Decoded: How we sense moonlight, daylight

New York : Neuroscientists have found that neurons in the retina of the eye share the job of sensing the ambient light level in our environment, enabling us to sense the overall degree of illumination.

The study revealed that in retina there are light-sensing neurons -- known as M1 ganglion cell photoreceptors -- which are specialised for "non-image" vision that is used to set our body clocks, regulate sleep and control hormone levels.
Although the cells appear visually indistinguishable from each other, they are tuned to respond to different light levels, and take turns signalling to the brain as these levels change.

As a result, the brain gets information about light intensity from the identities of the cells that are active, not just signal size, the researchers said.

"The bottom line is that nerve cells have more in their toolkit than we previously thought, and divide labour in ways we didn't expect," said Michael Do, from the F.M. Kirby Neurobiology Centre at Boston Children's Hospital. "Some cells signal vigorously in twilight and others in full daylight. Together, they cover a broad range of light intensities in the environment," added Elliott Milner, a doctoral student at Harvard University, in the paper reported in the journal Cell.

Further, as the light level goes up, a protein called melanopsin in the M1 cells captures more and more photons of light.
This causes the voltage across the cell membrane to become more positive, that is, it "depolarises".

As the voltage becomes more positive, the cell generates more electrical spikes (also known as action potentials), which are the signals that are sent to the brain.

However, in certain disorders like epilepsy, the cell loses its ability to fire spikes when the membrane voltage gets too positive.

This system may have evolved to help the brain distinguish light levels more precisely, based on which cells are "talking" and not only their general volume. It also may conserve energy, the researchers noted.