
The lens is encased in a capsular-like bag and suspended within the eye by tiny guy wires called zonules.
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Structure and Function of the Eye

11. Structure and Function of the Eye
Lens: Focusing Light
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Structure and Function of the Eye

12. Structure and Function of the Eye
Lens: Sight Defects
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Structure and Function of the Eye

13. Structure and Function of the Eye
Vitreous
Three chambers of fluid: Anterior chamber (between cornea and iris), Posterior chamber (between iris, zonule fibers and lens) and the Vitreous chamber (between the lens and the retina)
The first two chambers are filled with aqueous humour whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humour.
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Structure and Function of the Eye

14. Structure and Function of the Eye
Choroid
The choroid lies between the retina and sclera.  It is composed of layers of blood vessels that nourish the back of the eye.
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Structure and Function of the Eye

15. Structure and Function of the Eye
Retina
The retina is a very thin layer of tissue that lines the inner part of the eye.
It is responsible for capturing the light rays that enter the eye.  Much like the film's role in photography.  
These light impulses are then sent to the brain for processing, via the optic nerve.
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Structure and Function of the Eye

16. Structure and Function of the Eye
Optic Nerve
The optic nerve transmits electrical impulses from the retina to the brain.
It connects to the back of the eye near the macula.
The visible portion of the optic nerve is called the optic disc.
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Structure and Function of the Eye

17. Optic Nerve: Blind Spot
Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot.
Take a piece of paper and draw a dot and 10 cm to the left an x.
Close your right eye and hold the paper at arms length.
Look at the dot and move the paper towards you.
What happens to the X?
It disappears into the blind spot!
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Structure and Function of the Eye

18. Blind Spot Test Close Left Eye and Look at the Dot
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Structure and Function of the Eye

19. Structure and Function of the Eye
Macula
The macula is located roughly in the centre of the retina, temporal to the optic nerve.
It is a small and highly sensitive part of the retina responsible for detailed central vision.
The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.
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Structure and Function of the Eye

20. Structure and Function of the Eye
Macula: Fovea Test
Your fovea is the most sensitive part of the retina.
It has the highest concentration of cones, but a low concentration of rods.
This is why stars out of the corner of your eye are brighter than when you look at the directly.
But only your fovea has the concentration of cones to perceive in detail.
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Structure and Function of the Eye

21. Fovea Test Look at the star and try to read the letters
A B G T J I N K O J * U I L S W Q A M N
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Structure and Function of the Eye

22. Macula: Fovea Test
To show this draw a dot on a piece or paper. On each side of the dot write 10 capital letters.
AGSHDEDHJS*DHSJEKSEJD
Stare at the dot and with out moving your eyes see how many letters your can read.
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Structure and Function of the Eye

23. Macula: Concentration of Photoreceptors
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Structure and Function of the Eye

24. Retina: Photoreceptors
There are two types of photoreceptors in the retina: rods and cones.
The retina contains approximately 6 million cones.
The cones are contained in the macula, the portion of the retina responsible for central vision.
They are most densely packed within the fovea, the very centre portion of the macula.
Cones function best in bright light and allow us to appreciate colour.
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Structure and Function of the Eye

25. Structure and Function of the Eye
Retina: Rods
Rods allow us to perceive light and dark but not colour.
They are very sensitive and are involved in vision at low light intensities.
There function depends on the light stimulus being detected by a photopigment called Rhodopsin.
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Structure and Function of the Eye

26. Rods: Structure-Outer Segment
The outer segment contains the pigment rhodopsin in flattened membranous vesicles called lamellae.
There may be up to 1000 of these lamellae.
The outer region is connected to the inner region by a narrow region containing cytoplasm and a pair of cilia.
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Structure and Function of the Eye

27. Rods: Structure-Inner Segment
The inner segment contains a large number of mitochondria which provide ATP to resynthesis rhodopsin.
It also contains polysomes which is where the production of rhodopsin occurs.
At the base of the inner region is a synapse connected to a bipolar neurone.
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Structure and Function of the Eye

28. Structure and Function of the Eye
Rods: Rhodopsin
Rhodopsin consists of a proteins opsin combined with retinal, a derivative of vitamin A.
Retinal can exists in either cis or trans isomers.
Light causes the retinal to convert from cis to trans which can no longer bond to opsin and the retinal detaches.
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Structure and Function of the Eye

29. Rods: Potential Generation
This stimulus causes hyperpolarisation of the rod cells.
The cell membranes become less permeable to Na+ ions and the ions are actively pumped out of the inner segment.
This causes the rod to become negatively polarised as a result the neurotransmitter glutamate stops being released.
This causes an action potential to be generated in the attached bipolar neurone.
In the dark Na+ ions diffuse back in.
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Structure and Function of the Eye

30. Rods: Rhodopsin Resynthesing
ATP is used to resynthesis rhodopsin, but this takes time.
Rhodopsin breaks down in bright light so when a person goes from dark to light their rods are ‘bleached’ of rhodopsin.
They then must wait for the Rhodopsin to be resynthesised to see again.
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Structure and Function of the Eye

31. Structure and Function of the Eye
Retina: Cones
Cones allow us to perceive colour.
They have fewer membranous vesicles than rods, and these are formed by infolding of the outer membrane.
They only work effectively in light conditions.
Click to move to the next screen.
Stare at the yellow screen for 30 seconds
What happens when it changes to white?
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Structure and Function of the Eye

32. Structure and Function of the Eye
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Structure and Function of the Eye

33. Structure and Function of the Eye
Retina: Cones
Why does this happen?
There are three types of a pigment called iodopsin which detects red, blue or green light.
This is called Trichromatic Theory.
If one type of cone is stimulated for a long time the chemicals used to sense the light are depleted.
When you then look at white light you see all colours except yellow.
Many cones are linked to one ganglion cell, but only one rod is joined to each ganglion cell.
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Structure and Function of the Eye
A2 Biology. Unit 4.

34. What Colour are these Words?
Red
Blue
Black
Green
Yellow
Orange
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Structure and Function of the Eye

35. Distribution of Cone Types
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Structure and Function of the Eye

36. More Facts (Off Syllabus)
The cones are not evenly distributed being in the ratio of roughly 40:20:1 for RGB respectively. The green cones are most sensitive and the blue the least.
Thus, it is easier to discriminate between colors in the red-yellow-green-cyan regions of the spectrum than in the blue region.
Thus blue should be avoided in text and other graphic elements where recognition is very important.
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Structure and Function of the Eye

37. Eyes and Animation (Off Syllabus)
There is a .4 second delay in response from the time the image falls on the retina. The sensation may persist for up to 2 minutes.
This after image effect is exploited by film, TV and computer monitors which display a rapid succession of still images, which are refreshed before the perceptual image has decayed.
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Structure and Function of the Eye

38. Eye Disorders (Off Syllabus)
About one in ten men and one in a hundred women experience some form of divergent colour perception.
The most common is confusion between red and green. This can cause some problems in colour matching.
The genes for colour blindness located on the X chromosome.
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Structure and Function of the Eye

39. Structure and Function of the Eye
Colour Blindness Test
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Structure and Function of the Eye

40. Structure and Function of the Eye
Colour Blindness Test
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Structure and Function of the Eye

41. Structure and Function of the Eye
Colour Blindness Test
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Structure and Function of the Eye

42. Structure and Function of the Eye
Colour Blindness Test
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Structure and Function of the Eye

43. Structure and Function of the Eye
Colour Blindness Test
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Structure and Function of the Eye

44. Cross Section of the Orbits of the Eye
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Structure and Function of the Eye

45. Structure and Function of the Eye
Web Links
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Structure and Function of the Eye