Physiology of vision

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

PHOTORECEPTORSCones• 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]

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

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

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

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

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

Serial processing• Successive cells in VP from photoreceptors to cells of LGB are involved

in complex analysis of imagesParallel 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

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

Visual preception• It is a complex integration of Light sense ,Form sense, Sense of

contrast and Colour sense

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

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

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

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

Colour sense• Ability to distinguish between different colors excited by lights of

different wavelengths• It s the function of cones

Theories of colour visionTrichromatic 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

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

Thank you

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