The Visual System - Phototransduction

Question 1

in isotopic conditions, what light wavelengths is the retina most sensitive to?

Answer 1

500nm.

Question 2

in photopic conditions, what wavelengths of light is the retina most sensitive to?

Answer 2

560nm.

Question 3

what photopigment do rods contain?

Answer 3

rhodopsin.

Question 4

what wavelengths of light does rhodopsin absorb?

Answer 4

500nm.

Question 5

what are the 3 types of cones?

Answer 5

short,
medium,
long.

Question 6

what wavelengths of light do short cones absorb?

Answer 6

420nm.

Question 7

what wavelengths of light do medium cones absorb?

Answer 7

530nm.

Question 8

what wavelengths of light do long cones absorb?

Answer 8

560nm.

Question 9

what photopigment do cones contain?

Answer 9

opsin.

Question 10

photoreceptors are unusual because are hyperpolarised by light.
- what does this mean?

Answer 10

they are resting at -30mV, so are more depolarised than normal neuron resting potential,
when light is shined, they become hyperpolarised / more negative.

Question 11

in the outer segment of the photoreceptors, and in the dark, there are lots of cGMP.
-what does this mean?

Answer 11

more cGMP to bind to cGMP-gated channels,
Na+ enters the photoreceptor cell,
this keeps the cell depolarised in the dark.

Question 12

in the inner segment, how does the cell make sure it isn’t too depolarised?

Answer 12

there is an exit of K+ from the cell.

Question 13

in the outer segment, photons are absorbed by photopigments.

-what does this mean?

Answer 13

causes a decrease in cGMP because there is less available to bind to cGMP-gated channels,
so channels remain shut,
Na+ cannot enter the cells,
the cell becomes hyperpolarised as there are fewer positive ions entering the cell.

Question 14

how is rhodopsin activated by light?

Answer 14

retinal part of rhodopsin absorbs the light,
this causes an alpha subunit of a G-protein to exchange GDP for GTP,
it translocate an enzyme and activates it,
this reduces cGMP levels,
closes Na+ channels.

Question 15

why can rods not process bright light?

Answer 15

they become easily saturated.

Question 16

what happens to cGMP when rhodopsin is bleached?

Answer 16

cGMP levels are so low that no additional hyperpolarisation can occur.

Question 17

why are cones not easily saturated?

Answer 17

they are used in bright light anyway,

are capable to adapt.

Question 18

after hyperpolarisation, photoreceptors begin to adapt whilst still in light.
-what does this mean?

Answer 18

if the light got brighter then the cones would be able to respond again there would be another hyperpolarisation.

Question 19

how does this light adaption occur in the dark?

Answer 19

Ca2+ normally enters the cell via the cGMP channels,
this blocks an enzyme that normally converts GTP to cGMP,
this reduces the production of cGMP, so closes the ion channels.

Question 20

how does light adaption occur in the light?

Answer 20

channels are shut so there is an increase in breakdown of cGMP into GMP,
Ca2+ cannot enter the cell,
not block on conversion of GTP into cGMP,
causing gradual binding to channels and depolarisation,
meaning if the light increases then there is more channels to shut and cause hyperpolarisation.

Question 21

if a bipolar cell hyperpolarises from the reduced glutamate release it is a…

Answer 21

OFF bipolar cell.

Question 22

if a bipolar cell depolarises from the reduced glutamate release it is a…

Answer 22

ON bipolar cell.

Question 23

what type of receptors do OFF bipolar cells express?

Answer 23

ionotropic glutamate receptors.

Question 24

what type of receptors do ON bipolar cells express?

Answer 24

metabotropic glutamate receptors.

Question 25

bipolar cells have centre-surround organisation.

-what does this mean?

Answer 25

they have a receptive field that provides input to bipolar cells directly.