Fig. 5.

Which geometric features of axons are we potentially sensitive to with dMRI? Starting from a realistic axon shape segmented from 3-dimensional electron microscopy (Lee et al., 2019), coarse-graining over the increasing diffusion length L(t) first highlights the caliber variations (beads or varicosities) at the ρ ~ 1 μm scale, followed by the undulations with wavelength of the order λ_u ~ 30 μm. The fully coarse-grained “stick” compartment of negligible radius is characterized by one-dimensional Gaussian diffusion.