Think of the smell of an orange, a lemon, and a grapefruit. Each has
strong acidic notes mixed with sweetness. And yet each fresh, bright
scent is distinguishable from its relatives. These fruits smell similar
because they share many chemical compounds. How, then does the brain
tell them apart? How does the brain remember a complex and often
overlapping chemical signature as a particular scent?
Researchers at Cold Spring Harbor Laboratory (CSHL) are using the fruit
fly to discover how the brain integrates multiple signals to identify
one unique smell. It's work that has broader implication for how flies -
and ultimately, people - learn. In work published in Nature Neuroscience,
a team led by Associate Professor Glenn Turner describes how a group of
neurons in the fruit fly brain recognize multiple individual chemicals
in combination in order to define, or remember, a single scent.
The olfactory system of a fruit fly begins at the equivalent of our
nose, where a series of neurons sense and respond to very specific
chemicals. These neurons pass their signal on to a group of cells called
projection neurons. Then the signal undergoes a transformation as it is
passed to a body of neurons in the fly brain called Kenyon cells.
Kenyon cells have multiple, extremely long protrusions that grasp the
projection neurons with a claw-like structure. Each Kenyon cell claw is
wrapped tightly around only one projection neuron, meaning that it
receives a signal from just one type of input. In addition to their
unique structure, Kenyon cells are also remarkable for their
selectivity. Because they're selective, they aren't often activated. Yet
little is known about what in fact makes them decide to fire a signal.
Turner and colleague Eyal Gruntman, who is lead author on their new
paper, used cutting-edge microscopy to explore the chemical response
profile for multiple claws on one Kenyon cell. They found that each
claw, even on a single Kenyon cell, responded to different chemicals.
Additional experiments using light to stimulate individual neurons (a
technique called optogenetics) revealed that single Kenyon cells were
only activated when several of their claws were simultaneously
stimulated, explaining why they so rarely fire. Taken together, this
work explains how individual Kenyon cells can integrate multiple signals
in the brain to "remember" the particular chemical mixture as a single,
distinct odor .
Turner will next try to determine "what controls which claws are
connected," which will provide insight into how the brain learns to
assign a specific mix of chemicals as defining a particular scent. But
beyond simple odor detection, the research has more general implications
for learning. For Turner, the question driving his work forward is:
what in the brain changes when you learn something?
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