Tags: Entrainment, Light therapy, Melanopsin, Melatonin, Retinal ganglion cells
It has long been known that light and dark affect the daily as well as seasonal rhythms of living things. Early in the 1900s it was assumed that humans’ daily rhythms were less affected than those of “lower” beings, but that attitude was proved wrong.
In the 1980s it was noted that some totally blind people entrained perfectly to the 24-hour cycle while many did not. Until some time in the 1990s, it was not known if entrainment occurred by light to the eyes or to the skin. One research team claimed to have proven that light to the backs of the knees effected entrainment, but neither they nor other researchers could duplicate those results, which later were withdrawn.
It is now clear that we and other animals entrain primarily by light to the eyes, though secondary cues such as social activity, feeding times etc. also play a role. It is also clear that our eyes contain not only rods and cones for vision but also the recently discovered light sensitive ganglion cells for the entrainment of circadian rhythms. The light sensitive pigment in these cells is melanopsin.
There it stands, but there are always new questions to be answered. One is:
Do the light sensitive ganglion cells contribute to vision,
and do the rods and/or cones contribute to
the non-visual effects of light?
If the ganglion cells’ photosensitivity contributes to vision at all, it is very minimally. But a study* published this month by well-known researchers suggests that the cones do affect entrainment, with variations dependent on the timing and intensity of the light. The practical implications include the question of whether the use of blue-blocking goggles in the evening, so-called “dark therapy”, really does allow normal flow of melatonin as intended. [An aside: mightn’t there be a more direct way to find this out?] In addition, the jury is still out on what color light – white, blue or perhaps green – is best to use in light therapy.
In this study in Boston, more than 50 human subjects each spent 9 days in an laboratory environment free of time cues. Semi-recumbent, totally confused about the time of day, and with their melatonin rhythms individually determined, half of them were exposed to blue (460nm) light and half to green (555nm) for 6.5 hours starting shortly after their own melatonin secretion started.
It is well-known that blue light (including about 460 to 482nm) excites melanopsin leading to the suppression of melatonin. The question here is whether the green light may have similar or equal effects. Green was chosen because the human visual system is most excitable at green (555nm).
In the figure, A and B are two subjects. Each person’s normal pattern of melatonin secretion is repeated on the left and on the right in black. On the left, blue light acutely suppresses A’s melatonin, while green light only delays B’s for a good hour. The next night, as we see on the right, the pattern of melatonin secretion has been phase-shifted by the previous night’s light exposure about equally for the two subjects (horizontal red line). From previous knowledge, one would have expected an appreciably greater delay after the blue light than the green.
The authors write:
“Our data … raise the possibility that activation of cone photoreceptors in the late evening by relatively low-illuminance light sources, such as liquid crystal display monitors, table lamps, and dimmable lamps, may delay the circadian clock and therefore contribute to the high prevalence of delayed sleep phase disorder.” [My emphasis.]
“[B]locking short-wavelength light with blue-blocking goggles may not always be effective in preventing undesired circadian responses based on our finding that longer-wavelength light is able to induce robust phase-shift responses.”
“Our findings have implications for the development and optimization of light therapies for a number of disorders, including circadian rhythm sleep disorders.…”
* Spectral Responses of the Human Circadian System
Depend on the Irradiance and Duration of Exposure to Light.
Joshua J. Gooley, Shantha M. W. Rajaratnam, George C. Brainard,
Richard E. Kronauer, Charles A. Czeisler and Steven W. Lockley
Science Translational Medicine, 12 May 2010
See also these older posts:
(posted by D )
Next post: #54. Take a Nap!
Tags: Melanopsin, Retina, Retinal ganglion cells, Retinohypothalamic tract, Rods and cones, SCN, Suprachiasmatic nuclei, Suprachiasmatic nucleus
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.
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)
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