The fruit fly’s complete brain wiring map provides insight into the human brain: shot

Scientists have created the first detailed wiring diagram of an insect brain.

The brain, from a fruit fly larva, contained 3,016 neurons connected by 548,000 synapses, the team reported Thursday in the journal Science.

Previous wiring diagrams, known as connectomes, were limited to worms and tadpoles with only a few hundred neurons and a few thousand synaptic connections.

The fruit fly larva’s connection is an important advance because in many ways it is “closer to a human brain than the others,” said Joshua Vogelstein, study author and associate professor of biomedical engineering at Johns Hopkins University.

For example, “there are regions that correspond to decision-making, there are regions that correspond to learning, there are regions that correspond to navigation,” Vogelstein says.

But the challenges scientists faced in creating the fruit fly larva’s connectome show how far they still have to go to map a human brain, which contains more than 80 billion neurons and hundreds of trillions of synapses.

“The brain is the physical object that makes us who we are”

Researchers have focused on connectomes because a brain is so much more than just a collection of neurons.

“The brain is the physical object that makes us who we are, Vogelstein says. And to fully understand that object, he says, you need to know how it’s wired.

Mapping the complete human connectome is still many years away. So in the meantime, the researchers hope that this new wiring map of the fruit fly can provide clues about how all brains, for example, learn and remember and control an animal’s behavior.

The brain of a fruit fly larva, like a human brain, has a right and a left side. But when researchers mapped the connections in the insect brain, “it was a surprise where one’s right and left sides are,” Vogelstein says.

In humans, the right and left sides of the brain can have very different wiring. Circuits involved in speech tend to be on the left, for example, while circuits that recognize faces tend to be on the right.

A “landmark first reference”

The new map will help scientists study how learning changes the brain, how brain wiring differs by sex, and how wiring changes during an animal’s development.

“This is the landmark first reference that we can use to compare everything else,” says Vogelstein.

This complete map of neural connections took a large team more than a decade to complete and involved painstaking science.

The team began by cutting a single tiny brain, the size of a grain of salt, into thousands of very thin sections.

“You don’t screw it up at all, because if you make a mistake, you pretty much have to throw the whole brain out and start over,” says Vogelstein.

The team used an electron microscope to take a picture of each disc. Tracing the connections from one neuron to another required powerful computers and specialized computational tools.

These tools are enough to track millions of connections, Vogelstein explains, but not the trillions of connections found in a human brain.

So researchers at the Allen Institute in Seattle are working on an easier next goal: mapping the connection to a mouse. And even that is a huge challenge, says Nuno Maçarico da Costa, an associate investigator at the Allen Institute in Seattle, who was not involved in the study of fruit fly larvae.

“We started by trying to map the connectivity of a millimeter cube of mouse cortex, which is a kind of grain of sand, but which has a billion connections – 100,000 neurons and 4 kilometers of cable,” says da Costa.

It took 12 days just to cut open the one small cube, which represents only about one five-hundredth of a complete mouse brain, he says.

Despite the difficulty, mapping more complex brains is worth the effort, da Costa says, because it could ultimately help scientists understand how a human brain might be affected by disorders like schizophrenia.

“If your radio breaks,” says da Costa, “if someone has a wiring diagram of your radio, they’ll be in a better position to fix it.”

A human connectome will also help scientists answer some fundamental questions, like how we learn and why we behave the way we do, he says.

“Every idea, every memory, every movement, every decision you’ve ever made comes from the activity of neurons in your brain,” says da Costa. “And this activity is an expression of this structure.”