Array tomography is a state-of-the-art imaging system invented by Stanford University researchers. It allows researchers to count the myriad connections between nerve cells, as well as to catalog those connections’ surprising variety.

A typical healthy human brain contains about 200 billion nerve cells, or neurons, linked to one another via hundreds of trillions of tiny contacts called synapses. It is at these synapses that an electrical impulse traveling along one neuron is relayed to another, either enhancing or inhibiting the likelihood that the second nerve will fire an impulse of its own. One neuron may make tens of thousands of synaptic contacts with other neurons, said Stephen Smith, PhD, senior author of a paper published Nov. 18 in Neuron.

The new imaging system allows researchers to “travel” through the 3-D mosaic of neurons and observe different colors corresponding to different synaptic types just as a voyager might transit outer space and note the different hues of the stars dotting the infinite blackness. And to make a movie of the whole trip.

This level of detailed visualization has never been achieved before. “The entire anatomical context of the synapses is preserved. You know right where each one is, and what kind it is. Observed in this manner, the brain’s overall complexity is almost beyond belief”, said Smith. “One synapse, by itself, is more like a microprocessor —with both memory-storage and information-processing elements — than a mere on/off switch. In fact, one synapse may contain on the order of 1,000 molecular-scale switches. A single human brain has more switches than all the computers and routers and Internet connections on Earth,” he said.

Researchers plan to use array tomography to tease out more such distinctions within classes of synapses. That should accelerate neuroscientists’ progress in, for example, identifying the synapses that are lost after traumatic brain injury, or in neurodegenerative disorders such as Alzheimer’s.