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

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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 lengthHigh frequency

Long wave lengthlow 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

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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

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View from farsighted eyes

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View from nearsighted eyes

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Close-up of the retina

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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

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View of the retinaRamon y Cajal, Nobel 1906

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Flow of visual information in the retina

Vertical Connections

Back of eye Front of eye back of eyePhotoreceptor Cell---Bipolar Cell---Retinal Ganglion Cell---Brain

Horozontal Connections

Horozontal Cells- connect photoreceptors and bipolar cellsAmacrine Cells- connect bipolar cells and retinal ganglion cells

light

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The optic nerve creates a hole in the retina

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Revealing your blind spot

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Photoreceptor cells are the light sensors

Back of eye

Front of eye

120 million 6 million

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Rods and cones absorb different wavelengths of light

One class of rods: blue/green sensitiveThree classes of cones: S=blue sensitive

M=green sensitiveL=red sensitive

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The fovea is the focal point of the retina•Packed with cones,

•no light scattering

•High acuity

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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 microscopy19

Distribution of Rods and Cones

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Fundamental differences between rods and cones

Rods Cones

High sensitivity to light, specialized for night vision

Low sensitivity, specialized for day vision

Achromatic Chromatic

Low acuity---not in the fovea

High acuity---in the fovea

slow response fast response

High Amplification Lower Amplification

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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

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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

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Different opsins recognize different wavelengths

We have 4 different opsins

Rods: Rhodopsin: blue/green sensitive pigmentCones: S opsin: blue sensitive

M opsin: green sensitiveL opsin: red sensitive

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Appearance to a trichromat

Appearance to a proteranope (no red)

Appearance to a deuteranope (no green)

Appearance to a tritanope (no blue)

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Test for color blindness

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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

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