MODULE SUPPLEMENT: NEUROLOGICAL SYSTEM
Why Changes in Structure, Metabolism, and Function Occur

Why do these changes in brain structure, metabolism, and function occur?

Several non-mutually exclusive ideas have been proposed. One focuses on the impact of free radicals (Durany, et al., 1999; Halliwell, 2001) (See also The Free Radical Theory of Aging, note: this link will open in a new browser window which you can close to return here), while others emphasizes problems related to altered cellular handling of calcium or the accumulation of amyloid. 

As noted in the theories of aging modules, free radicals have an extra electron in their outer ring and are highly reactive. 

However, while all cells and tissues in the body can experience oxidative damage, Halliwell (2001) notes that there are at least 9 reasons why the brain is especially sensitive. First, it consumes a lot of oxygen. Second, it uses glutamate extensively which, if its levels rise because of impaired energy metabolism, can raise intracellular calcium to toxic levels. Third, many neurotransmitters can be auto-oxidized. For example, the metabolism of dopamine, levodopa, and norepinephrine create free radicals. Forth, iron can be involved in the production of free radicals and is found throughout the brain. Fifth, the membranes of neurons contain a lot of lipids that are susceptible to free radical lipid peroxidation (at least in vitro). Sixth, general brain metabolism can generate free radicals. Seventh, the brain's defenses against free radicals are relatively low in general. Eighth, microglia, like other phagocytic cells, can produce free radicals as part of its routine functioning. And Ninth, cytochromes P450 can be found in certain areas of the brain and apparently can produce free radicals. 

Beliefs in the impact of free radicals underpins some of the approaches to preventing dementing conditions or minimizing their progression, such as the use of Vitamin E (Klatte, et al. 2003; Masaki, et al, 2000) as well as other nutritional approaches.