Flashcards in Exam 1 Remington- Corneal Physiology Deck (106)
What are the 4 functions of the cornea?
Transmits light, absorbs short wavelength UV, refracts light, and proper corneal function requires minimal scattering and distortion
Cornea scatters less than __% incident light.
Cytoplasm of epi and endo cells in the cornea contain what?
Corneal crystallins, water soluble proteins
What do corneal crystallins do?
They make cytoplasm homogenous and decrease light scattering
What range does the cornea transmit?
UV 310 nm to IR 2500 nm
What is the reason for photokeratitis?
The cornea absorbs short wavelength UV
Inflammation caused by absorption of UV
What are causes of photokeratitis?
Welder's arc, tanning beds, and snowblindness
What is the refraction of light of the cornea determined by?
Curvatures of the various surfaces, interfaces between different indices of light, and length of pathway through the structures
Proper corneal function requires what?
Minimal scattering and distortion, no blood vessels, proper hydration
Relative corneal deturgescenes means the corneal is ___% water
Corneal hydration is dependent on what?
Temperature, swells when cooled and returns to normal at body temperature
What is the role of tears in maintaing proper hydration?
As tears evaporate the tear fluid becomes more concentrated, tear film osmolarity increases relative to the cornea, the hypertonicity of tear film draws water from cornea and the blink response is elicited because hypertonicity causes a slight sting and thus tear film is restored
What is the role of the epithelium in maintaining proper hydration?
ZO barrier prevents intercellular water influx from tears, ions move across both surfaces, aquaporins are water transport channels
What do channels of the epithelium allow?
Channels allows Na+ to pass into epi from tears and Cl- to pass into tears
What are major driving forces for water transport?
Cl- extrusion and Na+ influx
The co-transporter mechanism moves what ions in the cornea?
Na, K, and Cl ions
Na+/K+ ATPase pumps located in ______ ______ of the cornea actively move ions
Aquarporins are present where?
Epithelial cell membranes
There is a ________ flow of water across the plasma membrane in the cornea.
What is the role of the stroma in maintaining proper hydration?
GAGs are present in ground substance surrounding collagen fibers, increasing hydration leads to decreasing transparency
What is the role of GAGs in the stroma of the cornea?
Due to anionic characteristics they bind with water- there is an inherent tendency to imbibe water and to swell
Water content of the stroma should be approximately ___%
The normal water content of the sclera is ___%, if it falls below ___% or above ___% the sclera becomes lucent
68, 40, 80
What is the role of endothelium in the cornea in maintaining proper hydration?
These cells have high metabolic activity, apparent by large number of mitochondria and Golgi apparatus, slow leak of fluids and solutes from the aqueous, the Na+/K+ ATPase pump is located in the basolateral membrane of the endothelium
How does the Na+/K+ ATPase pump located in the basolateral membrane of the endothelium act?
Pumps act at a steady rate and the rate of leakage is dependent upon the pressure gradient across the endo
What are the ions that cotransporters move across membranes?
Na, K, Cl, HCO3
Where are aquaporins located in the endothelium?
In the endo cell membrane
What are 4 measures of endothelial function?
Coefficient of variation CV, cell density, polymegathism, and pleomorphism
What is the coefficient of Variation CV?
Indicates the degree of uniformity among endo cells
What is the CV equation?
Sd of mean cell area/ mean cell area
What is normal CV?
As CV increases there is an _____ in amount of variation in size
What is cell density?
Number of cells per unit area
How many cells in young?
4000-3000 cells/ mm^2 in young
How many cells by age 80?
2000 cell/ mm^2
How many cells necessary for normal function?
Variation in cell size
Variation in cell shape
What is the most efficacious shape?
Endothelial stress resulting from CL wear, disease, age, surgery etc. can lead to what?
Endothelial remodeling, including change in size, shape, or both
Corneal ATP is generated by what?
Glucose enters cornea primarily from what?
Oxygen enters cornea primarily from what?
In closed lid, oxygen is mostly from what?
Utilizing anaerobic metabolism ~__ ATPs are produced per molecule of glucose
Under aerobic conditions ~___ ATPs per molecule of glucose
Cell proliferation includes what?
Nutrients, including glucose and amino acids
Some glucose goes through the ____ ____ shunt providing building blocks for nucleic acid synthesis
What are 2 manifestationss of corneal edema?
Change in dimension and change in appearance
What happens when there is a change in dimension in corneal edema?
Diameter remains same, thickness increases
How does the thickness increase in corneal edema?
Stromal matrix accumulates fluid, fiber diameter remains the same, swelling directed posteriorly, increase in corneal hydration positively linearly correlated with corneal thickness
What is the change in appearance in corneal edema?
Normally ~1% light scattering, with fluid retention light scattering increases, cornea becomes cloudy, buckling of Descemet's occurs and striae (folds) are seen
What causes epithelial damage?
Loss of ZO, localized area of swelling and haziness, causes abrasion, injury
What causes endothelial damage?
More serious, implies loss of active pump mechanism, caused by disease, surgery, injury, poorly fit contact lens may allow stagnation of tears
What are 3 clinical manifestations of endothelial damage?
Formation of guttata, endothelial cells thin, and apical junctions deteriorate
What is Fuch's dystrophy?
Problem with cellular function, results in loss of endo function
What are manifestations of Fuch's dystrophy?
Guttata form, stromal edema, and epithelial microcysts
What is metabolic edema?
Hypoxic condition causes an increase in anaerobic metabolism, concentration of lactate increases, osmotic balance changes and induces stromal edema
What is the normal range of IOP?
What is borderline IOP?
What is high IOP?
> 25 mmHg
What is very high IOP?
> 50 mmHg
What happens with very high IOP?
Endo transport system overwhelmed, must be treated quickly
What are 3 effects of corneal edema?
Hypoxia affects cellular function and causes decrease in intracellular pH, scarring can occur with dysfunction in collagen formation, neovascularization and new vessel growth
What is acidification?
What is neovascularization?
Body's defense mechanism to provide a source of O^2
What is treatment of corneal edema?
Eliminate cause, temporary clearing with hypertonic solution of glycerin instillation
Corneal injury induces activation of what molecules that initiate and direct corneal repair?
Integrins, cytokines, and growth factors
Facilitate and maintain cellular function
Facilitate cellular communication
Mediate proliferation and differentiation
What happens in the epithelium in corneal wound healing?
Mitosis stops, cells at wound edges lose hemidesmosome attachments, basal cells migrate, when defect is covered protein synthesis increases, mitosis resumes and cell proliferation replaces lost cells, after the wound is healed hemidesmosomes reformed, cell membrane and BM are linked via a biochemical bond prior to reformation of hemidesmosomes, epithelium generally heals in 24-48 hours with hemidesmosomes reformed (usually no scarring evident)
What happens when basal cells migrate in the epithelium?
Epidermal growth factor is released after injury and enhances cell migration and proliferation
If BM injured, normal adhesion may take ___ months to be complete
What happens to Bowman's layer in corneal wound healing?
Cannot be regenerated, is replaced by epithelial or stromal tissue
What happens to stroma in corneal wound healing?
Increase in number of keratocytes, collagen deposition in stroma, stromal healing not as rapid as epithelial healing
What happens to collagen deposition in stroma during corneal wound healing?
Fiber arrangement not as regular, diameter of regenerated fibers is greater than normal, scar may result
How long does stromal healing take?
May take up to 48 months for tensile strength to approach normal
What happens in Descemet's membrane during corneal wound healing?
Is very resistant to injury, will be secreted by the stromal fibroblasts and the endothelium
What happens to endothelium during corneal wound healing?
Migration to cover area, cells thin and remodel into hexagonal shape, pump and barrier functions are reestablished
What is the cause of neovascularization?
Disease, inflammation, or lack of oxygen
New vessels sprout from what in corneal neoascularization?
What happens to the new vessels in corneal neovascularization?
Enzymes degrade BM of capillary, endothelial cells migrate, endothelial cells proliferate to form new vessels
_____ _____ have been identified in the normal cornea
____ _____ are released in some diseases
What are ghost vessels?
Vessels do not resorb, but when the situation that produced them is reversed, they no longer carry blood
Will ghost vessels be visible with slit lamp?
Sensory nerves terminate in free nerve endings tightly surrounded by ____ ____
Nerve ending pattern changes as the retract, reinsert or shift position apparently with ___ ___ ___
Epithelial cell turnover
Sensory innervation is ____ times higher than any other epithelial tissue
Sensory nerves have a ______ effect
Neurotrophic (influence corneal metabolism)
In animal studies denervation be ablating ablating the ophthalmic division of the trigeminal leads to what?
Increased epithelial layer permeability, decreased cellular adhesion, impaired wound healing, reduced mitosis, and reduced cell migration
What is the primary sensation in corneal sensitivity?
Is corneal sensitivity high or low?
Contact lenses _____ corneal sensitivity
How is corneal sensitivity clinically measured?
Blink response, atheiometer
What is temperature corneal sensitivity like?
Registers changes but does not accurately recognize hot/cold
In central corneal damage, normal nerve pattern is present by what week?
What happens in peripheral corneal damage?
Reinnervation takes longer than 60 days and density of pattern is lower than normal
What type of fibers have been identified in the cornea?
The fibers in the cornea may affect what?
Cl ion channels
Substances such as ______ and ______ have been identified in the cornea
Acetylcholine and acetylcholinesterase
Why are acteylcholine and acetylcholinesterase in the cornea?
Transmitters for pain, contribute to tissue reaction that occurs in response to pain, mediate inflammatory response, play a role in transport processes and affect cellular permeability, regulate cell mitosis (perhaps because ACh can stimulate cGMP production and cGMP stimulates mitosis
The sclera is a _____ tissue