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 Short wave length High frequency Long wave length low 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 RodsCones High sensitivity to light, specialized for night vision Low sensitivity, specialized for day vision AchromaticChromatic Low acuity---not in the fovea High acuity---in the fovea slow responsefast response High AmplificationLower Amplification 11

The fovea is the focal point of the retina Packed with cones, no light scattering High acuity 12

Fovea: mostly cones, small inner segment Periphery: Cone inner segments are larger and appear as islands in a sea of smaller rods Where rods and cones are located in the retina Cross-sections of the retina 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 15 Nomenclature for visual receptors Receptor == GPCR, opsin Ligand == chromophore, retinal, pigment Receptor bound to ligand == rhodopsin

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

Appearance to a trichromat Appearance to a proteranope (no red) Appearance to a deuteranope (no green) Appearance to a tritanope (no blue) 22 Life with three versus two color receptors

Test for color blindness 23

Genes encoding red and green opsins are on X chromosome ON Chapter 29 OFF ONOFF 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) 24

Recombination between red and green opsins causes color-blindness Chapter 29 normal Errors in DNA replication 7-8% of males are colorblind 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 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