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