For decades, biology textbooks have stated that eyes communicate with
the brain exclusively through one type of signaling pathway. But a new
discovery shows that some retinal neurons take a road less traveled.
New research,
led by Northwestern University, has found that a subset of retinal neurons
sends inhibitory signals to the brain. Before, researchers believed the eye
only sends excitatory signals. (Simply put: Excitatory signaling makes neurons
to fire more; inhibitory signaling makes neurons to fire less.)
The Northwestern
researchers also found that this subset of retinal neurons is involved in
subconscious behaviors, such as synchronization of circadian rhythms to
light/dark cycles and pupil constriction to intense bright lights. By better
understanding how these neurons function, researchers can explore new pathways
by which light influences our behavior.
“These
inhibitory signals prevent our circadian clock from resetting to dim light and
prevent pupil constriction in low light, both of which are adaptive for proper
vision and daily function,” said Northwestern’s Tiffany Schmidt, who led the
research. “We think that our results provide a mechanism for understanding why
our eye is so exquisitely sensitive to light, but our subconscious behaviors
are comparatively insensitive to light.”
The research was published in the May 1 issue of the journal Science.
Schmidt is an
assistant professor of neurobiology at Northwestern’s Weinberg College of Arts
and Sciences. Takuma Sonoda, a former Ph.D. student in the Northwestern
University Interdepartmental Neuroscience program, is the paper’s first author.
To conduct the
study, Schmidt and her team blocked the retinal neurons responsible for
inhibitory signaling in a mouse model. When this signal was blocked, dim light
was more effective at shifting the mice’s circadian rhythms.
“This suggests
that there is a signal from the eye that actively inhibits circadian rhythms
realignment when environmental light changes, which was unexpected,” Schmidt
said. “This makes some sense, however, because you do not want to adjust your
body’s entire clock for minor perturbations in the environmental light/dark
cycle, you only want this massive adjustment to take place if the change in
lighting is robust.”
Schmidt’s team
also found that, when the inhibitory signals from the eye were blocked, mice’s
pupils were much more sensitive to light.
“Our working
hypothesis is that this mechanism keeps pupils from constricting in very low
light,” Sonoda said. “This increases the amount of light hitting your retina,
and makes it easier to see in low light conditions. This mechanism explains, in
least part, why your pupils avoid constricting until bright light intensifies.”
Journal article: https://science.sciencemag.org/content/368/6490/527
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