0806 - The Retina and Visual Acuity Flashcards
Name the different cell types in the retina and the function of each.
Ganglion cells - send axons to the brain.
Interneurons (bipolar/amacrine/horizontal) - filter and relay electrical signals from photoreceptors to ganglion cells, improving signal/noise ratio. Bipolar cells invert signal (on to off)
Photoreceptors - Receive light and transduce it into an electrical signal (rhodopsin)
RPE - Absorb stray light, phagocytose disks of outer segments of rods.
Muller cells - maintain neural environment, buffer ions, monitor for inflammation.
Microglia - (resident macrophages/dendritic) - normally quiescent.
What are the three primary classes of bipolar cells?
Blue - connected to blue cones
Midget - connected to red and green cones.
Diffuse bipolar - connect to magnocellular pathway (M-pathway).
Some bipolar cells turn on in response to light, some turn off (remembering that photoreceptors turn off).
What are the 3 major classes of ganglion cells?
Parasol - big - 20% of all. Connect to diffuse bipolar cells, peripheral retina, detect change in contrast, not colour.
Midget - small - 90% of all. Central vision, red/green light, Parvocellular (P-pathway)
Bistratified - less than 10% of all, uniformly distributed, blue/yellow colour pathway.
What are the different types of photoreceptors and their spectral sensitivities? How are they distributed?
Rods - most active in low light (scotopic), peak sensitivity just under 500nm
Cones - S- (blue, 430nm), M- (green, 530nm), L (red, 560nm) (photopic vision)
Combined with other cell types, red/green light most important for daylight vision.
Photo transduction shuts off glutamate release instead of firing it up.
What is the difference between vision in bright and low light levels.
IN low (scotopic) light, you use rods (rhodopsion). In high (photopic) light, you use cones (S,M,L).
Outline the anatomy of the retina, including blood supply.
Optic disk with superior and inferior temporal retinal vessels coming out either side of horizontal meridian. Macula is central to vision, with fovea in centre of macular vertical meridian. Axons run across surface, but skirt around macula.
Why is R/G colourblindness more common in males than females?
Red arose as a mutation of green, both are carried on X chromosome.
Outline the distribution of photoreceptors in the retina.
10^8 rods, 4.5-5m cones. Cones concentrated in fovea (except blue - absent), rods across the rest, lowering in macula and absent in fovea.
What are the key anatomical features in the retina that affect visual acuity?
Best at fovea (concentration of cones)
Determined by number and type of cells (particularly photoreceptors), amount of convergence (less or none is better, hence midget cells good) onto bipolar cells, Density of ganglion cells at that point reflect number of pathways going into the brain (even though absent from fovea - get it from perimeter). So the affect of a lesion in an area depends on ganglion cell distribution in the area - could be devastating in macula, or unnoticed in peripheral retina.
Ultimately, gets worse as you get away from fovea.
What does parallel processing mean?
Ability of the brain to process incoming stimuli of varying type and quality at the same time and unite them into a field of view.
What non-visual brain functions are mediated by light?
Circadian rhythm/melatonin release, so sleep/wake.
Outline the pathway from the retina to the visual cortex.
Retina, optic tract, chiasm, thalamus (dLGN), optic radiations, V1. Colour and spatial discrimination (parvo pathway) then placed into ventral aspect (temporal lobe), and motion goes in dorsal stream (magno pathway)
Which nucleus in the thalamus is responsible for light processing onto the visual cortex?
Dorsal lateral geniculate nucleus. Stratified by function, and mapped.
