Here is a really interesting article from Sciencecentral.com which discusses the work that brain scientist Anat Biegon at Brookhaven National Laboratory and her colleagues are doing.

When a person suffers a brain injury, there is a large outflow of glutamate, a chemical neurotransmitter, which binds to certain receptors on nerve cells, killing them and possibly resulting in memory loss. This all takes place within the first hour after a brain injury.