23/ visual system: phototransduction

Question 1

rod vs cone vs retinal ganglion photopigments

Answer 1

• rod: rhodopsin
• cone: S, M, L opsins - short medium long wavelength
• retinal ganglion: melanopsin (tells us night/day not vision, circadian rhythm)

Question 2

how do photoreceptors propagate an ap

Answer 2

• unusual - hyperpolarised by light
• in dark: cGMP-gated (cGMP = ligand) non selective cation channels are open. allow Na+ influx (dark current) to depolarise photoreceptors
• in light: light decreases cGMP levels, closes channels and preventing Na+ influx, hyperpolarising receptors

Question 3

voltage trace memb potential of photoreceptors in light/dark

Answer 3

• -30mV in dark, down to -60mV in light
• in continued light becomes less hyperpolarised - adaptation

Question 4

what regulated hyperpolarisation in photoreceptors to stop it becoming too extreme

Answer 4

potassium - can always move down its conc grad (into cell?)

Question 5

how many photons are needed to evoke sensation of light in humans

Answer 5

5-7

Question 6

what happens in rhodopsin/opsins during transduction

Answer 6

• activated by light
• stimulates g protein transducin to become transducing GTP
• the alpha subunit activates the enzyme phosphodiesterase PDE
• PDE reduces cGMP levels, closing Na+ channels
• signal amplification - enzyme cascade

Question 7

saturation of response in bright light rods vs cones

Answer 7

• rods: can’t process bright light, easily saturated. rhodopsin is bleached. cGMP levels so low that no additional hyperpolarisation can occur
• cones: not saturated easily, used in bright light

Question 8

light adaption in dark and light - calcium

Answer 8

• dark: ca2+ enters and blocks guanylyl cyclase. reduces cGMP production, so some ion channels close
• light: channels shut so ca2+ can’t enter cells. block on guanylyl cyclase is released. more cGMP produced = more channels open

Question 9

on vs off bipolar cells

Answer 9

• upstream: photoreceptor hyperpolarises to light so reduced glutamate release
• bipolar cell hyperpolarises = off bipolar cell
• or bipolar cell depolarises = on bipolar cell
• bipolar cells use dif receptors

Question 10

bipolar cells receptive field

Answer 10

• retinal ganglion cells will only fire aps when specific areas of the retina are illuminated
• receptive fields are centre surrounded