Figure 1.

Important microstructural factors in a bundle of axons (blue). At the individual level, axon diameter and myelin thickness influence the speed of action potential propagation along the axon. The g‐ratio combines these factors to describe conduction efficiency of a fiber. The coherence of fibers, along with the density or packing of axons in a region, are additional factors influencing the effectiveness of information transfer along groups of white matter fibers, often grouped into major tracts based upon their shared termination points [Color figure can be viewed at http://wileyonlinelibrary.com]