Vision L2

Question 1

dscribe the strucutr eof cone and rods

Answer 1

Rods and cones both consist of an outer segment specialised for transduction, joined to an inner segment (with more normal cellular machinery) by a connecting cilium.

Rod outer segments (ROS) consist of stacked membranous discs containing the visual pigment, and enzymes of the transduction cascade.

Cone outer segments consist of continuous folds of invaginating lamellae.

Question 2

describe rhodopsin

Answer 2

The visual pigment, rhodopsin is a membrane protein with 7 transmembrane helices embedded in the disk membrane: a prototypical G-protein coupled receptor.

Question 3

what dies rhodopsiin bind/

Answer 3

Rhodopsin binds a chromophore molecule, 11-cis retinal in a specific binding pocket.

peak absorbtion of 500nm

Question 4

chromophore in cone cells?

Answer 4

idemnticle chromophore - different charge interactions tuen absorbtion to different wave lengths

Question 5

descriube Photoisomerisation of the photopigment

Answer 5

Following absorption of a single photon the chromophore isomerises from 11-cis to all-trans retinal.

This photoisomerization leads to a catalytically active form of rhodopsin known as metarhodopsin II, often referred to simply as R*.

Question 6

describe how R* returns to the R state

Answer 6

After R* has played its role, all-trans retinal dissociates slowly (100-1000 s) from the opsin: rhodopsin is now in its so-called bleached form and must be regenerated before it can be used again.

Regeneration cannot take place within the photoreceptor itself, but instead is performed in the retinal pigment epithelium (RPE), the cells of which encompass the apical processes of the outer segments.

Question 7

describe the rpocess of photopigment generation

Answer 7

In order to regenerate the photopigment,

1. retinal is reduced to all-trans-retinol
2. transported out of the photoreceptor into the RPE
3. where it is converted back to 11-cis- retinal.
4. Finally it is transported back to the photoreceptors, where it rejoins the bleached opsin to form rhodopsin.
5. After bright light it can take 30 minutes or more before rhodopsin is fully regenerated.

Question 8

Retinal is a derivative of vitamin A (11-cis-retinol), and deficiency leads to ….

Answer 8

Retinal is a derivative of vitamin A (11-cis-retinol), and deficiency leads to night blindness.

Question 9

photopigment regeneration in cones?

Answer 9

An alternative pathway via Müller cells

allows rapid pigment regeneration in cones.

Question 10

Phototransduction takes place via a
G protein-coupled cascade, which culminates in the ______ of cyclic GMP (a negative internal transmitter).

Answer 10

Phototransduction takes place via a
G protein-coupled cascade, which culminates in the destruction of cyclic GMP (a negative internal transmitter).

Question 11

describe the process of phototransduction

Answer 11

1. R* activates heterotrimeric GTP-binding protein (G protein) called transducin
2. Transducin and rhodopsin diffuse freely in the disc membrane, and by random collisions the active rhodopsin can activate >100 transducin molecules.
3. Each transducin molecule activates one effector enzyme, a phosphodiesterase: (PDE).
4. PDE hydrolyzes cyclic GMP (cGMP), present in high concentrations in the dark, into the inactive 5’GMP.
5. The rod plasma membrane contains a high density of cyclic nucleotide gated cation channels which are opened by cGMP and so are continuously open in the dark
6. The reduction in the concentration of cGMP results in closure of channels, leading to a hyperpolarizing response.
7. Guanylyl cyclase (GC) then resynthesises cGMP to terminate the response.

Question 12

light results in the cell becoming

Answer 12

hyperpolarised

Question 13

describe how amplification is used in photoreceptrs

Answer 13

A characteristic feature of such second messenger cascades is the possibility of amplification: a single rhodopsin molecule activates ~150 G proteins s-1, and each molecule of PDE hydrolyzes ~600 cGMP molecules s-1.

The resulting reduction in cGMP concentration (~105 molecules hydrolysed per photon) results in the closure of several hundred channels, (3% of the channels open in the darkness) blocking the entry of ~106 cations.

This explains how a single photon can give rise to a measurable signal in one rod.

Question 14

what is Retinitis pigmentosa

Answer 14

Hereditary retinitis pigmentosa (RP) is a progressive hereditary retinal degeneration afflicting 1 in 3000 people.

There is a gradual onset of night blindness in adolescence, leading to loss of all peripheral vision by adulthood, and in extreme cases total blindness.

There is no single cause for hereditary RP, but about 5-10% of cases are caused by mutations in the gene for rhodopsin.

Many other molecules of the transduction cascade are affected in other cases of hereditary retinal disease including PDE and the light-sensitive channel.

Question 15

describe the dark current

Answer 15

In darkness, when the cGMP concentration is high, many channels are open and there is a continuous circulating dark current consisting of Na+ and Ca2+ ions flowing into the outer segment, depolarizing the cell to around -30 mV.

The circuit is completed by outward current in the inner segment, mainly through K+ channels. When the channels carrying the dark current close during illumination, K+ continues to exit via channels in the inner segment, resulting in a hyperpolarization. A Na/K pump maintains the ionic gradients

Question 16

Answer 16

Question 17

describe graded hyperpolarisation?

Answer 17

Flashes of increasing intensity close more and more channels causing a graded hyperpolarization, saturating at around - 75 mV when all the cGMP-gated channels are closed.

Question 18

describe light adaptation?

Answer 18

Sensitivity is reduced by background illumination

Question 19

can the dark current be completely supressed by bright light?

Answer 19

yep

Question 20

light adapt - defined as a….

Answer 20

light adapt - defined as a reduction in sensitivity as the steady light intensity increases.

Question 21

Photoreceptor adaptation is believed to be largely mediated by …

Answer 21

Photoreceptor adaptation is believed to be largely mediated by calcium ions.

Question 22

Answer 22

Question 23

describe the rpocess of photoreceptor adaptation throguh Ca2+ ions

Answer 23

1. Photoreceptors saturate when most of the cGMP has been hydrolysed and all of the channels close.
2. To prevent this cGMP must be resynthesised via the enzyme guanylyl cyclase (GC), which is inhibited by Ca2+.
3. Ca2+ entering by the light-sensitive channels sets up a regulatory feedback loop.
4. In darkness Ca2+ enters via the cyclic nucleotide- gated channels, inhibiting GC.
5. When cGMP is reduced by light, the channels close, less Ca2+ enters the cell, but continues to be extruded by a Na/Ca/K exchanger.
6. The resulting drop in Ca2+ concentration relieves the inhibition of GC so that more cGMP is synthesised to counteract the effect of excitation

Question 24

The basic processes of transduction and adaptation appear to be the same in rods and cones; however, with important quantitative differences. First, cones are ~___ less sensitive, than rods, and cannot detect single photons.

Answer 24

The basic processes of transduction and adaptation appear to be the same in rods and cones; however, with important quantitative differences. First, cones are ~50x less sensitive, than rods, and cannot detect single photons.

Question 25

cones are ~50x less sensitive, than rods, and _____ detect single photons.

Answer 25

cones are ~50x less sensitive, than rods, and cannot detect single photons.

Question 26

which is faster rods or cones?

Answer 26

cones are much faster

Question 27

a. single cone alone cannot tell anything about colour

Answer 27

True needs conparison

Question 28

Colour vision is therefore achieved by ....

Answer 28

Colour vision is therefore achieved by comparing the outputs of different cones.

Question 29

We can define the colour by ....

Answer 29

We can define the colour by the ratio of excitation in the three cone classes.

Question 30

The red opsin gene evolved by duplication of the green opsin gene ~\_\_\_ million years ago in Old World monkeys

Answer 30

The red opsin gene evolved by duplication of the green opsin gene ~35 million years ago in Old World monkeys

Question 31

is the red opsin gene near the grren one? why?

Answer 31

The red and green opsin genes thus share extensive homology (98% identical) and are adjacent to each other on the X-chromosome.

Question 32

are there any blue cones i nthe fovear

Answer 32

Loss of the blue pigment gene (tritanopia) is, in contrast, rather rare. Blue cones comprise only ~10% of cones, and are excluded from the foveola, resulting in foveal tritanopia.

Question 33

fill in gaps

Answer 33

Question 34

distribution of blue cones

Answer 34

Question 35

fat

Answer 35

mamba