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Physiology of vision

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ophthalmology

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Physiology of vision

  1. 1. Mechanism • Initiation of vision { phototransduction} • Processing and transmission of visual sensation • Visual perception
  2. 2. PHOTORECEPTORS Cones • Higher threshold receptors for photopic vision n colour vision Rods • Low threshold receptors for night vision (scotopic vision) Rhodopsin/ visual purple • visual pigment present in rods • Rhodopsin = opsin + retinine[11-cis retinal/vit A aldehyde]
  3. 3. Phototransduction  Phenomenon of conversion of light energy into nerve impulse  Light falls on retina cause photochemical changes trigger cascade of biochemical reaction gen of electrical impulses rhodopsin bleaching  Photochemical changes rhodopsin regeneration
  4. 4. In the photoreceptors, • Light falls on rods -> decomposition of rhodopsin( all trans retinal and opsin) • Opsin activates transducin-> sodium channels close ->Hyperpolarization -> decrease inhibitory neurotransmitter release from the rod • Bipolar cells excited • Ganglion cells • action potential initiated • transmit to the brain –> vision occurs
  5. 5. Rhodopsin regeneration • 11-cis retinal is regenerated from All trans retinal and vit A from blood • 11-cis retinal +opsin = rhodopsin • Regeneration process is independent of light
  6. 6. Mechanism • Initiation of vision{ phototransduction} • Processing and transmission of visual sensation • Visual perception
  7. 7. Processing and transmission of visual sensation • ganglion cells -> neurons of lateral geniculate body ->primary visual cortex • Visual image is analyzed by both serial and parallel processing
  8. 8. Serial processing • Successive cells in VP from photoreceptors to cells of LGB are involved in complex analysis of images Parallel processing • Starting from ganglion cells 2 pathways • Magnocellular pathway – large magno/M cells • Parvocellular pathway – small parvo/P cells • This 2 pathaways are for analyzing diff features of an image
  9. 9. Mechanism • Initiation of vision{ phototransduction} • Processing and transmission of visual sensation • Visual perception
  10. 10. Visual preception • It is a complex integration of Light sense ,Form sense, Sense of contrast and Colour sense
  11. 11. Light sense • Awareness of the light • Light minimum-minimum brightness required to evoke a sensation of light • Visual adaptation-The ability of the visual system to allow good visibility in different lighting situation • dark adaptation visual adaptation light adaptation
  12. 12. Dark adaptation • Ability of the eye to adapt itself to decreasing illumination • Rods are used more in dim light [scotopic vision] • Delayed dark adaptation [retinitis pigmentosa,vit A deficiency] Light adaptation • Process by means of which retina adapts itself to bright light • Very quick • strictly speaking light adaptation is merely the disappearance of dark adaptation
  13. 13. Form sense • Ability to discriminate between the shape of the object • Cones play a major role, therefore form sense is most acute at fovea • Visual acuity is a measure of form sense
  14. 14. Sense of contrast • Ability of the eye to perceive slight changes in the luminance between regions which are not separated by definite borders • Loss of contrast results in fogginess of vision
  15. 15. Colour sense • Ability to distinguish between different colors excited by lights of different wavelengths • It s the function of cones
  16. 16. Theories of colour vision Trichromatic theory [Young-Helmholtz] • Three kinds of cones • One is more sensitive to long wavelength[red], • One to medium wavelength[green] • One to short wavelength[blue] • All other colours are perceived by combination of this
  17. 17. The opponent colour theory of Herings • Three sets of receptor system – red-green, blue-yellow and black-white • Each system fn as an antagonist pair • Stimulation of one of an opponent pair produces excitation of that receptor system and inhibition on other • Eg Red light stimulates red receptors and simultaneously inhibits green
  18. 18. Thank you
  19. 19. Binocular single vision • The coordinated use of both eyes so as to produce a single mental impression • It is acquired during 6 months. By 6 years it is well developed

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