1. The Visual System Part 2

2. The Retina

3. Photoreceptors –Rods –Cones Bipolar cells Ganglion Cells –axons converge at optic disk –axons constitute optic nerve Horizontal cells Amacrine cells

4. Importance of abundant membrane

5. Visual Acuity 120,000,000 rods and 6,000,000 cones per retina 1,200,000 retinal ganglion cells & axons 105:1 convergence ratio Fovea: cones only, 1:1 highest visual acuity but poor sensitivity Counting fingers and Reading the Writing on the Wall

7. Demonstration of Blind Spot (diagram)

8. Differences in spectral sensitivity due to 4 types of opsin. Rods have 1 type; Each type of cone has another type. Trichromatic Theory of Color Vision Web-based color deficiency test “Blue” cones“Green” cones“Red” cones

9. Stargazing Finding your car in a dark parking lot Choosing socks

10. “Bleaching” of photopigments

11. The Dark Current

13. Phototransduction In the DARK, rod is depolarized due to influx of Na+ (called dark current) In the LIGHT, rod is hyperpolarized cGMP keeps Na+/Ca++ channels open Light results in decrease of cGMP and thus closure of ion channels, and Hyperpolarization of cell, and Reduced release of Neurotransmitter Glutamate

14. Hubel and Weisel Experiments Retinal Ganglion Cells Lateral Geniculate Neurons Visual Cortex Neurons

15. YouTube Videos

17. Properties of Cortical Neurons Simple –respond to stationary bar of light in certain orientation Complex –respond to moving bar of light in certain orientation Hypercomplex –respond to moving bar of light of a certain length in a certain orientation

18. Parallel Processing in the Cortex Motion sensitive Color and shape sensitive Story of woman at street crossing

19. Why so much emphasis on the Visual System?