doodleconnects.

WHAT THE RESEARCH SHOWS

Every wire,
mapped.

A brain is a network of connected cells.

A connectome is the full map of those connections: which neuron links to which. Scientists have now drawn complete maps for a worm and a fruit fly, and a cubic millimetre of mouse brain.

Compare the maps Read the short answer
A researcher seen from behind at a desk in a dim lab, looking at a large screen showing a colourful tangle of reconstructed nerve cells
AI-generated image, not a photograph of a real person or place.
01 Slice it thin02 Image every slice03 Trace every wireWorm, fly, then mouse

THE SHORT ANSWER

Three maps,
each bigger.

Each map traces every neuron and every connection in a piece of nervous system.
The tools grew from notebooks to AI and crowds.

THE WORM

302 neurons, 1986

The first complete connectome mapped all 302 neurons of the 1 mm roundworm C. elegans and about 7,000 connections between them.

Compare the maps

THE FLY

A whole adult brain, 2024

The FlyWire map of a fruit fly brain details over 50 million connections between more than 130,000 neurons, the largest complete adult connectome yet.

See the numbers

THE METHOD

Slices, AI and proofreaders

Brains are imaged slice by slice with electron microscopes. Software traces the neurons, and people check its work for errors.

How it is done

COMPARE THE MAPS

How big
is each map?

Pick a connectome to see what it covers. The bar shows how many connections it contains, from thousands to hundreds of millions.

Figures from NIH (NINDS and NIMH) releases and the MICrONS project page.

Map size

Choose a map
Roundworm, 1986302 neurons

The whole nervous system of the worm C. elegans: all 302 neurons and the 7,000 connections between them, published by Sydney Brenner and colleagues.

Fruit fly brain, 202450 million+ links

The first complete wiring map of an adult fruit fly brain, with over 50 million connections between more than 130,000 neurons, published in Nature.

Mouse cortex, 2025523 million synapses

A cubic millimetre of mouse visual cortex with an estimated 200,000 cells and 120,000 neurons. Automated detection found more than 523 million synapses.

The mouse dataset covers one small block of brain, not a whole brain; the worm and fly maps cover the whole nervous system or brain.

TWO NUMBERS WORTH KNOWING

A long wait,
then a leap.

One figure on how long the first map stood alone,
one on how much a speck of brain holds.

THE GAP · ROYAL SOCIETY COMMENTARY

Before new studies like it were possible

26years

A 2015 commentary in Philosophical Transactions B says twenty-six years passed after the worm map before developments, notably more powerful computers, made new studies of this kind possible.

MOUSE CORTEX · MICRONS

Synapses found in about a cubic millimetre

523million

The MICrONS dataset spans a 1.4 × 0.87 × 0.84 mm block of mouse cortex. It is the largest connectomics dataset by number of cells and connections detected, as of 2025.

NIH says the fly map's methods could be used to map larger brains, and that the BRAIN Initiative is funding efforts to map mammalian and human brains.

A clear agar plate under a laboratory microscope, with tiny pale worms visible on its surface
AI-generated image, not a photograph of a real person or place.

FOUR STEPS TO A MAP

From slices
to a wiring diagram.

The basic method has not changed since the worm: see every cell membrane and synapse, then follow each neuron through the slices.

  1. Cut very thin slicesSynapses and cell membranes are too small for a light microscope, so the tissue is cut into thousands of ultrathin sections.
  2. Image every sliceEach section is photographed with an electron microscope, then the images are stitched together in 3D.
  3. Let software traceAI and other algorithms follow each neuron and its long branches through the stack of images.
  4. Have people check itThe software makes mistakes, so trained people proofread it. FlyWire opened this work to scientists around the world.

WHY A WORM

Small enough
to finish.

In Cambridge in the 1960s, Sydney Brenner looked for an animal with genetics, behaviour and a nervous system small enough to map completely. The fruit fly, with some 100,000 neurons, seemed too complex then.

He chose C. elegans, a transparent worm about 1 mm long with only about 1,000 cells. The finished manuscript arrived at the Royal Society in 1984 in six loose-leaf notebooks, labelled ‘The mind of a worm’.

A close-up of a small fruit fly with red eyes resting on a slice of ripe banana
AI-generated image, not a photograph of a real person or place.

QUESTIONS PEOPLE ASK

Frequently asked

What is a connectome?

It is a wiring diagram of a nervous system: a map of its neurons and the connections, or synapses, between them. The fly connectome also records the type of neurotransmitter each neuron sends.

What was the first connectome?

In 1986 Sydney Brenner and colleagues published a map of all 302 neurons in the roundworm C. elegans and the 7,000 synapses between them.

How big is the fruit fly connectome?

NIH says it details over 50 million connections between more than 130,000 neurons, and calls it the largest and most complete connectome of an adult animal created.

Who checked the fly map?

A consortium of fruit fly labs called FlyWire, with scientists around the world, proofread the computer tracing. NIMH says only 15% of neurons had been proofread in 2021.

Has a human brain been mapped this way?

Not yet. The largest dataset so far, from MICrONS, covers about a cubic millimetre of mouse cortex. NIH says the BRAIN Initiative is funding efforts to map larger mammalian and human brains.

SOURCES

Where the facts come from

Each figure on this page comes from the sources below. Check them for the full detail and context.

  1. NIMH press release: Researchers fully map neural connections of the fruit fly brain (October 2024)
  2. NINDS: BRAIN Initiative researchers complete groundbreaking map of the fly brain
  3. Emmons (2015), The beginning of connectomics, Philosophical Transactions of the Royal Society B (PMC)
  4. MICrONS Explorer: Cubic millimeter functional connectome