W21-L2: Retina Flashcards
What is visual acuity?
Ability to resolve fine detail tested by Snellen chart
What do the two numbers in a snellen chart refer to?
Top number is test distance eg 6 meters, bottom number is the distance they have to be to see what a normal person would see at 6 meters
What is normal vision, legal blindness and the minimum acuity for driving?
6/6 is normal vision, 6/60 is legal blindless, 6/12 is minimum for driving
What optical factors affect visual acuity?
- Pupil size
- Clarity of optical media (eg cataracts)
- Refractive errors
At scotopic light what is the best visual acuity possible?
6/60
What are the normal neurones of the retina?
Rods, cones, Horizontal cells, Bipolar cells, amacrine cells and ganglion cells
What are the two synaptic layers of the retina?
- Outer plexiform layer
- Inner plexiform layer
Which photoreceptor is more sensitive to light, the rod or the cone?
Rod
Is there more cones or rods in the retina?
Rods, 100 million vs 5 million
Why is the vision with rods worse than cones?
Connectivity
What is the through pathway of the retina?
Photoreceptors–> Bipolar Cells –> Ganglion cells
What are the two lateral interactions that occur in the retina?
Outer retina=Horizontal cells
Inner retina= Amacrine cells (axonless)
Both inhibitory
Where are bipolar cells located?
Inner nuclear layer
What are the types of bipolar cells?
Two:
- ones that hyperpolarise when light is on the retina (off BCs)
- ones that depolarize when light is on the retina are called “on” BCs
What are the type of ganglion cells and what do they release?
ON, OFF, M and P and Release Glutamate
What is interesting about the receptor field properties of ganglion cells?
light in different parts of the receptor fields produces different rates of ganglion firing
What are the photopigments and what are they all attached to?
Rods contain rhodopsin
Cones contain one of three cone-opsins
All are bound to vitamin A
How do photoreceptors function?
- Photoreceptors are hyperpolarized by light.
- Use glutamate as their neurotransmitter.
- Respond to light with graded changes in membrane potential (not Action Potentials)
How does a photoreceptor function in the dark?
cGMP gates a sodium channel causing continuous influx of sodium ions which causes depolarization of the cell.
How does a photoreceptor function in the light?
In the light, cGMP breaks down to GMP:
- cGMP no longer gates the sodium channels
- Flow of Na ions ceases
- Cell is hyperpolarized
What is the process known as phototransduction?
Light activates rhodopsin
Initiates a cascade of events that ultimately leads to the closure of cGMP gated sodium channels.
Rh->Transducin->PDE->breaks down cGMP.
Closure of sodium channels>hyperpolarization
What creates the centre-surround response of the retina?
The way the retina is wired, which gives a central response and a surround response (horizontal cells)
What creates the surround response in the retina?
Horizontal cells receive input from many photoreceptors and provide output to other photoreceptors.
In the surround response which cells depolarize and which hyperpolarize?
Photoreceptors depolarize and ganglion cells hyperpolarizes
What is melanoma associated retinopathy?
Rare complication of melanoma where antibodies are produced directed against ON bipolar cells.
