Where is the fovea centralis? What does it contain and what is its function?
Central point of retina with only cones and no rods. High levels of visual acuity.
Contains axons of “ganglion cells” but no receptor cells
Optic disc “Blind spot”
What is the macula lutea? What gives it its color? What is one of its functions?
Yellow area surrounding fovea.
Color = lutein and zeaxanthin (carotenoids)
Both pigments are antioxidants and absorb excess blue and UV light (protective)
What does the central artery of retina do? Describe its path.
Blood supply to retina.
Branch of internal carotid a.
Thru optic tract and exits onto inner surface of retina between retina and vit. humor.
Order these retinal layers from outer to inner eye. Photoreceptors (rods and cones) Ganglion cells Retinal pigmented epithelium Neural cells (bipolar, horizontal)
Outer -- Retinal pigmented epithelium Photoreceptors (rods and cones) Neural cells (bipolar, horizontal) Ganglion cells Inner --
Describe structure of rods and cones, including pigments and transmitters.
Outer segment w/ stacks of vesicular discs containing pigments.
Iodopsin - cones
Rhodopsin -rods
Inner segment w/ nucleus and synaptic terminal that releases glutamate
Part of visible light spectrum that rod and cone pigments respond to?
450-700 nm
How to photoreceptors respond to light?
Hyperpolarize and release less glutamate
- Light converts 11-cis-retinal to trans-retinal.
- Causes opsin to activate PDE via transducin (G protein)
- PDE reduces cGMP –> closes Na+ and Ca+ channels
- Hyperpolarizes cell
- Less glutamate released.
Rod or Cone?
Photopic - vision in bright light
Cones
Rod or Cone? High sensitivity (more pigment, night vision)
Rods
Rod or Cone?
Low acuity, not present in fovea
Rods
Rod or Cone?
Sensitive to direct axial sunlight
Cones
Rod or Cone?
Good spatial resolution
Cones
Rod or Cone?
Chromatic - three types of pigmented cells respond to different parts of the spectrum
Cones
Does peripheral vision have color?
Yes. Thin layer of cone cells evenly distributed on rest of retina
What features of the fovea allow it to have the highest possible level of visual acuity?
- Only cones - favor acuity and color
- Cones more exposed to light by outward dispersal of ganglion and other integrative cells
- Each cone activates single ganglion
T or F?
Choroid is behind retina so that light reception is not affected by ganglion and bipolar cells.
False. Light reception not affected by b.v.s in choroid.
T or F?
Inner layer retinal cells get nutrients from smaller retinal arteries
True
T or F?
Pigment turnover in photoreceptors requires high O2 levels so RPE close to choroid (blood supply).
True
T or F?
Light is refracted by ganglion and bipolar cells.
False. Light passes thru ganglion and bipolar cells w/o distortion b/c they have same refractive index as vit humor.
What are the functions of the RPE?
- Visual acuity (limit reflection of light back to PRs)
- Antioxidant (needed b/c absorp. of blue light increases ROS)
- Maintain photoreceptor excitability (trans-retinal is reformed into cis-11-retinal)
- Nutrient (transports glucose and retinal to PRs)
- Phagocytosis of photoreceptor cell debris from light reception
What is retinitis pigmentosa?
Deficient uptake of membrane fragments (metabolic debris), which accumulate and separate the receptors from the choroid, leading to anoxia and cell death
Why is the fovea avascular?
Reduces vascular interference with high acuity.
How does the fovea receive nutrients? Get rid of waste?
Capillaries encircle fovea to supply it.
No veins. Extravasated salt & water is transported from the extracellular space into the choroid veins via membrane transporters in the RPE.
How does retinal detachment occur?
The contact bet. neural retina and RPE is mechanically unstable. In detached retina, neural retina tears away from RPE.
Macular degeneration can detach the retina.
How is vision affected by macular degeneration?
Blindness in fovea/center
What is the difference between “wet” and “dry” macular degeneration? Which one is also called exudative? Which one is more serious?
Wet = Abrupt lesion of b.v. that separates RPE and receptor cells (exudative, more serious) Dry = Slow build up of metabolic waste separates choroid from receptor cells and interferes with nutrient supply to receptor cells.
What is the role of bipolar, horizontal, and amacrine cells?
Integrate information from receptor cells and activate ganglion cells
What is the role of ganglion cells?
Output of retina. Transmit visual information to thalamus, superior colliculus, and other brain stem areas.
Where do ganglion cells converge?
Converge at optic disc to form optic nerve
The optic nerve/tract consists of …
- axons of ganglion cells
- CNS ganglia
- central artery and veins of retina
What cells make up the CNS glia and what do they do?
Oligodendricytes myelinate the axons
Astrocytes surround cell bodies and dendrites; contact blood vessels
What is papilledema?
Increased CSF pressure limits venous return from retina
Pressure extends around optic nerve and causes edema (bulge) under optic disc
What would you see through an opthalmoscope in a person with papilledema?
Blurred margins of optic disc.
Dilated tortuous veins
What are two symptoms of papilledema that you would not be able to see through an opthalmoscope?
Expanded CSF space around optic nerve.
Axoplasmic stasis of ganglion cells
Would you see increased, decreased, or normal intraocular pressure in papilledema?
Normal
Why is the optic disc called the “blind spot”
No receptor cells, no cells to respond to light.
Why don’t we have a blank circle in our vision due to the blind spot?
Interpolation via saccadic eye movements or by activity in visual cerebral cortex.
