connectome is a map of every neuron in a nervous system and every synapse between them. Cajal drew single cells; connectomics tries to draw all of them at once, by slicing tissue into sections thinner than a cell and tracing every fibre through the stack.

the question

If you knew every connection, would you know how the brain works? Or is the wiring diagram to the brain what a street map is to a city's traffic?

the classic finding

The first complete connectome took more than a decade of hand tracing: the 302 neurons of the worm Caenorhabditis elegans (White et al., 1986). It has been studied ever since, and it did not by itself explain the worm. Bargmann and Marder showed why: chemical signals that are not on the map reconfigure the same circuit into different ones from moment to moment [paraphrase, not a quotation].

what held up, and what did not

Machine learning made the tracing fast enough to scale. The whole brain of an adult fruit fly, about 140,000 neurons, was mapped and published in 2024 (Dorkenwald et al., 2024)(Schlegel et al., 2024). A cubic millimetre of human temporal cortex, removed during surgery, was imaged in full (Shapson-Coe et al., 2024). A cubic millimetre of mouse visual cortex was imaged after its activity had been recorded while the mouse watched films, so structure and function could be compared cell by cell (MICrONS Consortium, 2025).

A warning came from engineers. Jonas and Kording took a 1970s microprocessor, whose wiring is known exactly, and applied the standard methods of neuroscience to it. They did not recover how it works [paraphrase, not a quotation].

what it looks like in machines

Every artificial network comes with its connectome for free: every weight is known. That has not made them easy to understand, which is the strongest argument that a map is necessary and not enough. See interpretability.

what is still open

Whether a mouse or human connectome is reachable, how much variation there is between individuals, and what level of description turns a map into an explanation.

in the tour: the room called "The wiring diagram" has a placard called "Three hundred and two" ···

Reading list

1. White et al. (1986). The structure of the nervous system of the nematode Caenorhabditis elegans. Philosophical Transactions of the Royal Society of London. B, Biological Sciences. doi:10.1098/rstb.1986.0056
2. Bargmann & Marder (2013). From the connectome to brain function. Nature Methods. doi:10.1038/nmeth.2451
3. Dorkenwald et al. (2024). Neuronal wiring diagram of an adult brain. Nature. doi:10.1038/s41586-024-07558-y
4. Schlegel et al. (2024). Whole-brain annotation and multi-connectome cell typing of Drosophila. Nature. doi:10.1038/s41586-024-07686-5
5. Shapson-Coe et al. (2024). A petavoxel fragment of human cerebral cortex reconstructed at nanoscale resolution. Science. doi:10.1126/science.adk4858
6. MICrONS Consortium (2025). Functional connectomics spanning multiple areas of mouse visual cortex. Nature. doi:10.1038/s41586-025-08790-w
7. Jonas & Kording (2017). Could a Neuroscientist Understand a Microprocessor?. PLOS Computational Biology. doi:10.1371/journal.pcbi.1005268

see also

	Santiago Ramón y Cajal — one cell at a time
	brains and models compared
	timeline: neuroscience