The trauma that causes a concussion/mTBI can injure the brain's neurons in a variety of ways that are not easily identified using standard neuroimaging techniques. One kind of damage is called diffuse axonal injury (DAI), in which axons (the output fibers of neurons) are twisted, stretched, or severed.
Watch this video from Georgia Health Sciences University illustrating diffuse axonal injury:
Understanding Diffuse Axonal Injury
Diffuse axonal injury affects nerve fibers, which can lead to a disruption in nerve communication that affects a person's physical and cognitive abilities.