Connectome

A connectome is a map of the neural connections in a nervous system: an accounting of which neurons synapse onto which others, and how strongly. Olaf Sporns, Giulio Tononi, and Rolf Kotter coined the term in 2005, drawing an analogy to the genome. A genome specifies an organism's genetic makeup; a connectome specifies the wiring of its nervous system. Connectomes are built at different scales, from the regional connectivity visible in diffusion MRI down to synapse-level reconstructions of individual neurons from electron microscopy. The two scales answer different questions.

Complete synapse-level connectomes now exist for a handful of small nervous systems. C. elegans (302 neurons) was mapped by hand starting in the 1980s. The adult fruit fly Drosophila brain followed in 2024 (FlyWire, 139,255 neurons, 54.5 million synapses). In September 2026, researchers led by HHMI's Janelia Research Campus completed the connectome of the male fly's full central nervous system, brain and nerve cord together, at over 166,000 neurons and 125 million synapses, the largest wiring diagram by neuron count to date. In mammals, the MICrONS project reconstructed one cubic millimeter of mouse visual cortex at synaptic resolution alongside the neurons' recorded activity as the animal watched video (Nature, 2025). A 2024 pilot mapped part of human temporal cortex.

Connectomics is the structural half of the whole brain emulation roadmap and a general tool in neuroscience for tracing circuits from sensory input to motor output. Whole-brain maps remain limited to small nervous systems. Cost per reconstructed neuron has fallen from roughly $16,500 in the 1980s to about $100 for recent zebrafish work, driven by AI-assisted proofreading. But data volume and the manual correction of automated reconstructions both still scale with neuron count, and a whole mammalian brain remains a data problem measured in petabytes to exabytes.