xliv. Rods and cones and the “new” ipRGC

30 July 2009 at 20:16 | Posted in Circadian rhythm | 9 Comments
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We learned in middle school that there are two, and only two, types of light sensitive cells in the retina, rods and cones, right?   Right, that’s what we learned.  Could be the science teachers are still saying that, since the third type was discovered within the last decade. 

Mammalian retina

Mammalian retina

The mammalian retina consists of many layers.  One might think that light would first strike the rods and cones, the photosensitive cells we use for vision.  But our retina is “inside out” compared to the more logical layout found in the octopus and its relatives; light in our eyes must travel through the many retinal layers to reach our rods and cones.  

One of the first layers the light reaches is composed of the one and a half million ganglion cells, most of which are involved in processing visual (image forming) information.  Fewer than 25000, some say just a couple thousand, of these cells are themselves sensitive to light. They function as light meters and they function much more slowly than the rods and cones, not registering abrupt fluctuations in light intensity.  These cells project their axons to several brain centers including the suprachiasmatic nuclei, SCN, the “body clock” through the retinohypothalamic tract.  They thus provide the major clue for the adjustment of the body clock.  The incoming information about light intensity is also used to adjust pupil size (narrowing of pupils in bright light) and to regulate physical activity and melatonin synthesis. 

Newly discovered, they are called by many names:

  • intrinsically photosensitive Retinal Ganglion Cells (ipRGC, also pRGC)
  • photosensitive ganglion cells
  • melanopsin-containing retinal ganglion cells
  • melanopsin-expressing retinal ganglion cells (mRGC)

 Melanopsin jpg

Late in the previous century, scientists weren’t sure that there existed ipRGCs, and those who thought that they do exist were arguing about what opsin, what pigment, they use.  Is it  melanopsin or one of the cryptochromes, which also respond to blue light?  One argument against melanopsin was that it resembles invertebrate opsins and differs from other opsin photopigments found in vertebrates.  

Again, as with our hormone melatonin, it was research on specialized light-sensitive cells of frog skin which provided answers.  

It has been known for a while that even when vision is lost, the light-sensitive ganglion cells may function perfectly.  Recent research on mice at Salk Institute shows that the opposite also is true.  A way was found to knock out the ipRGCs while leaving the rods and cones alone.  The mice became arrhythmic, but still could see. 

One of the researchers speculates:  “It is entirely possible that in many older people a loss of this light sensor is not associated with a loss of vision, but instead may lead to difficulty falling asleep at the right time.”

Update:  I’ve just discovered a wonderful post, Why can’t human eyes detect all wavelengths?, on the blog of Xenophilius Lovegood (!?).  Xeno, claiming to be “a slightly mad scientist”, explains the physical / chemical / electrical changes in the rods and cones as they react to light.  He also has a bit about the ipRGCs.  Recommended.


Next post:  xlv.  Some helpful links



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  1. Hi. This is a great blog! I work at the Laboratory of Human Chronobiology at Weill Cornell Medical College with Scott Campbell and Patricia Murphy. We are conducting an NIH-funded study of the physiological, behavioral, and molecular bases of DSPS in collaboration with Mike Young of Rockefeller University. If you or anyone else with DSPS would be interested in participating in the study, or for more information about it, please give us a call at (914)997-5825. The study involves living continuously in an apartment in our lab for 20 consecutive days and nights in the absence of time cues (free-run protocol) but participants sleep on their own schedules for most of the study (ie., not forced desychrony). Thanks!

  2. Hello Melanie, and thanks for the kind words. As you can see, I’m not looking to be the most popular blog, but I’d really like to reach those with a need to know! Getting the diagnosis was such a revelation that I was “high” for six months. So many things about me were explained.

    I’ll write you privately about participation in the study and I’ll copy your comment to the niteowl list. Several people have participated “long distance” in a study at Northwestern and another at San Diego. We’ve also just heard from a Brian Curtis at the UofUtah working with Christopher R. Jones and looking for people with severe ASPS & DSPS who have family members with same. We’re always delighted to hear about studies which may lead to help for people like us down the road. Of course just getting information about circadian rhythm disorders “out there” is helpful in itself.

    (And we often send off rants when we see advice saying that normalizing by way of nothing more than sleep hygiene is just so easy. Aaargh.)

  3. I would love to live in a room for 20 days where nobody bothered me, nobody criticized me, and I was just allowed to live, and sleep. Where can I find out more about the room and who would qualify, and what I could bring with me into the room, how big it is….etc? I need to exercise a lot to stay sane. Would I be able to flip the bed over to do chin-ups and sit-ups, etc. I have medication, I suppose that would need to be considered……

    Are you serious? You are looking for “volunteers?” What kind of paper work would I need from my current sleep doctor(s) etc.?

    That sounds like a freaking vacation to me!

    I am eating right now so that I can take medicine to force myself asleep, and will pray i make it to work functioning, like I do every day….etc, etc. Have to make a car payment because I “forgot.” Need to get off of here so I can try to explain myself a bit to my “girlfriend of the quarter-year,” etc.

  4. Ok…I just commented on something from Aug. 11th 2009. How appropriate is THAT! I am laughing. Oh, my…OK. Funny. =)

  5. No problem being “late”, John. I consider all my posts to always be current and interesting 🙂
    Such studies as mentioned in a comment above are going on all the time at various places. Many of them only accept unmedicated, non-smoking, very healthy young men who haven’t crossed more than one timezone the last several months. Such subjects are easiest to compare, and on a campus they are easy to find. But occasionally they want to study people who have DSPS / Non-24. There are various study protocols, and some sound more bothersome than others (like being kept awake, semi-reclining in very low light for 56 hours, blood samples taken every 30 minutes — and things like that…).
    I, too, have fantasized about spending 3 weeks or so living alone, all expenses paid, if their testing isn’t too cumbersome! Just give me a minute to talk a library into lending me a couple dozen books all at once.

  6. […] Some more links: Blindfolding the blind and Rods and cones and the “new” ipRGC […]

  7. […] a much simpler explanation of ipRGCs (the third type of light sensitive cell in the […]

  8. Hi. Great blog you have here, with lots of good information. I am doing something similar, but on more subjects, therefore more superficial. I am looking for a diagram of the retina and yours is great. Is it copyrighted and/or may I use it — with credit given, of course.

  9. My bad conscience: using illustrations without attribution. You’ve sent me on (an interesting) hunt for that image, and I’ve found it! When I started studying circadian rhythms, the most thorough – while understandable – texts I found were by Bora Zivkovic aka “Coturnix“. While trying to finish up his PhD, Bora blogged prolifically on the subject. (In his earliest texts in English, it was easy to see that he is an immigrant to the US. He learned fast!) One of his blogs, likely the first one, is called “Circadiana” and the image you ask about appeared in his “Clock Tutorial #7” of 6th February 2005. Thanks for making me search for it!

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