Even the mildest form of a traumatic brain injury, better known as a concussion, can deal permanent, irreparable damage. Now, an interdisciplinary team of researchers at the University of Pennsylvania is using mathematical modeling to better understand the mechanisms at play in this kind of injury, with an eye toward protecting the brain from its long-term consequences. Their recent findings, published in the Biophysical Journal, shed new light on the mechanical properties of a critical brain protein and its role in the elasticity of axons, the long, tendril-like part of brain cells. This protein, known as tau, helps explain the apparent contradiction this elasticity presents. If axons are so stretchy, why do they break under the strain of a traumatic brain injury? Tau's own elastic properties reveal why rapid impacts deal permanent damage to structures within axons, when applying the same force more slowly causes them to safely stretch. This illustration shows that the protein tau acts as "cross ties" for the microtuble "train tracks" that run the length of the axon. The tracks can stretch under slow application of force, but rapid impacts transfer the energy to the tau cross ties, causing them to snap.