神経科学：マウスの脳地図が構造と機能の関係性を明らかにする

A detailed map of mouse brain cell structure and connections, offering insights into how they relate to activity in the mouse brain, is revealed in a package of papers from the MICrONS (Machine Intelligence from Cortical Networks) consortium, published in Nature and Nature Methods this week. From just one cubic millimetre of mouse brain — about the size of a chia seed — around 84,000 neurons, half a billion synapses, and approximately 5.4 kilometres of neuronal wiring are traced with newly developed artificial intelligence tools. Although only a small fraction of the brain is represented, the high-resolution connectivity maps help to describe how the brain is organized and how the different types of cell work together.

Brains are made up of a network of cells, including neurons, which are activated by stimuli and connected by synapses. The basis of cognitive function involves the interplay between neuronal activation and the connectivity of cells. To learn more about mammalian brain circuitry, the MICrONS consortium generated a massive high-resolution map of neural shape and wiring from the visual cortex of a single mouse and explored how these related to function and genetic identity.

The researchers first recorded the activity of around 75,000 neurons in a mouse running on a treadmill and watching visual images. The mouse was modified in a way that makes its neurons emit a fluorescent protein when the neurons are active. These data were then matched to a reconstructed connectivity map of just one cubic millimetre of mouse visual cortex. Electron microscopy of the sample identified more than 200,000 cells, around 84,000 neurons and 524 million synaptic connections. Papers in the package describe the tools for identifying different cells, analysing the connections, and linking the wiring diagrams to neuronal activity. Together, the work provides a description of specific parts of the brain and offers insights into how they work together.

The mouse visual cortex is interesting to study, as it shares similarities with other mammals, including humans, but this is only one region of the brain — and a small portion at that. Wider maps are needed to study complete circuits, which will require advances in tools and approaches in the field of connectomics, the authors note. “Despite these limitations, this work marks a major leap forwards and offers an invaluable community resource for future discoveries in neuroscience,” write Mariela Petkova and Gregor Schuhknecht in an accompanying News & Views. They describe the MICrONS project as “the most comprehensive dataset ever assembled that links mammalian brain structure to neural activity in an active animal”.