How the eye sees. Properties of light. The light-gathering parts of the eye. 1. Properties of light. 2. The anatomy of the eye. 3.

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1 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

2 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

3 View from nearsighted eyes 7 The optic nerve creates a hole in the retina 8 Revealing your blind spot 9

4 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

5 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

6 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

7 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

8 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

9 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 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 A 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

10 Color vision is different for different organisms 28