the structure and function of the eye a2 biology. unit 4

The Structure and The Structure and Function of the Function of the

EyeEyeA2 Biology. Unit 4.A2 Biology. Unit 4.

19/04/2319/04/23 Structure and Function of the EyeStructure and Function of the Eye 22

Learning ObjectivesLearning Objectives

Be able to name the structures of the eye.Be able to name the structures of the eye. Be able to explain the function of the parts Be able to explain the function of the parts

of the eye.of the eye. Be able to explain how rods and cones Be able to explain how rods and cones

function.function.


Windows to the WorldWindows to the World

Eyes are photosensitive organs Eyes are photosensitive organs designed to detect light and designed to detect light and pass on the information as pass on the information as electrical signal to the brain.electrical signal to the brain.

Vision such as ours and Vision such as ours and animals are know as ‘camera’ animals are know as ‘camera’ like vision.like vision.

Camera like vision has evolved Camera like vision has evolved at least seven separate times at least seven separate times through out natures history. through out natures history.


The External Structure of the EyeThe External Structure of the Eye


Structure of the EyeStructure of the Eye


ScleraSclera

The The sclerasclera, , commonly known commonly known as "the white of the as "the white of the eye," is the tough, eye," is the tough, opaque tissue that opaque tissue that serves as the eye's serves as the eye's protective outer protective outer coat. coat.


The IrisThe Iris

The coloured part of the eye The coloured part of the eye is called the is called the irisiris. It controls . It controls light levels inside the eye light levels inside the eye similar to the aperture on a similar to the aperture on a camera. camera.

The round opening in the The round opening in the centre of the iris is called the centre of the iris is called the pupil. pupil.

The iris is embedded with tiny The iris is embedded with tiny muscles that dilate (widen) muscles that dilate (widen) and constrict (narrow) the and constrict (narrow) the pupil size. pupil size.


The CorneaThe Cornea

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


ConjunctivaConjunctiva The The conjunctivaconjunctiva is the is the

thin, transparent tissue thin, transparent tissue that covers the outer that covers the outer surface of the eye. surface of the eye.

It begins at the outer It begins at the outer edge of the cornea, edge of the cornea, covers the visible part of covers the visible part of the eye, and lines the the eye, and lines the inside of the eyelids. inside of the eyelids.

It is nourished by tiny It is nourished by tiny blood vessels that are blood vessels that are nearly invisible to the nearly invisible to the naked eye. naked eye.


LensLens The purpose of the The purpose of the lenslens

is to focus light onto the is to focus light onto the back of the eye. back of the eye.

The nucleus, the The nucleus, the innermost part of the lens innermost part of the lens is surrounded by softer is surrounded by softer material called the material called the cortex. cortex.

The lens is encased in a The lens is encased in a capsular-like bag and capsular-like bag and suspended within the eye suspended within the eye by tiny guy wires called by tiny guy wires called zonules. zonules.


Lens: Focusing LightLens: Focusing Light


Lens: Sight DefectsLens: Sight Defects


VitreousVitreous

Three chambers of fluidThree chambers of fluid: : Anterior chamberAnterior chamber (between cornea and iris), (between cornea and iris), Posterior chamberPosterior chamber (between iris, zonule fibers and lens) and the (between iris, zonule fibers and lens) and the Vitreous chamberVitreous chamber (between the lens and the (between the lens and the retina)retina)

The first two chambers are filled with aqueous The first two chambers are filled with aqueous humour whereas the vitreous chamber is filled humour whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humour. with a more viscous fluid, the vitreous humour.


ChoroidChoroid

The The choroidchoroid lies lies between the retina between the retina and sclera. It is and sclera. It is composed of composed of layers of blood layers of blood vessels that vessels that nourish the back of nourish the back of the eye. the eye.


RetinaRetina

The The retinaretina is a very thin is a very thin layer of tissue that lines layer of tissue that lines the inner part of the eye. the inner part of the eye.

It is responsible for It is responsible for capturing the light rays capturing the light rays that enter the eye. Much that enter the eye. Much like the film's role in like the film's role in photography. photography.

These light impulses are These light impulses are then sent to the brain for then sent to the brain for processing, via the optic processing, via the optic nerve. nerve.


Optic NerveOptic Nerve

The The optic nerveoptic nerve transmits electrical transmits electrical impulses from the impulses from the retina to the brain.retina to the brain.

It connects to the It connects to the back of the eye near back of the eye near the macula.the macula.

The visible portion of The visible portion of the optic nerve is the optic nerve is called the optic disc. called the optic disc.


Optic Nerve: Blind SpotOptic Nerve: Blind Spot

Where the optic nerve meets the retina there are Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot.no light sensitive cells. It is a blind spot.

Take a piece of paper and draw a dot and 10 cm Take a piece of paper and draw a dot and 10 cm to the left an x.to the left an x.

Close your right eye and hold the paper at arms Close your right eye and hold the paper at arms length.length.

Look at the dot and move the paper towards Look at the dot and move the paper towards you.you.

What happens to the X?What happens to the X? It disappears into the blind spot!It disappears into the blind spot!


Blind Spot TestBlind Spot TestClose Left Eye and Look at the DotClose Left Eye and Look at the Dot


MaculaMacula

The The maculamacula is located is located roughly in the centre of the roughly in the centre of the retina, temporal to the optic retina, temporal to the optic nerve.nerve.

It is a small and highly It is a small and highly sensitive part of the retina sensitive part of the retina responsible for detailed responsible for detailed central vision.central vision.

The fovea is the very centre The fovea is the very centre of the macula. The macula of the macula. The macula allows us to appreciate detail allows us to appreciate detail and perform tasks that and perform tasks that require central vision such require central vision such reading.reading.


Macula: Fovea TestMacula: Fovea Test

Your fovea is the most sensitive part of the Your fovea is the most sensitive part of the retina.retina.

It has the highest concentration of cones, It has the highest concentration of cones, but a low concentration of rods.but a low concentration of rods.

This is why stars out of the corner of your This is why stars out of the corner of your eye are brighter than when you look at the eye are brighter than when you look at the directly.directly.

But only your fovea has the concentration But only your fovea has the concentration of cones to perceive in detail.of cones to perceive in detail.


Fovea TestFovea TestLook at the star and try to read the lettersLook 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 A B G T J I N K O J * U I L S W Q A M N


Macula: Fovea TestMacula: Fovea Test

To show this draw a dot on a piece or To show this draw a dot on a piece or paper. On each side of the dot write 10 paper. On each side of the dot write 10 capital letters.capital letters.

AGSHDEDHJS*DHSJEKSEJDAGSHDEDHJS*DHSJEKSEJD Stare at the dot and with out moving your Stare at the dot and with out moving your

eyes see how many letters your can read.eyes see how many letters your can read.


Macula: Concentration of Macula: Concentration of PhotoreceptorsPhotoreceptors


Retina: PhotoreceptorsRetina: Photoreceptors

There are two types of photoreceptors in the There are two types of photoreceptors in the retina: rods and cones.retina: rods and cones.

The retina contains approximately 6 million The retina contains approximately 6 million cones.cones.

The cones are contained in the macula, the The cones are contained in the macula, the portion of the retina responsible for central portion of the retina responsible for central vision.vision.

They are most densely packed within the fovea, They are most densely packed within the fovea, the very centre portion of the macula.the very centre portion of the macula.

Cones function best in bright light and allow us Cones function best in bright light and allow us to appreciate colour. to appreciate colour.


Retina: RodsRetina: Rods

Rods allow us to perceive light and dark Rods allow us to perceive light and dark but not colour.but not colour.

They are very sensitive and are involved They are very sensitive and are involved in vision at low light intensities.in vision at low light intensities.

There function depends on the light There function depends on the light stimulus being detected by a stimulus being detected by a photopigment called photopigment called RhodopsinRhodopsin..


Rods: Structure-Outer SegmentRods: Structure-Outer Segment

The outer segment The outer segment contains the pigment contains the pigment rhodopsinrhodopsin in flattened in flattened membranous vesicles membranous vesicles called called lamellaelamellae..

There may be up to 1000 There may be up to 1000 of these of these lamellaelamellae..

The outer region is The outer region is connected to the inner connected to the inner region by a narrow region region by a narrow region containing cytoplasm and containing cytoplasm and a pair of cilia.a pair of cilia.


Rods: Structure-Inner SegmentRods: Structure-Inner Segment

The inner segment contains a large The inner segment contains a large number of mitochondria which provide number of mitochondria which provide ATP to resynthesis rhodopsin.ATP to resynthesis rhodopsin.

It also contains polysomes which is where It also contains polysomes which is where the production of rhodopsin occurs.the production of rhodopsin occurs.

At the base of the inner region is a At the base of the inner region is a synapse connected to a bipolar neurone.synapse connected to a bipolar neurone.


Rods: RhodopsinRods: Rhodopsin

Rhodopsin consists of a proteins Rhodopsin consists of a proteins opsinopsin combined with combined with retinalretinal, a derivative of , a derivative of vitamin A.vitamin A.

Retinal can exists in either Retinal can exists in either cis cis or or transtrans isomers.isomers.

Light causes the retinal to convert from Light causes the retinal to convert from ciscis to to transtrans which can no longer bond to opsin which can no longer bond to opsin and the retinal detaches.and the retinal detaches.


Rods: Potential GenerationRods: Potential Generation

This stimulus causes hyperpolarisation of the This stimulus causes hyperpolarisation of the rod cells.rod cells.

The cell membranes become less permeable to The cell membranes become less permeable to NaNa++ ions and the ions are actively pumped out of ions and the ions are actively pumped out of the inner segment.the inner segment.

This causes the rod to become negatively This causes the rod to become negatively polarised as a result the neurotransmitter polarised as a result the neurotransmitter glutamate stops being released.glutamate stops being released.

This causes an action potential to be generated This causes an action potential to be generated in the attached bipolar neurone.in the attached bipolar neurone.

In the dark NaIn the dark Na+ + ions diffuse back in.ions diffuse back in.


Rods: Rhodopsin ResynthesingRods: Rhodopsin Resynthesing

ATP is used to resynthesis rhodopsin, but ATP is used to resynthesis rhodopsin, but this takes time.this takes time.

Rhodopsin breaks down in bright light so Rhodopsin breaks down in bright light so when a person goes from dark to light their when a person goes from dark to light their rods are ‘bleached’ of rhodopsin.rods are ‘bleached’ of rhodopsin.

They then must wait for the Rhodopsin to They then must wait for the Rhodopsin to be resynthesised to see again.be resynthesised to see again.


Retina: ConesRetina: Cones

Cones allow us to perceive colour.Cones allow us to perceive colour. They have fewer membranous vesicles than They have fewer membranous vesicles than

rods, and these are formed by infolding of the rods, and these are formed by infolding of the outer membrane.outer membrane.

They only work effectively in light conditions.They only work effectively in light conditions. Click to move to the next screen.Click to move to the next screen. Stare at the yellow screen for 30 secondsStare at the yellow screen for 30 seconds What happens when it changes to white?What happens when it changes to white?


Retina: ConesRetina: Cones

Why does this happen?Why does this happen? There are three types of a pigment called There are three types of a pigment called

iodopsiniodopsin which detects red, blue or green light. which detects red, blue or green light. This is called This is called Trichromatic TheoryTrichromatic Theory.. If one type of cone is stimulated for a long time If one type of cone is stimulated for a long time

the chemicals used to sense the light are the chemicals used to sense the light are depleted.depleted.

When you then look at white light you see all When you then look at white light you see all colours except yellow.colours except yellow.

Many cones are linked to one ganglion cell, but Many cones are linked to one ganglion cell, but only one rod is joined to each ganglion cell.only one rod is joined to each ganglion cell.


What Colour are these Words?What Colour are these Words?

RedRed

BlueBlue

BlackBlack

GreenGreen

YellowYellow

OrangeOrange


Distribution of Cone TypesDistribution of Cone Types


More Facts (Off Syllabus)More Facts (Off Syllabus)

The cones are not evenly distributed being in the The cones are not evenly distributed being in the ratio of roughly 40:20:1 for RGB respectively. ratio of roughly 40:20:1 for RGB respectively. The green cones are most sensitive and the The green cones are most sensitive and the blue the least.blue the least.

Thus, it is easier to discriminate between colors Thus, it is easier to discriminate between colors in the red-yellow-green-cyan regions of the in the red-yellow-green-cyan regions of the spectrum than in the blue region. spectrum than in the blue region.

Thus blue should be avoided in text and other Thus blue should be avoided in text and other graphic elements where recognition is very graphic elements where recognition is very important.important.


Eyes and Animation (Off Syllabus)Eyes and Animation (Off Syllabus)

There is a .4 second delay in response There is a .4 second delay in response from the time the image falls on the retina. from the time the image falls on the retina. The sensation may persist for up to 2 The sensation may persist for up to 2 minutes.minutes.

This after image effect is exploited by film, This after image effect is exploited by film, TV and computer monitors which display a TV and computer monitors which display a rapid succession of still images, which are rapid succession of still images, which are refreshed before the perceptual image has refreshed before the perceptual image has decayed. decayed.


Eye Disorders (Off Syllabus)Eye Disorders (Off Syllabus)

About one in ten men and one in a About one in ten men and one in a hundred women experience some form of hundred women experience some form of divergent colour perception.divergent colour perception.

The most common is confusion between The most common is confusion between red and green. This can cause some red and green. This can cause some problems in colour matching.problems in colour matching.

The genes for colour blindness located on The genes for colour blindness located on the X chromosome.the X chromosome.


Colour Blindness TestColour Blindness Test


Cross Section of the Orbits of the Cross Section of the Orbits of the EyeEye


Web LinksWeb Links

http://www.stlukeseye.com/Anatomy.asp http://www.accessexcellence.com/AE/AEC

/CC/vision_background.html http://webvision.umh.es/webvision/index.ht

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