“You have less brain than a fly” is said as an insult, but the simplicity of this organ has allowed scientists to unravel the first complete map of this insect’s brain
The complete map of connections between neurons in a fly brain, called a connectome, took 12 years of painstaking work and shows the location of the 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 cells can send chemical messages to each other that in turn trigger electrical signals that run along the wiring between neurons.
Researchers have identified networks through which neurons on one side of the brain send data to the other, second study published in the journal Science. The team also classified 93 distinct types of neurons, which differ in their shape, proposed function and how they connect to other neurons.
The scientists cut the brain of a fly larva into 5,000 sections and took microscopic images of each section.
This study is the first to map the entire central brain of an insect. In 2020, another research group published a partial connectome from an adult fruit fly containing 25,000 neurons and 20 million synapses. But scientists only have complete connectomes for three other, simpler organisms: a nematode, a sea squirt larva, and a marine worm larva. According to Joshua Vogelstein, study co-author and director and co-founder of the NeuroData Laboratory at Johns Hopkins University, each of these connectomes contains a few hundred neurons and lacks the distinct cerebral hemispheres already seen in insects and mammals.
More than 80 people helped build the new connectome, Michael Winding, first author of the study and research associate in the Department of Zoology at the University of Cambridge, told Live Science via email. To do this, the scientists cut the brain of a fly larva into 5,000 sections and took microscopic images of each section. The images were stitched together to form a three-dimensional volume. The team then analyzed the images, identified the individual cells and manually traced their wiring.
He resulting map surprised researchers. For example, scientists tend to think that neurons send messages along long wires called axons and receive messages along shorter, more 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 account for about one-third of the synapses in the larval fly brain.
The connectome was also surprisingly “shallow,” meaning incoming sensory information passes through very few neurons before reaching one involved in motor control, which can direct the fly to perform physical behavior. To achieve this level of efficiency, the brain has built-in “shortcuts” between circuits that somewhat resemble those in the most advanced artificial intelligence systems.
One of the limitations 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. These dynamics affect the way information flows through the brain.
Still, the connectome opens the door to many future advances, such as more energy efficient artificial intelligence systems and a better understanding of how humans learnwhich in essence is not very different from what flies do.