Woodpeckers like the acorn woodpecker of California, which hits the wood with its bill at speeds of 12 to 15 miles an hour, manage to keep their wits because their brains are held firmly in a case, which acts as a shock absorber.
The bird’s body also moves in a single plane like a metronome, avoiding the rotational forces that would twist and tear its brain loose from its moorings.
The human brain does not enjoy this kind of protection. Instead, it is encased in a sac of fluid.
Thus when a cyclist hits the pavement, for example, the impact can make the brain bounce around in its sac, hitting first one side of the skull and then the other. That motion can tear nerve fibers and cause permanent damage.