Eye cell sets body clock

The eye contains three types of photoreceptor

Scientists have discovered a new type of light sensitive cell in the eye that appears to play an important role in setting the body's internal 24-hour clock.

The finding could explain how blind people still seem able to set their body clock to the rhythm of night and day.

The body's clock controls sleep and alertness, and even regulates body temperature.

Upsetting it is what prompts jet lag, the daytime lethargy and fatigue caused by quickly crossing many time zones.

It had been thought that only two types of cell - rods and cones - had the ability to turn light energy into electrical responses.

Now we have to rethink how the retina works and how the brain understands what is going on in the visual world

Dr David Berson

The new type of photoreceptor has been discovered by a team from Brown University, Rhode Island, US.

It is found deeper in the eye's retina than rods and cones and looks remarkably different, more like the underside of a canopy of twisted tree branches.

The scientists have dubbed the new cell "an intrinsically photosensitive ganglion cell".

It also turns light energy directly into brain signals.

The researchers believe it is these signals that govern the body's 24-hour clock.

Network of fibres

The cells send out nerve fibres which travel within the optic nerve and connect with the clock region in the brain.

Lead researcher Dr David Berson said: "We think this population of cells plays a role in setting the circadian clock and probably in a variety of other functions where all the brain needs to know is how bright it is.

"It is a visual system that runs parallel to the one we have been thinking about all these years.

"Now we have to rethink how the retina works and how the brain understands what is going on in the visual world."

The cells were found in rats, but the scientists believe there is a strong likelihood that humans carry exactly the same type of cell.

The scientists went looking for the cells in an effort to explain why some people who are functionally blind - whose rods and cones do not work - can still adjust their biological rhythms to match the day and night of the external world.

Dye-filled cells

The researchers injected a fluorescent dye into the tiny part of a rat's brain that governs the 24-hour clock cycle.

The dye traveled back to the new photoreceptors in the eye.

The researchers then found the dye-filled cells, recorded their electrical activity, and found that they continued responding to light whether or not they were connected to the retina or brain.

A photoreceptor is a cell in the eye that contains a chemical called a photopigment that changes its properties in response to light.

This change triggers a cascade of biochemical reactions and an electrical response in the photoreceptor. This signal then moves along a pathway between the cells and the brain.

The research is published in the journal Science.