As a person ages, their brain undergoes a number of changes :
Some nerve cells, called neurons, in various regions of the brain die, although the neurons most important to learning usually live on.
Some neurons, and the fibers that connect them to other neurons, shrink and degenerate. This tends to especially occur areas of the brain that are important to learning, memory, planning, problem solving, and other complex mental activities.
Twisted fibers, called tangles, develop within neurons and protein plaques develop in the areas surrounding neurons.
Tiny structures inside neurons that metabolize energy for cell functions become more susceptible to damage. Inflammation (swelling) of the brain increases, which cause damage to nerve cells.
Oxidative stress increases. This is caused by the release of special molecules, called free radicals, from normal cellular processes. This can lead to nerve cell damage and death.
In healthy older people, the impact of these changes may be modest, resulting in various degrees of age-related memory decline. In people who have Alzheimer’s, however, some of these changes are much more extreme, with devastating consequences. Determining how the brain changes under normal aging conditions and what relevance these have to Alzheimer’s is an important area of research.