Retina Flashcards
What is visual acuity?
the ability to resolve fine detail
What factors limit visual acuity?
neural factors
optical factors
How do we test visual acuity?
recognition of letters on a Snellen or LogMAR chart
What does 6/60 and 6/12 mean?
these individuals can see at 6m what a normal person sees at 60m = legally blind
can see at 6m what a normal person sees at 12m -> needed for driving
What are the optical factors affecting VA?
Pupil size
Clarity of the optical media (ie how transparent the cornea is)
Refractive errors (error in focussing of the light by the eye)
What is scotopic vision? what photoreceptors are most active?
low light levels (ie darkness) RODS are responsible for vision
What are the neural factors affecting VA?
?
What is photopic vision? what photoreceptors are involved?
vision under well lit conditions -due to CONES
What is the structure of the retina - name the 6 types of neurons involved
Layered structure with 6 types of neurons -> Ganglion cells, Bipolar cells, Horizontal cells, Amacrine Cells, Rods and Cones
What are the photoreceptor cells? where are they located?
Rods & Cones
Located in the last layer of the retina furtherest from the light source.
What are the two synaptic layers of the retina called?
Outer plexiform layer, and Inner plexiform layer
What do Rods do?
function in dim light & provide black and white vision. Only 1 type, Very sensitive and NOT found in the fovea
What do Cones do?
function in well lit conditions and are involved in the perception of colour. 3 Types (red, green, blue), less sensitive and densest in the fovea
Are there more rods or cones in the retina? which ones is bigger?
RODS outnumber Cones 20:1
Cones are bigger
What are the main layers of the retina?
GC layer, Inner plexiform layer, Inner nuclear layer, Outer plexiform layer, outer nuclear layer, Photoreceptor layer, Pigmented epithelium
What is the general pathway through the 6 different neuron types in the retina?
photoreceptors synapse with bipolar cells which synpase with GCs, and HC and AC create lateral interactions at each synapse (HC between Ph +BCs, and AC between BC + GCs)
How are bipolar cells classified?
10 different types: depending on whether they receive signals from Rods (1 type) or Cones (9 types)
What is the response of BCs when light falls on the retina?
OFF BCs - hyperpolarise
ON BCs - depolarise
How do horizontal cells function?
receive input from and provide input to photoreceptors. use GABA (inhibitory) - respond to light be HYPERPOLARISING
How do Amacrine cells function?
axonless cells that are important for lateral inhibition - release glycine and GABA onto bipolar and GCs
What is unique about the GCs?
only neuron in the retina that fires action potentials, release glutamate
What is the receptive field or a cell?
area of retina that when stimulated with light changes that cell’s membrane potential
What are photopigments?
proteins that change conformation when light hits it
What photopigments do Rods contain?
Rhodopsin
What photopigments do Cones contain?
3 different kinds of cone-opsins
What is the role of Vit A in vision?
Vit A binds opsins (photopigments) -> essentiat for photoreceptor function
What happens to photoreceptors when light hits them?
respond to light with graded changes in membrane potential -> become hyperpolarised -> so they release LESS glutamate released
What is the mechanism by which photoreceptors become hyperpolarised in response to light?
light activates rhodopsin which leads to a siganlling pathway resulting in PDE activation which breaks down cGMP to GMP- stopping the cGMP mediated opening of the Na channel -> this stops Na+ influx into the photoreceptor -> hyperpolarized (more negative)
