THE AVERAGE LIFE SPAN IN THE UNITED STATES in 1900 was about 50 years. By 2015, it was approximately 77 years for men and 82 for women (Figure 64–1). The average is even higher in 30 other countries. These increases result largely from a reduction in infant mortality, the development of vaccines and antibiotics, better nutrition, improved public health measures, and advances in the treatment and prevention of heart disease and stroke. Because of increased life expectancy, along with the large cohort of “baby boomers” born soon after World War II, the elderly are the most rapidly growing segment of the US population.
The human life span is increasing. The average life span in the United States has increased rapidly over the past 100 years. (Adapted from Strehler 1975; Arias 2004.)
Increased longevity is a double-edged sword since age-related cognitive alterations are increasingly prevalent. The magnitude of the change varies widely among individuals. For many, the alterations are mild and have relatively little impact on the quality of life—the momentary lapses we jokingly call “senior moments.” Other cognitive impairments, although not debilitating, are troubling enough to hinder our ability to manage life independently. The dementias, however, erode memory and reasoning and alter personality. Of these, Alzheimer disease is the most prevalent.
As the population ages, neuroscientists, neurologists, and psychologists have begun to devote more energy to understanding age-related changes in the brain. The primary motivation has been to find treatments for Alzheimer disease and other dementias, but it is also important to understand the normal process of cognitive decline with age. After all, age is the greatest risk factor for a wide variety of neurodegenerative disorders. Understanding what happens to our brains as we age may not only improve the quality of life for the general population but may also provide clues that will eventually help us vanquish seemingly unrelated pathological changes.
With this in mind, we begin this chapter with a consideration of the normal aging of the brain. We then turn to the broad range of pathological changes in cognition, and finally focus on Alzheimer disease.
The Structure and Function of the Brain Change With Age
As we grow old, our bodies change—our hair thins, our skin wrinkles, and our joints creak. It is no surprise then that our brain also changes. Indeed, the widespread behavioral alterations that occur with age are signs of underlying alterations in the nervous system. For example, as motor skills decline, posture becomes less erect, gait is slower, stride length is shorter, and postural reflexes often become sluggish. Although muscles weaken and bones become more brittle, these motor abnormalities result in large part from subtle processes that involve the peripheral and central nervous systems. Sleep patterns also change with ...