Traumatic brain injury

Traumatic brain injury is the damage the brain takes from a mechanical force coming from outside. It can be a direct blow to the head, it can be rapid acceleration and deceleration, as in a crash, where the brain is shaken inside the skull even though nothing strikes it, or it can be a penetrating wound that goes through the bone.

What matters is that the damage does not all happen at once. It arrives in two stages. The primary injury is the one at the instant of impact: whatever gets displaced inside the skull. Then the secondary injury begins, a neurotoxic cascade that unfolds later, especially if there is hypoxia, meaning a lack of oxygen, hypotension, meaning low blood pressure, or raised intracranial pressure. The impact opens the wound; what happens next decides how big it ends up.

That second part has a specific mechanism. Damaged neurons spill glutamate, a neurotransmitter that in normal amounts is used for communication and in excess is toxic. Then ions pour in, those ions activate enzymes inside the cell, and the enzymes end up breaking down the membranes and the cell's structure. The result is death of neurons and edema, which is swelling from fluid building up.