Intelligent design, my ass.
Thanks to its bad neighborhood the poor brain is being constantly banged about. Every so often it gets plastered against its membranous sac, typically when a head meets an rapidly moving object such as a sidewalk or a baseball bat. This is not good for something with "the consistency of custard". Evolution has struggled to adjust (emphases mine) ...
... axons are remarkably elastic. They can stretch out slowly to twice their ordinary length and then pull back again without any harm. Axons are stretchy due in part to their flexible internal skeleton. ... When an axon stretches, these microtubules can slide past one another. If the movement is gradual, the microtubules will immediately slide back into place after the stretching stops, with no harm done.
If Smith delivers a quick, sharp puff of air, however, something else entirely happens. Instead of recoiling smoothly, the axon develops kinks. Over the next 40 minutes, the axon gradually returns to its regular shape, but after an hour a series of swellings appears. Each swelling may be up to 50 times as wide as the normal diameter of the axon. Eventually the axon falls apart.
These kinks form, Smith believes, when microtubules are stretched so rapidly that they snap ... Normally, enzymes inside neurons are constantly taking apart microtubules and building new ones with the recycled parts. But now the enzymes attack the broken ends of the microtubules, causing the internal structure of the axon to dissolve...
... Smith’s findings could shed light on a common but puzzling brain trauma known as diffuse axonal injury. This happens when people experience sudden accelerations to the brain—from a bomb’s shock waves, for example, or from whiplash in a car crash ... When pathologists perform autopsies on people with diffuse axonal injury, they see severed axons with swollen tips, just like what Smith sees in his experiments.
Smith’s research also suggests that even mild shocks to the brain can cause serious harm. ... A moderate stretch to an axon, Smith recently found, causes the sodium channels to malfunction. In order to keep the current flowing, the traumatized axons start to build more channels.
Smith suspects that such a mended axon may be able to go on working, but only in a very frail state. Another stretch—even a moderate one—can cause the axon to go haywire ... The axon dies like a shorted-out circuit.
... Preliminary brain studies show that axons are still vulnerable even months after an initial stretch...
Just in case you're not depressed enough yet, wherever you read "axons" substitute the phrase "young axons". Any wagers on how well older axons stretch? Also note that "even months after" doesn't mean they're not vulnerable "years after".
It's interesting, after reading this article, to search PubMed with the phrase "microtubule amyloid axonal injury". A 2006 paper looked at animal model transient accumulation of neurotoxic amyloid precursor protein after injury. Amyloid protein has, of course, long been associated with Alzheimer's dementia. Head injury is also strongly associated with dementia risk; head injury avoidance is about the only "intervention" known to reduce the risk of Alzheimer's disease. (Don't make too much of this injury/amyloid connection though, researchers have been banging on it since the 1990s. It's not straightforward.)
Short of radical genetic engineering, or spending our lives watching TV with thickly padded carpets, what can we do about our fragile brains? Sure, football is dead. Yes, soccer will lose the header. Sure we can change the rules of hockey. Yes, horseback riding is almost as crazy as riding donorcycles. But, really, have you watched any TV lately? There are worse things than dementia.
Today's helmets are not the answer. Current bicycle helmet designs, for example, don't materially change the rate of anterior impact deceleration. Their primary benefit is to facilitating head gliding and reduce abrasions; they aren't designed to reduce the deceleration injuries that matter -- without severing our wimpy cervical spines. (On road bikes effectiveness is further diminished by paradoxical automobile driver behavior.)
We need to revise our sports (so long NFL), but we also need much better helmets. Air bags anyone?