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Nearly
all species have some ability to detect light. At least three types of
cells in the human retina allow us to see images or distinguish
between night and day. Now, researchers at the Johns Hopkins
School of Medicine have discovered yet another type of cell that can
sense light and contribute to vision.
In the journal Nature, the
team of Johns Hopkins
neuroscientists report their discovery that retinal horizontal
cells - nerve cells once thought only to talk to neighboring
nerve
cells - are light sensitive
themselves.
"This
is mind-boggling," said
neuroscientist King-Wai Yau. "For more than 100 years, it's been known
that rod cells and cone cells are responsible for sensing
light, and
therefore, vision.
"Then, about seven years ago, another light
sensor was discovered in
the retina, revealing a third type of light-sensitive cells
in
mammals, so we set out to look at whether this was true in other
vertebrates as well."
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Focusing
their efforts on the
melanopsin light sensor, which is responsible for sensing day and night
but barely involved -- in mammals, at least -- in seeing images, Yau's
team looked for melanopsin-containing cells in other
vertebrates, and
found some in the retinal horizontal cells in goldfish and catfish.
Catfish contain two types of retinal horizontal cells: those
that
connect to cone cells, which respond to bright light, and
those that
connect to rod cells, which respond to dim light. The team
took
electrical readings from single isolated retinal horizontal
cells.
They found that light caused a change in electrical current
in cone
horizontal cells but not in rod horizontal cells.
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Horizontal
cells,
says Yau,
allow cross-talk between neighboring photoreceptor cells, allowing
these cells to compare the light they sense, a process necessary for
the brain to see images.
"The brain processes what it sees in context to the surroundings. This
allows our brain to see borders and contours; horizontal
cells are the
reason why we can recognize and see a face, for example." |
Testing light at
different
wavelengths, the team found that the fish horizontal cells are
thousands of times less light sensitive than their partner cone cells.
"The bottom line is that the light effect on the horizontal
cells is
subtle, perhaps to allow the eyes of these animals to
fine-tune their
functions to different ambient light conditions. But that these
horizontal cells are light sensitive at all is a very surprising
finding and changes how we think about retinas as a whole."
Source: The Solomon H. Snyder Department of Neuroscience
at Johns Hopkins School of Medicine
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