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


ix. Melatonin and the effect of light

26 November 2005 at 16:49 | Posted in Circadian rhythm | Leave a comment
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Oops, sorry.  Got a little ahead of myself there.  Melatonin should have been explained before that more technical stuff.
Melatonin is a hormone produced and secreted by the pineal, a tiny, midbrain gland which doesn’t seem to have any other tasks.
The pineal takes its orders from a nearby organ which Mary Poppins surely should have sung about, the suprachiasmatic nuclei.  However you choose to pronounce that, it’s a whole dance tune in itself.  The SCN (for short) is our central biological clock.  If totally isolated from outside cues about time, it will keep running on its own “almost a day” (= circadian) cycles virtually indefinitely.  Cells from mammalian SCN do so, in fact, in a dish.  
The body clock needs to be reset daily.  The SCN receives (or receive, if you prefer) information from the retina directly along a pathway called the retino-hypothalamic tract.  And yes, the SCN is located in the hypothalamus, in the brain. 
The vital information which the SCN processes, is sent to it by special light-sensitive cells in the retina in the back of the eye.  This information about light received at the retina has nothing to do with vision.  It tells our system when day and night are.  There can be other cues as well, such as mealtimes and activities, but light is by far the strongest.  
The rhythms of core body temperature (CBT) and melatonin secretion, at the least, need to be coordinated with each other and with the cycle of light/dark to allow us to fill our need for one long sleep-session, preferably at night.  
The level of melatonin in the blood during the (subjective) day is (or should be) near zero.  As CBT falls during an individual’s night, then starts rising again about two hours before wake-up time, our melatonin level rises during the first half of the night, to drop off during the second.  However, the two curves are not mirror images of one another.  The melatonin level rises quite sharply and then stays on a plateau.  It starts falling a couple of hours before the CBT bottoms out.  Here’s how this is shown in post no. xiii:  


Bright light banishes melatonin from the blood and stops / delays the secretion of it.  

A person with a normal circadian system who cooperates with nature’s signals will sleep at the same time every night.  The schedule might look like this:

  • 8 or 9 p.m., melatonin secretion starts, perhaps not measurable yet (DLMO, Dim Light Melatonin Onset)
  • 10 p.m., calming down, lights are dim
  • 10:30, feet are hot, person feels sleepy
  • 11 p.m., asleep
  • 3 a.m., melatonin level starts receding
  • 5 a.m., core body temperature minimum
  • 7 a.m., wake up, and light exposure banishes the remainder of the melatonin 

We all know people who’d rather sleep 10 p.m. to 6 a.m. or midnight to 8 a.m.  Those schedules are within the normal range.  

When I was a kid, medicine and science thought that the normal range should be achievable for everyone.  It is not.


Next:  x. Other animals


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