The brain connections of a single insect no longer contain much uncharted territory.
Every nerve cell—and virtually every connection between them—in the brain of fruit fly larvae has now been mapped, researchers report in Science 10 March . This is the most complex circuit of the entire brain that has been created.
Previously, only three organisms—the sea squirt and two types of worms—had brain circuitry fully tuned to this resolution. But the brain of these creatures has only a few hundred neurons. Scientists conducting the new study wanted to understand a much more complex brain.
Fruit flies ( Drosophila melanogaster ) share a wide range of behaviors with humans, including the integration of sensory information and learning. Larvae perform almost all of the same actions as adult flies, except for a few, such as flying and mating, but have smaller brains, allowing for much faster data collection.
The idea for this project was born 12 years ago, says neuroscientist Marta Zlatik of the MRC Laboratory of Molecular Biology in Cambridge, England. At the time, she and her colleagues used an electron microscope to image the entire brain of fruit fly larvae. They then stitched these images together in a computer and manually traced each neuron to create a 3-D visualization of the cells. Finally, the team found the connections that carry information between cells, and even identified the sending and receiving ends.
The researchers identified more than 3,000 neurons and about 550,000 connections, known as synapses.
This three-dimensional visualization of fruit fly larval neurons was created from serial electron microscope images. The spheres represent each nerve cell body, and the elongated tails are the branches that send and receive information.
Neurons transmit information to each other in circuits. Examining patterns of neuronal connectivity—not just directly connected partners, but also connections between connected cells, etc.—revealed 93 different types of neurons. Classes were consistent with previous groups characterized by form and function. And nearly 75 percent of the neurons with the most connections were linked to the brain’s learning center, indicating the importance of animal learning.
The researchers hope that the work will serve as a blueprint for fellow scientists studying brain circuitry. “Now we have a reference map,” says Zlatik.