Connectome

A connectome is a map of all the connections in a nervous system. Think of a city map, but with brain cells as the junctions and the connections between them as the streets.

How does it work?

Brain cells, also called neurons, talk to each other via contact points called synapses. A connectome records which cell is connected to which other cell, and how strong that connection is.

To make such a map, brain tissue is cut into extremely thin slices. Each slice is photographed with an electron microscope. Computers put those thousands of images back together into a three-dimensional model, tracing every cell from start to finish. This searching and tracing is nowadays done with the help of AI, because by hand it would take centuries.

The first complete connectome was that of a small worm with 302 brain cells. Researchers have also managed it for a fruit fly: the FlyWire project mapped the first complete connectome of an adult female fly in 2024, with some 140,000 brain cells. For a human brain, with around 86 billion cells, such a map does not exist yet.

Why it matters

A connectome shows how a brain is wired, and that raises new questions. Which cells drive which behaviour? How does the wiring differ between individuals? Researchers, for example, tracked all the brain cells in the head of a tiny worm at the same time for the first time. In males and hermaphrodites, nearly all sensory cells turned out to respond differently to smells and tastes, even though that difference does not show up in how the cells are connected to each other (link).

A connectome can also serve as a blueprint for a computer model. For example, the AI company Odyssey had a computer model of a fruit fly's brain move through an environment that its world model Odyssey-3 invents on the spot (link). This is a first demonstration, and researchers still want to find out how much of the movement really comes from the rebuilt brain.