Transcript The Structure and Function of the Eye

The Structure and
Function of the
Eye
A2 Biology. Unit 4.
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
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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 Test
Close 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 Test
Look at the star and try to read the letters
ABGTJINKOJ*UILSWQAMN
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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.
AGSHDEDHJS*DHSJEKSEJD
 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
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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
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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
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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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Retina: Cones
 Why does this happen?
 There are three types of a pigment called
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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 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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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_background.html
 http://webvision.umh.es/webvision/index.ht
ml
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