Function and Blood Supply of the Lens Flashcards
The lens transmits visible light, which is in the range of?
400-700 nm
Three types of light entering the earth and what absorbs each type
UVC- completely absorbed by ozone. None reaches earth.
UVB- Absorbed by the cornea
UVA- Most is absorbed by the lens, except 3% that still reaches the retina. It is the least harmful type.
Refractive properties of the eye depends on
- Refractive index (refractive index is higher in the nucleus and lower in the cortex bc there are more crystallines in the nucleus than the cortex)
- Change in refractive index between the lens and surrounding environment
- Length of optical path
- Surface curvature. During accommodation, the surface curvature of the lens changes.
How does the lens and ciliary body change shape to accommodate at near/ view distant objects.
- Ciliary body contracts (stands up), zonules relax, lens becomes more rounded for accommodation.
- Ciliary body relaxes (sits down, thin), zonules are pulled tight and lens becomes more elongated for distance vision.
With age, why is the ability for the lens to change its curvature impaired?
- Ciliary body moves progressively inward towards the lens.
- Lens thickens, which means that when the ciliary body contracts for accommodation, it won’t have much effect on the lens since the lens is already thick and rounded as is.
- Lens becomes more rigid due to the nucleus stiffening.
Axial length vs equatorial length of the lens
Axial length is from anterior to posterior. 4mm.
Diameter is across the equator (side to side). Adult is about 10mm, and infants is 6.5mm.
Does axial length increase or decrease with accommodation?
Increases. Becomes fatter.
Does the diameter increase or decrease with accommodation?
Decreases
Does the radius of curvature increase or decrease with accommodation?
Decreases, which means the lens steepens and the power increases.
What 6 things contribute to lens transparency?
- Lack of blood vessels.
- Orderly arrangement of lens fibers with uniform spacing
- Few cellular organelles in the visual axis, which prevents scattering of light.
- High concentration of crystallins, which keep other proteins orderly so the lens stays transparent.
- ATPase pumps remove excess water by pumping Na+ into the anterior chamber.
- Presence of antioxidants- Vitamin C (Ascorbic acid) and glutathione.
What are crystallins?
Water soluble proteins that comprise over 90% of the total cellular protein of the lens.
Types alpha, beta, and gamma. Beta and gamma are not well understood. But alpha crystallins function as molecule chaperones- they keep other proteins orderly, especially when they become unfolds, to prevent aggregation and keep the lens orderly.
What are the two antioxidants present in the lens?
Glutathione- tripeptide Ascorbic acid (Vitamin C)
Can neutralize free radicals that will affect DNA and result in protein denaturation and aggregation.
Chromophore
Yellow pigment of the lens that increases with age to cause opacification
With age, the lens looses transparency. Which two factors increase and which two factors decrease, resulting in cataracts?
Increases
- Chromophore concentration increases- yellow pigment that causes DNA damage and aggregation.
- Epithelial cell membrane permeability increases. Allows more solutes and water to enter epithelial cells, resulting in the swelling of lens fibers and disruptment of orderly arrangement.
Decreases
- Glutathione concentration decreases. Gutathione is an antioxidant. With lower concentrations, more free radicals are able to disrupt DNA and cause protein aggregation.
- Alpha crystallin concentrations decrease. By age 45, no Alpha crystallins are left. This results in protein aggregation.
The lens is avascular. Where does it receive its nutrients from?
From the aqueous humor in the posterior chamber. Simple diffusion of hydrophobic and small or uncharged polar molecules can enter the cell. Larger polar molecules and ions must be transported by facilitated diffusion.
