A blow to the head, such as a quarterback might receive from a defender's helmet, causes the brain not only to move forward and/or back in the skull but also to rotate, which distorts the axons that connect the brain cells, or neurons.
1. In a normal neuron, the axon, which is protected by a myelin sheath, is not broken or otherwise distorted.
2. After a concussive blow, an axon might twist or bend, interrupting communication between neurons.
3. If a concussion is severe enough, the axon swells and disintegrates. Less severely damaged axons return to normal.