Each traumatic brain injury has a primary and secondary injury associated with it. The primary injury refers to what happens to the brain and skull at the moment of impact. The effect of this impact is related to the unique structure of the skull and the brain.
The bone of the skull is very hard, while the brain is very soft. When a traumatic brain injury occurs, such as a car crash or a fall, the impact may cause the skull to move very quickly. The brain moves at a different speed because it weighs less than the skull; as a result the brain may hit the bone of the skull, causing bruising and bleeding. Sometimes, the impact can be so severe that the brain bounces back and forth inside the skull, causing very severe bruising and bleeding.