Scientists have traced all 54.5 million connections in a fruit fly’s brain
By tracing every single connection between nerve cells in a single fruit fly’s brain, scientists have created the “connectome,” a tool that could help reveal how brains work.
At some point of the brain of a unique fruit fly, nerve cells weave themselves together, enabling flight, mating, eating, sound asleep and every other activity of her fly life. Now, in nine papers published October 2 in Nature, scientists report the first complete map of her nerve cells — all 139,255 of them, to be exact — and their fifty four.5 million connections.
This complete-brain map, traced over years with painstaking precision, is tiny but exquisite: It holds 149.2 meters of neural wiring, all tidily packed into a brain concerning the scale of a poppy seed. As such, this map shows how neural information may float among cells in Drosophila melanogaster, an animal that’s simpler than a human but complex enough to remain mysterious to people seeking to grab its brain.
“This work is completely fascinating,” says neuroscientist Olaf Sporns of Indiana University in Bloomington. Back in 2005, he and his colleagues coined the term “connectome,” an accounting of the connections between nerve cells, or neurons (SN: 2/7/14). At some point of the nearly two decades since then, scientist have mapped more connectomes, including those of male and hermaphrodite C. elegans worms, a larval fruit fly, small bits of mouse and human brains, and element of an adult fruit fly’s brain (SN: Three/9/23; SN: Eight/7/19; SN: 5/23/24). This most up-to-date fruit fly connectome is the biggest of its sort.
“When connectomics first got started, making a map like the one presented at some point of this work appeared just like science fiction,” Sporns says. “And now, amazingly, here it's a ways.”
The project involved electron microscopy images of greater than 7,000 thin slices of a female fruit fly’s brain and machine learning that aligned the complex tendrils of neurons, tracing cells through different slices. Machine learning got the researchers within striking distance of your complete connectome. “But humans are still required to correct the errors,” says Sven Dorkenwald, a computational neuroscientist who worked on the project at Princeton University and who is now on the Allen Institute for Brain Science and the University of Washington in Seattle. Hundreds of individuals from greater than 50 laboratories proofread the map with human eyes, making certain that cells’ shapes were as they . It become a significant job, from start to finish.
“Did we think it become going to take this long, like, almost two decades later we'd have the fly connectome? Probably now now not,” says Sebastian Seung, a computational neuroscientist at Princeton University. “But overly optimistic people drive progress.”
At some point of the early days, working on a connectome map “become a contrarian thing to do,” Seung says. “Most people will thought it become crazy. There were two objections. One is that it's a ways now now not it's possible you may, and the second is that even when you were successful, the information may well be useless.”
But already, the information have proven their utility, revealing cellular details and juicy hints about how brains work. For example, there are simplest two CT1 neurons in your complete fly brain, every of which is involved with sensing changes in light and motion. Every neuron stretches across a whole eye and makes a major collection of synapses — greater than 148,000, the map shows.
The other analysis sorted some neurons into classes normally often often referred to as “integrators,” which receive a tremendous collection of messages from other cells, or “broadcasters,” which send signals to a tremendous target market. These megaphone cells will help signals spread, but in selective ways.
And with the connectome now mapped, scientists have begun to construct computer models of how information flows in the brain. “You start with the connections between neurons, and you utilize that to enable you build a simulation of a network,” Seung says. “It’s an extremely obvious approach but you couldn’t do it when you didn’t have the connectome.”
One new study, as an instance, shows how taste neurons can activate other downstream cells. And that’s just the starting place, Seung says. “My joke for the science fiction enthusiasts is that one fly did be sacrificed for this experiment, but this fly may most probably reside forever in simulation.”
Sporns also looks to the future: “I foresee a future where connectome maps will change into much more comprehensive and detailed, soon to consist of brains of vertebrates like mouse and human,” he says. Those maps may help answer big questions about brain connectomes — whether or not they’re variable among individuals, if they alter over time, and whether or not they are going to help predict behaviors.
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