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Physiology of the Eye > Cornea > Flashcards

Cornea Flashcards

1
Q

What factors influence the health of the cornea?

A

preocular tear film
aqueous humor
IOP
eyelids

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2
Q

What three structures furnish the metabolic needs of the avascular cornea?

A

aqueous humor
limbal capillaries
tears

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3
Q

Where do the corneal epithelium and endothelium receive their oxygen supplies from?

A

epithelium - tears

endothelium and deep stroma - aqueous humor

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4
Q

Where does the corneal epithelium receive its glucose stores from?

A

limbal capillaries and tears (receives most of its glucose form the stroma)

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5
Q

Characterize glucose metabolism in the corneal epithelium under normal conditions.

A

The epithelium converts glucose to glucose-6-phosphate (first step in aerobic glycolysis), after which 85% is further metabolized into pyruvate. Most of the pyruvate is then converted into lactate, however some is sent through the citric acid cycle (Kreb’s cycle) to generate ATP

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6
Q

Characterize glucose storage in the corneal epithelium.

A

Stored as glycogen to be used during stressful times. If depleted, healing (cellular migration) is inhibited.

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7
Q

Whey does the corneal swell slightly when the eyelids are closed during sleep?

A

The corneal epithelium must revert to anaerobic glycolysis when the lids are closed due to reduced oxygen delivery. This leads to increases in lactic acid production, and build up of lactic acid in the corneal leads to corneal hydration, thus swelling.

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8
Q

How does the pentose phosphate shunt affect/contribute to corneal epithelial metabolism.

A

Produces nicotinamide-adenine dinucleotide phosphate (NADPH), ribose-5-phosphate and reduced triphosphate-pyridine.

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9
Q

What is the importance of NADPH in corneal epithelial metabolism?

A

resource for free radical control.

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10
Q

What is the importance of Ribose-5-phosphate in corneal epithelial metabolism?

A

used in nucleic acid synthesis of DNA/RNA.

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11
Q

What is the importance of reduced triphosphate-pyridine in corneal epithelial metabolism?

A

used by the corneal epithelium for lipid synthesis (cell membranes).

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12
Q

What are the glucose needs of the corneal endothelium vs the epithelium?

A

approximately 5 times greater.

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13
Q

What is the main metabolic pathway for the corneal endothelium?

A

anaerobic glycolysis. (citric acid and pentose phosphate pathways also involved)

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14
Q

Complete loss of the corneal epithelium results in a what % increase in corneal thickness?

A

200

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15
Q

Complete loss of the corneal endothelium results in a what % increase in corneal thickness?

A

500

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16
Q

What happens to corneal thickness in the dog after eyelid opening at two weeks of age, and what is the proposed reason for this?

A

Corneal thickness decreased until approximately 6 weeks of age, which is though to correspond to the time it takes for complete maturation of corneal endothelial cells.

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17
Q

In the dog, after 6 weeks of age what happens to corneal thickness?

A

It increases until approximately 30 weeks of age.

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18
Q

Cation concentration of the cornea is controlled by what enzyme, and what happens if this enzyme is blocked?

A

Na+, K+ ATPase. If blocked the cornea becomes over hydrated (corneal edema after death??)

19
Q

What are the proposed mechanisms of corneal edema seen with increased IOP?

A
  • Influx of aqueous humor (fluid being pushed into the cornea)
  • compromised endothelial function
  • disruption of collagen arrangement via physical stretching of lamellae
  • combination of the above
20
Q

Glycine, proline and hydroxyproline are found in high concentrations in what corneal structure?

A

corneal collagen

21
Q

What is the rate of oxygen consumption in the human cornea?

A

3.5-4 uL/cm per hour

22
Q

Characterize the function of GAG’s in the corneal stroma.

A

They act as anions which bind cations and water. The GAG’s are found in the inter fibrillar and interstitial space of the stroma, attach to collagen fibrils and proteins, and appear to maintain the regular arrangement of fibrils and lamellae in the stroma by controlling distribution of water. (the concentration of GAG’s and arrangement of collagen lemallae are different among the cornea and sclera, which accounts for one being transparent and the other being opaque)

23
Q

What types of collagen have been reported in the human cornea?

A 
types 1 and 6 (Bowman's layer)
type 1 (50-55%)
type 3 (1-2%)
type 4 (stroma)
type 5
type 8
type 9
type 12 (Descemet's membrane)
24
Q

What is the predominant GAG of the corneal stroma?

A

keratan sulfate (nonsulfated chondroitin sulfate, chondroitin-4-sulfate and dermatin sulfate are also present)

25
Q

What are the differences in water absorption and distribution of keratan sulfate and chondroitin sulfate in the cornea?

A
  • keratan sulfate absorbs 2-3 times as much water as chondroitin sulfate, however chondroitin sulfate holds onto the water it absorbs 8-9 times better.
  • keratan sulfate is located in the posterior stroma closer to the endothelium, and where it facilitates water movement from aqueous to cornea.
26
Q

What is the difference in canine corneal sensitivity relative to skull type?

A

dolichocephalic - most sensitive
mesaticephalic - intermediate sensitivity
brachycephalic - least sensitive

27
Q

Where is corneal sensitivity greatest, centrally or peripherally?

A

peripherally

28
Q

What happens to myelin around corneal nerves as the enter the cornea from the limbus?

A

the myelin is lost progressively as the nerves traverse to the central portion.

29
Q

Briefly describe the corneal limbal plexus?

A

0.8-1mm wide, ringlike band surrounding the peripheral cornea.

30
Q

What are the two portions of the corneal limbal plexus?

A
  • periscleral zone: out of the two zones, and is predominantly perivascular
  • pericornealzone: inner of the two zones, denser and more highly branched.
31
Q

Characterize the penetration of corneal nerves from the limbal plexus into the corneal stroma.

A

14-18 regularly spaced, radially directed stromal bundles, each bundle containing 30-40 axons. Axons undergo repetitive dichotomous branching to innervate the entire cornea, which penetrate the basal corneal epithelium and arborize into structures termed “epithelial leashes”.

32
Q

What are the physiologic results of “reflex uveitis” from corneal nerve stimulation?

A

miosos, hyperemia, ocular hypertension, increased protein levels in the aqueous humor.

33
Q

What appear to be the humoral mediators of “reflex uveitis”?

A

prostaglandins, histamine and acetylcholine.

34
Q

What is the proposed role of Substance P in corneal nerve physiology?

A

It does not appear to be involved in nociception, however may play a role as a neurogenic mediator of the inflammatory response.

35
Q

What type of receptors found in corneal epithelial cells appear to be activators of cyclic nucleotides, which turn on enzyme systems in epithelial cells?

A

Beta-Adrenergic recpetors

36
Q

What are corneal cyrstallines?

A

transparent lens-like proteins found in the corneal stroma.

37
Q

Which cells synthesize corneal crystallines?

A

corneal fibroblasts

38
Q

What happens to corneal crystallines after corneal injury?

A

Synthesis decreased significantly, which coincides with corneal opacity or “haze”. Remodeling of the wound and increased transparency coincides with increased synthesis of corneal cyrstallines.

39
Q

Regarding the penetration of a topically applied drug to the ocular surface (conjunctiva and cornea), what is the most important characteristic?

A

solubility

40
Q

What must a drug possess in order to effectively penetrate the cornea, and why?

A

the drug must possess both lipophilic (nonpolar) and hydrophilic (polar) properties. This is because the cornea is a lipid - water - lipid structure.

41
Q

What is the path of least resistance for a drug applied to the ocular surface?

A

cornea

42
Q

What may happen to drugs that penetrate the globe through the sclera or conjunctiva?

A

Since these structure are not the path of least resistance, the drug likely looses integrity if it passes through these structures.

43
Q

The conjunctival and scleral route of drug application can also be an effective route for drugs to enter what internal ocular structures?

A

iris and ciliary body

Physiology of the Eye (10 decks)

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