3,016 neurons and 548,000 synapses, the first insect brain map

Image credit: Eye of Science/Science Photo Library

Scientists have created the first complete map of the insect brain, including all neurons and synapses. This is a landmark achievement in understanding how the brain processes sensory information streams and translating them into action. The paper was published March 9 in Science.

Drosophila is an important model animal, and the brains of Drosophila melanogaster larvae are smaller than poppy seeds. This study provides the first to provide a map of brain wiring in Drosophila melanogaster, known as connectomesomes, depicting 3,016 neurons and 548,000 synapses in its larval brain.

“Now that we have a reference brain map, we can look at changes in connectivity in Alzheimer’s, Parkinson’s, and any degenerative disease model.” Co-author Marta Zlatic, a neuroscientist at the University of Cambridge in the United Kingdom, said.

So far, scientists have only mapped the connectomes of Caenorhabditis elegans, Dunaliella sand silkworms, and the larvae of the glass sea squirt.

Drosophila is an ideal model for connectome studies because scientists have sequenced their genomes and the bodies of the larvae are transparent. Fruit flies also exhibit complex behaviors, including learning, navigating, processing odors, and weighing the risks and benefits of an action.

“It’s manageable for current technology.” Chung-Chuang Lo, a computational neuroscientist at Tsinghua University in Taiwan, said.

“If it had been in the ’80s when work on Caenorhabditis elegans was still going on, it would have been impossible to ask Drosophila about this project.” Co-author Albert Cardona, a neuroscientist at the University of Cambridge, said.

The researchers spent a year and a half capturing brain images of a 6-hour-old fruit fly larvae with nanometer-resolution electron microscopy. They then used computer-assisted programs to pinpoint neurons and synapses and spent months manually examining them.

The researchers identified 3,016 neurons, 93 percent of which were paired with partner neurons in the other brain hemisphere. Most unpaired neurons are Kenyon cells, key neurons in the centers of learning and memory.

The researchers then traced the twisted connections of each neuron and annotated 548,000 synapses, which can be divided into 4 types. Kei Ito, a neuroscientist at the University of Cologne in Germany, said: “It’s really time-consuming and labor-intensive. ”

Most connectome research efforts include one type of connection — the dendrites of one neuron from another — while ignoring axon-to-axon or dendritic-to-dendritic connections. “Now we need to rethink them, so a new computational model of the nervous system should be created.” Lo said.

The brain wiring diagram of the study showed that the insect brain is multilayered, with pathways of different lengths connecting the brain’s input and output. “It’s a nice nested structure.” Co-author Michael Winding, a neuroscientist at the University of Cambridge in the UK, said.

But some brain networks have “shortcuts” that can skip a few layers. The authors believe that such shortcuts could improve the brain’s computing power and compensate for the limited number of neurons. They also found that 41 percent of brain neurons formed a “circulatory circuit” that provided feedback to upstream “partners.”

These shortcuts and loops are similar to state-of-the-art artificial neural networks used in AI research.

The current atlas only provides data for a single animal, and Lo says the next step is to map the brain of an adult fruit fly, which is more complex and has more neurons. The authors say advances in technology will allow for the eventual mapping of other species. (Source: Wang Fang, China Science News)

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