I just finished writing an article about electroencephalography (EEG) — a non-invasive and fairly common technique that measures human brain waves through electrodes fitted on the scalp. Because the electrodes are pretty far away (centimeters, anyway) from the neurons underneath, each brain wave actually represents the aggregated activity of tens of millions of neurons, all firing in synchrony.

Scientists estimate that the entire brain holds about 100 billion neurons.

Now, with those scales in mind, think of the mind-bogglingly enormous task presented to a scientist who wants to map how individual neurons are connected to each other. “It’s a bit like taking a giant plate of spaghetti and, without unraveling it, trying to figure out which strand goes where,” says Ju Lu, a postdoctoral researcher in molecular and cellular biology at Harvard. “Except in this case, each strand of spaghetti has up to 37 branches.”

But that’s exactly what Lu and a team of other neuroscientists from Harvard and MIT have done! As published yesterday in PLoS Biology, Lu’s team used genetic engineering with fluorescent proteins to map ‘connectomes’ — complete neural circuit diagrams — in mouse muscle tissue. It was painstaking work that went along at about a half millimeter per hour. But check out the incredible results:

In the first image, each individual neuron takes a different color. In the second image, the branches are color-coded based on the number of muscle fibers innervated by each axon. For more details, check out the press release and the open-access paper in PLoS Biology