After a decade, Scientists Unveiled the Fly’s Brain in Stunning Detail

After a decade, Scientists Unveiled the Fly’s Brain in Stunning Detail

Scientists have mapped the connections of 140,000 neurons in the brain of the fruit fly Drosophila melanogaster.

The fruit fly’s brain is smaller than a poppy seed, but in this tiny space it has enormous complexity. More than 140,000 neurons are wired together by more than 490 feet of wires, the length of four blue whales, spaced closely together.

Hundreds of scientists have described these connections in stunning detail in a series of papers published Wednesday in the journal Nature. The electrical circuitry will be a boon to researchers who have been studying the nervous system of the fly species Drosophila melanogaster for generations.

Previously, the tiny worm was the only adult animal whose brain had been completely reconstructed, with just 385 neurons in the entire nervous system. The new fly map is “the first time we have a complete map of any complex brain,” said Mala Murthy, a Princeton neuroscientist who helped lead the work.

Other researchers said that analyzing the neural connections in the fly’s brain would reveal principles applicable to other species, including humans, whose brains have 86 billion neurons.

In one of the new studies, scientists have solved the mystery of how sensory signals travel through the brain and prompt it to issue commands. They created a computer simulation of a fly’s entire brain. When presented with artificial taste sensations, the artificial brain gave signals to stick out its tongue.

Sebastian Sung, another project leader at Princeton, said the simulation reminded him of a longstanding hypothesis that “mind uploading” might allow us to transfer our brains into a computer.

“Mind uploading was science fiction, but now mind uploading – at least in the fly – is becoming accepted science,” Dr. Sung said.

The mapping began in 2013 when Davy Bock, a neuroscientist at the Janelia Research Campus of the Howard Hughes Medical Institute in Virginia, and his colleagues immersed the brain of an adult fly in a chemical bath, turning it into a solid block. They removed a very thin layer from the top of the block and took pictures of it with a microscope.

The scientists then cut off another layer and took another picture. To photograph the whole brain, they took 7,050 slices and about 21 million pictures.

Dr. Sung and his colleagues also developed software to interpret these images. They programmed computers to recognize cross-sections of neurons in each image and fold them into three-dimensional cell shapes.

It took scientists more than a decade to create the first high-resolution model of a fly’s brain. Based on the different neuron shapes, Gregory Jefferis, a neurobiologist at the University of Cambridge, and his colleagues classified the cells into 8,453 different types, compiling the largest catalog of cell types in any brain. (Scientists have identified only 3,300 cell types in the human brain.)

By tracing neurons on a map, Dr. Murthy and her colleagues gained insight into what these different cell types do.

Some types of neurons, for example, command walking flies to stop. The researchers found that one circuit stops flies by blocking commands to walk, while the other stops flies by strengthening the joints of their legs.

Philip Shiu and his colleagues at the University of California, Berkeley, used the map to create a virtual fly brain, with simulated neurons transmitting signals to connected cells.

Dr. Shiu’s team tested the simulated brain by seeing how it responded to food. The fly’s tongue-like proboscis is covered with sugar-sensitive neurons. The researchers activated them and watched as signals traveled through the fly’s brain.

The artificial brain did what a real brain would do: It commanded the proboscis to stick out so the fly could eat. And if the virtual fly tasted sugar only with the right side of its trunk, the brain sent commands to tilt it to the right.

Anita Devineni, a drosophila expert at Emory University who was not involved in the project, said she relied on the new resource to plan new experiments.

“We use it in everything we do,” she said.

Dr. Murthy and her colleagues hope to use the fly map to uncover fundamental rules of the complex brain, such as how the wiring of neurons allows signals to spread quickly throughout the brain. But they also recognize that the larger brain may not follow all the same rules.

Now the researchers are embarking on a much more ambitious map: a mouse brain that contains about 1,000 times more neurons than a fly.

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