Whether or not brain cells are damaged or destroyed in a head injury depends on the force of impact, according to the Mayo Clinic. Another factor that affects brain cell damage is the type of injury event.
In some cases, damage to brain cells occurs only in the area of the brain that is directly underneath the point of impact on the head. A severe blow or jolt can cause multiple areas of damage if the brain moves back and forth within the skull. The Mayo Clinic states that a severe hit that causes the head to rotate can tear the structure of brain cells.
The Mayo Clinic also notes that a large explosion, such as those in combat situations, can cause widespread damage to brain cells.