When the head is struck, it often moves until it is abruptly brought to a stop against a solid object. At this moment, the brain continues moving for a brief instant until it hits the bony prominences inside the now stationary skull. The skull does not give with this impact. The brain does. This is often accompanied by a stretch of the nerve endings. Sometimes this “stretch” goes beyond the nerve’s limit, and microscopic nerve (axonal) tears occur. These are not detected by regular x-rays. Sometimes, the injury to the moving brain takes place at a site opposite the point at which the skull was initially struck. This type of injury is called “contra-coup,” as opposed to a “coup” injury occurring on the same side as the initial impact (coup = blow, French; contra-coup = opposite the blow). Traumatic lesions, whether they are the product of closed or penetrating injuries, and whether they are coup or contra-coup, may be said to have direct effects, and secondary effects. Direct Effects and Secondary Effects will be addressed in the next post.

https://www.pathology.vcu.edu/WirSelfInst/trauma.html