After 12 years of work, a huge team of researchers from the UK, US and Germany has completed the largest and most complex brain map yet, describing every neural connection in the brain of a fruit fly larva.
Although it falls far short of the size and complexity of a human brain, it still contains a respectable 548,000 connections between a total of 3,016 neurons.
The mapping identifies the different types of neurons and their pathways, including interactions between the two sides of the brain and between the brain and the ventral nerve cord. This brings scientists closer to understanding how the movement of signals from neuron to neuron leads to behavior and learning.
“If we want to understand who we are and how we think, part of it is to understand the mechanism of thinking,” says Joshua T. Vogelstein, biomedical engineer at Johns Hopkins University.
“And the key to that is knowing how neurons connect to each other.”
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To create this magnificent multifunctional map, dubbed the connectome, researchers have scanned thousands of slices of baby fruit fly brain with a high-resolution electron microscope. They then stitched the images together and added them to already collected data, meticulously labeling every single connection between neurons.
This includes both the cells that communicate with each other in each cerebral hemisphere and those that communicate between the two hemispheres, making it possible to study in depth the interactions throughout the brain.
The cerebral hemispheres have unique and important functions, but how they integrate and use information from both sides for complex behavior and cognition is not well understood.
“The way the brain’s circuitry is built affects the calculations that the brain can perform,” explains neuroscientist Marta Zlatic of the University of Cambridge.
“But up to this point we haven’t seen the structure of a brain other than the roundworm Caenorhabditis elegansthe tadpole of a lower chordate and the larva of a marine annelid, all of which have several hundred neurons.”
Recently, scientists have made significant strides in mapping the human brain and tracking neuronal activity in mice, but the focus has been on specific regions, and current technology is still not advanced enough to develop a connectome for larger animals like humans to finish.
However, Zlatic explains: “All brains are similar – they are all networks of interconnected neurons – and all brains of all species have many complex behaviors to perform: they all have to process sensory information, learn, choose actions, navigate their environment, choose food, conspecifics recognize, escape predators, etc.”
fruit flies (Drosophila melanogaster) are a popular scientific research model because of their easy-to-study characteristics, their complex but compact brains, and because they share many biological similarities with us humans.
In particular, it was found that the connection structures observed by the researchers are most repetitive among the incoming and outgoing neurons in the part of the brain that enables us to learn and to remember what we have learned.
They also found that some of the identified functions worked similarly to some machine learning computer networks.
“What we learned about the code for fruit flies will have implications for the code for humans,” says Vogelstein. “That’s what we want to understand – how to write a program that leads to a human brain network.”
As a next step, the team proposes to learn more about the neural structure involved in specific behavioral functions such as learning and decision-making, and to study the activity of the entire connectome while the insect is active.
The first attempt to map a brain was a 14-year study by C. elegans that started in the 1970s. It provided an incomplete map of the roundworm’s brain and eventually earned the scientists a Nobel Prize.
“It’s been 50 years and this is the first brain connectome. It’s a flag in the sand that we can do this,” says Vogelstein.
“Up until then, everything worked.”
The research was published in the journal Science.