C. elegans connectome

A connectome is a complete wiring diagram of a nervous system: a map that names every neuron and draws every connection between them. The C. elegans connectome is that map for a tiny soil roundworm called Caenorhabditis elegans, and it was the first one ever finished for a whole animal. Because the adult hermaphrodite worm has exactly 302 neurons, the same count in every individual, its entire nervous system could be charted neuron by neuron.

Producing it was painstaking handwork. In the 1970s and 1980s a team led by John White and Sydney Brenner sliced the worm into thousands of slivers thinner than a soap bubble, photographed each one under an electron microscope, and traced by eye which neuron touched which across the whole stack of images. The result, published in 1986, was nicknamed The Mind of a Worm and gave biology its first full circuit diagram of a living creature.

This map turned the worm into a foundational model for circuit neuroscience, the study of how connected groups of neurons produce behavior. Knowing every cell and link, researchers can ask precisely how a fixed circuit generates feeding, mating, or fleeing from danger, and they can test their answers by switching single neurons on or off. It remains the reference and the inspiration for the much harder work of mapping larger brains, such as those of flies and mice.