(See also the aging brain and nervous system.)
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. Some areas of the brain decrease in size by up to 1% per year in some people but without any loss of function. Thus, age-related changes in brain structure do not always result in loss of brain function. However, a decrease in brain function with aging may be the result of numerous factors that include changes in brain chemicals (neurotransmitters),changes in nerve cells themselves, toxic substances that accumulate in the brain over time, and inherited changes. Different aspects of brain function may be 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 later.
Intellectual performance—the ability to process information (regardless of speed)—is usually maintained if no underlying neurologic or vascular 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 disease.
As people age, the number of nerve cells in the brain may decrease, although the number lost varies greatly from person to person, depending on the person’s health. In addition, some types of memory are more vulnerable to loss, such as memory that holds information temporarily. However, the brain has certain characteristics that help compensate for these losses.
Redundancy: The brain has more cells than it needs to function normally. Redundancy may help compensate for the loss of nerve cells that occurs with aging and disease.
Formation of new connections: The brain actively compensates for the age-related decrease in nerve cells by making new connections between the remaining nerve cells.
Production of new nerve cells: Some areas of the brain may produce new nerve cells, especially after a brain injury or a stroke. These areas include the hippocampus (which is involved in the formation and retrieval of memories) and the basal ganglia (which coordinate and smooth out movements).
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 damage to blood vessels leading to vascular 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 and release neurotransmitters is impaired, 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 (see figure 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 mellitus).
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.