Jessica Hamzelou, reporter
(Image: Davi D. Bock, Wei-Chung Allen Lee and colleagues/Nature)
This overlapping rainbow of connecting cells represents new insights into how mammalian brains work. The techniques used to create this three-dimensional map of a tiny chunk of mouse brain could help neuroscientists understand the connections that make up our own brains.
To create this map - which shows the neurons in a piece of mouse visual cortex just 8 thousandths of a cubic millimetre in volume - Clay Reid and his colleagues at Harvard Medical School in Boston combined two imaging techniques.
First, the group measured the activity of neurons in the visual cortex of a living mouse in order to spot the cells that were excited by visual information. They then killed the mouse and cut the visual cortex into 1215 slices, before using an electron microscope to capture more than 3 million images of the slices. Finally they stitched the pictures together to form this 3D image using a computer capable of handling the 30 to 40 terabytes of information.
That was the relatively easy part. Next, Reid's team painstakingly traced the connections made by 10 of the neurons that were active in the first stage of the study. That involved tracing 240 axons and dendrites through the tangled mess created by other axons and dendrites in the 3D image.
The results are already shedding light on the detailed behaviour of the brain. The team found that neurons that inhibit brain activity received input from all the excitatory neurons in the chunk. That finding offers new insight into the debate over whether these neurons receive specific or random inputs, and may inform research into conditions characterised by a lack of brain activity inhibition, such as epilepsy, says Reid.
In another paper just published, Winfried Denk and his colleagues at the Max Planck Institute for Medical Research in Heidelberg, Germany, used the same approach to map the neurons of the mouse retina.
Together, the two papers pioneer a new area of research coined "functional connectomics" that promises a better understanding of how mammalian brain networks communicate.