Chapter 9

Chapter 9 - Chapter 9: Retinal Processing and Early Vision...

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Chapter 9: Retinal Processing and Early Vision - “Retina is derived from Latin “rete”, meaning “net”. - All vertebrates have a retina. - Retinal cells are actually neurons- outgrowths of the CNS occurring during embryonic develop’t. - Newton believed that light produced vibrations on the retina (an advance from the older view that the lens was the main instrument of vision). A. The photoreceptor array - Along the retinal margin just within the eye’s inside wall = the array of photoreceptors, which are embedded in the fibrous matrix that holds the eye together. - Photoreceptors = transducing elements of vision. - They lie directly in front of a layer of cells called the pigment epithelium : cells containing pigmented material that absorbs stray light missed by rods/cones or scattered; albinos lack this pigmented material. (Does that mean their vision is darker or somehow impaired?) 1. Rods and cones - Rods are by far more numerous: a retina usually contains ~120 million rods and ~6 million cones. - Rods serve as the front-end of a visual system that mediates night vision; cones = day vision. Distribution of photoreceptors across the retina - Retina parts: 2 halves- nasal retina and temporal retina (toward nose, toward temple). - Midpoint between the 2 halves contains a small pit called the fovea . - Point in nasal retina through which retinal nerve fibers exit eyeball = optic disk . - Osterberg’s study showed the following properties: o No photoreceptors whatsoever at the optic disk; thus, this small spot on the nasal retina is blind and the optic disk is also called the blind spot . o Cone density is low in peripheral retina but very high at the fovea (7 000/sq. mm at periphery, 150 000/sq. mm at fovea. o Opposite pattern w/ rods: at ~5mm from the fovea on each side (retinal eccentricity) is the max density of rods; none at the fovea. 2. Visual Transduction ( phototransduction ) - Photopigment in photoreceptors of all mammals = rhodopsin . - Rods and cones can be separated into an inner and an outer segment. o Outer is embedded in the fibrous matrix and contains large concentration of rhodopsin. o Rod outer segments contain stacks of discs; cone outer segments have infoldings: rhodopsin molecules are located within membrane of these structures. Dark current - In total dark, rhodopsin is in an inactive state. - But, in absence of any light, there is a steady flow of Na+ ions into the outer segment through cGMP-gated sodium channels = process called dark current . - Entry of positive Na+ is accompanied by outward flow of K+ ions in the inner segment, which maintains a net balance of ionic charges. - To prevent excess of both Na+ and K+, a sodium-potassium pump exchanges the ions oppositely. -
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Chapter 9 - Chapter 9: Retinal Processing and Early Vision...

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