Summary of article from the San Francisco Chronicle
February 8, 2002
"Secret to Body's 24-Hour Time Clock is In the Eye"
by Carl T. Hall
A new light sensing cell has been discovered in the eye that keeps the body's 24-hour clock, or circadian rhythm, on track. New research is beginning to elucidate how the body detects how much light is in the environment.
The new class of retinal photoreceptors contain spidery cells with photopigment in them, known as melanopsin. These cells only encode the amount of light entering the eye, no other information. This so-called circadian light-relay system is geared to the subtle, slow changing rhythms of the sun and seasons, unlike the visual system which is sensitive to even very dim light.
Recent research came about because it was discovered in the mid-1990's that blind people could still adjust their circadian clocks with a shift in day-night cycles.
Melanopsin was first discovered in the photosensitive skin of amphibians and was later discovered in the eyes of mammals. Direct evidence for this parallel system of light detection is still not available for humans, but the human body does produce the melanopsin protein.
The signals from these light-sensing cells go into the suprachiasmatic nucleus (SCN) region of the brain. The SCN is known to help regulate mood and alertness. The sensors alert the SCN about how much light is available in the environment, which in turn affects sleep, wakefuleness, body temperature, and hormone levels. The SCN then synchronizes the rhythms of these physiological variables with the rhythms of day and night. The same nerve cells may also affect the pupil of the eye, widening or narrowing it depending on the amount of light available.
Although this is a very exciting area of research, little is still known about this photosensing pathway. Future studies are currently in the works to see what happens in lab animals that lack the melanopsin gene.
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