Augustine G. DiGiovanna, a biology professor at Salisbury State University and the author of Human Aging: Biological Perspectives, offers the following answer:
Image: THE WHOLE BRAIN ATLAS, HARVARD UNIVERSITY
The constant activity of nerve cells in the brain requires an ongoing supply of chemical energy. Nerve cells, also called neurons, get this energy by breaking down glucose in a long series of reactions. Most of the energy emerges from the last few reactions, which require oxygen. The brain receives both glucose and oxygen through the blood, which is pumped up to the brain by the heart via arteries in the neck. (The pulse of blood in one of these arteries, the carotid, can be felt by pressing your fingers gently on the side of your neck). The arteries branch out at the brain, carrying blood over and through the organ.
If the supply of oxygen to the brain cells is inadequate, the neurons cannot break down glucose completely. As a result, they do not obtain enough energy for their activities; also, the partial breakdown of glucose yields harmful waste products. These conditions not only prevent nerve cells from functioning normally, but can also injure and kill them. Further damage may occur later when the blood flow is restored (reperfusion injury) and free radicals form. Either way, this damage causes brain malfunction, which becomes evident as the signs and symptoms of a stroke.
Strokes are also called cerebrovascular accidents (CVAs) because they result from abnormalities in the heart or blood vessels, which cause either a reduction in blood flow to areas in the brain or hemorrhaging within the brain itself. The first are called ischemic strokes and the second, hemorrhagic strokes. Strokes are classified based on the timing of the signs and symptoms, which vary widely depending on where and how badly the brain is damaged. An ischemic stroke having signs and symptoms that disappear within 24 hours is called a transient ischemic attack (TIA). TIAs often reoccur because blood flow to the same brain area is reduced and may eventually be followed by more serious strokes. In a reversible ischemic neurological deficit (RIND), the signs and symptoms subside more slowly. In a progressive stroke, signs and symptoms develop gradually and get worse over time. If the signs and symptoms develop quickly and show very little or no improvement over time, a completed stroke has occurred.
Most strokes result from atherosclerosis in arteries either within the brain or leading from the heart to the brain. Atherosclerosis leads to ischemic strokes by making arteries narrow, rough, stiff and weak. Narrowness reduces blood flow directly. Roughness reduces blood flow by causing the formation of blood clots (thrombi), which partially or completely block the vessel. Stiffness causes inadequate blood flow by preventing arteries from dilating when a region of the brain becomes more active and needs more oxygen or when blood pressure drops. Weakness in arteries can lead to reduced blood flow if the weak area bulges outward, forming an aneurysm. Though the region with the aneurysm is initially wider than normal, blood swirling in aneurysm can clot. As the clot enlarges, it can partially or completely block blood flow through the artery. Blood clots in arteries may break free of the vessel wall (embolize)--in which case flowing blood can push them into smaller arteries, where they block blood flow completely. Emboli formed by atherosclerotic plaque can also block arteries.