Stroke: What is a stroke?

The Basic Process

Blood Flow Blockage. The brain receives about 25% of the body's oxygen, but it cannot store it. Brain cells require a constant supply of oxygen to stay healthy and function properly. Therefore, blood needs to be supplied to the brain continuously through two main arterial systems:

• The carotid arteries come up through either side of the front of the neck. (The pulse of a carotid artery can be felt by placing the fingertips gently against either side of the neck right under the jaw.)
• The basilar artery forms at the base of the skull from the vertebral arteries, which run up along the spine, join, and come up through the rear of the neck.
• A reduction of, or disruption in, blood flow to the brain is the primary cause of a stroke. Blockage for even a short period of time can be disastrous and cause brain damage or even death.
• A stroke is usually defined as two types:
• Ischemic (caused by a blockage in an artery).
• Hemorrhagic (caused by a tear in the artery's wall that produces bleeding in the brain).

The consequences of a stroke, the type of functions affected, and the severity, depend on where in the brain it has occurred and the extent of the damage.

Cell Death. In addition to oxygen cut-off to the brain, other factors are involved in the cycle of events leading to brain cell death after a stroke. The process is not altogether clear, but one hypothesis is as follows:

• When blood is cut off in the brain, proteins known as excitatory amino acids (such as glutamate and glycine) are released. These amino acids literally excite nerve cells and, when over produced, can kill them.
  
• These proteins open channels in the membranes that cover neurons (brain cells), allowing large amounts of calcium to flow in.
  
• Calcium reacts within the neurons to release harmful substances that damage cells.
  

Natural particles released by the body's chemical processes, called oxygen-free radicals, may also play a role in the cell damage caused by a stroke. (Some oxygen-free radicals under study are called lipid peroxides.)

Another hypothesis focuses on an enzyme called PARP:

• PARP ordinarily makes minor molecular repairs.
  
• Confronted with substantial injury, however, PARP over-responds and takes up a substance called ATP, the major source of energy in cells.
  
• The cells die.
  
• Within minutes of a stroke, the zone of initial cell death is surrounded by additional damaged and dying brain cells. This process can continue for hours, leading to brain damage, perhaps irreversible.