The brain is a network: Its function depends critically on its connectivity. A neuron’s role in the brain depends far less on its spatial position than on how it connects to other neurons. And those connections, or synapses, are buried among a dense tangle of thin, highly branched axons and dendrites, the neuronal wires. Each just a micron or so thick, the wires form circuits that may span many millimeters or more. Understanding the brain means understanding its structure over many orders of magnitude in length.
For more than a century, neuroscientists have selectively stained small numbers of neurons in their entirety, so that their axons and dendrites, viewed through a light microscope, could be distinguished from those of their neighbors. More recently mice and other animals have been genetically engineered so that some or all of their brain cells synthesize proteins that fluoresce in various colors.1 (See also...
