The Special Senses Vision - 3

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The Special SensesVision - 3

• Professor A.M.A Abdel Gader
• MD, PhD, FRCP (Lond., Edin), FRSH (London)
• Professor of Physiology, College of Medicine
• King Khalid University Hospital
• Riyadh, Saudi Arabia

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• Color (Photopic) Vision
• Young - Helmholtz theory
• The Trichromatic theory

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History of color vision

• Newton (1704) used a prism to show that
• sunlight was composed of light with all
• colors in the rainbow. He defined it as the
  spectrum.

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History of color vision

• Primary colors

723-647
575-492
492-450
Thomas Young 1807 primary colors when mixed gtgtgt
white or any other color
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Mixing colors
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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

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Cone wavelength ranges
S
M
L
Relative absorption
700
400
500
600
Wavelength (nm)
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Photopic vision (CONES)

• Cone pigments three kinds

565
535
440
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Photopic visionYoung Helmholtz theory

• Color vision is subserved by
• three types of cones, each containing a
  photoreceptor pigment most sensitive to one
  primary color
• Cones (contain red-sensitive pigment)
• Cones (contain green-sensitive pigment)
• Cones (contain blue-sensitive pigment)
• in the fovea centralis

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Cone wavelength ranges
S
M
L
Relative absorption
700
400
500
600
Wavelength (nm)
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Color Blindness

• Weakness or total blindness in detecting a
  primary color
• Definitions
• Trichromats see the 3 1ry colors
• Dichromats blind to one 1ry color
• Monochromats have color pigment

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Color Blindness cont.

• Prot Red
• Deuter . Green
• Trit Blue
• Anamoly weakness
• Protanamoly
• Deuteranamoly Trichromats
• Tritanamoly

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Color Blindness cont.

• Anamoly weakness
• Anopia . Total loss
• Protanopia
• Deuteranopia Dichromats
• Tritanopia

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Trichromatic/dichromatic color vision
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Color Blindness cont.

• Prevalence
• males .8
• females . 0.4
• Inheritance sex-linked
• due abnormal gene in the X chromosome

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Electrophysiology of Vision
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• Light
• Change in photopigment
• Metarhodopsin II
• Activation of transducin
• Activation of phophodiesterase
• Decrease IC cyclic GMP
• Closure of Na channels
• Hyperpolarization of receptor
• Action potential in optic nerve fibres

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Photoreception
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Bleaching and Regeneration of Visual Pigments
Figure 17.15
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Electrophysiology of Vision

• Electric recording in Retinal cells
• Bipolar cells Hyper- Depolarization
• Horizental cells Hyper- Depolarization
• Amacrine cells Depolarizing potential
• Ganglion cellsDepolarizing potential

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