Sensory Transduction & Ion Channels: Photoreceptors

Question 1

Key receptor types

Answer 1

Photoreceptors
Mechanoreceptors
Thermoreceptors
Nociceptors
Chemoreceptors

Question 2

What is the Electrophysiology of cone photoreceptor at rest?

Answer 2

· Photoreceptor cells leak K+ all the time, producing a negative internal potential
· Resting membrane potential is -45mV, and these cells are more depolarised than normal nerve cells (-70mV) even at rest due to Na+ channels in the outer segment being open by default, allowing for the influx of Na+, and there is also glutamate being released from synaptic terminal
· Glutamate can act to depolarise of hyperpolarise the post-synaptic cell, depending on whether there is a metabotropic (inhibitory) or ionotropic (excitatory) receptor on that cell

Question 3

Why is a membrane potential of -45mV beneficial for cone photoreceptors?

Answer 3

This membrane potential is set up at an intermediate level (-45mV) so it can respond equally well to increases and decreases in brightness over a particular photoreceptor

Question 4

What is the Electrophysiology of cone photoreceptor in response to increased light?

Answer 4

· If the light striking the outer segment gets brighter, some of the Na+ channels close, causing the cells to become more negative inside (hyperpolarisation) as there is no more Na+ influx
· This prevents the release of glutamate from the synaptic terminals

Question 5

What is the Electrophysiology of cone photoreceptor in response to decreased light?

Answer 5

· If the region striking the outer segment gets darker, more Na+ channels open, allowing for the influx of Na+ and depolarising the cell
· More glutamate is released from synaptic terminal

Question 6

What is the response of a cone photoreceptor like to brief flashes of light of different intensities?

Answer 6

There is a peak in the response by a photoreceptor to brief flashes of light

This peak allows us to follow a rapidly released stimulus, because it is generated by a very rapid response to the light flash, followed by a rather rapid termination (rapid onset and offset)

Question 7

What allows for the photoreceptor Na+ influx at rest?

Answer 7

cGMP opens Na+ channels in outer segment, allowing for Na+ influx

Question 8

How many cGMP molecules are needed to open each Na+ channel?

Answer 8

2 cGMP to hold each Na+ channel

Question 9

What is the photopigment in photoreceptors?

Answer 9

Photoreceptors contain a photopigment made of a protein called opsin and a chromophore (light-sensitive molecule) called retinal (11-cis retinaldehyde) on the membrane disc in the outer segment.

Question 10

What is retinal (11-cis retinaldehyde) made of?

Answer 10

11-cis retinaldehyde (retinal) is made up of a carbon ring and a carbon tail. All of the carbon-carbon links that make the tail are in the trans-configuration except for one in the 11th location, which is in the cis-configuration. The cis-configuration is less stable than the trans-configuration.

Question 11

Explain Transduction- initiation of the light response.

Answer 11

When light strikes this photopigment:

1) The unstable 11-cis bond in retinal ruptures, and it reforms in the more stable trans-configuration, resulting in all-trans retinaldehyde
2) Opsin is now linked to all-trans retinaldehyde, which acts as an agonist and activates the photopigment. This acts as a G-protein coupled receptor
3) A single opsin can activate many G-proteins (transducins)
4) The ⍺ portion of the activated transducin dissociates from the β and 𝛾 subunits and activates an enzyme called cGMP phosphodiesterase (PDE6), which is responsible for destroying cGMP. This enzyme converts cGMP into GMP, reducing the concentration of cGMP
5) There is a fall in intracellular cGMP, and cGMP diffuses away from the Na+ channels on the membrane, closing the Na+ channels

Question 12

How is the response to a single photon amplified?

Answer 12

• Each opsin can activate many G-proteins (transducins)
• PDE6 has an extremely high rate of activity meaning there is a significant drop in cGMP levels for each photon

This amplification explains the rapid onset of the visual response

Question 13

Explain Transduction- termination of the light response.

Answer 13

We need to stop the activated photopigment from activating any more G-proteins (transducins).

1) Enzyme called rhodopsin kinase phosphorylates the opsin, which slows down the opsin, allowing a protein called arrestin to bind to it and inactivate the opsin
2) Retinal is then removed and converted to retinol
3) Consequentially, the activated photopigment is not produced, and therefore transducins (G-proteins) are not activated either
4) No activated transducins means cGMP phosphodiesterase is not activated either
5) Another enzyme called guanylate cyclase takes GTP and converts it into cGMP, restoring cGMP levels, which reopen the Na+ channelsa

Question 14

Describe the Control of cGMP levels.

Answer 14

The enzymes cGMP phosphodiesterase and guanylate cyclase are both very active even at rest, and light tips the balance one way or the other in terms of activity.

The 2 enzymes keep each other in balance and have a reciprocal control system

Question 15

After termination of the light response, how is the photoreceptor system reset back to electrophysiology at rest?

Answer 15

1) Dephosphorylation of the opsin
2) Removal of arrestin
3) Another molecule of 11-cis retinaldehyde attaches to the opsin, ready to respond to the next incoming photon

Question 16

How is the response to light graded?

Answer 16

Response to light is GRADED

• increasing intensity = more photons = increased hyperpolarisation
• but, this is only up to a point and the cell saturates

Just-noticeable difference (JND) and saturation are two needs that are in conflict. In order to be able to distinguish the difference in brightness/intensity, there needs to be a big difference in their response magnitude. The bigger the difference, the more quickly the cell will saturate.
-so, there has to be something that prevents this saturation, especially since we can see over an enormous range of light brightness

Question 17

How do we extend the range of a photoreceptor sensory system to prevent saturation?

Answer 17

One way you can extend the range of a photoreceptor sensory system is to have different photoreceptors to respond to different parts of the range (CONES vs RODS)

-Fractionation: different receptors with different ranges (or adaptation of same receptor)

Question 18

What is the Rods sensitivity to light?

Answer 18

Rods are very sensitive and are therefore absolutely key to seeing below light level (below twilight).

Question 19

What is the Cones sensitivity to light?

Answer 19

Cones are less sensitive to light and can therefore see over a huge range of illumination levels
-not sensitive enough to see below twilight level

Question 20

Where is there overlap in the range of illumination levels between cones and rods?

Answer 20

There is a little bit of overlap between rods and cones, and their range is round about twilight, but they mostly function separately.

Question 21

How do photoreceptors react when someone is put in a dark room?

Answer 21

Initially, because it they can’t see a thing and threshold is very high, sensitivity to light starts very low
-high intensity of light required for detection

Sensitivity to light increases very quickly and cones start to adapt (biochemical adaptation)
-lower intensity of light required for detection

Sensitivity carries on increasing for the next 5 minutes, and then plateus

Then, rods adapt to the dark and sensitivity to light increases again
-lower and lower intensity of light required for detection

Question 22

Why is cone adaptation essential?

Answer 22

It is the main mechanism that allows cones to see over a huge range of illuminations because they keep resetting their sensitivity

Question 23

Do rods have biochemical adaptation above twilight levels?

Answer 23

No

Question 24

Why do rods not have much biochemical adaptation above twilight levels?

Answer 24

As they are very sensitive, they have used up their photopigment in light, which is still being replaced, but not quickly enough to make up for how quickly it is being removed.

In the dark however (as seen in the picture), rods start to restore their photopigments again, albeit from 0 (hence why rod adaptation is longer than cone), but eventually their sensitivity passes that of the cones and they carry on building their stores for about 25 minutes. This is why you can see well in dim illumination.

Question 25

How do cones respond to a decrease in illumination?

Answer 25

If you turn your eyes and look at something that is black and keep your eyes fixed on that black object, it will reduce the amount of light striking the opsin:

• Reduce the amount of activation of the opsins/photopigment
• Reduce the activation of transducins
• Reduce the activation of cGMP phosphodiesterase
• cGMP levels build up which will open more Na+ channels
• Cell depolarises

Question 26

How do cones respond to a long-term reduction in illumination?

Answer 26

BIOCHEMICAL ADAPTATION

• lack of light hitting the photoreceptor will cause it to adapt. The way it adapts is via Ca2+ influx.
• Ca2+ that is entering when the cell is depolarised attaches to guanylate cyclase activating protein (GCAP) and interferes with it → guanylate cyclase activity slows down
• reduces cGMP binding to channels, meaning less channels are open
• Ca2+ also attaches via calmodulin to the Na+ channels, reducing their likelihood of being open

Therefore, the Na+ channels are more closed by the reduced cGMP activity and the Ca2+-calmodulin action, restoring membrane potential (-45mV) to resting level even though light levels remain low

Ca2+ also binds to recoverin, slowing down the activity of rhodopsin kinase- therefore each photopigment remains active longer, and activates more transducins, prolonging and increasing response to each photon

Question 27

What is the Cone response to reduced illumination and long-term reduction in illumination?

Answer 27

In summary, looking at a darker object depolarises the cell by opening the Na+ channels and causing depolarisation of the cell.

But, if you carry on looking at it (long-term reduction in illumination), Ca2+ closes these channels again (adaptation), returning the membrane potential back to what it was (-45mV), even though the light hasn’t changed.

Question 28

How is photopigment regenerated?

Answer 28

All-trans retinol that is produced at the end of the visual cycle is exported to the pigment epithelium which converts it to 11-cis retinal, sending it back to the photoreceptors.

There are a second set of cells called the Muller cells which does exactly the same thing, regenerates photopigment, however, it produces 11-cis retinol, and not 11-cis retinal.

Question 29

Why can’t Muller cells produce 11-cis retinal, but only 11-cis retinol?

Answer 29

Because only cones contain the enzymes that can convert 1–cis retinol to 11-cis retinal

Question 30

Why can’t rods cope with bright light levels?

Answer 30

Muller cells are taking all-trans retinol from both the rods and cones, converting it into 11-cis retinol, which only the cones can use and convert to 11-cis retinal.

This is quite possibly the reason why the rods can’t cope with bright light levels because they can’t regenerate the photopigment fast enough, unlike the cones which can because they can regenerate photopigment via both pigment epithelium and also Muller cells.

Question 31

What is the Structural difference between rods and cones?

Answer 31

Rods:

• long outer segments with densely stacked discs
• capable of capturing every photon passing along their length

Cones:
-outer segments are much shorter and contain less photopigment

Question 32

What are the Responses of rods and cones?

Answer 32

Rods
-sensitivity boosted by having a prolonged response. Their equivalent of rhodopsin kinase doesn’t stop the rod response as quickly as it does in the cones, so the activated rhodopsin lasts for much longer and therefore builds up a much bigger response per photon, but at the expense of very poor temporal sensitivity.

Cones:
-terminate the response very quickly. They can therefore follow a fast flicker, but at the expense of sensitivity.

Question 33

What is the Saturation in rods and cones?

Answer 33

Rods have limited adaptation and a restricted supply of 11-cis retinal, which is probably why their responses saturate and become non-functional at light levels > twilight.

Cones have very efficient adaptation and very rapid supply of 11-cis retinal (because Muller cells are regenerating photopigment too), therefore they can function without saturation over a huge range of light levels.