The answer lies in our understanding of the structures of these organs: We have detailed maps of many systems, such as the lungs, but we are far from having a comprehensive one of the brain. With detailed maps, we can detect when something is amiss; without them, we don’t even know where to start looking for issues. This foundational knowledge is crucial for understanding how the brain functions. This is why considerable effort is invested in mapping the brain. In neuroscience, this field is known as connectomics.
Connectomics seeks to chart the brain’s neural connections and understand their implications. Yet, the path to fully mapping the human brain is dauntingly complex. With approximately 86 billion neurons and 100 trillion synapses, making a complete map feels almost like science fiction. The technology we currently possess is simply not advanced enough to accomplish this within any foreseeable timeline. Recently, however, a significant milestone was achieved with the complete mapping of the brain of Drosophila melanogaster, the common fruit fly, an organism with “only” 140,000 neurons – 607,142 times fewer than those in the human brain. Even at this scale, the project took over a decade, suggesting that mapping the human brain could take centuries.
The breakthrough fruit fly connectome study, published in Nature, was a monumental interdisciplinary effort. Hundreds of researchers across neuroscience, computer engineering, and artificial intelligence collaborated to map the 140,000 neurons and 54 million synapses in the fruit fly brain. AI was essential, helping to label the vast number of components across 21 million captured images. Over 146 labs and 122 institutions worldwide participated, underscoring the scale and significance of this work.
This comprehensive wiring diagram not only illuminates the structure of the fruit fly’s brain but also provides insights relevant to understanding more complex brains, including our own. Connectomics aims to map all synapses and neural pathways and understand how they integrate across different brain regions. Scientists believe that there’s a direct relationship between brain structure and the behaviours it elicits; in other words, the arrangement and connections of neurons influence cognitive and behavioural functions. Mapping these connections brings us closer to unlocking the mechanisms behind brain function.
According to Professor Lucas, mapping the fruit fly is particularly exciting as it is the most complex connectome mapped to date. Previously, the only fully mapped adult brain belonged to a worm with just 385 neurons. The fruit fly, which shares 60% of its DNA with humans, performs several functions similar to ours. While this may seem modest compared to the 96% we share with chimpanzees, some of the shared DNA relates to learning, Down syndrome, and even jet lag. The hope is that understanding the fruit fly’s neural wiring will inform studies of the human brain.
However, like any scientific endeavour, connectomics has its limitations. Human brains are uniquely structured, meaning a connectome from one person will not necessarily match another’s. Therefore, the insights gathered from a single connectome cannot be universally applied carelessly. Professor Lucas emphasises that while the connectome reveals much about neural connections, it offers limited information on the nature of those connections. It cannot easily capture whether these connections are strong or weak or how they may change over time. He suggests viewing the connectome as a starting point, one that requires ongoing modifications and annotations as our understanding deepens.
The D. melanogaster connectome represents a major step in neuroscience, offering foundational insights that may one day enhance our understanding of the human brain. As researchers delve deeper into the brain’s intricacies, these efforts hold the promise of new approaches to treat neurological diseases. Connectomics stands at the intersection of basic research and clinical potential, aiming to transform our approach to the brain’s complexities and, ultimately, the treatment of brain disorders.
