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’

http://community.webshots.com/photo/1502654433035450872CBJCFz

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