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their lifetimes. Overexposure to glucocorticoids also increases the similar increases in blood pressure and blood muscle flow when
number of neurons damaged by stroke. Moreover, prolonged performing a lab test. This confirmed that hostility scores do
exposure before or immediately after birth can cause a decrease not predict the biological response to simple mental tasks.
in the normal number of brain neurons and smaller brain size. Then the researchers added harassment to the test by lead-
The immune system, which receives messages from the ner- ing the subjects to believe that their performances were being
vous system, also is sensitive to many of the circulating hor- unfairly criticized. Men with high hostility scores showed much
mones of the body, including stress hormones. Moderate to larger increases in muscle blood flow and blood pressure, and
high levels of glucocorticoids act to suppress immune function, slower recovery than those with low hostility scores. Scientists
although acute elevations of stress hormones actually facilitate found that harassed men with high hostility scores have larger
immune function. increases in levels of stress hormones. Thus, if you are a hostile
Although acute stress-induced immunoenhancement can person, learning to reduce or avoid anger could be important to
be protective against disease pathogens, the glucocorticoid- avoid cardiovascular damage.
27
Aging
ablo Picasso, Georgia O Keefe and Grandma people, or four percent of the population, were older than age
Moses, artists. Louise Nevelson, sculptor. Albert 65, and typically they were ill. In 1990, when life expectancy was
Einstein, physicist. Giuseppe Verdi, musician. more than 75 years, 30 million people, or 12 percent of the pop-
Robert Frost, poet. Each of these great minds ulation, were older than age 65. A generation ago, frailty was
worked di"erently, but they all shared an out- seen among people in their 60s; today it is more typical among
Pstanding trait: They were creative and produc- those in their 80s. Moreover, few people challenged the notion
P
tive in old age. They defied the popular notion that aging always that aging meant inevitable brain decline because scientists
leads to a pronounced decline and loss of cognitive ability. knew little about the brain or the aging process. Today s under-
Neuroscientists now believe that the brain can remain rel- standing of how the normal brain ages comes from studies of
atively healthy and fully functioning as it ages, and that diseases the nervous system that began decades ago and are just now
are the causes of the most severe decline in memory, intelligence, bearing results. Modern technologies now make it possible to
verbal fluency and other tasks. Researchers are investigating the explore the structure and functions of the brain in more depth
normal changes that occur over time and the e"ect that these than ever before and to ask questions about what actually hap-
changes have on reasoning and other intellectual activities. pens in its aging cells.
It appears that the e"ects of age on brain function vary Thus, neuroscientists are increasingly able to distinguish
widely. The vast majority of people get only a bit forgetful in between the processes of normal aging and disease. While some
old age, particularly in forming memories of recent events. For changes do occur in normal aging, they are not as severe as sci-
example, once you reach your 70s, you may start to forget names entists once thought.
or phone numbers, or respond more slowly to conflicting infor- All human behavior is determined by how well the brain s
mation. This is not disease. However, other individuals develop communication systems work. Often a failure in the cascade of
senile dementia, the progressive and severe impairment in men- one of these systems results in a disturbance of normal func-
tal function that interferes with daily living. The senile demen- tions. Such a failure may be caused by an abnormal biochemi-
tias include Alzheimer s and cerebrovascular diseases and a"ect cal process or by a loss of neurons.
about one percent of people younger than age 65, with the inci- The cause of brain aging still remains a mystery. Dozens of
dence increasing to nearly 50 percent in those older than 85. In theories abound. One says that specific  aging genes are
a small, third group, that includes the Picassos, Nevelsons and switched on at a certain time of life. Another points to genetic
others, mental functioning seems una"ected by age. Many peo- mutations or deletions. Other theories implicate hormonal
ple do well throughout life and continue to do well even when influences, an immune system gone awry and the accumulation
they are old. The oldest human, Jeanne Calment, was consid- of damage caused by cell byproducts that destroy fats and pro-
ered to have all her wits during her 122-year lifespan. teins vital to normal cell function.
It s important to understand that scientific studies measure
Aging neurons
trends and reflect what happens to the norm. They don t tell what
happens to everybody. Some people in their 70s and 80s func- The brain reaches its maximum weight near age 20 and slowly
tion as well as those in their 30s and 40s. The wisdom and expe- loses about 10 percent of its weight over a lifetime. Subtle
rience of older people often make up for deficits in performance. changes in the chemistry and structure of the brain begin at
The belief that pronounced and progressive mental decline midlife in most people. During a lifetime, the brain is at risk
is inevitable was and still is popular for several reasons. For one, for losing some of its neurons, but neuron loss is not a normal
until the 20th century, few people lived to healthy old ages. In process of aging. Brain tissue can respond to damage or loss of
1900, when life expectancy was about 47 years, three million neurons in Alzheimer s disease or after stroke by expanding
28
Frontal cortex
Parietal cortex
Basal forebrain
tists debate whether this circuitry is changed only by neuron
Striate cortex
atrophy or whether some neuron loss over time also is in-
evitable. In any event, when the circuitry begins to break down,
remaining neurons can respond by expanding their roles.
THE AGING BRAIN. Studies of people who have died contradict the Learning conditions may dictate what happens to brain [ Pobierz całość w formacie PDF ]