Visual system- retina to cortex Flashcards
What are horizontal and amacrine cells thought to be involved in
Setting up centre-surround visual fields, enabling lateral inhibition to provide greater contrast
What allows high acuity in the fovea for cones
Often a 1:1 relatinoship between cones/bipolar cells/ganglion cells meaning low convergence, small receptive fields, densely packed
What is sacrificed for high acuity in cones at the fovea
Sensitivity
What reduces acuity but improves sensitivity at the peripheral retina for cones
More convergence of cone info -> bipolar cell info -> ganglion cell info
Why does it make sense for our cone periphery to be high sensitivity but low acuity
It is more important our periphery is sensitiive so we can notice things, then turn and use our foveally dominated vision for detail
How sensitive vs high acuity are the rods
Low acuity, very high sensitivity
Why is rod vision low acuity but high sensitivity
Info from many rods converges onto a single rod bipolar, which is compressed again by ganglion cells
Why does convergence reduce acuity
The brain doesn’t know which one of the 15-30 rods that connect to each bipolar cell the signal came from before they were all pooled
Sensitivity and accuracy of scotopic vs photopic vision
Scotopic vision- very sensitive, not very accurate
Photopic vision- very accurate, not very sensitive
How do amacrine cells affect rod vision during mesopic conditions
Amacrines link rod and cone pathways via gap junctions, so the rod signals can piggyback onto the cone bipolar pathways to provide add info to the cone pathway to make it more sensitive (Wassle et al, 1995)
What are amacrine cells sometimes called for their role in mesopic vision
Piggyback cells
What is the result of the ganglion cells receiving a mix of info from rods and cones in mesopic conditions
Vision has a balance between acuity and sensitivity
What is the pathway taken by info in mesopic conditions in the retina
Rods ->Rod bipolars -> All amacrine cells -> Cone bipolars -> Ganglion cell
What conformational change marks the start of phototransduction
Photons induce a confirmational change in retinal (cis retinal -> trans retinal), which activates the opsin (a GPCR) that activates the G protein transducin
What is the opsin in rods
Rhodopsin
What are the opsins in cones
Photopsins- 3 types; red, green, blue
What is the effect of the activation of G protein transducin in phototransduction
Transducin triggers phosphodiesterase (PDE) to hydrolyse cGMP, reducing cGMP levels which closes cGMP-gated channels
What is the effect of cGMP-gated channels closing in phototransduction
The photoreceptor hyperpolarises in a graded manner, as Na+ can no longer enter through the channel
What is the result of graded hyperpolarisation of photoreceptors due to phototransduction
Corresponding graded reduction in the rate of neurotransmitter release onto bipolar cells
When taking intracellular recordings from a single cone cell from a turtle retina stimulated with increasing amount s of light- what is the effect of light
Light hyperpolarises the cone, at the highest light levels the response is saturated and the receptor is said to be bleached
What is the sensitivity to bleaching of rods vs cones
Rods- more sensitive, bleach at ambient light levels meaning they don’t function in daylight
Cones- less sensitive, are fully active even in bright sunshine
What 4 mechanisms terminate the phototransduction cascade
Inactivation of rhodopsin, inactivation of transducin, inactivation of phosphodiesterase (PDE), activation of guanylate cyclase
Phototransduction cascade termination- what causes inactivation of rhodopsin
Rhodopsin is phosphorylated by the opsin kinase, then arrestin binds to phosphorylated rhodopsin, completely deactivating it- resuming dark current
What happens to rhodopsin in the dark
Rhodopsin regenerates after being bleached