ophthalmology

2. Mechanism
• Initiation of vision { phototransduction}
• Processing and transmission of visual sensation
• Visual perception

4. 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]

5. 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

9. 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

10. 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

11. Mechanism
• Initiation of vision{ phototransduction}
• Processing and transmission of visual sensation
• Visual perception

12. 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

13. 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

14. Mechanism
• Initiation of vision{ phototransduction}
• Processing and transmission of visual sensation
• Visual perception

15. Visual preception
• It is a complex integration of Light sense ,Form sense, Sense of
contrast and Colour sense

16. 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

17. 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

18. 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

19. 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

20. Colour sense
• Ability to distinguish between different colors excited by lights of
different wavelengths
• It s the function of cones

21. 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

22. 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

23. Thank you

24. 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