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:
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 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. Also, the remaining nerve cells function less well. However, the brain has certain characteristics that help compensate for these losses.
Thus, people who have had a brain injury or stroke can sometimes learn new skills, as occurs during occupational therapy.
People can influence how quickly brain function declines. For example, physical exercise seems to slow the loss of nerve cells in areas of the brain involved in memory. Such exercise also helps keep the remaining nerve cells functioning. On the other hand, consuming two or more drinks of alcohol a day can speed the decline in brain function.
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 of the arteries to the brain (cerebrovascular disease). This disease is more likely to occur in people who have smoked for a long time or who have high blood pressure, high cholesterol, or high blood sugar (diabetes mellitus) that is not controlled by lifestyle changes or drugs. These people may lose brain cells prematurely, possibly impairing mental function. As a result, the risk of dementia at a relatively young age is increased.
As people age, the disks between the back bones (vertebrae) become hard and brittle, and parts of the vertebrae may overgrow. As a result, the disks lose some of their capacity to cushion, so more pressure is put on the spinal cord and on the branches of the nerves that emerge from it (spinal nerve roots). The increased pressure may injure nerve fibers at the point where they leave the spinal cord. Such injury can result in decreased sensation and sometimes decreased strength and balance.
As people age, peripheral nerves may conduct impulses more slowly, resulting in decreased sensation, slower reflexes, and often some clumsiness. Nerve conduction can slow because the myelin sheaths around nerves degenerate. Myelin sheaths are layers of tissue that insulate nerves and speed conduction of impulses (Fig. 3: Typical Structure of a Nerve Cell).
Degeneration also occurs because as people age, blood flow decreases, nearby bones overgrow and put pressure on the nerves, or both. Age-related changes in function can become more noticeable when the nerves are injured by something else (for example, by diabetes).
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 its cell body, located in or near the spinal cord, is undamaged. This self-repair process occurs more slowly and incompletely in older people, making older people more vulnerable to injury and disease.
Last full review/revision December 2014 by Steven A. Goldman, MD, PhD