13 Visual

13 Visual - VISION page 1 AC Brown A7b INTRODUCTION A. Role...

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VISION AC Brown page 1 A7b INTRODUCTION A. Role of the visual system: sense photons emitted by or reflected from objects in the environment and to interpret the resulting afferent input. This is accomplished by four processes: 1. Formation of a small image of the environment on retina (which contains the photoreceptor afferent neurons) by the optical apparatus of the eye 2. Transduction of the image into a pattern of action potential discharge by retinal neurons (retinal transduction) Note: Transduction is possible only for light within a limited wavelength range, approximately 400-700 nm (nanometers); the exact range depends on whether the eye is light adapted (photopic vision) or dark adapted (scotopic vision) 3. Conduction of the resulting action potentials to regions of the central nervous system (CNS) responsible for conscious sensation of visual input (visual pathways) 4. Extraction of the properties of the visual image: shape, color, brightness, movement, etc. (visual cortex)
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VISION AC Brown page 2 A7b OPTICS A. Image Formation in the Eye Note: image formation depends on focusing the light rays by bending (refraction) as the light passes through spherical surfaces. 1. Refracting surfaces a. cornea b. front lens surface b. rear lens surface Note: The most refraction occurs at the cornea. However, the lens surfaces are important because the lens can change its power, thereby permitting focusing on near objects. 2. Eye aqueous and vitreous humors a. secreted by the ciliary bodies; drained by the canal of Schlemm b. normally at a positive pressure of 15±3 mmHg; positive pressure helps to maintain shape of eye c. excess pressure (due, for example, to blockage of fluid drainage) is termed glaucoma , and can interfere with retinal circulation and axoplasmic transport, leading to retinal degeneration and blindness 3. Result: small, inverted image of the external world is formed on the retina Note: light incident on the retina that is not absorbed by the retinal photoreceptors is absorbed by melanin in the pigment epithelium in back of the retina, preventing photons that pass through the retina from being reflected back into the eye, which would otherwise blur the retinal image Note: The lens is normally transparent but if protein precipitation occurs, light passing through the lens Is scattered ( cataract ), resulting in blurring; major causes: aging, UV exposure, diabetes (diabetes also can cause diabetic retinopathy due to small artery disease)
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VISION AC Brown page 3 A7b OPTICS (continued) B. Accommodation 1. Definition: variations in eye refracting power in order to focus objects at different distances onto the retina 2. Mechanism When the ciliary muscles of the lens are relaxed, the tension developed by the zonule fibers of the suspensor ligament flatten the lens, reducing its power to a minimum. When the ciliary muscles contract, they take up some of the suspensory ligament force, permitting the lens to bulge passively, thereby increasing its bending power. 3.
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This note was uploaded on 01/03/2012 for the course BIO 308 taught by Professor Acbrown during the Spring '10 term at Portland.

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13 Visual - VISION page 1 AC Brown A7b INTRODUCTION A. Role...

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