This New Brain Map Shows Every Nerve Cell in an Adult Male Fruit Fly's Central Nervous System. Here's Why That Matters

Researchers have mapped an adult male fruit fly’s brain and nerve cord—the equivalent of our spinal cord—in unprecedented detail, charting 166,700 nerve cells in the insect’s central nervous system and millions of connections between them. While the insect’s brain is only the size of a poppy seed, the new wiring diagram can provide gargantuan insights. It has already helped scientists trace neural paths between sensory inputs and behavioral outputs, explore how different genes affect brain circuits and look at sex differences between male and female flies.

It can even help researchers better understand the brain wiring in humans. The new central nervous system map and some of the discoveries it has led to were published in three studies in the journal Cell and another in the journal Current Biology on September 3. Need to know: Central versus peripheral nervous systems The central nervous system consists of the brain and spinal cord (or nerve cord in fruit flies).

The peripheral nervous system is the network of nerves branching from the spinal cord. They help you sense the world around you. That information gets relayed to the brain, which then integrates it and sends commands back out to the peripheral nervous system, resulting in actions like movement.

Fruit flies—a colloquial name for bugs that belong to the Drosophila genus—previously made headlines two years ago when researchers released a map of an adult female fly’s brain. With almost 140,000 nerve cells, or neurons, it represented the most complete brain diagram of any organism ever made. This “connectome” also included more than 54.5 million synapses, the connections between neurons that allow them to communicate with one another. (Each neuron can send branching projections to multiple cells.) Earlier this year, the female fruit fly map was expanded to include the nerve cord.

Now, scientists have done it again. Like with the female brain, the adult male insect’s central nervous system was cut into extremely thin slices that were then scanned with a powerful electron microscope, which uses a beam of electrons to magnify objects. An artificial intelligence-based computer algorithm then took the resulting millions of digital images and stitched them into a 3D replica.

It captured each neuron and more than 300 million synapses. Human experts then checked and cleaned up the A.I.’s work—which has been revolutionary for cellular mapping projects. The people will never go away, but the people will not need to scale with the number of neurons, which would be economically not feasible.” The new connectome has already enabled researchers to compare male and female fly brain circuitry, exploring differences between the sexes within one species, called sexual dimorphism.