Photoreceptors + phototransduction

Question 1

what are photoreceptors

Answer 1

rods and cones

Question 2

what is the lifecycle of photoreceptors

Answer 2

12 days
- must be constantly removed and regenerated
- translation of opsin to new disks only takes a few hours

Question 3

when are rods more sensitive

Answer 3

respond better at lower light levels

Question 4

what is the ratio of rods to bipolar cells

Answer 4

many rods converge onto the same bipolar cells
- make larger receptive fields that are located in the peripheral visual field
- high innervation ratio (lots of receptors per cell)
magnocellular

Question 5

what is the ratio of cones to bipolar cells

Answer 5

single cone connects to single bipolar cell
- small receptive fields located in the central visual field
- low innervation ratio (one receptor to one cell)
parvocellular

Question 6

what is rod saturation

Answer 6

rods can’t become any more active
(as hyperpolarised as possible)
- happens around indoor lighting levels

Question 7

when do photoreceptors hyperpolarlise

Answer 7

when stimulated by photons

Question 8

when are NTs sent by photoreceptors

Answer 8

sent in the dark
stop in the light

Question 9

what is dark current

Answer 9

influx of positive ions in the dark
(current flows in the dark)
more positive in the dark

Question 10

what is the membrane potential in the dark vs light

Answer 10

starts at -40 in the dark and becomes more negative with light

Question 11

why does light make the cell more negative

Answer 11

channels close - no Na+ and Ca2+ flowing into the cell anymore
(K+ leak channels open all the time)

Question 12

why are receptive fields larger in the periphery

Answer 12

many rods converge onto one bipolar cell
(high innervation ratios)
= magnocellular receptive fields

Question 13

why are receptive fields smaller in the fovea

Answer 13

one cone to one bipolar cell
(low innervation ratios)
= parvocellular receptive fields

Question 14

what is centre surround arrangement

Answer 14

(looks like a target)
- on centre types respond when the middle is light and the outside is dark
- makes visual receptive fields sensitive to contrast

Question 15

what are the steps of phototransduction (7)

Answer 15

1. photon interacts with retinal opsin
2. opsin changes shape
3. cascade of events which activates phosphodiesterase (PDE)
4. PDE breaks down cGMP
5. reduced cGMP concen closes the cGMP gated channels, reducing inward Na+ and Ca2+ current
6. hyperpolarisation of photoreceptor
7. reduction in NT (glutamate) release

Question 16

what reduces Na+ and Ca2+ current in phototransduction

Answer 16

reduced concen of cGMP closing cGMP gated channels
(caused by activation of phosphodiesterase breaking down cGMP)

Question 17

what does phototransduction lead to

Answer 17

hyperpolarisation of the photoreceptor
- reduction in glutamate release