Scientists map a fly’s entire brain
A team of scientists has just mapped every cell and every connection in the brain of an adult organism . That map is called a “connectome”, and the organism was a fruit fly .
A fruit fly is about 3 millimeters long and lives, on average , between forty and fifty days. Still , the detailed mapping of the fly’s brain could power a greater understanding of the human brain in the future.
To understand why, let’s have a quick biology lesson .
A neuron is a type of specialized cell . Neurons are the building blocks of our whole nervous system , including the brain. A synapse is a small gap between two neurons.
Here’s how a brain cell communicates with another: The first neuron releases a chemical , called a neurotransmitter , into the synapse—remember, that’s the empty space between cells. So a brain cell releases a chemical into the space next to it. The chemical then connects to a nearby neuron. This second neuron can then generate its own signal and release chemicals into the next synapse, forming a chain . The chain can be long or short, depending on where the message needs to go. That’s how information travels through your nervous system.
In the average human brain—listen to this—in the average human brain, there are about 86 billion neurons. Eighty-six billion cells. And there are 100 trillion synapses, on average. That means, on average, each of those 86 billion cells has a connection to over 1,000 nearby cells.
The connections between the neurons are an important part of why animals behave the way they do. If you touch a hot stove , the neurons connect your sensory receptors to your muscles , and you pull your hand away . If you take a bite of a mango , your neurons will tell your brain that’s it’s sweet and familiar; your brain may then tell your muscles to keep eating. But if your stomach is full, your brain may tell your muscles to slow down.
This is good. But our neurons and synapses also contribute to behavior that we don’t want: they can be part of the reason why some fall into addiction ; why some people’s memory fails them as they grow older; why some people develop tremors or have difficulty with their balance .
Understanding how all our 86 billion cells interact with each other is a long way off . But you have to start somewhere.
In 2019, scientists created a connectome—the brain map—of a very simple worm ; its brain has just 300 neurons. Then, in early 2023, scientists published the connectome of a larval fruit fly—this is a fruit fly in its very early days. It has about 3,000 neurons. Next, came a connectome of a region of an adult fruit fly’s brain, with 27,000 neurons.
You see where this is going. So now, in 2024, a different team of scientists has mapped the connectome of an adult fruit fly with 140,000 neurons and 55 million connections. The size of these brain maps is getting bigger and bigger.
A fruit fly might not seem like a complex organism; you might think there isn’t much we can learn from something so small. But don’t underestimate a fruit fly! The fly’s brain might be small, but it processes sound , smell , touch , and vision . It uses that information to search for food, to learn about and remember its surroundings , to seduce a romantic partner , to avoid predators , and to warn others of threats .
In other words, fruit flies do a lot of the things that we do, albeit on a smaller scale . So studying the fruit fly’s brain can still reveal basic principles of how organisms make decisions and move through the world.
Now that a full connectome has been developed, the process can be scaled up to study larger animals. A mouse’s brain is about 500 times bigger than a fruit fly’s, and a mouse is closer to humans in the evolutionary family tree . Understanding how a mouse’s brain works would allow researchers to make stronger hypotheses about how human brains work. This work is already underway . Several scientific teams are starting to build out a connectome of a mouse, piece by piece .
This is long, hard, and expensive work. But if other, similar projects are any guide, the cost and time required to map sections of a brain will only decrease with time.
Jeff’s take
If you’ve heard of the Human Genome Project—the connectome is like another piece to the puzzle for what makes us human . Our genes are unchanging parts of our DNA and they affect things like our appearance , body structure , susceptibility to certain diseases . They contribute to our personality . The Human Genome Project was a map of our genes, which we are born with.
A connectome is a map of a brain, which is partially the result of genes, and also partially the result of experiences. Put these two together and we’ll have a much better understanding of what it means to be alive.
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