What Role Do Cone Cells and Iodopsin Play in Colour Vision?

[Wrichik Basu]

As per Wikipedia, there are three types of cone cells. Quoting Wikipedia:

Humans normally have three types of cones. The first responds the most to light of long wavelengths, peaking at about 560 nm ; this type is sometimes designated L for long. The second type responds the most to light of medium-wavelength, peaking at 530 nm, and is abbreviated M for medium. The third type responds the most to short-wavelength light, peaking at 420 nm, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively, depending on the individual.

I searched a bit more, and found that there are three different types of iodopsin pigment in these cells. Quoting Wikipedia again:

In humans there are 3 different iodopsins (rhodopsin analogs) that contain the protein-pigment complexes photopsin I, II, and III.

The 3 types of iodopsins are called erythrolabe(photopsin I + retinal), chlorolabe(photopsin II + retinal), and cyanolabe(photopsin III + retinal).

These photopsins have absorption maxima for red ["erythr"-red] (photopsin I), green ["chlor"-green] (photopsin II), and bluish-violet light ["cyan"-bluish violet] (photopsin III).

Can you say what reaction occurs for each of the pigments when they are exposed to light of wavelength of the maxima?

 

[BillTre]

This is from the Wikipedia entry on photopsin:

Opsins are Gn-x protein-coupled receptors of the retinylidene protein family. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change in the protein that activates photopsin and promotes its binding to G protein transducin, which triggers a second messenger cascade.

These are all closely related protein-chromophore complexes. I would expect that their steps in transduction (changing the light signal into a neural signal) would be the same, but the protein differences would be involved in tuning the the light sensitivity to different wavelengths.

It should be noted that although the receptors have different maximums, their overall responses overlap.
Neural mechanisms further tune the neural response further down neural pathways (probably) through an opponent process (mutual inhibition).
This is what our brains are aware. We are not directly aware of the events in the retina.
In different animals, these interactions can occur in the retina, in the brain, or both.
The differences in the color we see are due to differences in the neural pathways activated not the detailed chemistry of the transduction process in the receptor cells.

Another wikipedia article (visual phototransduction) has more details on the process, including the cellular response.
The second messenger affects membrane currents in a non-intuitive manner.
There is normally (in the dark) an ongoing current across the membrane which releases a transmitter the inhibits downstream neurons. The second messagers inhibit this release of the inhibitory neurotransmitter, thereby activating the downstream cells. Some call this disinhibition.

 

[Wrichik Basu]

@BillTre Thanks, that was really helpful.

I have a question arising out of the second Wikipedia article you sighted:

The article clearly outlines all the steps involved in the production of the nerve signals. But it does not say anything about how the process differs for different photopsins. It should differ, otherwise all signals would look the same, and our brain would have no idea about colour. Can you throw some light on the different mechanisms for different photopsins? I need to know how the signals produced differ from one another.

 

[BillTre]

The article clearly outlines all the steps involved in the production of the nerve signals. But it does not say anything about how the process differs for different photopsins. It should differ, otherwise all signals would look the same, and our brain would have no idea about colour. Can you throw some light on the different mechanisms for different photopsins? I need to know how the signals produced differ from one another.

The difference in the signals for different colors (or other attributes) is maintained (and distinguished by the brain) because different individual neurons that carry the signals for the different colors. The different neurons then connect up appropriately to other areas of the brain so the signals are maintained as different information flows.
Neural connections in the brain can be very specific at a very detailed level.
Not all the details of how this is done in neural development are known at this time.

 

[Wrichik Basu]

The difference in the signals for different colors (or other attributes) is maintained (and distinguished by the brain) because different individual neurons that carry the signals for the different colors. The different neurons then connect up appropriately to other areas of the brain so the signals are maintained as different information flows.
Neural connections in the brain can be very specific at a very detailed level.
Not all the details of how this is done in neural development are known at this time.

Let me summarise what I have learned so far. Please correct me if I'm wrong, because all the knowledge has been derived from Wikipedia.

1. The retina has three different types of cone cells in addition to the rods. The three types have peaks at colours RGB, and they contain the photopigments Photopsin 1, Photopsin 2 and Photopsin 3 respectively, along with retinal.

2. In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarized at about -40 mV, leading to glutamate release which inhibits excitation of neurons.

The depolarization of the cell membrane in scotopic conditions opens voltage-gated calcium channels. An increased intracellular concentration of Ca2+ causes vesicles containing glutamate, a neurotransmitter, to merge with the cell membrane, therefore releasing glutamate into the synaptic cleft.

3. A lot of reactions occur when light falls on the retina. They're all summarised here. With all those reactions, the "signal" is generated, which is essentially the depolarization and repolarization of neurones.

4. The difference in colour occurs because there are separate neurones carrying information about colour to the brain. Other processes in the brain tell us about colour, but essentially a single neurone will carry signal of a single colour. Signals in the neurones do not vary for different colours; instead, separate neurones are excited for different colours. For colours other than RGB, more than one type of cone cell is excited, and separate neurones carry information to the brain. The brain then processes information and tells us about colours.

5. The story of intensity of light is related to this. The more intense the light, larger is the "signal" generated by cone cells for that wavelength. As per the summation principle, a "larger" signal gets higher priority at the synapse, and we get to understand the intensity of light.

 

[BillTre]

Yes @Wrichik Basu, that sounds correct to me.

There are also other channels of information for other kinds of visual information that gets extracted at various stages, like lines and circle-surround areas.
There are also non-image kinds of visual information (circadian rhythms (day-night timing)) which can be detected from different places in the body (retina, pineal gland, and/or parapineal glad.

 

[Wrichik Basu]

@BillTre Thanks again for the clarification.

I need to know one more thing. In 1967, George Wald received the Nobel Prize for his work in phototransduction, which is known as Wald's Visual Cycle today. Say, I want to work with cone cells or iodopsin. How do scientists work with them? How did Wald work with cone cells back in 1967? He also measured the potential in hyperpolarized state and depolarized stare. How can I do these today? In short, how do I work with cone cells and Photopsin? Do I extract them from the eye of a dead person?

 

[BillTre]

He also measured the potential in hyperpolarized state and depolarized stare. How can I do these today?

This would usually be done with microelectrodes. This would be Neurophysiology.
You would do some thing like dissect out the part of the animal containing the cells,
put it in a dish in some way that you keep it alive and well
have this set-up over an microscope, and
poke it with electrodes.
If you can see, it you can poke it, is a saying.
Alternatively, you could use a fluorescent voltage indicator (not as fast or detailed as electrodes).

Say, I want to work with cone cells or iodopsin. How do scientists with with them

Depends what you want to do:
They would take a prep like described above, or say an isolation of cells or cell parts in some way gives you a more pure prep for say biochemistry.
Nowadays, you could study these things with molecular biology and genetics (which involves whole organisms).

I'm not that familiar with his work, but this sounds like doing biochem/molecular biology on a prep of cell parts (you can isolate only the photorecptor part of the cell is isolated. There are methods that can do this. You can do EM (electrode microscopy), biochem, and physiology on these things.
Similar with genetics: mutations of different steps (molecular biology, physiology, sequence differences).

How can I do these today? In short, how do I work with cone cells and Photopsin? Do I extract them from the eye of a dead person?

Actually, you could get human material, if you got funding and worked through an eyebank, which would be where human tissue would come from.

I work for an eyebank now, which is the only reason I know this.
I would guess, more than 90% of the recoveries I do, are for use in treatments of human eye disease (mostly cornea recoveries, not whole eyes recoveries).
The rest are usually whole eye recoveries, mostly in the Portland area (head quarters), I mostly do Eugene and three smaller cities, not Portland, where they have a big lab, and can get really fresh material for complex tissue processing. I would expect there to be a higher percentage of eyes used for research in the Portland area, because the eyes would be able to get to the eyebank lab more quickly, from a more local area recovery. They some this out at headquarters, in great detail.
Big cities, with eyebanks located there, will be able to do this. Our Eyebank just opened a new branch in Boston, which is a big place for biomedical research, and probably access to human material.
They also have big medical schools, with lots of research, that could be making new human treatments and operations.

From the corneas, the Portland eyebank (Lions VisionGift) makes a product that can treat 2 corneas for every 1 cornea collected), the transplantation is a well worked out and simple operation. Cornea cells have a slow metabolic rate, are not vascularized and therefore don't depend on the circulation to stay alive.
They can be collected, still alive, 24 hours after a person dies, and transplanted, as live cells, to have a medical effect, on some patient.
In there normal state, these cells, make sheet over the back side of the cornea, and using pumps, control the amount the of fluid in the gel-like cornea, to keep it optically good. They don't need a lot of energy to just sit around a while so they can survive. Most of the other cells (with a higher metabolic rates) are not so lucky.
Most neurons would be dead in minutes or less.
So would larger structures like organs.
Therefore, organ transplants are based on getting organs from a small subset of pre-dead (very/terminally ill) patients, where:

• the family and doctors agree,
  
• legal consent received,
  
• there are no restrictions from the Medical Examiner (ME),
  
• there is some immediate research or medical need

Then the patient can be un-plugged, he dies, and some organ is extracted, put on ice, either taken to a lab or hospital.
The organ extraction, is done in an Operating Room (higher tech, in some ways, than most labs (not like Monte Python!)).
In other cases, the whole eye is collected, sent to headquarters, and the researcher's desired parts (like the retina or something) are taken out in some way for research.
So that is what is possible, in situations like that.
If you worked in a lab that was funded and able to take advantage of its services (had some project to do), you could get this kind of thing to study.

Working with non-human mammals, you could more easily get research material from a slaughter house.

Genetics would require working with organisms that could be easily kept alive and bred, in lots of copies (thousands of separately bred lines).
Such as, from not like human, to very much like human (as is commonly perceived IMHO):

• single cells
  
• a microscopic worm or fruit fly
  
• a little fish
  
• mice
  
• humans

Humans might involve sampling populations or medical studies.
Human research would be more close to medical work.
More close to humans, means you get a lot more ethical issues and limitations on what experiments/observations are possible.

Working with different animals (or other organisms), you have increasingly reduced limitations as you work with (approximately) with smaller, simpler, and more stupidly thought of animals (like: flies, microscopic worms, single cells)).
You have more research possibilities (for many reasons), with increasingly more freedom in what you can study with organisms, as you are further removed from human.
Little flies, microscopic worms, single cells are extremes organisms to study, with fewer such limits.
With these animals, you can make mutations, breed lines, make crosses to generate some genetically specific animal you want to study.
Ask very detailed, well controlled questions.

These let you ask a lot more detailed questions about things, but they would usually be further away from direct study of human biology.
Many pick different points on this continuum to work.