PLoS Biol. 7, e1000032 (2009)

When neuroscientists measure parts of nervous systems, they do so statistically, pooling data about the activity of many neurons at once. But researchers based in Cambridge, Massachusetts, have painstakingly mapped every neuron involved in innervating six mouse interscutularis muscles — muscles that allow mammals to wiggle their ears.

Jeff Lichtman of Harvard University and his colleagues used these six 'connectomes' to compare the innervation of paired tissues on the left and right sides of the same creatures. The wiring was strikingly different, underscoring the flexible nature of mammalian neural development. Many of the neurons were also 25% longer than required to form the connections that they did. That is odd because nerve cells are metabolically expensive.