With this brain map we are one step closer to total fruit fly simulation

by Ana Lopez

In a way, fruit flies are just like us. They have eyes, legs, nervous system and they like fruits. However, unlike us, they only have a few thousand neurons in their brains, which means that scientists can map not only all cells, but also all the connections between them. , if you think about it, actually a human being.

Perhaps I’m exaggerating our similarities to fruit flies, commonly referred to by their scientific name, Drosophila (melanogaster, though that part isn’t usually necessary), but there’s a reason we use them in many biological experiments. You may not think you look much like one of these creatures, but you definitely look more like a fruit fly than a bacteria or dinoflagellate. Understanding even a relatively simple animal like Drosophila teaches us a lot about animals and life in general.

Despite being perhaps the best-understood organisms along with yeast, a single drosophila is still orders of magnitude too complex to simulate every aspect of. Hell, we’re having trouble simulating a single cell properly. However, if you think of a creature not as a gestalt, but as a collection of interrelated systems, you can start taking a bite out of the elephant.

The most recent bite, from a team led by Cambridge University biologists, is a “synapse-by-synapse map” of a larval drosophila brain. With 3,016 neurons and 548,000 synapses, it is 10 times the complexity of the last organism whose brain has been mapped, a member of Congress. (Actually, it was one of the worst kinds of worms, a ringworm. Humans have about 86 billion neurons and almost countless synapses.)

The fruit fly larva is not a fly, of course, but it is already an advanced creature, with adaptive behavior, structures analogous to adult fly brains, short- and long-term memory, and other expected brain functions. In addition, they are easier to catch. More importantly, it has “a compact brain with several thousand neurons that can be imaged at the nanoscale by electron microscopy (EM) and the circuits can be reconstructed in a reasonable time frame”, as the article published today in Science states. In other words, it’s the right size and not too weird.

The brain was sliced ​​into incredibly thin layers and imaged via EM, and the resulting slices were carefully examined to see how neurons and axons and other cellular structures progressed between them. “We developed an algorithm to track brain-wide signal propagation across polysynaptic pathways and analyze feedforward (from sensory to output) and feedback pathways, multisensory integration, and interhemisphere interactions,” they write.

Serial section electron microscopy volume revealing Drosophila brain structure. Image Credits: Michael Winding

The result is the model you see, it looks like a snail with a clown’s wig on (I need not add that this is not what it looks like in vivo).

Of course, there are a lot of interesting observations about the way the brain is organized, from nested recurrent loops, multisensory integration, hemisphere interactions and all that good stuff. But having a complete connectome of a complex living thing is fundamentally exciting for everyone in that space – there’s a lot you can do if you have a decent simulation of a brain. While previous studies have replicated individual subsystems or smaller brains, this is the largest and most complete characterization to date and as a 3D digital resource it will almost certainly be used and cited across the discipline.

Some of these things are even found in artificial neural networks; studying how such complex behaviors are produced by such a sparsely populated brain could “perhaps inspire new machine learning architectures”.

Interestingly, we already have a detailed mechanical model of the adult fly’s body and movements, and while the question is obvious, the answer is no: we can’t put this brain inside that body and say we have the whole body have simulated. thing. But maybe next year.

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