Scientists have unveiled the first complete map of an insect’s brain.
The full map, called a connectome, took 12 years of painstaking work to construct and shows the location of 3,016 neurons in the brain of a fruit fly larva (Drosophila melanogaster). Between these brain cells are 548,000 connection points, or synapses, where the cells can send chemical messages to each other which, in turn, trigger electrical signals that travel through the cells’ wiring.
The researchers identified networks through which neurons on one side of the brain send data to the other, the team reported March 9 in the journal Science (opens in a new tab). The team also classified 93 distinct types of neurons, which differ in shape, proposed function and how they connect to other neurons.
The new connectome is notable for its comprehensiveness, experts told Live Science.
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“This study is the first to be able to map the entire central brain of an insect and thus characterize all the synaptic pathways of all neurons”, Nuno Macarico da Costa (opens in a new tab) And Casey Schneider-Mizell (opens in a new tab)members of the Seattle-based Allen Institute for Brain Science’s Neural Coding group, who were not involved in the initiative, told Live Science in a joint email.
In 2020, another research group published a partial connectome of an adult fruit fly (opens in a new tab) which contained 25,000 neurons and 20 million synapses. But scientists only have complete connectomes for three other organisms: a nematode, a larval ascidian and a larval sea worm. Each of these connectomes contains a few hundred neurons and lacks the distinct brain hemispheres seen in insects and mammals, the study’s co-lead author said. Joshua Vogelstein (opens in a new tab)director and co-founder of the NeuroData laboratory at Johns Hopkins University.
More than 80 people helped build the new connectome, study first author Michel Bobinage (opens in a new tab), a research associate in the University of Cambridge’s Department of Zoology, told Live Science in an email. To do this, the scientists thinly sliced a larval fly brain into 5,000 sections and took microscopic images of each slice. They reconstructed these images to form a 3D volume. The team then pored over the images, identified individual cells within them, and manually traced their threads.
The resulting map surprised scientists in several ways.
For example, scientists tend to think that neurons send outgoing messages through long wires called axons and receive messages through shorter branching wires called dendrites. However, there are exceptions to this rule, and it turns out that axon-to-axon, dendrite-to-dendrite, and dendrite-to-axon connections make up about a third of the synapses in the larval fly brain, Winding said.
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The connectome was also surprisingly “shallow”, meaning that incoming sensory information passes through very few neurons before being passed to the one involved in motor control, which can direct the fly to perform a physical behavior, Vogelstein said. To achieve this level of efficiency, the brain has built-in “shortcuts” between circuits that somewhat resemble those in advanced AI systems, Winding said.
One limitation of the connectome is that it doesn’t capture which neurons are excitatory, meaning they cause other neurons to fire, or inhibitory, meaning they make neurons less likely to fire, said Schneider-Mizell. These dynamics affect how information flows through the brain, he said.
Still, the connectome opens the door to many future advances, such as more energy-efficient AI systems and a better understanding of how humans learn, Vogelstein said.
“Humans do things like make decisions, learn, navigate the environment, eat,” he said. “And so do flies. And there’s good reason to think that the mechanisms that flies have to implement these kinds of cognitive functions also exist in humans.”