The Effects of Aging on the 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. 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 at least age 80 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 usually decreases, although the number lost varies greatly from person to person, depending on the person’s health.
Also 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.
Spinal Cord
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.
Written by: Steven A. Goldman, MD, PhD. Published in the Merck Manual, Consumer version.