the special senses vision - 3 professor a.m.a abdel gader md, phd, frcp (lond., edin), frsh (london)...
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The Special SensesVision - 3
Professor A.M.A Abdel GaderMD, PhD, FRCP (Lond., Edin), FRSH (London)
Professor of Physiology, College of Medicine &King Khalid University Hospital
Riyadh, Saudi Arabia
Color (Photopic) Vision‘Young - Helmholtz theory’‘The Trichromatic theory’
History of color visionNewton (1704) used a prism to show thatsunlight was composed of light with allcolors in the rainbow. He defined it as the
spectrum.
History of color vision
Primary colors:
723-647 575-492 492-450
Thomas Young 1807:primary colors: when mixed >>>
white or any other color
Mixing colors
Photopic vision (CONES)
Helmholtz ..1860:The three primary colors are
perceived by three photoreceptor pigments (with
broad absorption curves)
White light is produced by mixing three colours
Cone wavelength ranges
S M L
Wavelength (nm)
Rela
tive a
bso
rpti
on
400 500 600 700
Photopic vision (CONES)
Cone pigments: three kinds
565 535 440
Photopic visionYoung Helmholtz theory
Color vision is subserved bythree types of cones, each containing a
photoreceptor pigment most sensitive to one primary color
1. Cones (contain red-sensitive pigment)2. Cones (contain green-sensitive
pigment) 3. Cones (contain blue-sensitive pigment)
in the fovea centralis
Cone wavelength ranges
S M L
Wavelength (nm)
Rela
tive a
bso
rpti
on
400 500 600 700
Color Blindness
Weakness or total blindness in detecting a primary color:
Definitions:1. Trichromats: see the 3 1ry colors
2. Dichromats: blind to one 1ry color
3. Monochromats: have color pigment
Color Blindness –cont.
•Prot …… Red•Deuter …. Green•Trit …… Blue•Anamoly …weakness
•Protanamoly •Deuteranamoly Trichromats•Tritanamoly
Color Blindness –cont.
•Anamoly …weakness•Anopia …. Total loss
•Protanopia •Deuteranopia Dichromats•Tritanopia
Trichromatic/dichromatic color
vision
Color Blindness –cont.
•Prevalence:males ………….8%females …. 0.4%
Inheritance: sex-linked due abnormal gene in the X
chromosome
Electrophysiology of Vision
Light
Change in photopigment
Metarhodopsin II
Activation of transducin
Activation of phophodiesterase
Decrease IC cyclic GMP
Closure of Na channels
Hyperpolarization of receptorAction potential in optic nerve fibres
Photoreception
Figure 17.15
Bleaching and Regeneration of Visual Pigments
Electrophysiology of Vision
Electric recording in Retinal cells:•Bipolar cells: Hyper- & Depolarization
•Horizental cells: Hyper- & Depolarization
•Amacrine cells: Depolarizing potential
•Ganglion cells:Depolarizing potential
