Ocular Physiology and Histology Flashcards
What are the 4 basic layers of the eye?
Fibrous: cornea & sclara
Vascular: iris, ciliary body, choroid
Neurologic: retina
“other”: lens, aqueous humor, vitreous
What are the three major histological divisions of the eye?
anterior segment, posterior chamber, posterior segment
What is the name of the junction between the sclara, cornea, and conjunctiva?
the limbus
What are the 3 layers of the posterior segment?
outer: sclera
middle: choroid
inner: retina
What are the major preretinal factors of vision?
- refraction and focusing
- accommodation (ciliary body can change curvature and position of lens)
What are the 4 layers of the cornea?
1.epithelium
2. stroma
3. descemets membrane
4. endothelium
What are the 4 main functions of the tear fim?
- bend light
- protect and nourish ocular surface
- remove debris and pathogens
- immune surveillance
What are the 3 layers of tear film? functions?
- lipid layer: prevents evaporation
- aqueous layer: majority of tear film
- mucin layer: promotes adherence and spreading on eye
What is the difference between the palpebral and bulbar conjunctiva?
palpebral covers inner eyelids
bulbar covers globe up to cornea
What is the junction between palpebral and bulbar conjunctiva?
fornix
What are the 4 main characteristics of the cornea?
- avascular: nutrition from aqueous humor and tear film
- highly ordered for transparency: absorb/scatter UV light, transmit light in visible spectrum
- thin!: 0.5-0.6mm in dogs and cats
- highly innervated
Why is it difficult to deliver ocular drugs?
- tear film washes away topical medications
2.The cornea is meant to function as a barrier - Alternating hydrophobic and hydrophilic layers make absorption difficult
What are the main components of the corneal stroma
-keratocytes
-ground substance
-collagen fibers
Why do the collagen fibers of the corneal stroma have to be precisely arranged?
to limit light scatter
Why is descemet’s membrane critical?
it is relatively resistant to degradation by proteases and it is the last line of defense in bad corneal ulcers
What critical function does the corneal endothelium perform?
it is involved in the active maintenance of the osmotic gradient between the corneal stroma and aqueous humor
How is the lens held in place?
by zonula fibers attaching to the ciliary processes
describe epithelial regeneration in the lens
-cells at anterior pole divide and move toward equator
-at equator cells are pushed toward the center of the lens by cells following behind them
-cells elongate and lose organelles
-fibers meet at suture lines
What does lens transparency depend on?(3)
precise protein composition
precise spatial arrangement
intact fiber membranes
Where does the lens derive its nutrients?
the aqueous humor
What disease occures when alterations to the lens proteins or fiber membranes occur?
cataracts
What is the vascular/muscular tunic of the eye called?
uvea
What are the 6 main functions of the uvea?
1.regulate light entering the eye
2.produces aqueous humor
3.nutritional support, waste removal
4. stabilizes lens
5. helps focus light on retina
6. maintains blood aqueous barrier
What are the two structures of the anterior uvea?
-Iris
-Ciliary body
What is the structure of the posterior uvea?
choroid
What are the main functions of aqueous humor?
-provides glucose, other substances to avascular lens and posterior cornea
-removes metabolites and toxins
Where is aqueous humor produced?
the non pigmented ciliary epithelium
How is aqueous humor drained from the eye?
-via the conventional outflow pathway: throguh pupil to iridocorneal angle or uveosclaral pathway: absorbed across iris base and out via superchorodial space
What is a cause of glaucoma?
changes to aqueous humor drainage pathway
What is the function of the choroid?
-metabolic support for the retina
-contains the reflective tapetum
What is the retinal pigmented epithelium and its function
technically part of the retina but functionally like the choroid. Its roles are to provide nutrition and oxygen to outer retina, dispose of waste, thermal protection, regeneration of photopigments, part of blood retinal barrier
What are the chain of events that leads to light absorption by photoreceptors?
-Na channel closures
-membrane hyperpolarizes
-decrease in glutamate release
(opposite to how neurons work)
What are the roles of interneurons in the retina
-increased contrast
-suppression of surrounding photoreceptors
-inhibition or amplification of data
Describe the function/pathway of retinal ganglion cells
-receive processed signal from interneurons
-axons from RGC form the optic nerve and run to brain
-# of RGC determines visual acuity
What is the name of the exit for the optic nerve?
lamina cribrosa
