Differential contributions of microtubules and spectrins to axonal mechanics

Axons grow to extreme lengths and hence are subjected to large stretch deformations during limb or other bodily movements. Axons in the brain too undergo significant deformations even during normal activities, and can be damaged causing concussion or traumatic brain injury during head impacts. Have axons evolved special strategies to withstand large deformations? To this end, we investigate the mechanical responses of microtubules and the actin-spectrin periodic scaffold in axons. Our experiments and theoretical modelling suggests that microtubules are able to relax mechanical stress by unbinding of crosslinks, whereas the spectrin scaffold can buffer excess tension by unfolding of spectrin repeats--triply folded alpha helices connected by linker domains. This, in effect, makes the parallel array of microtubules behave elastically at short times (sudden deformations) and fluid-like at long times. The spectrin scaffold, on the other hand, behaves as a non-linear viscoelastic solid. We'll discuss the functional consequences of these differential contributions and how it might help us better understand axonal susceptibility to injuries like concussion, traumatic brain injury and stretch injuries to nerve fibers.