How the eye sees 1.Properties of light 2.The anatomy of the eye 3.The cells that transmit light information from the retina to the brain 4.Visual pigments 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 Visible Spectrum We detect only a small portion of the electromagnetic spectrum light from 740 nm (red) to 370 nm (blue) 3

The Light Gathering Parts of the Eye 44

Focusing light on the retina 5

The retina is a point-to-point map of the visual field But the visual field is inverted! 6

Errors in focusing 7

View from farsighted eyes 8

View from nearsighted eyes 9

Close-up of the retina 10

Structure of the eye The Basic Retinal Circuit 1. Receptor Cells (rods and cones) 2. Bipolar Cells 3. Ganglion Cells Different cells in the retina Back of eye Front of eye 4. Horozontal Cells 5. Amacrine Cells 6. Pigment cells 11

View of the retina Ramon y Cajal, Nobel

Flow of visual information in the retina Vertical Connections Back of eye Front of eye back of eye Photoreceptor Cell---Bipolar Cell---Retinal Ganglion Cell---Brain Horozontal Connections Horozontal Cells- connect photoreceptors and bipolar cells Amacrine Cells- connect bipolar cells and retinal ganglion cells light 13

The optic nerve creates a hole in the retina 14

Revealing your blind spot 15

Photoreceptor cells are the light sensors Back of eye Front of eye 120 million 6 million 16

Rods and cones absorb different wavelengths of light One class of rods: blue/green sensitive Three classes of cones: S=blue sensitive M=green sensitive L=red sensitive 17

The fovea is the focal point of the retina Packed with cones, no light scattering High acuity 18

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 19

Distribution of Rods and Cones 20

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 21

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 22

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 23

Different opsins recognize different wavelengths We have 4 different opsins Rods: Rhodopsin: blue/green sensitive pigment Cones: S opsin: blue sensitive M opsin: green sensitive L opsin: red sensitive 24

Appearance to a trichromat Appearance to a proteranope (no red) Appearance to a deuteranope (no green) Appearance to a tritanope (no blue) 25

Test for color blindness 26

Genes encoding red and green opsins are on X chromosome 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 27