Lens, Uvea, Vitreous

lens provides ____ of total dioptric power of eye

1/3

what type of stimulation starts the process of accommodation

parasympathetic

what does parasympathetic stimulation do in the lens

causes contraction of the ciliary muscle

when the ciliary muscle contracts, what happens to the lens zonules

decrease in tension

during accommodation lens anterior pole moves (forward/backward), and anterior curvature (increases/decreases)

forward, increases

during accommodation lens posterior pole moves (forward/backward), and posterior curvature (increases/decreases)

backward, increases

during accommodation, lens thickness (increases/decreases) and anterior chamber depth (increases/decreases)

increases, decreases

during accommodation, lens diameter (increases/decreases) and lens power (increases/decreases)

decreases, increases

what does accommodation do to IOP and why

• initially can cause decrease in IOP when CM contraction pulls on the scleral spur and opens up the TM

• can then cause an increase in IOP due to a decrease in AC depth when the anterior pole moves forward

what can happen to narrow angle patients during accommodation and why

pupillary block

AC depth decreases and lens thickens

lens can push forward on iris and block the TM outflow at the angle due to narrow chamber depth

what kind of drugs can cause pupillary block in narrow angle patients

give a drug example

miotics like pilocarpine

accommodation in triad of miosis, convergence, and accommodation can lead to pupillary block

lens is (vascular/avascular)

avascular

lens has the largest concentration of ______ of any structure in the body

proteins

where does lens get glucose and oxygen

aqueous

lens is so packed with proteins that it needs to get oxygen and glucose elsewhere

what 2 lens functions does the lens need glucose and oxygen for

• producing new lens fibers and proteins

• maintaining the Na+/K+ ATPase

role of the Na+/K+ ATPase in the lens

maintaining lens dehydration and transparency by balancing water and ions in the lens

how does the Na+/K+ ATPase work in the lens

• epithelial cell pump moves Na+ into the aqueous humor and K+ into the lens

• water follows Na+ into the AH

water follows _____ out of the lens to maintain lens transparency

Na+

what ion enters the lens via the Na+/K+ ATPase

K+

over 70% of glucose used by the lens is produced through _____________

anaerobic glycolysis

what part of the lens is supplied by glucose via aerobic metabolism

lens epithelium only

what is the first step of aerobic and anaerobic respiration

conversion of glucose to glucose 6-phosphate via hexokinase enzyme

what happens if hexokinase is not available to convert glucose to glucose 6-phosphate

glucose is converted to sorbitol via aldose reductase

what happens if excess sodium accumulates in the lens

osmostic gradient is created that favors movement of water into the lens

causes lens swelling, lens fiber damage, and cataracts

__________ converts glucose to glucose 6-phosphate

__________ converts glucose to sorbitol

hexokinase

aldose reductase

why do diabetic patients experience cataract formation

excess glucose leads to sorbitol accumulation in the lens

accumulation of sorbitol causes lens damage and early cataracts

other than cataract formation, what side effect of sorbitol accumulation in the lens do diabetic patients experience

lens swelling that causes refractive shift

what is the main protector against oxidative damage in the lens

glutathione

how does glutathione protect against oxidative damage in the lens

acts as a reducing agent (donates electron) to detoxify hydrogen peroxide

how does the lens get glutathione

• transported into the lens from the aqueous

• synthesized by lens epithelial cells and superficial fiber cells

what happens to glutathione diffusion with age

glutathione diffusion decreases with age

factor in cataract formation

another name for vitamin c

ascorbic acid

is ascorbic acid found in higher quantities in the lens or aqueous

much higher in the lens

role of ascorbic acid in the lens

helps protect the lens from oxidative damage

2 main proteins in the lens

glutathione

ascorbic acid

how is oxidative damage detrimental to the lens

it reduces lens clarity

how do lens fiber cells help maintain lens transparency

• minimize light scatter because they lack membrane-bound organelles

• packed very close together and uniformly spaced

• cytoplasm of lens fiber cells have crystallins that minimize light scatter via destructive interference

what ion needs to be limited inside the lens to prevent cataract formation

Ca2+

list factors that help maintain lens transparency

• Na+/K+ ATPase pumps that move K+ into the lens and Na+ out of the lens

• avascular nature of lens to minimize light scatter

• lens fiber cell arrangement, lack of membrane-bound organelles, and crystallin content

• transport processes that monitor and limit Ca2+ concentration in lens

what forms the embryonic nucleus

primary lens fibers of the posterior epithelium

what facilitates growth of the lens after the embryonic nucleus is formed

secondary lens fibers of the anterior epithelium

where does mitosis of lens fiber cells occur

germinative zone of the anterior lens epithelium

what happens to lens fiber cells after mitosis

fiber cells move from germinative zone through transition zone and into lens equator, where fiber elongation occurs

where does lens fiber elongation occur

lens fiber cells

during lens fiber elongation what happens to lens fiber cells

they lose their membrane bound organelles and gain crystallins (allows for lens transparency)

in what part of the lens aerobic respiration occur

anterior lens epithelium

what part of the lens has the greatest metabolic demand and why

anterior lens epithelium

this is the area where mitosis of fiber cells occurs

the _______________ has the greatest demand of all lens components and therefore contains a significant amount of __________ to provide energy for mitosis

anterior lens epithelium

mitochondria

how are nutrients provided to the anterior lens epithelium

aqueous humor travels over the lens and provides nutrients

______________ is responsible for transporting nutrients from the AH to the lens

anterior lens epithelium

crystallins are (soluble/insoluble) lens proteins

soluble

what happens to crystallins with age and why

crystallins decrease and insoluble lens proteins increase

due to increase in cross-linking between lens fiber cells

what happens when there is increased cross linking in the lens

insoluble lens protein aggregates form

alters amount of water in the lens

__________ within the lens decrease a lot with age

alpha crystallins

by what age are there no more alpha crystallins in the lens

45

molecular chaperones

alpha crystallins

prevent degradation of other crystallins

what happens when alpha crystallins in the lens begin to degrade

lens fiber cells begin to degrade and cataracts can start to form

how much does lens thickness increase per year

0.02 mm

what happens to lens diameter with age

relatively stable after teen years

what happens to anterior and posterior lens capsule thickness with age

anterior: increases with age

posterior: relatively stable with age

where is the lens capsule the thickest and thinner

thickest: anterior mid-peripheral part of the lens (pre-equatorial)

thinnest: posterior pole

(T/F): Lens is the thickest BM in the body

true

what type of collagen makes up the lens

type IV

what happens to lens radius of curvature with age

decreases (lens becomes more convex)

how should refractive error change with increased lens thickness and why doesn’t it

since lens becomes more convex (+), patient should become more myopic with age

change in gradient refractive index of lens prevents this from happening

what happens to AC depth with age

decreases as anterior lens moves further forward

state if amino acids, glutathione, Na+, Ca2+, H2O, and crystallins increase or decrease with age

amino acids: decrease

glutathione: decrease

Na+: increase

Ca2+: increase

H2O: increase

crystallins: decrease

what happens to nuclear fibers with age

lose nucleus and organelles

become more yelloww

where does nuclear sclerosis begin

embryonic nucleus

expands to fetal and adult nucleus

what is the center of the lens called

embryonic nucleus

what part of the lens has the highest refractive index and why

embryonic nucleus

has the highest concentration of crystallins

what demarcates the boundaries of the fetal nucleus

y sutures

main function of iris

regulates pupil size

benefits of small pupil size

reduces spherical and chromatic aberrations

increases depth of field

(miosis/mydriasis) is more difficult with age

mydriasis

2 results of aging of the iris

• increased pigment deposition on the anterior structures of the eye

• increased resistance to dilation due to aging of the iris sphincter and dilator muscles

what part of the iris causes increased pigment deposition with age

posterior pigmented iris epithelium

2 main functions of the ciliary body

• accommodation via ciliary muscle

• production and secretion of AH

what part of the CB contributes to AH drainage

ciliary stroma via uveoscleral meshwork

what happens to AH formation with age

decreases

what happens to CB contraction with age

does not decrease

why does accommodation decrease with age (presbyopia)

changes in the lens

not in the contraction of the CB

2 main functions of the choroid

• provides nutrients to outer retinal layers

• has pigment that absorbs excess light that passes through RPE

where is the suprachoroidal space and what is it used for

space on top of the choroid between the choroid and sclera

site of passageway for nerves and arteries from the back of the eye to the front

also used for suprachoroidal space injections

the choroid has a (higher/lower) protein content than the retina

higher

what does the protein gradient between the retina and choroid do

since the choroid has higher protein content, the protein gradient allows absorption of excess water from the retina into the choroid

what happens to bruch’s membrane with age

thickens

what happens to the choriocapillaris with age

decreases in thickness

what happens to over choroid thickness with age

decreases

list 4 functions of the vitreous

• transparent medium for light

• UV filter for the retina

• structure and cushion for the globe (absorbs external forces/vibrations)

• storage for ions and nutrients for the retina and lens

what wavelength of light does the vitreous filter out

300-350

how does the vitreous impact drug absorption

gel-like consistency of vitreous decreases the bioavailability of topical drugs in the posterior segment

total volume of the eye

5 mL

total volume of the vitreous

4 mL (80% of the eye)

weight of the vitreous

4 grams

what is the main type of collagen in the vitreous

type 2

what is the vitreous composed of

99% water, type 2 collagen fibrils, hyaluronic acid

what is hyaluronic acid

a non-sulfated GAG

what is the role of hyaluronic acid in the vitreous

supports collagen fibrils and helps maintain proper spacing, maintains vitreous viscosity

vitamin C concentration is (higher/lower) in the vitreous than blood plasma

higher

vitreous has more vitamin C