Transcript VISION -I

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‫أترى حين أفقأ عينيك‬
‫ثم أثبت جوهرتين مكانهما‪..‬‬
‫هل ترى‪..‬؟‬
‫هي أشياء ال تشترى‪..‬‬
‫أمل دنقل‬
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Photoreceptors (Rods and cones)
 Consist of:
1) Outer segment (facing choroid)

Detects light stimulus. It contains visual
pigment (iodopsin in cones & rhodopsin in
rods) that forms 40 % of this segment.
2) Inner segment
Contains metabolic machinery of cell
3) Nucleus
4) Synaptic terminal (facing bipolar cells)
 Transmits signals generated in photoreceptor on
light stimulation to dendrites of bipolar cells. 4

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Rods
Cones
120 millions / retina
6 millions / retina
At the periphery
At Center (Fovea centralis)
Rhodopsin pigment
Photopsin (3types).
More convergence
(200 rods : 1 bipolar)
One to one connection
More sensitive to light
(even to 1 photon).
Less sensitive to light.
Less accurate
(less visual acuity).
More accurate
(Clear vision)
More functioning at night
(Scotopic vision)
More functioning in day
(Photobic vision)
Vision in shades of grey
Color perception
(3 types for Red, Green & Blue)
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Photopigments
 Undergo chemical alterations when activated by
light
 Consists of 2 components:
1- Opsin
 Protein that is integral part of disc plasma
membrane
2- Retinal
 Derivative of vitamin A
 Light-Absorbing part of photopigment
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Photopigments
1- Rod pigment:

Rhodopsin that Absorbs all visible wavelengths
2- Cone pigments (color pigments):

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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PHOTORECEPTORS RESPONSE TO LIGHT
PHOTORECEPTOR POTENTIAL
Chemical response to light
phototransduction
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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 becomes more sensitive to light (a decline in
visual threshold).
Due to  regeneration of photosensitive pigments in
both rods and cones. Retinal and opsins are converted
back into photosensitive pigments (It is nearly maximal
in about 20 minutes).
Regeneration of rhodopsin in rods is completed in 20
minutes. Rods play the major role in dark adaptation.
LIGHT ADAPTATION
 It occurs when we shift suddenly from a dark to a
bright light place. Immediately, light seems very
intense and annoying with reflex closure of eye lids.
But within 5 min, normal sensation returns again.
 Then the eyes adapt to increased illumination & the
visual threshold rises. There is a decline in
sensitivity of retina to light.
 Due to breakdown of photosensitive pigments in
both rods and cones. The photosensitive chemicals
are reduced to retinal & opsins (bleaching).
COLOUR VISION
 It is the sense of discrimination of various
wavelengths of visible spectrum (400 - 700
nm).
 Each color has a specific wave length.
 Shortest wave length is that of violet color.
 Longest wave length is that of red color.
 Color vision is mainly the function of Cones
(highly developed in fovea centralis).
Trichromatic (=Young-Helmholtz) theory
 There are 3 types of cones. Each one is maximally
sensitive to one of the 3 primary colors.
1st cone contains pigment maximally sensitive to blue
violet (440 nm).
2nd cone contains pigment maximally sensitive to
green of spectrum (535 nm).
3rd cone contains pigment maximally sensitive to red
of spectrum (570 nm).
 Any colored object stimulates the 3 cones but in an
unequal manner. Number of impulses arising from
these cones to visual cortex differ in frequency
according to the color. This difference gives the
specific sensation by that color.
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 Equal stimulation of all 3 cones gives sensation of white.
 Color perception is a retinal function.
 Color translation is a cortical function (area 17).
COLOR BLINDNESS
Causes:
• Most commonly hereditary (recessive sex - linked) and
affecting males (8%) more than females (0.4 %).
• Very rarely acquired e.g. post encephalitis.
• Achromatopsia = color blindness due to lesion in V8 in
visual cortex.
Types: According to Young - Helmholtz theory, Color
blindness is classified to:
1-Trichromate: Patient has 3 cones but one of them is
weak.
 Protanomaly: weakness in red color perception.
 Deuteranomaly: weakness in green color perception.
 Triatanomaly: weakness in blue color perception.
2- Dichromate: One cone is totally absent & other two are
present.
 Protanopia: if cone of red color is absent.
 Deuteranopia: if cone of green color is absent.
 Tritanopia: if cone of blue color is absent.
3- Monchromate:
 Patient has only one cone system and matches the
different colors as various degrees of grey color (cone
monochromatism).
 Or Patient has no cone system at all  color & day
blindness (rod monochromatism).
Test of color vision:
 Ishihara's chart test: Colored printed figures and each
figure is printed by different colored small circles on a
background of colored circles.
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Red green color blindness is most common.
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.
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