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Physiology of Vision (2)
Dr. Abdelrahman Mustafa Department Basic Medical Sciences
Division of Physiology Faculty of Medicine Almaarefa Colleges
بسم الله الرحمن الرحيم
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Learning ObjectivesBy the end of this lecture, You should able to •Contrast the transduction process for rods and the three types of cones,•Describe the visual pathway •Predict the visual field defects resulting from the following•Describe the topographic representation of the visual field.•Describe the processing of information in the visual cortex, and the consequence of a lesion in the higher visual association areas.
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THE RODS AND CONES
• GRNETAL FEATURES• These are photoreceptors of the
retina • Telereceptor• Slowly to never adapt • Contain specific photosensitive
pigments which are chemically changed on exposure to light
• Such changes initiate action potential in the optic nerve fibers
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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 part2- Retinal:Derivative of vitamin ALight-Absorbing part of photopigment
– Rod pigment: Rhodopsin that Absorbs all visible wavelengths
– 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
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Visual Pigments: RODS• Retinal + Opsin = Rhodopsin• Functions only in dark, dim light & peripheral vision• Regenerate only in dark or dim light situations
RHO DOPSIN OPSIN RETINAL
(DIM Light)
Impulse
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Visual Pigments: Cones• Retinal + Red, Green or Blue Opsin = Red, Green
or Blue visual pigments• Function only in bright light (daylight)• Provide sharp color images
Red Cone Red Opsin RETINAL
(Light)
Impulse
Green Cone Green Opsin RETINAL Impulse
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DARK
rod cell Na+ channels open
membrane depolarizes
Na+ inflow stimulates glutamate release
glutamate inhibits bipolar cells
LIGHT
rod cell Na+ channels close
membrane hyperpolarizes
no Na+ inflow prevents glutamate release
bipolar cells initiate action potential
visual pathway started
NO signal in optic nerve
DARK VERSUS LIGHT ACTION POTINAL CONDITIONS(TRASDUCTION)
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Dark versus light conditions
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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
:1-Trichromate: Patient has 3 cones but
one of them is weak. 2Dichromate: One cone is totally
absent & other two are present.3- Monchromate:• Patient has only one cone system
and matches the different colors
tested By Ishehara Chart
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Visual Fields
• Each eye has two areasFor vision NASAL FIELDS For front of vision Pathway through Temporal
nerve CONTRA LATERALY
TEMPORAL FIELDS For surrounding vision Pathway through NASAL
nerves IPSILATERALY
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visual pathway• 1)Optic disc• Collection of nerve fibers that transmit AP• 2)Optic Nerve • Ipislateral nasal nerve (same eye)• Ipsilateral temporal nerve (same eye)• 3)Optic chiasma• Nasal nerves crosses to go to the opposite side of the
brain DECASSATION • 4)Optic Tract • Ipislateral temporal nerve (same eye)• Contaletral nasal nerve (other eye)
• 5) Lateral geniculonate Body • The LGN contains a topographic representation of
what the retina “sees”.• This retinotopic map is sent to the visual cortex.
• 6)Optic Radiation • From LGN to visual cortex • Ipislateral temporal nerve (same eye)• Contaletral nasal nerve (other eye)
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visual pathway• 7)Primary visual cortexArea 17 (=Primary visual area)
- Concerned with the
appreciation of visual sensations.
Area18(=Secondaryvisualarea/
visual association area)
- Concerned with correlation and
integration of visual sensations.
Area 19 (=Occipital eye field)
- Concerned with movement of
the eye.
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point sourceof light
Fovea
RetinaVitreous
IrisLens
Cornea
Optic nerve
Right eye
nasal temporal
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Ganglioncells
Conephotoreceptors
Conebipolar cells
Optic axons
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Optic nerve
Opticchiasm
Opticradiation
Visualcortex
Optic tract
Lateralgeniculatenucleus
Superiorcolliculus
Hypo-thalamus
Pretectum
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Whole brain
ventral surface Optic nerve
Optic tract
Optic chiasm
LateralGeniculateNucleus
Optic
radiation
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Lesions of Visual Pathway
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Lesions of Visual Pathway
V1 V1
LGN LGN
Optic Chiasmbitemporal hemianopaia
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Q1
• Which of the following NOT feature of cons receptor
• A) about6 millions / retina• B)Found at At Center (Fovea centralis)• C) has Photopsin (3types).• D)More sensitive to light.
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Q2
• Transduction of dark vision inhibited by• A)glutamate releasing • B)Na influx• C)Bipolar cell block • D) Hyperoplrization
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Q3
• Which of the following DOSE NOT considered as visual pathway Station
• A)Optic disc• B)Optic tract • C)Medial geniculate Body of the Thalamus• D)Visual cortex
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Q4
• Decassation of nasal nerve occur in:• A)Optic disc• B)Optic chisma• C)Thalamus• D)Visual cortex
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References
• Human physiology by Lauralee Sherwood, 7th edition
• Text book physiology by Guyton &Hall,12th edition
• Text book of physiology by Linda .s contanzo,third edition
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