THE VISUAL SYSTEM: EYE TO CORTEX

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1 THE VISUAL SYSTEM: EYE TO CORTEX o o o 1. The Eyes o Structure 2. The Retina o Cone & Rod Vision 3. Visual Transduction by Rhodopsin o 4. From Retina to Primary Visual Cortex o 5. Color Blindness o 6. Vitamin A = Retinol

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3 Color Vision Trichromatic Vision: Red, Green, Blue

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5 A human retina has approx. 6 million cones and 120 million rods. Light Light

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7 Color Vision: Cone Cells

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9 The fovea centralis (the term fovea comes from the Latin, meaning pit or pitfall) is a small, central pit composed of closely packed cones in the eye. It is located in the retina. The fovea is responsible for sharp central vision which is necessary in humans for activities where visual detail is of primary importance, such as reading and driving

10 optic chiasm

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13 A A photomicrograph from a monkey.

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19 Introduction Retinal - Aldehyde form of Vitamin A (Retinol)

20 Introduction: Sources of Vitamin A (Retinol) Humans cannot synthesize retinol from scratch Must ingest it directly or one of its precursors Animal sources of preformed vitamin A Liver, Milk, Butter, Cheese, Egg Yolk Pro-vitamin A Carotenoids - convertible into vitamin A in the body Beta-Carotene: Carrots, Spinich, Kale, Collard Greens, Sweet Potatoes, Pumpkin, Cantaloupe

21 Introduction: The Eye Retina Rod Cells Cone Cell Photoreception takes place in Rod and Cone cells of the retina

22 Rhodopsin - Opsin : Transmembrane G-Protein receptor - Retinal: Chromaphore responsible for visible light absorption - Retinal + Opsin = Rhodopsin

23 Rhodopsin Sensory rhodopsin II (rainbow colored) embedded in a lipid bilayer(heads red and tails blue) with Transducin below it. G t α is colored red, G t β blue, and G t γ yellow.

24 Introduction: Phototransduction 11-Cis Retinal A photon is absorbed by Rhodopsin which causes a conformational change in retinal

25 Introduction: Phototransduction All-Trans Retinal The conformational change allows a G-protein to replace GDP with GTP - initiates transduction pathway.

26 Chemistry: Retinal Binding Mechanism of Schiff Base (Imine) formation Lysine side chain ends in -NH2

27 Vitamin A, Beta-Carotene, and Cancer Surveys suggest that diets rich in vitamin A and betacarotene can lower the risk for cancer (especially lung cancer) However, one study was stopped because subjects with increased beta-carotene had a 46% higher risk of dying from lung cancer Another study showed that smokers were more likely to develop lung cancer if they took betacarotene supplements Beta-carotene supplements are not advisable except in rare situations

28 Polar Bear Liver One ounce of polar bear liver contains enough vitamin A (retinol) to kill a person!

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38 Color Blindness Test What do YOU see?

39 Both the normal and those with all sort of color vision deficiencies read it as 12.

40 The normal read this as 8. Those with redgreen deficiencies read this as 3. Those with total color blindness cannot read any numeral.

41 The majority of the normal and those with total color blindness cannot read any numeral. The majority of those with red-green deficiencies read this as 5.

42 The normal read this as 26. A Color Blind person may only see a 2 or 6 depending on the type of poor color perception

43 The normal color vision person will see both colors that make up the path back to the X A color blind person may only see the upper or lower half.

44 In tracing the winding line between the two X's, the normal trace the bluish-green line, but the majority of those with color vision deficiencies are unable to follow the line or follow a line different from the normal one.

45 In tracing the winding line between the two X's, the normal trace the line connecting the bluish-green and the yellowish-green, those with red-green deficiencies trace the line connecting the bluish-green and purple, and those with total color blindness cannot trace any line.

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