BIO: We should celebrate the discoveries about biological clocks that won three researchers a Nobel prize. Their work gives us a window on how genes set our clocks, while jet lag hands us the result in personal experience. But what happens between genes and jet lag? The overall story starts with sleep and finishes with proteins under genetic control. That’s good, but what goes on in the brain?
With the help of an excellent diagram from the jet lag article, shown at bottom, let’s begin with retinal ganglion cells. You have been studying the ones that channel activity from the rods and cones, but there’s another kind that sense light directly. These photosensitive retinal ganglion cells send signals about day-night cycles to the hypothalamus along the retinohypothalamic tract, in particular to the suprachiasmatic nucleus or SCN. Daylight makes the SCN tell the pineal gland to stop making melatonin, which would put us to sleep. This light-entrained system works fine for most of us, although people who live near the poles rely more on the intrinsic gene-generated rhythms, because who wants to stay awake all summer and sleep all winter?
How do blind people maintain normal circadian rhythms? They have normal light-dark cycles because of a visual sense that we are not aware of consciously.
There are retinal ganglion cells that act as photoreceptors for our daily rhythms. They use a special pigment called melanopsin to absorb light. Instead of forming an image, they pass on information about the timing of light and dark to the suprachiasmatic nucleus in the hypothalamus along a phylogenetically ancient pathway. In other words, this stimulation probably doesn’t reach the lateral geniculate nucleus by the usual visual pathway (optic nerve, optic tract), yet in some people the information may eventually reach the visual cortex with rather baleful results.
The retina is mostly clear–transparent–except for blood vessels and the photopigments in the rods and cones, which light reaches only after it has already passed through most of the retina. So light can reach these ganglion cells almost undiminished. The ganglion cells also receive some input from rods and cones, but this influence is still not understood well.