The brain reaches its maximum size (by weight) in early adult life.1 It decreases by about ten percent over the remainder of the life span. It ways about three pounds and contains about one hundred billion brain cells (neurons). There are about a million billion connections (synapses) linking those cells together. As a person ages the number of synapses generally decreases, but the commonly cited figure of 50,000 cells a day is no longer believed by most neuroscientists. The loss of neurons that does occur is not evenly distributed across the brain. There is little or no significant loss in many cortical regions used in normal cognition.
More important than the loss of neurons and the thinning of synaptic connections that occurs as we age, is the loss of cells from cluster of cells (nuclei) about the size of a pinhead located in the brain stem. This brain stem is about the length of an adult forefinger. The neuroscientist Paul Coleman calls these nuclei “juice machines.” They send ascending fanlike projections to many parts of the cortex. The brains neurotransmitters travel along these projections. Reductions in levels of these neurotransmitters leads to many of the infirmities that inflict us as we age: memory loss, depression, decrease in overall mental sharpness, and inefficient mental processing. Fortunately these infirmities can be improved by drugs that increase these neurotransmitters.
Although the loss of neurons occurs normally with aging, this loss can be compensated for by increases in the networking capacity of the remaining neurons. Although the number of neurons decreases from birth onward, fewer but stronger and more enduring connections form among the remaining neurons (see the healthymemory blog posts “HAROLD,” “Is Dementia an Inevitable Part of Aging,” and “Hope for an Aging Population: STAC”).
“This capacity to compensate for the loss of its components makes the brain the only known structure in the universe that works more efficiently despite a loss of its components. To this extent the brain is unique among both biological and mechanical structures: over the years it doesn’t ‘wear out’.”2