The human brain is a crowded place. It contains an estimated 100 billion neurons, connected to one another by a million billion synapses. Neuroscientist Olaf Sporns of Indiana University, cellular biologist Jeff Lichtman of Harvard, and computational neuroscientist Sebastian Seung of MIT are mapping these connections, aiming to eventually create a "connectome" of an entire brain.
Each neuron can be thought of as a cellular decision maker: It receives a multitude of signals and decides which ones to pass on and which to shrug off. Figuring out the interactions of all these decision makers would move researchers closer to creating models of how we perceive our surroundings and store memories. "One of the biggest hypotheses in neuroscience is the idea that memories are stored in the connections between neurons," Seung says. With a connectome, individual memories might be read from neural connections.
To tackle such a vast challenge, Sporns traces the major pathways that bridge different regions of the human cortex. Working with mice, Lichtman and Seung image one neuron at a time. So far, only the lowly nematode has had its nervous system completely mapped, and it has a mere 300 neurons--a neurological hamlet compared with the massive megalopolis of the human brain.
How To Wire An Ear
To trace individual neurons, Jeff Lichtman of Harvard uses engineered mice whose DNA includes jellyfish genes so that the cells contain fluorescent proteins. Under blue light, the neurons glow green, and computer software assigns each of the 200 green neurons a different color. The "tree" of neurons at right carries signals that control a muscle connecting the skull to the ear. Individual muscle fibers appear as bundles at the tips. Comparing how this simple neural pathway changes between baby and adult mice sheds light on how brains rewire over time, Lichtman says.