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. 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:

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 Depression Depression is a feeling of sadness and/or a decreased interest or pleasure in activities that becomes a disorder when it is intense enough to interfere with functioning. It may follow a recent... read more , stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction). Symptoms occur suddenly... read more , an underactive thyroid gland (hypothyroidism Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more ), and degenerative brain disorders such as Alzheimer disease Alzheimer Disease Alzheimer disease is a progressive loss of mental function, characterized by degeneration of brain tissue, including loss of nerve cells, the accumulation of an abnormal protein called beta-amyloid... read more .

As people age, the number of nerve cells Nerves The peripheral nervous system consists of more than 100 billion nerve cells (neurons) that run throughout the body like strings, making connections with the brain, other parts of the body, and... read more 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.

Thus, people who have had a brain injury or stroke can sometimes learn new skills, as occurs during occupational therapy Occupational Therapy (OT) Occupational therapy, a component of rehabilitation, is intended to enhance a person's ability to do basic self-care activities, useful work, and leisure activities. These activities include... read more .

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 Blood Pressure High blood pressure (hypertension) is persistently high pressure in the arteries. Often no cause for high blood pressure can be identified, but sometimes it occurs as a result of an underlying... read more , high cholesterol Dyslipidemia Dyslipidemia is a high level of cholesterol and/or triglycerides or a low level of high-density lipoprotein (HDL) cholesterol. Lifestyle, genetics, disorders (such as low thyroid hormone levels... read more , or high blood sugar (diabetes mellitus Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Symptoms of diabetes may... read more ) 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 Vascular Dementia Vascular dementia is loss of mental function due to destruction of brain tissue because its blood supply is reduced or blocked. The cause is usually strokes, either a few large ones or many... read more 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 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.