Ischemic Cascade
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The ischemic (ischaemic) cascade is a series of biochemical reactions that take place in the brain and other aerobic tissues after seconds to minutes of ischemia (inadequate blood supply).[1] This is typically secondary to stroke, injury, or cardiac arrest due to heart attack. Most ischemic neurons that die do so due to the activation of chemicals produced during and after ischemia.[2] The ischemic cascade usually goes on for two to three hours but can last for days, even after normal blood flow returns.[3]
A cascade is a series of events in which one event triggers the next, in a linear fashion. Thus “ischemic cascade” is actually a misnomer, since in it, events are not always linear: in some cases, they are circular, and sometimes one event can cause or be caused by multiple other events.[4] In addition, cells receiving different amounts of blood may go through different chemical processes. Despite these facts, the ischemic cascade can be generally characterized as follows:
1. Lack of oxygen causes the neurons normal process for making ATP for energy to fail.
2. The cell switches to anaerobic metabolism, producing lactic acid.
3. ATP-reliant ion transport pumps fail, causing the cell to become depolarized, allowing ions, including calcium (Ca++), to flow into the cell.
4. The ion pumps can no longer transport calcium out of the cell, and intracellular calcium levels get too high.
5. The presence of calcium triggers the release of the excitatory amino acid neurotransmitter glutamate.
6. Glutamate stimulates AMPA receptors and Ca++-permeable NMDA receptors, which open to allow more calcium into cells.
7. Excess calcium entry overexcites cells and causes the generation of harmful chemicals like free radicals, reactive oxygen species and calcium-dependent enzymes such as calpain, endonucleases, ATPases, and phospholipases in a process called excitotoxicity.[5][6] Calcium can also cause the release of more glutamate.
8. As the cells membrane is broken down by phospholipases, it becomes more permeable, and more ions and harmful chemicals flow into the cell.
9. Mitochondria break down, releasing toxins and apoptotic factors into the cell.