CORNEAL PHYSIOLOGY

-Riddhi Solanki

B.OptomAditya Jyot Institute Of

Optometry

Layers of cornea

Primarily concerned with:The sources of energy which fuel the cornea’s 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 junctions

Communication 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)

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

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

Carbon dioxide efflux

Open eye from cornea & aqueous humor into tear film Closed eye

into the aqueous humor

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%)

Glucose consumption

30-90mg /hr40-60% of total consumption is by the epithelium

Glucose metabolic pathwayEMBEDEN-MAYERHAOFF

PATHWAY produces lactate (anaerobic)+2ATP

TRICARBOXYLIC 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

Corneal transparency: stroma

Transmits 90% of incident lightPotentially a non-transparent layerFibrils: n=1.47Ground substance: n=1.37Regular fibril spacing :60 µm

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)

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 spacesSattler’s vielhaloes

Histochemical examination has shown presence of the endothelium of enzymes

needed for glycolysis & krebs cycle

The endothelial pump

Drug Permiability Across Cornea

Lipid And Water Solubility Of The DrugMolecular SizeWeight And Concentration Of The DrugsIonic Form Of The Drugs