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Aging neurons

Today’s understanding of how the normal brain ages comes from studies of the nervous system that began decades ago and are just now bearing results. Modern technologies now make it possible to explore the structure and functions of the brain in more depth than ever before and to ask questions about what actually happens in its aging cells. Thus, neuroscientists are increasingly able to distinguish between the processes of normal aging and disease. While some changes do occur in normal aging, they are not as severe as scientists once thought.

All human behavior is determined by how well the brain’s communication systems work. Often a failure in the cascade of one of these systems results in a disturbance of normal functions. Such a failure may be caused by an abnormal biochemical process or by a loss of neurons.

The cause of brain aging still remains a mystery. Dozens of theories abound. One says that specific “aging genes” are switched on at a certain time of life. Another points to genetic mutations or deletions. Other theories implicate hormonal influences, an immune system gone awry and the accumulation of damage caused by cell byproducts that destroy fats and proteins vital to normal cell function.

The brain reaches its maximum weight near age 20 and slowly loses about 10 percent of its weight over a lifetime. Subtle changes in the chemistry and structure of the brain begin at midlife in most people. During a lifetime, the brain is at risk for losing some of its neurons, but neuron loss is not a normal process of aging. Brain tissue can respond to damage or loss of neurons in Alzheimer’s disease or after stroke by expanding dendrites and refining connections between neurons. A damaged brain neuron can readjust to damage only if its cell body remains intact. If it does, regrowth can occur in dendrites and axons. When neurons are completely destroyed, nearby surviving neurons can compensate, in part, by growing new dendrites and connections.

In the first large studies to follow the same group of normal healthy humans for many years, scientists have uncovered unexpected results. They report declines in some mental functions and improvements in others. In one study, the speed of carrying out certain tasks became slower, but vocabulary improved. Several studies found less severe declines in the type of intelligence relying on learned or stored information, compared with the type that uses the ability to deal with new information.

This research is supported by animal studies in which scientists found that changes in mental function are subtle. For example, in rodents and primates in which only minor brain abnormalities can be detected, certain spatial tasks, such as navigating to find food, tend to become more difficult with age.

The aging brain is only as resilient as its circuitry. Scientists debate whether this circuitry is changed only by neuron atrophy or whether some neuron loss over time also is inevitable. In any event, when the circuitry begins to break down, remaining neurons can respond by expanding their roles.

Learning conditions may dictate what happens to brain cells. Studies of rats shed light on some of the changes that occur in brain cells when the animals live in challenging and stimulating environments. In tests of middle-aged rats exposed to these environments, researchers found that dendrites in the cerebral cortex, which is responsible for all conscious activity, developed more and longer branches when compared with rats housed in isolated conditions. Another study showed that brain cells in rats given acrobatic training had greater numbers of synapses per cell than rats given only physical exercise or rats who were inactive. The scientists concluded that motor learning generates new synapses. Physical exercise, however, improved blood circulation in the brain.

Other scientists report that rats reared in a stimulating environment made significantly fewer errors on a maze test than did similar rats kept in an isolated environment. Moreover, the stimulated rats showed an increase in brain weight and cortical thickness when compared with the control animals.

Older rats tend to form new dendrites and synapses as do younger animals in response to enriched environments. But the response is more sluggish and not as large. Compared to younger rats, the older rats have less growth of the new blood vessels that nourish neurons.

While much has been learned about the aging brain, many questions remain to be answered. For instance, does the production of proteins decline with age in all brain neurons? In a given neuron, does atrophy cause a higher likelihood of death? How does aging affect gene expression in the brain—the organ with the greatest number of active genes? Are there gender differences in brain aging that may be due to hormonal changes at menopause?

Neuroscientists speculate that certain genes may be linked to events leading to death in the nervous system. By understanding the biology of the proteins produced by genes, scientists hope to be able to influence the survival and degeneration of neurons. (From the book Brain Facts: a Primer on the Brain and Nervous System, 2002)

Exercise 7. Are the following statements true of false, according to the text? Explain your opinion, correct the false statements.

1. Mental decline with age is caused by loss of neurons.

2. Healthy aging is accompanied by a considerable loss of memory.

3. Nerve cells can both age and regenerate with time.

4. Neurons respond very slowly to the change of environment and lifestyle.

5. Age can bring some improvement of mental activity.


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