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Ocular Phys Block 9 > Corneal Function > Flashcards

Corneal Function Flashcards

1
Q

Primary function of the cornea

A

Refract light

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2
Q

Factors affecting refraction in the cornea

A
  • curvature of anterior surface
  • change in RI from air to cornea (tear film)
  • corneal thickness
  • curvature of posterior surface
  • change in RI from cornea to aqueous
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3
Q

What is the 1st refractive surface

A

Actually the tear film

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4
Q

Total power of the eye

A

60D

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5
Q

Power of the cornea

A

40-48D

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6
Q

How much power of the eye does the cornea have

A

2/3-3/4

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7
Q

What kind of focusing ability does the cornea have

A

Fixed focus. Squinting can actually change the corneal focus though

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8
Q

What is the second primary function of the cornea

A

Transmit light

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9
Q

How does the cornea transmit light

A

Minimal scattering
Minimal distortion
Maximal transmission

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10
Q

Keys to maximize light transmission

A
  • smooth optical surface (tear film)
  • regularity of surface epithelial cells
  • absence of blood vessels
  • arrangement of correct spatial arrangement of collage fibrils in stroma
  • less than 1% light scatter (majority at epithelium and endothelium)
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11
Q

How much light scatter int he cornea

A

1%

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12
Q

How to protect your cornea for long term vision

A

Minimize risk of infection
Prevent damage to the retina
Avoid penetrating injury
Swift healing mechanism

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13
Q

What is one of the most sensitive tissues in the human body

A

Cornea

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14
Q

Innervation of cornea

A
  • 200-600x greater than skin
  • 20-40x greater than roots in teeth
  • ophthalmic division of trigeminal nerve (60-80 long ciliary nerves)
  • unmyleinaed nerve endings
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15
Q

What is the cornea sensitive to

A

Touch
Temp (cold)
Chmicals

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16
Q

What does touching thecornea cause

A

Involuntary lid closure

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17
Q

What is the CV of the cornea

A

Endothelial integrity can be evaluated by determining the coefficient of variation (CV) of cell size

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18
Q

What is a normal CV for cornea

A

0.25

25% variation in shape and size of endothelial cells

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19
Q

What is a normal hexagonality of endothelial cells

A

69%

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20
Q

What is a normal endothelial cell count

A

2000 cells/mm2

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21
Q

What happens to endothelial cells as some of them die

A

Others will enlarge and try to take up some of the space

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22
Q

Why do we need to know endothelial cell count before surgeries

A

To make sure we are not causing damage to endothelium. It does not regenerate

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23
Q

Stroma makes up ______ of the cornea

A

90%

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24
Q

This is important in maintains corneal transparency int he stroma

A

Regularity of size of collagen fibrils (300A) and the spacing (550A) between collagen fibrils

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25
Q

What maintains the precise arrrangment of collagen fibrils in the cornea

A

The negatively charged molecules located around each collagen fibrils by their bonds with water molecules

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26
Q

Blood vessels and corneal transparency

A

Lack of blood and lymph vessels

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27
Q

What else is lacking in the cornea that helps with transparency

A

Lack of myelin sheaths

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28
Q

Tough layer of collagen and laminin that protects the underlying layers of the cornea

A

Bowmans layer

29
Q

Layers of the cornea

A 
Epithelial
Bowmans 
Stroma 
Dua 
Descemet's 
Endothelium
30
Q

This is acellular, 8-14 microns thick, and resistant to deformation, trauma, and foreign bodies (will scar)

A

Bowmans layer

31
Q

Dua’s layer of the cornea

A

A new 6th layer

  • 15 microns thick
  • anterior to descemets membrane
32
Q

What is the most important function of Dua’s layer

A

Withstands high pressures

33
Q

How much of the cornea is water

34
Q

How does the epithelium maintain corneal hydration

A

Tight junctions

35
Q

How does the stroma maintain corneal hydration

A

Anionic proteoglycans pull water in

36
Q

How does the endothelium maintain corneal hydration

A

Slowly leaks fluids into cornea

37
Q

Mechanism by which cornea remains hydrated (78% water content)

A

Corneal deturgescnence

38
Q

What is corneal hydration dependent on

A
  • barrier function of the epithelium and endothelium
  • the anionic characteristics of molecules within the stromal matrix that account for the tendency of the stroma to imbibe water
  • water and ion transport through the epithelial and endothelial cell membranes
39
Q

What is the movement of water out of the cornea from the stroma through the endothelium and into the aqueous or through the epithelium into tears mediated by

A

Ion flow and osmotic gradients

40
Q

As ions are exchanged in the cornea and the concentration is altered

A

Water passage follows, moving down its concentration gradient. Cl- extrusion and Na+ absorption are the major driving forces for water transport across the epithelium and endothelium

41
Q

Would you be concerned if you saw corneal edema following cataract surgery 5 min post op

A

No

Concerned 1-7 days after surgery if it is still present

42
Q

Phthisis bulbi

A

No longer maintinaing integrity and low pressure. No longer useful. Could remove and replace with artificial eye

43
Q

Small integral membrane porteins residing in the plasma membrane, some are water selective, others also transport glycerol

A

Aquaporins

44
Q

What do aquaporins form in the cornea

A

Bidirectional osmotic water transport channels across the plasma membrane

45
Q

What do aquaporins function as in the cornea

A

Not only as channels but have some role in cellular processes, particularly in cell migration

46
Q

IOP and cornea clouding

A

High IOP on the order of 50mm Hg or higher can move excessive water into the corneal stroma from the AC and overwhelm the endothelial transport system

47
Q

Corneal abrasion and corneal haziness

A

Causes a loss of the zonular occludens barrier results in a localized area of edema and haziness

48
Q

How can epithelial edema affect VA

A

Decrease it when it separates cells causing surface irregularities; it is uncomfortable and can be painful

49
Q

Extensive epithelial abrasion

A

Allow fluid entrance into the stroma

Bandage CL to help protect

50
Q

What is unique about the cornea

51
Q

Where does most oxygen to the cornea come from

52
Q

Where do nutrients come from to the cornea

A

Diffusion through tear film and active transport through aqueous

53
Q

What does the cornea relie on for healing

A

Peptide growth factors

54
Q

What is the fastest healing tissue in the body

55
Q

Corneal abrasions heal in

A

24-48 hours

56
Q

Corneal epithelium can be completely replaced in

57
Q

Do you get a lot of O2 when you sleep to the cornea

58
Q

Recurrent corneal erosion

A

Will feel a burning sensation when they wake up

59
Q

What part of the cornea consumes most of the oxygen

A

Endothelium and epithelium

-25-30x of that of the stroma

60
Q

Glycogen storage in the cornea

A

Epithelium can store glycogen

61
Q

Inflammatory or infective condition that compromises the integrity of the epithelium

A

Corneal ulcer

62
Q

What part of the cornea is hydrophobic

A

Epithelium and endothelium (non-polar compounds)

63
Q

What part of the cornea has a high water content

64
Q

What is the epithelium impermeable to

65
Q

Removal of epithelium can facilitate what

A

Uptake of water soluble drugs

66
Q

What can generally penetrate the cornea more easily

A

Lipid soluble materials

67
Q

Some drugs getting to the AC

A

Require high Cxn to reach AC. Heard for Abs to get into AC. Have to do IV or oral in high Cxn

68
Q

Loss of stability and structure in the cornea

A

Keratoconus
Ectasia
High or low pressire

69
Q

Corneal transplants

A

2,3 or more layers removed. Best to maintain persons endothelium

Ocular Phys Block 9 (5 decks)

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