the visual system: phototransduction

Question 1

what are the photopigments for cells in the retina?

Answer 1

rods = rhodopsin
cone = 3 varieties of opsins - S, M, L
retinal ganglion = melanopsin

Question 2

how do photoreceptors differ from most cells?

Answer 2

depolarised in their resting state and hyperpolarised by light

Question 3

what happens to photoreceptors in the dark?

Answer 3

cGMP-gated non-selective cation channels are open in the dark allowing Na+ influx known as the dark current to depolarise photoreceptors

Question 4

what happens in photoreceptors in light?

Answer 4

light decreases cGMP levels, closing the channels, preventing Na+ influx, hyperpolarising photoreceptors

Question 5

how does light cause a decrease in cGMP levels?

Answer 5

1. opsin activated by light
2. GDP exchanged for GTP on alpha subunit
3. alpha subunit activated enzyme phosphodiesterase (PDE)
4. PDE reduces cGMP levels, closing Na+ channels

Question 6

what is signal amplification?

Answer 6

a couple of photons can break down 1400 cGMP molecules as there is an enzyme cascade

Question 7

how does saturation occur in response to bright light?

Answer 7

limited amount of cGMP-gated channels, once they are all shut you can’t stop anymore Na+ getting in

Question 8

difference in saturation between rods and cones

Answer 8

rods - easily saturated, cannot process bright light
cones - not easily saturated, used in bright light

Question 9

how does light adaptation occur in the dark?

Answer 9

1. Ca2+ normally enters cell (through cation channels) and blocks guanylyl cyclase (responsible for converting GTP to cGMP)
2. reduced cGMP production, so closes some ion channels

Question 10

how does light adaptation occur in the light?

Answer 10

1. channels are shut so Ca2+ cannot enter cells
2. guanylyl cyclase no longer blocked so cGMP production resumed and more channels open

Question 11

difference between ON and OFF bipolar cells?

Answer 11

OFF = hyperpolarise (turn off) in response to light
ON = depolarise (turn on) in response to light

based on response to glutamate, photoreceptor hyperpolarises to light = reduced glutamate release

Question 12

why do ON and OFF bipolar cells have different responses to light?

Answer 12

use different receptors
OFF = ionotropic glutamate receptor (channel)
ON = metabotropic glutamate receptor (GPCRs)

Question 13

organisation of the bipolar cell receptive field

Answer 13

centre-surround organisation
- if photoreceptors in receptive field centre are stimulated they directly communicate with bipolar cell
- if photoreceptors in receptive field surround are stimulated they communicate with same bipolar cell via a horizontal cell