The natural process of cell growth, decline, and death. Although aging is an inevitable and familiar process, there is much about it that scientists can observe and even measure but do not fully understand. At what point does the body cease to replace cells that die? Is this a biological or chronological point? Why does it and aging occur at seemingly different rates in different people? Can an individual influence his or her rate of aging through lifestyle choices? Why and how? Why do apparently identical choices have different outcomes from one person to another? These and myriad other questions intrigue researchers and guide numerous research studies, yet the answers remain elusive.
Age-Related Health Changes
As the body ages, it becomes less resilient in its ability to resist and recover from the ailments common to everyday life. What were mild aches and discomforts in youth and middle age can become obstacles that interfere with favorite activities as well as the activities of daily living (ADLs). Many of these changes, as the best scientists understand them, are biological in nature. The body, as it grows older, changes in terms of the way it functions. Certain processes become slower, and others (such as menstruation) stop altogether. The likelihood of developing many common health problems arthritis, cancer, diabetes, and heart disease, to name just a few likewise increases with age. The reasons are likely a blend of genetics and environment (including lifestyle). Modern medicine has become more successful in treating these conditions, often delaying them for years and even decades. These interventions do not delay the aging process, however. Eventually the body’s tissues and organ systems wear out whether or not diseases develop.
Age-Related Changes in the Brain
As a function of aging, cells die throughout the body. This is true in the brain just as it is true elsewhere in the body. With increasing age, parts of the brain such as the hippocampus shrink. As well, neurotransmitters in the brain and throughout the body become less active; it takes more stimulation to initiate their actions. One result might be that cognitive functions such as deductive reasoning and memory recall take a little longer. Experience allows most people to function without much noticeable change, however. An older person might occasionally forget where he or she put the car keys, for example, but knows where the keys are likely to be and easily finds them. Age-related changes in the brain can be difficult to distinguish from health conditions (such as Alzheimer’s disease) involving brain functions or can compound the effects of disease or injury (such as with stroke).
Age-Related Parkinson’s Disease
Changes related to aging in the brain also take place in the substantia nigra, where the brain’s dopamine-producing neurons reside. Some scientists speculate that everyone who lives long enough will eventually experience enough depletion of these neurons to result in symptoms such as those that occur with Parkinson’s disease. But does this mean everyone who lives long enough (a significant variable in itself) will develop actual Parkinson’s disease? Or is Parkinson’s disease just a name given to what is really a natural and normal process of aging? Most researchers believe neither idea is entirely accurate, although there are components of evidence for both. One estimate is that every decade after age 40, the substantia nigra loses 10 percent of its dopamine-producing cells through the normal processes of aging. Yet the brain can function remarkably well with limited resources; scientists estimate that the substantia nigra can adequately compensate for dopamine depletion until it has as little as 20 percent of the dopamine-producing neurons of a healthy brain. Even though there is a correlation between aging and Parkinson’s disease, aging is not the only reason Parkinson’s disease occurs.
About a third of people older than age 75 have at least some of the symptoms of Parkinson’s disease, although often the symptoms are mild enough that the person does not seek treatment for them or they are discovered incidentally in relation to other health concerns. Most people diagnosed with Parkinson’s disease are older than age 60, and the progression of the disease for them is usually gradual. Most will die of causes other
than Parkinson’s disease. One theory as to the cause of Parkinson’s disease is that it is the consequence of accelerated aging, in which the neurons of the substantia nigra age more rapidly than cells elsewhere in the body to leave the brain “older” than the rest of the body. What triggers such an acceleration, if this is in fact what happens, remains a mystery. As well, it fails to explain early-onset Parkinson‘s disease and the vast variations among individuals in the course of the disease. Many of these same questions apply as well to other degenerative diseases that are more prevalent in old age.
See also accelerated aging; apoptosis;