How Stroke Affects the Brain
The effects of a stroke can be devastating. Sometimes brain damage from a stroke is too great, and the person dies or needs artificial life support. Strokes also cause death or damage to brain cells that results in long-lasting impairments of senses, motor skills, behavior, language ability, memory, and thought processes.
The good news is that experts are finding that the brain has great plasticity and powers of recuperation. The specific deficits and potential for recovery depend on which portions of the brain are damaged by the stroke and the type and severity of the stroke. Strokes can affect the following areas of the brain: brain stem, cerebellum, limbic system, and cerebrum.
Located at the base of the brain, the brain stem maintains basic life-supporting functions such as breathing, heart rate, blood pressure, and digestion. A major stroke in this part of the brain is usually fatal.
When people survive, they often require artificial life support. Since the brain stem also helps maintain consciousness, a major stroke in this area can lead to a coma. A coma can also occur when a stroke in the cerebrum, which surrounds the brain stem, causes swelling and puts pressure on the brain stem.
Located above the brain stem, the cerebellum controls coordination, balance, and posture. Strokes in this area cause clumsiness, shakiness, and difficulty controlling muscles. Long-term impairment includes coordination and balance problems, difficulty walking, and headaches. Strokes in the cerebellum are not common.
This group of structures above the cerebellum controls basic physiological processes, such as growth, reproductive cycles, and sleep cycles, as well as powerful emotions like rage, terror, hunger, and sexual desire.
Strokes in this area of the brain are rare, but when they occur basic human drives may be severely impaired and natural inhibitions may be lost.
Approximately two-thirds of strokes occur in the cerebrum, the largest and uppermost portion of the brain. Its outer layer of gray matter, known as the cerebral cortex, is the center of conscious thought, perception, voluntary movement, and integration of all sensory input.
The cerebral cortex is divided into two halves, or hemispheres, each of which is responsible for a different set of duties.
In most right-handed people, the right hemisphere specializes in spatial relationships, color perception, visual interpretation, and musical aptitude; the left half of the brain typically oversees analytical tasks (such as mathematical computation and logical reasoning) and linguistic tasks (such as comprehending words and formulating speech). In left-handed people, the hemispheres responsible for these duties are typically reversed. One exception is speech, which usually involves both hemispheres in left-handed people.
Each brain hemisphere also governs movement and sensory perception on the opposite side of the body. Thus, a person who has a stroke in the left hemisphere may experience paralysis or sensory deficits (such as numbness) on the right side of the body. Each hemisphere is further divided into four lobes:
1. Frontal lobe. This lobe is at the front of the brain, behind the brow. A part of the frontal lobe called the motor cortex is responsible for motor function. Damage to the motor cortex on one side of the brain can result in weakness or paralysis on the opposite side of the body. Expressive aphasia (difficulty speaking, writing, or gesturing) can occur when a stroke affects the frontal lobe in the dominant hemisphere (for example, the left hemisphere of someone who is right-handed).
2. Parietal lobe. Located behind the frontal lobe, the parietal lobe is responsible for receiving and interpreting sensory information from the body. When a stroke affects the parietal lobe, it can cause sensory loss (such as numbness) and vision problems on the side of the body opposite from the brain damage.
3. Temporal lobe. When a stroke affects the temporal lobe, situated at ear level and underneath both the parietal and frontal lobes, it can produce language problems known as aphasia (difficulty understanding speech, verbalizing thoughts, reading, or writing).
Memory loss also is a common consequence of a stroke in the temporal lobe. However, memory problems may be only temporary, since the temporal lobe on the other side of the brain can eventually compensate (unless both sides of the brain have been affected by the stroke).
4. Occipital lobe. Lying at the rear of the cerebral cortex, in the back of the skull, the occipital lobe is dedicated to the perception and interpretation of visual data delivered from the eyes via the optic nerve. An occipital lobe stroke can result in loss of the ability to recognize and interpret visual stimuli—faces, for example.
A stroke may also produce long-term problems—from sleep disturbances and emotional instability to poor judgment and depression.