The Structure and Function of the Eye

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The Structure and Function of the Eye

• A2 Biology. Unit 4.

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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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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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The External Structure of the Eye
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Structure of the Eye
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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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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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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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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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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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Lens Focusing Light
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Lens Sight Defects
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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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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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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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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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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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Blind Spot TestClose Left Eye and Look at the Dot
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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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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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Fovea TestLook 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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Macula Fovea Test

• To show this draw a dot on a piece or paper. On
  each side of the dot write 10 capital letters.
• AGSHDEDHJSDHSJEKSEJD
• Stare at the dot and with out moving your eyes
  see how many letters your can read.

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Macula Concentration of Photoreceptors
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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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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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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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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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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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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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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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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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(No Transcript)
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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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What Colour are these Words?

• Red
• Blue
• Black
• Green
• Yellow
• Orange

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Distribution of Cone Types
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More Facts (Off Syllabus)

• The cones are not evenly distributed being in the
  ratio of roughly 40201 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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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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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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Colour Blindness Test
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Colour Blindness Test
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Colour Blindness Test
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Colour Blindness Test
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Colour Blindness Test
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Cross Section of the Orbits of the Eye
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Web Links

• http//www.stlukeseye.com/Anatomy.asp
• http//www.accessexcellence.com/AE/AEC/CC/vision_b
  ackground.html
• http//webvision.umh.es/webvision/index.html