Calcium enters the cell through channels, or doors, opened by excitatory amino acids. Excitatory amino acids help brain cells communicate. Normally they are tightly regulated within a brain cell and only released in small amounts. When a brain cell dies, the entire quantity of these chemicals is released at once. They open calcium channels in other cells and keep them open too long.
Free radicals are extremely destructive molecules that are unneeded byproducts of normal metabolism.
The body has natural, highly effective defenses against damage caused by these toxic molecules. During injury, however, excess amounts of free radicals are generated that overwhelm the natural defenses. Particular targets of free radicals are the membranes of nerve cells already weakened and swollen. Free radical attack on these membranes made of fatty molecules, known as lipids, is called lipid peroxidation. When the outer cell wall degrades and bursts, the cell dies releasing its contents and beginning the cycle of destruction over on neighboring cells.