Download VISION-II

VISION -II
Dr.Mohammed Sharique Ahmed Quadri
Assistant Professor
Department Basic Medical Sciences
Faculty of Medicine
Almaarefa Colleges
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Astigmatism
• Condition in which curvature of cornea is not
uniform.
• Rays passing though meridian of greater
undergo greater refraction and focus in front
of rays passing through meridian having lesser
curvature.
• Hence the image at retina is not clear.
• Correction is by cylindrical lenses/ contact
lenses.
Layers of Retina
• Retina – receptor containing portion of retina is
actually an extension of the CNS
• Neural portion of retina consists of 3 layers of
excitable cells
– Outermost layer containing rods and cones (light
sensitive ends faces away from light)
– Middle layer of bipolar cells & associated interneurons
– Inner layer of ganglion cells
• Axons of ganglion cells join to form optic nerve
– Point on retina at which optic nerve leaves is the optic disc
» Region often called the blind spot because no image
can be detected here because of lack of rods and cones
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Test the Existance of Blind Spot
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Retinal Layers
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Eye
• Fovea
– Pinhead-sized depression in exact center of retina
– Bipolar and ganglion cells layer is pulled aside , so that
light can directly strikes photoreceptors
– Point of most distinct vision
– Has only cones
• Macula lutea
– Area immediately surrounding fovea
– Fairly high acuity ( high concentration of cones)
• Macular degeneration
– Leading cause of blindness in western hemisphere
– “doughnut” vision
– Loss of photoreceptors in macula lutea
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Photoreceptors
• Rod and cone cells
• Consist of 3 parts
– Outer segment (facing choroid)
• Detects light stimulus
– Inner segment
• Contains metabolic machinery of cell
– Synaptic terminal (facing bipolar cells)
• Transmits signal generated in photoreceptor on light
stimulation to next cells in visual pathway
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VISUAL RECEPTORS
Rods :- 120 million
maximum at periphery
Responsible for dim
light vision – (gray
vision) Scotopic vision.
Cones : 6 Million
maximum at Fovea
Centralis
- Responsible for
• Bright light vision
• Acuity of vision
• Color vision
3 types of cones
– Red /Blue/Green
Photoreceptors
Photopigments
• Undergo chemical alterations when activated
by light
• Consists of 2 components
– Opsin
• Protein that is integral part of disc plasma membrane
– Retinal
• Derivative of vitamin A
• Light-absorbing part of photopigment
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Photopigments
–Rod pigment
• Rhodopsin
–Absorbs all visible wavelengths
–Cone pigments (color pigment)
• Respond selectively to various
wavelengths of light
A single photon of light can activate a rod
whereas several hundred photons are
required to activate a cone
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RETINAL RECEPTORS RESPONSE TO LIGHT
Chemical response to light
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phototransduction
Properties of Rod Vision and Cone
Vision
DARK ADAPTATION :
If a person has been in brightly lighted surroundings for
a long time and then moves to a dark area the retina
slowly become more sensitive to light.
This decline in visual threshold is known as dark
adaptation.
The retinal and opsins are converted back into
photosensitive pigments. (It is nearly maximal in
about 20 minutes).
LIGHT ADAPTATION
• When one passes suddenly from a dim to a brightly
lighted environment, the light seems intensely & even
uncomfortably bright.
• Then the eyes adapt to increased illumination & the
visual threshold rises. This light adaptation occurs
over a period of about 5 minutes.
• The photosensitive chemicals in both the rodes &
cones are reduced to retinal & opsins (bleaching).
• The concentration of the photosensitive chemicals are
considerably reduced.
COLOUR VISION
• It is the function of Cones.
• Cones contain colour pigments made up of
Photopsin + 11 cis retinal.
Young - Helmholtz Trichromatic theory
of Colour Vision.
• 3 types of Cones are present.
• Each type contains a different pigment, which
respond maximally to a different colour
(wavelength)
3 primary colors – Blue, Green, Red.
• Blue Cones respond maximally to blue
colour with peak sensitivity to
wavelength 440 nm.
• Green Cones most sensitive to green
colour- 535 nm.
• Red Cones – red colour – 570 nm.
• All the other colors are sensed because
of combination of these 3 types of
cones.
Equal stimulation of all 3 cones causes the sensation of white color
COLOR BLINDNESS
• Classification of color blindness based on the
Trichromatic theory.
• Defect in red –
Prot
• Defect in green –
Deuter
Prefix
• Defect in blue –
Trit
Weakness for color
– Anomaly.
suffix
Absence of color perception – Anopia.
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Trichromats (Anomalous Trichromatic vision.)
These people have all 3 types of cones present but
one is weak.
Weakness for red – Protanomaly
Weakens for green – Deuteranomaly
Weakness for blue – Tritanomaly
Dichromats
Have only 2 cones system. One type of cone is
absent.
Absence of red color – Protanopia.
Absence of green color – Deutranopia
Absence of blue color – Tritanopia
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Monochromats
Have only 1 Cone system. Very rare.
Red green color blindness is most common.
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X linked recessive modes of inheritance.
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Females are carriers – males are sufferers.
Ishehara Chart
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Left eye blind
Bitemporal hemianopia
Heteronymous
Homonymous hemianopia
Homonymous hemianopia
with macular sparring
VISUAL CORTEX
Area 17 – Primary visual area
- Concerned with the appreciation of visual
sensations.
Area-18 – Secondary visual area/visual association area.
- Concerned with correlation and integration of
visual sensations.
Area-19 – Occipital eye field
- Concerned with movement of the eye.
In each visual cortex, both the eyes are represented in alternating
columns of cells called Ocular Dominance Columns.
Thus corresponding regions of the retina lie side by side close to
each other.