A2 Biology. Unit 4.


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.
18/04/2020 Structure and Function of the Eye 2


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.
18/04/2020 Structure and Function of the Eye 3

The External Structure of the Eye
18/04/2020 Structure and Function of the Eye 4

18/04/2020 Structure and Function of the Eye 5


The sclera , commonly known as "the white of the eye," is the tough, opaque tissue that serves as the eye's protective outer coat.
18/04/2020 Structure and Function of the Eye 6


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.
18/04/2020 Structure and Function of the Eye 7


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
18/04/2020 Structure and Function of the Eye 8


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.
18/04/2020 Structure and Function of the Eye 9


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.
18/04/2020 Structure and Function of the Eye 10

18/04/2020 Structure and Function of the Eye 11

18/04/2020 Structure and Function of the Eye 12


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.
18/04/2020 Structure and Function of the Eye 13


The choroid lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.
18/04/2020 Structure and Function of the Eye 14


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.
18/04/2020 Structure and Function of the Eye 15


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.
18/04/2020 Structure and Function of the Eye 16


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!
18/04/2020 Structure and Function of the Eye 17

Blind Spot Test
Close Left Eye and Look at the Dot
18/04/2020 Structure and Function of the Eye 18


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.
18/04/2020 Structure and Function of the Eye 19


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.
18/04/2020 Structure and Function of the Eye 20

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
18/04/2020 Structure and Function of the Eye 21


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.
18/04/2020 Structure and Function of the Eye 22

Macula: Concentration of
Photoreceptors
18/04/2020 Structure and Function of the Eye 23


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.
18/04/2020 Structure and Function of the Eye 24


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 .
18/04/2020 Structure and Function of the Eye 25


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.
18/04/2020 Structure and Function of the Eye 26


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.
18/04/2020 Structure and Function of the Eye 27


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.
18/04/2020 Structure and Function of the Eye 28


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.
18/04/2020 Structure and Function of the Eye 29


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.
18/04/2020 Structure and Function of the Eye 30


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?
18/04/2020 Structure and Function of the Eye 31

18/04/2020 Structure and Function of the Eye 32


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.
18/04/2020 Structure and Function of the Eye 33

Red
Blue
Black
Green
Yellow
Orange
Structure and Function of the Eye 18/04/2020 34

18/04/2020 Structure and Function of the Eye 35


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.
18/04/2020 Structure and Function of the Eye 36

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.
18/04/2020 Structure and Function of the Eye 37


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.
18/04/2020 Structure and Function of the Eye 38

18/04/2020 Structure and Function of the Eye 39

18/04/2020 Structure and Function of the Eye 40

18/04/2020 Structure and Function of the Eye 41

18/04/2020 Structure and Function of the Eye 42

18/04/2020 Structure and Function of the Eye 43

Cross Section of the Orbits of the
Eye
18/04/2020 Structure and Function of the Eye 44

 http://www.stlukeseye.com/Anatomy.asp
 http://www.accessexcellence.com/AE/AEC
/CC/vision_background.html
 http://webvision.umh.es/webvision/index.ht
ml
18/04/2020 Structure and Function of the Eye 45