Theories on Aging
Learning Outcomes
- Describe and compare theories of aging
Why do we age?
There are a number of attempts to explain why we age and many factors that contribute to aging. The peripheral slowing hypothesis suggests that overall processing speed declines in the peripheral nervous system, affecting the brain’s ability to communicate with muscles and organs. Some of the peripheral nervous system (PNS) is under a person’s voluntary control, such as the nerves carrying instructions from the brain to the limbs. As well as controlling muscles and joints, the PNS sends all the information from the senses back to the brain.
The generalized slowing hypothesis theory suggests that processing in all parts of the nervous system, including the brain, are less efficient with age. This may be why older people have more accidents. Genetics, diet, lifestyle, activity, and exposure to pollutants all play a role in the aging process.[1][2][3]
Cell Life
Cells divide a limited number of times and then stop. This phenomenon, known as the Hayflick limit, is evidenced in cells studied in test tubes which divide about 50 times before becoming senescent. In 1961, Dr. Hayflick theorized that the human cell’s ability to divide is limited to approximately 50-times, after which they simply stop dividing (the Hayflick limit theory of aging). According to telomere theory, telomeres have experimentally been shown to shorten with each successive cell division.[4]
Senescent cells do not die. They simply stop replicating. Senescent cells can help limit the growth of other cells which may reduce risk of developing tumors when younger, but can alter genes later in life and result in promoting the growth of tumors as we age (Dollemore, 2006). Limited cell growth is attributed to telomeres which are the tips of the protective coating around chromosomes. Each time cells replicate, the telomere is shortened. Eventually, loss of telomere length is thought to create damage to chromosomes and produce cell senescence.
Link to Learning
Watch this Ted talk by molecular biologist Elizabeth Blackburn on “The Science of Cells That Never Get Old.” Blackburn won a Nobel Prize for her pioneering work on telomeres and telomerase, which may play central roles in how we age.
Biochemistry and Aging
Free Radical Theory of Aging
The free radical theory of aging (FRTA) states that organisms age because cells accumulate free radical damage over time. A free radical is any atom or moleculewhich has a single unpaired electron in an outer shell. This means that as oxygen is metabolized, mitochondria in the cells convert the oxygen to adenosine triphosphate (ATP) which provides energy to the cell. Unpaired electrons are a byproduct of this process and these unstable electrons cause cellular damage as they find other electrons with which to bond. These free radicals have some benefits and are used by the immune system to destroy bacteria. However, cellular damage accumulates and eventually reduces