Flexible…Yet Fragile?It's long been known that brain cells have long, tendril-like parts called axons, which are notoriously elastic. The mystery is why these stretchy, flexible axons would break so easily under brain trauma.
The answer appears to lie with the protein that holds the axons together. The protein is called tau, and it binds together the tiny microtubules that make up an axon. While tau is very flexible, the microtubules are not. And those microtubules are what transport molecular cargo throughout the brain.
Dr. Douglas Smith, one of the members of the research team and director of the Penn Center for Brain Injury and Repair, explains it like this:
"One of the main things you see in the brains of patients who have died because of a TBI is swellings along the axons. When [the microtubules] break, there's an interruption in the flow of this cargo and it starts to accumulate, which is why you get these swellings."
The Protein MysteryThe next question the team had to answer was why the microtubules break. After all, the stretchy protein tau ought to work like a bungee cord, keeping the microtubules safe even when trauma occurs.But what they found was that tau is viscoelastic. Put another way, the protein acts a lot like Silly Putty.
"When you add stress to it slowly, it stretches a lot," said Vivik Shenoy, the engineering expert on the team, in a Phys.org article. "But if you add stress to it rapidly, like in an impact, it breaks. If you're in a situation where the tau doesn't stretch, such as what happens in fast strain rates, then all the strain will transfer to the microtubules and cause them to break."Nearly any brain trauma essentially causes the brain cells to stretch to accommodate the impact. What these findings indicate is that the speed of that stretch could be the most important determining factor in the severity of a traumatic brain injury.