Mapping the Information Highway in the Brain
It’s hard to imagine life without Google Maps nowadays. We use the interactive map daily to find out how to get from point A to point B. Wouldn’t it be great if there were an online map to guide neuroscientists to study the brain? In the newest issue of Cell, researchers from University of Southern California published a project mapping the neural networks of the mouse cortex, and they call it the Mouse Connectome Project (MCP).
The cerebral cortex is responsible for processing information related to cognition, motivation and emotion. To solve the puzzle of how the cortex carries out these functions, researchers have struggled to create a map that accurately shows the connections within the cortex. As you are aware from the recent hype about connectomics, neuroscientists have been making brain maps for a long time. So what is new about MCP?
Some of the oldest brain maps were made by neuroscientists focusing ona limited number of brain circuits, which were relevant to their own studies. It’s like if you made a map of the locations you have visited in New York City and the routes you took, but neglected the rest of the city and the streets you missed. Maps made this way don’t have enough information to cover all the roads in the city. Like the Google team that drove vans around to get street views, it takes systematic group effort to collect information that can make a map useable for a broader audience. MCP researchers spent years tracing the brain circuits to cover all over cortex in an organized fashion.
The MCP also used an advanced method to map the brain circuits better: double coinjection tract tracing. The researchers injected one anterograde tracer, which travels down the axons of the cell, and one retrograde tracer, which travels up toward the cell body, simultaneously to examine the input and output pathways of the cortex. This allows the researchers to see if the traffic from two cortex parts is one-way, two-way, or if there are any pit stops along the way. Having this directional information is essential to understand the brain. Although MCP is not the first online brain connection map, it is the first brain connectivity map using the double coinjection tract tracing method.
Lastly, the MCP project created a user-friendly interface, which you can find at www.mouseconnectome.org. Like GoogleMaps, you can search brain regions and switch between layers of different tracers. You can even look at individual experiments, just like street view.
With all the these features combined, MCP created an accessible map to understand the organization logic of the cortex. Based on the map, the researchers have identified eight cortex subnetworks that are relatively segregated. Four of the eight subnetworks are related to movement and sensation of the body regions. These networks have distinct regions for face, upper limb, lower limbs, the trunk and the whiskers. Two networks along the medial bank of the cortex form the medial subnetworks, which are responsible for the integration of auditory, visual and somatic sensory sensations. Cortical regions on the lateral sides of the cortex form subnetworks related to the sensation and homeostatic control of internal states, such as hunger, and also serve as a major site of convergence for cortical information routed to the hippocampus.
The Mouse Connectome Project has created a useful tool for all neuroscientists to explore the brain circuits of their own interests. With the anatomy data available, they can advance our understanding of brain circuits, which would help us better understand certain neurological diseases that alter brain circuits.
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Written by Xiaorui “Ray” Xiong
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References:
Zingg B., Hintiryan H., Gou L., Song M., Bay M., Bienkowski M., Foster N., Yamashita S., Bowman I. & Toga A. & Dong H.W. (2014). Neural Networks of the Mouse Neocortex, Cell, 156 (5) 1096-1111. DOI: 10.1016/j.cell.2014.02.023
Image courtesy Brian Zingg.
