Transcript Eye - BiologyMad

cornea
iris
pupil
lens
aqueous humour
suspensory ligaments
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ciliary muscle
The Retina
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The Retina
• Contains photoreceptor cells (rods
and cones) and associated
interneurones and sensory neurones.
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Visual Transduction
• This is the process by which light initiates a nerve
impulse.
• The structure of a rod cell:
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• Detection
of
light is
carried
out on i
the
membrane
disks
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• These disks contain thousands of molecules of
rhodopsin (photoreceptor molecule)
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Visual Transduction
• Rhodopsin consists of:
– Opsin (membrane bound protein)
– Retinal (covalently-bound prosthetic
group) sensitive part
• Retinal is made from vitamin A
• Retinal is the light sensitive part – exists in 2 forms: cis and trans forms
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Visual Transduction
• In the dark retinal is in the cis form.
• When it absorbs a photon of light it
quickly switches to the trans form.
• This changes the shape of the opsin
protein – a process called bleaching
Light- fast (ms)
Rhodopsin
with cis retinal
Rhodopsin
with trans retinal
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Visual Transduction
• The reverse reaction (trans to cis)
requires an enzyme reaction and is
very slow (taking a few minutes)
• This process requires ATP, as rhodopsin
has to be resynthesised
Light- fast (ms)
Rhodopsin
with cis retinal
Rhodopsin
with trans retinal
Dark - slow (mins)
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Visual Transduction
Bleaching of the rhodopsin in a rod cell
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Alters the permeability of the membrane to Na+
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nerve impulse
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sensory neurone in the optic nerve
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to the brain
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Visual Transduction
• Rhodopsin controls sodium channels
• Rhodopsin with cis retinal opens sodium
channels (absence of light)
• Rhodopsin with trans retinal closes
sodium channels (light)
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In the Dark…
• In the dark the channel is open  Na+ flow
in can cause rod cells to depolarise.
– Therefore in total darkness, the membrane of a
rod cell is polarised
• Therefore rod cells release
neurotransmitter in the dark
• However the synapse with bipolar cells is an
inhibitory synapse i.e. the neurotransmitter
stops impulse
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In the Light…
As cis retinal is converted to trans retinal, the
Na+ channels begin to close
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less neurotransmitter is produced. If the
threshold is reached, the bipolar cell will be
depolarised
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forms an impulse which is then passed to the
ganglion cells and then to the brain
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Rods and Cones
Rods
Outer segment is rod shaped
Cones
Outer segment is cons
shaped
109 cells per eye, distributed 106 cells per eye, found
throughout the retina, so used mainly in the fovea, so can
for peripheral vision.
only detect images in centre
of retina.
Good sensitivity
Only 1 type 
monochromatic vision
Poor sensitivity
3 types (R, G & B)  colour
vision
Many rods connected to one
bipolar cell  poor acuity =
poor resolution
Each cone is connected to
one bipolar cell  good acuity
= good resolution
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Colour Vision
• 3 different cone cells. Each have a
different form of opsin (they have the
same retinal)
• 3 forms of rhodopsin are sensitive to
different parts of the spectrum
– 10% red cones
– 45% blue cones
– 45% blue cones
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Colour Vision
• Coloured light will stimulate these 3 cells
differently - by comparing the nerve impulses from
the 3 kinds of cones the brain can detect any
colour
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Red light  stimulates R cones
Yellow light  stimulates R and G cones equally
Cyan light  stimulates B and G cones equally
White light  stimulates all 3 cones equally
• Called the trichromatic theory of colour vision
Colour Vision
• When we look at something the image falls
on the fovea and we see it in colour and
sharp detail.
• Objects in the periphery of our field of view
are not seen in colour, or detail.
• The fovea has high density of cones.
• Each cone has a synapse with one bipolar
cell and one ganglion  each cone sends
impulses to the brain about its own small area
of the retina  high visual acuity
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Accommodation
• Refers to the ability of the eye to alter its
focus so that clear images of both close
and distant objects can be formed on
the retina
– The lens shape can be altered by
suspensory ligaments and the ciliary
muscles. This adjusts the focus
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Accommodation
• Distant objects:
– Light rays are almost parallel so do not
need much refraction to focus onto the
retina.
– The lens therefore needs to be thin and
“weak” (i.e. have a long focal length).
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Accommodation
• Close objects:
– Light rays are likely to be diverging, so
need more refraction to focus them onto
the retina.
– The lens therefore needs to be thick and
“strong” (i.e. have a short focal length).
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The Iris
• Regulates the amount of light entering
the eye so that there is enough light to
stimulate the cones, but not enough to
damage them
• Composed of 2 sets of muscles:
– Circular and radial  have opposite affects
(antagonistic)
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The Iris
• By contracting and relaxing these muscles
the pupil can be constricted and dialeted
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The Iris
• Is under control of the autonomic
nervous system
– Sympathetic Nerve  pupil dilation
– Parasympathtic Nerve  pupil constriction
– The drug atropine inhibits the
parasympathetic nerve, causing pupil to
dilate
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The Iris
• The iris is a good example of a reflex arc:
stimulus More Light
receptor Rods and Cones
Sensory neurone
coordinator Brain
Motor neurone
effector
Iris muscles
response Pupil constricts