A connectome is a complete mapping of all connections, including every individual synapse and gap junction, between all neurons in a model organism's brain. In other words, a comprehensive circuit diagram of the brain. Current approaches to mapping the connectomes of model organisms employ serial block face scanning electron microscopy (SBEM) and serial-section transmission electron microscopy (ssTEM). The only connectome that has been mapped out to date has been from the flatworm, C. elegans (shown above; Credit: openworm.org), which has only 302 neurons. Candidate future connectomes include the fly, with around 10,000 neurons, and the mouse brain, with 80 - 100 million neurons.
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