Diverse ’Connectomes’ Hint at Genes’ Limits in the Nervous System

Cambridge, Mass. February 9, 2009 - Genetics may play a surprisingly small role in determining the precise wiring of the mammalian nervous system, according to painstaking mapping of every neuron projecting to a small muscle mice use to move their ears. These first-ever mammalian "connectomes," or complete neural circuit diagrams, reveal that neural wiring can vary widely even in paired tissues on the left and right sides of the same animal. Scientists at Harvard University and the Massachusetts Institute of Technology describe the work this week in the journal PLoS Biology, accompanied by vivid images depicting neurons that are strikingly treelike, but also tremendously varied. "We had expected to find a great degree of neural symmetry in the same mouse's two interscutularis muscles, but this isn't even close to true," says Jeff W. Lichtman, professor of molecular and cellular biology in Harvard's Faculty of Arts and Sciences. Curiously, the connectome of the mouse interscutularis - a muscle also found in dogs, rats, and other mammals that readily move their ears - reveals that some of its neurons are as much as 25 percent longer than is necessary. This casts doubt on a longstanding assumption among neuroscientists that neural wiring length is generally minimized to conserve space, energy, and resources.