The Visual System Part 2. The Retina Photoreceptors –Rods –Cones Bipolar cells Ganglion Cells –axons converge at optic disk –axons constitute optic nerve.

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Presentation transcript:

The Visual System Part 2

The Retina

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

Importance of abundant membrane

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

Demonstration of Blind Spot (diagram)

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

Stargazing Finding your car in a dark parking lot Choosing socks

“Bleaching” of photopigments

The Dark Current

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

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

YouTube Videos

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

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

Why so much emphasis on the Visual System?

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