When we get older, our minds shift in subtle yet observable ways. Sections of the cerebral cortex shrink as neurons retract the branched extensions that receive chemical signals.
But what is the root of these shifts? Many hypotheses exist to understand why neurons, and cells in general, age. They are as follows:
Oxidative Stress and DNA Damage
Gene expression changes can play a role in neuronal aging. Genes involved in synaptic plasticity are less present in older people's brains than in younger adults' brains. These genes are also more likely to display signs of damage.
DNA damage sustained over a lifetime can play a role in the aging of the brain and body. Every cell in your body has mitochondria, which serve as cellular power plants, carrying out chemical reactions that provide energy. Some of these metabolic reactions create dangerous byproducts known as free radicals, which are highly reactive molecules that, if left unchecked, can kill fats and proteins necessary for normal cell function, as well as damage DNA. To neutralize free radicals and avoid the oxidative stress they inflict, your body has built-in defense mechanisms. Unfortunately, as people get older, their defenses weaken, making them more susceptible to oxidative harm.
Oxidative damage appears to play a role in the age-related deterioration in memory and cognitive abilities, according to studies in humans and rodents. People with moderate cognitive impairment and Alzheimer's disease have more signs of oxidative damage in their brains than healthy people, and rats with more oxidative damage do worse on memory tests.
The brain is one of the most metabolically active organs in the body, requiring about 20% of the oxygen and fuel consumed by the body. Because of its high energy demands, the brain may be more prone to physiological changes associated with aging than other tissues. Although the brain's energy needs remain high, its energy supply is no longer able to keep up; the brain's capacity to consume and use glucose decreases, and mitochondrial metabolism slows.
Immune dysfunction
Immune system problems are often associated with the metabolic changes that accompany aging. Microglia, the brain's primary immune response cells, have a variety of functions, including protecting against bacteria, cleaning up cellular waste, and assisting in the maintenance and remodeling of synapses. These inflammatory reactions are beneficial, but an inflammatory state that lasts too long is detrimental to the brain's health.
With age, microglia become more aggressive, increasing the inflammatory response in the brain while decreasing the development of anti-inflammatory molecules that are beneficial. Excessive microglia activity has been shown in mice to affect cognitive ability.
Impaired protein recycling
Damaged proteins and molecules are usually broken down and recycled by cells. This mechanism, however, isn't perfect, and damaged molecules can accumulate in cells over time, preventing them from functioning normally. In Alzheimer's disease, for example, defective proteins accumulate in the brain and clump together. Outside of brain cells, amyloid proteins form plaques, and tau protein tangles form within brain cells.
In a healthy brain, the accumulation of proteins and other cell components also leads to cellular degeneration. Since brain cells are not replaced as often as cells in other sections of the body, such as bone marrow, intestinal lining, and hair follicles, they may be more susceptible to the buildup of damaged molecules. Furthermore, the cellular machinery involved in the breakdown and recycling processes deteriorates with age, lowering the performance of waste collection systems.
While some of the changes that occur in the brain are an unavoidable part of the aging process, they are not entirely beyond our control. Poor cardiovascular health, for example, is likely to play a role in the aging brain's changes. On the other hand, the same things that are good for your heart — eating a balanced diet, exercising regularly, and reducing stress — are also good for your brain as you grow older.
