Effects of Aging

Brain: Brain function varies normally as people pass from childhood through adulthood to old age. During childhood, the ability to think and reason steadily increases, enabling a child to learn increasingly complex skills. During most of adulthood, brain function is relatively stable. After a certain age, which varies from person to person, brain function declines. Different aspects of brain function are affected at different times. Short-term memory and the ability to learn new material tend to be affected relatively early. Verbal abilities, including vocabulary and word usage, may begin to decline at about age 70. Intellectual performance--the ability to process information (regardless of speed)--is usually maintained until about age 80 if no neurologic disorders are present. Reaction time and performance of tasks may become slower because the brain processes nerve impulses more slowly. However, the effects of aging on brain function may be difficult to separate from the effects of various disorders that are common among older people. These disorders include depression, stroke, an underactive thyroid gland (hypothyroidism), and degenerative brain disorders such as Alzheimer's disease.

As people age, the number of nerve cells in the brain usually decreases, although the number lost varies greatly from person to person, depending on the person's health. As the number of nerve cells decreases, new connections are made between remaining nerve cells. These connections may help compensate for the loss of nerve cells. In addition, the brain has more cells than it needs to function normally--a characteristic called redundancy. Redundancy may also help compensate for the loss of nerve cells that occurs with aging and disease. Furthermore, new nerve cells may form in some areas of the brain, even during old age.

As people age, blood flow to the brain may decrease by an average of 20%. The decrease in blood flow is greater in people who have atherosclerosis affecting the arteries to the brain (cerebrovascular disease), which may be associated with smoking, high cholesterol levels, diabetes, or high blood pressure. The decreased blood flow that results from these conditions can cause nerve cells in the brain to be lost prematurely. Consequently, brain function may decline prematurely.

Spinal Cord: As people age, the bone of the spine may overgrow, putting pressure on the spinal cord. As a result, the number of axons in the spinal cord decreases, leading to a slight decrease in sensation.

Peripheral Nerves: As people age, peripheral nerves may conduct signals more slowly. Usually, this effect is so minimal that no change in function is noticeable. Also, the peripheral nervous system's response to injury is reduced. When the axon of a peripheral nerve is damaged in younger people, the nerve is able to repair itself as long as the cell body is undamaged. This self-reparation occurs more slowly and incompletely in older people than in younger people, making older people more vulnerable to injury and disease.