Researchers have long said they won’t be able to understand the brain until they can put together a “wiring diagram” – a map of how billions of neurons are interconnected. Now, researchers at the Salk Institute for Biological Studies have jumped what many believe to be a major hurdle to preparing that chart: identifying all of the connections to a single neuron.
In the March 1 issue of the journal Neuron, the researchers describe how they modified the deadly rabies virus, turning it into a tool that can cross the synaptic space of a targeted nerve cell just once to identify all the neurons to which it is directly connected.
“We’ve wanted to do this for a very long time and finally found a way to make it possible,” says the study’s senior author, Edward M. Callaway, Ph.D., a professor in the Systems Neurobiology Laboratories. “It will offer us an unprecedented view of the brain.”
The problem neuroscientists are confronted with “is akin to a computer user who tries to figure out how the machine’s electronic chip works by looking down at it; there is no way to figure out how things are connected,” Callaway says. “If you were given a wiring diagram, you could begin to understand how the chip moves electricity and how that operates the computer.”
Neuroscientists also want to deconstruct the flow of electrical signals in the extraordinarily complex architecture of the brain and then correlate these neural circuits with such brain functions as perception and behavior. But these circuits are difficult to unravel because dozens of different neuronal types are entangled within a precisely connected network, and even neighboring neurons of the same type differ in connectivity and function.
So, researchers have been trying to figure out the pattern of connections typical of a type of neuron, to see which other cell types they connect with and how those connections are configured. To do this, they need a tracer that can tease apart the chain of directly connecting neurons, identifying them one by one. .....
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