physiology of cornea

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  • CORNEAL PHYSIOLOGY -Riddhi Solanki B.OptomAditya Jyot Institute Of Optometry

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  • Layers of cornea

    8.bin

  • Primarily concerned with:The sources of energy which fuel the corneas metabolic activity.Corneal transparency & its maintenance.

  • Corneal PermeabilityWater Endothelium> EpitheliumOxygen Derived from atmosphereCarbon dioxide Permeability is 7times more than oxygen

  • Corneal permeability & other substancesSodium- endothelium > epithelium 100 timesGlucose & amino acids:metabolically activeAssociated molecules fluorescein

  • Low sodium permeabilityRelatively impermeable to water,lactic acid,amino acid, glucose,& large moleculesRelatively impermeable to associated and fat soluble entities

  • Role of cell junctionsCommunication Electrical couplingBarrier to - electrolytes

    - fluid - macromolecules

  • General classification of junctionsOccluding or tightAdheringEach is further divided into shape & size of cell contact - zonulae(belts) - fasciae (bands) - maculae(focal)

    9.bin

  • FibronectinCell surface glycoproteinInvolved in cell adhesion to surfacesReleased beneath generating epitheliumSynthesized by corneaFound in basal & apical surface of cultured endothelium cells.

  • Oxygen supply to corneaIt is the most important metaboliteDrives the supply from several surfaces

    - atmosphere via tear film - capillaries of the limbal region - aqueous humor via corneal endothelium - capillaries of palpebral conjunctiva

  • Carbon dioxide effluxOpen eye

    from cornea & aqueous humor into tear film Closed eye

    into the aqueous humor

  • 10.bin

  • Metabolism corneal energy by carbohydrate metabolismGlucose enters cornea from aqueous humorEnergy:ATP

    anaerobic:breakdown of glucose into lactic acid aerobic: breakdown of glucose by TCA cycle into carbon dioxide & water

  • Sources of glucose to corneal epitheliumAqueous humor (90%)Limbal blood vessels & tears(less than 10%)

    11.bin

  • Glucose consumption30-90mg /hr40-60% of total consumption is by the epithelium

  • Glucose metabolic pathwayEMBEDEN-MAYERHAOFF PATHWAY produces lactate (anaerobic)+2ATPTRICARBOXYLIC ACID CYCLE aerobic HEXOSE MONOPHOSPHATE SHUNT aerobic produces NADPH,CO2,H2O& 6ATP

  • Lactic acidNot metabolized by corneaRemoved by diffusion into aqueous humorAccumulation results in epithelial & stromal oedemaHypoxia doubles lactic acid concentration resulting in an osmotic gradient

  • CORNEAL TRANSPARENCYFactors Affecting Corneal Transparency:

    Corneal Epithelium and Tear Film Arrangement of Stromal LamellaeCorneal VascularisationCorneal Hydration

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  • Corneal transparency: stromaTransmits 90% of incident lightPotentially a non-transparent layerFibrils: n=1.47Ground substance: n=1.37Regular fibril spacing :60 m

  • Diffraction theory of MauriceDepends on ordered arrangement of collagen fibrilsTransparency is maintained if the disruption is less than few wavelengthScattering effect increases as swelling increases(fibrils become larger optically)

  • Corneal swellingLactate & metabolite accumulation- osmotic gradient causes water imbibitionHydrophilicity of GAGs causes a natural water imbibitionSwelling sleep is due to : hypoxia (50%) lower water osmolarity increased temperature & humidity

  • It effects:Change in refractive index of intra & extracellular spacesSattlers vielhaloes

  • Histochemical examination has shown presence of the endothelium of enzymes needed for glycolysis & krebs cycle

  • The endothelial pump

  • Drug Permiability Across CorneaLipid And Water Solubility Of The DrugMolecular SizeWeight And Concentration Of The DrugsIonic Form Of The Drugs

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