How Light Sensors in Eye Work

Neuro-scientists have unravelled how newly discovered light sensors in the eye detect light and communicate with the brain.
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These light sensors are a small number of nerve cells in the retina that contain melanopsin molecules.

Unlike conventional light-sensing cells in the retina-rods and cones, melanopsin-containing cells are not used for seeing images.

Instead, they monitor light levels to adjust the body’s clock and control constriction of the pupils in the eye, among other functions.

“These melanopsin-containing cells are the only other known photoreceptor besides rods and cones in mammals, and the question is, how do they work,” said Michael Do, a postdoctoral fellow in neuro-science at Johns Hopkins.

“We want to understand some fundamental information, like their sensitivity to light and their communication to the brain,” he informed.

Using mice, the team first tested the light sensitivity of these cells by flashing light at the cells and recording the electrical current generated by one cell.

They found that these cells are very insensitive to light, in contrast to rods, which are very sensitive and therefore enable us to see in dim light at night, for example.

According to Do, the melanopsin-containing cells are less sensitive than cones, which are responsible for our vision in daylight.

“The next question was, what makes them so insensitive to light? Perhaps each photon they capture elicits a tiny electrical signal. Then there would have to be bright light-giving lots of captured photons for a signal large enough to influence the brain. Another possibility is that these cells capture photons poorly,” said Do.

To figure this out, the team flashed dim light at the cells. The light was so dim that, on average, only a single melanopsin molecule in each cell was activated by capturing a photon.

They found that each activated melanopsin molecule triggered a large electrical signal. Moreover, to their surprise, the cell transmits this single-photon signal all the way to the brain, said a Johns Hopkins release.

Yet the large signal generated by these cells seemed incongruous with their need for such bright light. “We thought maybe they need so much light because each cell might also contain very few melanopsin molecules, decreasing their ability to capture photons,” said King-Wai Yau, a professor of neuroscience at Hopkins.

When they did the calculations, the research team found that melanopsin molecules are 5,000 times sparser than other light-capturing molecules used for image-forming vision.

“It appears that these cells capture very little light. However, once captured, the light is very effective in producing a signal large enough to go straight to the brain,” said Yau.

Sources: The Telegraph (Kolkata, India)

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