How the eye sees 1. Properties of light 2. The anatomy of the eye 3. Visual pigments 4. Color vision 1 Properties of light Light is made up of particles called photons Light travels as waves speed of light = wavelength X frequency short wave length = high frequencey Long wave length low frequency Short wave length High frequency 2 The light-gathering parts of the eye 43
The retina is a point-to-point map of the visual field But the visual field is inverted! 4 Errors in focusing 5 View from farsighted eyes 6
View from nearsighted eyes 7 The optic nerve creates a hole in the retina 8 Revealing your blind spot 9
Photoreceptor cells are the light sensors Back of eye Front of eye 120 million 6 million 10 Fundamental differences between rods and cones Achromatic Rods High sensitivity to light, specialized for night vision Chromatic Cones Low sensitivity, specialized for day vision Low acuity---not in the fovea slow response High acuity---in the fovea fast response High Amplification Lower Amplification 11 The fovea is the focal point of the retina Packed with cones, no light scattering High acuity 12
Where rods and cones are located in the retina Cross-sections of the retina Fovea: mostly cones, small inner segment Periphery: Cone inner segments are larger and appear as islands in a sea of smaller rods Electron microscopy 13 Distribution of rods and cones 14 Rods and cones have different visual receptors The visual receptors are G Protein-Coupled Receptors seven transmembrane regions hydrophobic/ hydrophilic domains conserved motifs chromophore stably attached to receptor (Schiff s base Lys296 in TM7) thermostable Nomenclature for visual receptors Receptor == GPCR, opsin Ligand == chromophore, retinal, pigment Receptor bound to ligand == rhodopsin 15
Interactions between the chromophore and the opsin alter photon absorbance 16 The light catcher is 11-cis-retinal covalently attached to opsin GPCR Vitamin A derivative Binds light, changes conformation from 11-cis to all-trans 17 Pigment cells recycle retinal 18
Trichromatic Theory of color vision: all colors are combinations of responses in three primary receptors (Red, Green, Blue or Long, Medium, Short) 19 Different opsins recognize different wavelengths We have 4 different opsins Rods: Rhodopsin: blue/green sensitive pigment Cones: S cones have S opsin: blue sensitive M cones have M opsin: green sensitive L cones have L opsin: red sensitive 20 Life with three versus two color receptors 21
Life with three versus two color receptors Appearance to a trichromat Appearance to a proteranope (no red) Appearance to a deuteranope (no green) Appearance to a tritanope (no blue) 22 Test for color blindness 23 Genes encoding red and green opsins are on X chromosome red and green opsins are next to each other on the X a cone cell expresses either the red or the green opsin the locus control region (LCR) is a promoter that stably turns on either the red or the green opsin males have 1X. females have 2X (X inactivation ensures 1 opsin per cell) ON OFF OFF ON Chapter 29 24
Recombination between red and green opsins causes color-blindness red and green opsins are next to each other on the X they share 96% sequence identity this makes them prone DNA copying errors males have 1X: if they inherit an opsin mutation, they are colorblind females have 2X: they need to inherit 2 mutant opsins to be colorblind normal Errors in DNA replication 7-8% of males are colorblind Chapter 29 25 Polymorphism in red pigment affects color discrimination 60% of males have S180, 40% have A180 About 50% of women are heterozygotes with one S180 and one A180 26 How to go from a dichromat to a trichromat (ie, from a non-primate mammals and New World monkeys to Old World monkeys and humans) 27
Color vision is different for different organisms 28