The visual system: phototransduction

Question 1

What is rhodopsin?

Answer 1

Rod photopigment

Question 2

What is S, M and L opsin?

Answer 2

Cone photopigments

Question 3

What is melanopsin

Answer 3

Retinal ganglion photopigment

Question 4

Where do retinal ganglion cells feed into and regulate?

Answer 4

They feed into the suprachiasmatic nucleus
- Regulate circadian rhythms

Question 5

Where is there a greater number of long wavelength cones in the eye?

Answer 5

In the retina

Question 6

What are photoreceptors hyperpolarised by?

Answer 6

By light

Question 7

What state are photoreceptors in at rest?

Answer 7

Depolarised

Question 8

What does a photoreceptor release at rest?

Answer 8

Glutamate

Question 9

Why do photoreceptors hyperpolarise in the light?

Answer 9

Na+ causes depolarisation in rods at rest
- Na+ channels are kept open due to second messenger cGMP
- Light reduces cGMP, causing the Na+ channels to close
- So the membrane potential becomes more negative and hyperpolarises

Question 10

What is rhodopsin made of?

Answer 10

Retinal and opsin

Question 11

What is cGMP produced by?

Answer 11

Guanylyl cyclase

Question 12

How does retinal change when in light?

Answer 12

Becomes bleached (from purple to yellow)
- Due to a conformational change

Question 13

Describe phototransduction in terms of rhodopsin

Answer 13

Rhodopsin is activated by light
- G protein transducin becomes transducin GTP
- Alpha subunit activates phosphodiesterase (PDE)
- PDE reduces cGMP levels, closing Na+ channels

Question 14

Describe phototransduction in terms of rhodopsin

Answer 14

Rhodopsin is activated by light
- G protein transducin becomes transducin GTP
- Alpha subunit activates phosphodiesterase (PDE)
- PDE reduces cGMP levels, closing Na+ channels

Question 15

Absorption of 1 photon lead to the amplification of —– cGMP molecules and suppresses a current of —- Na+ ions

Answer 15

1400 cGMP
1 million Na+ ions

Question 16

Why are rods not able to process bright light?

Answer 16

They are easily saturated

Question 17

What happens to rhodopsin in bright light?

Answer 17

Rhodopsin becomes bleached
- cGMP levels are so low that no additional hyperpolarisation can occur

Question 18

How do photoreceptors respond to extended periods of bright light?

Answer 18

Photoreceptors initially hyperpolarise but gradually depolarise with continuous bright light
- Adaptation allows view to increase in light intensity

Question 19

What effect does Ca2+ have on cGMP?

Answer 19

In the dark, Ca2+ enters cells
- Ca2+ blocks guanylyl cyclase, reducing cGMP production
- Causing some ion channels to close

Question 20

How are glutamate levels affected by hyperpolarisation?

Answer 20

There is a reduced glutamate release during photoreceptor hyperpolarisation

Question 21

What is an off bipolar cell?

Answer 21

A hyperpolarised bipolar cell
- Turns off in response to light
- Expresses ionotropic glutamate receptors

Question 22

What is an on bipolar cell?

Answer 22

A depolarised bipolar cell
- Turns on in response to light
- Expresses metabotropic glutamate receptors

Question 23

When will retinal ganglion cells fire action potentials?

Answer 23

When specific areas of the retina are illuminated

Question 24

What is the organisation of bipolar cells?

Answer 24

Centre surround organisation
- Photoreceptors and the receptive field centre surround the bipolar cells