physiology of cornea
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CORNEAL PHYSIOLOGY
-Riddhi Solanki
B.OptomAditya Jyot Institute Of
Optometry
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Layers of cornea
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Primarily concerned with:The sources of energy which fuel the cornea’s metabolic activity.Corneal transparency & its maintenance.
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Corneal PermeabilityWater Endothelium> EpitheliumOxygen Derived from atmosphereCarbon dioxide Permeability is 7times more than oxygen
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Corneal permeability & other
substancesSodium- endothelium > epithelium 100 timesGlucose & amino acids:metabolically activeAssociated molecules fluorescein
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Low sodium permeabilityRelatively impermeable to water,lactic acid,amino acid, glucose,& large moleculesRelatively impermeable to associated and fat soluble entities
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Role of cell junctions
Communication Electrical couplingBarrier to - electrolytes
- fluid - macromolecules
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General classification of
junctionsOccluding or tightAdheringEach is further divided into shape & size of cell contact
- zonulae(belts) - fasciae (bands) - maculae(focal)
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FibronectinCell surface glycoproteinInvolved in cell adhesion to surfacesReleased beneath generating epitheliumSynthesized by corneaFound in basal & apical surface of cultured endothelium cells.
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Oxygen supply to cornea
It 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
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Carbon dioxide efflux
Open eye from cornea & aqueous humor into tear film Closed eye
into the aqueous humor
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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
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Sources of glucose to corneal epitheliumAqueous humor (90%)
Limbal blood vessels & tears(less than 10%)
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Glucose consumption
30-90mg /hr40-60% of total consumption is by the epithelium
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Glucose metabolic pathwayEMBEDEN-MAYERHAOFF
PATHWAY produces lactate (anaerobic)+2ATP
TRICARBOXYLIC ACID CYCLE aerobic
HEXOSE MONOPHOSPHATE SHUNT
aerobic produces NADPH,CO2,H2O& 6ATP
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Lactic acidNot metabolized by corneaRemoved by diffusion into aqueous humorAccumulation results in epithelial & stromal oedemaHypoxia doubles lactic acid concentration resulting in an osmotic gradient
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CORNEAL TRANSPARENCYFactors Affecting Corneal
Transparency:
Corneal Epithelium and Tear Film Arrangement of Stromal LamellaeCorneal VascularisationCorneal Hydration
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Corneal transparency: stroma
Transmits 90% of incident lightPotentially a non-transparent layerFibrils: n=1.47Ground substance: n=1.37Regular fibril spacing :60 µm
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Diffraction theory of Maurice
Depends 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)
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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
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It effects:Change in refractive index of intra & extracellular spacesSattler’s vielhaloes
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Histochemical examination has shown presence of the endothelium of enzymes
needed for glycolysis & krebs cycle
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The endothelial pump
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Drug Permiability Across Cornea
Lipid And Water Solubility Of The DrugMolecular SizeWeight And Concentration Of The DrugsIonic Form Of The Drugs
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