Connectomics: Mapping Every Connection

From the 302-neuron worm in 1986 to a complete fruit-fly brain in 2024, a cubic millimetre of human cortex and mouse visual cortex in 2025: how scientists map neural wiring synapse by synapse, and what the maps reveal.

The word connectome was coined in 2005 (by Olaf Sporns and colleagues, and independently by Patric Hagmann) for a complete map of neural connections. At the finest scale — every synapse between every neuron — mapping requires electron microscopy, enormous computing and painstaking verification.

The worm

In 1986 John White, Sydney Brenner and colleagues published The Structure of the Nervous System of the Nematode Caenorhabditis elegans, based on thousands of thin sections photographed by electron microscope and traced by hand over more than a decade. It mapped all 302 neurons and roughly 7,000 connections — the first complete connectome.

The fly

In October 2024 the FlyWire consortium, led by Mala Murthy and Sebastian Seung at Princeton with hundreds of contributors (including citizen scientists), published the first complete wiring diagram of an adult fruit-fly brain in a package of papers in Nature: about 140,000 neurons and more than 50 million synapses, with over 8,000 cell types. Researchers used it to simulate circuits and predict, for example, which neurons drive feeding.

A cubic millimetre of mouse and human

In 2024 a Harvard–Google team led by Jeff Lichtman and Viren Jain published (in Science) a reconstruction of a cubic millimetre of human temporal cortex removed during epilepsy surgery: about 57,000 cells and 150 million synapses in 1.4 petabytes of data, revealing rare neurons with exceptionally strong connections. In April 2025 the MICrONS project published in Nature a functional connectome of about a cubic millimetre of mouse visual cortex: roughly 200,000 cells and over 500 million synapses, combined with recordings of how about 75,000 of those neurons responded while the mouse watched videos — linking wiring to function.

How it's done

Tissue is cut or milled into slices only tens of nanometres thick and imaged by electron microscopes, generating petabytes of images. Machine-learning algorithms segment neurons and detect synapses; human "proofreaders" fix errors.

Why it matters — and its limits

Connectomes reveal wiring rules impossible to see otherwise and constrain models of computation. But wiring alone doesn't determine function: synapse strengths, neuromodulators, gap junctions and plasticity also matter. A full human connectome at synaptic resolution would require data on the order of a zettabyte; a whole mouse brain is the next major goal. At larger scales, the Human Connectome Project (launched 2009–2010) mapped region-to-region pathways in living people using MRI.