Aging of the CNS begins before birth, as programmed cell death (apoptosis) determines which structures will remain in the brain. Neurons continually die throughout life.
Effects of
Aging on the Brain
Aging of the CNS begins before
birth, as programmed cell death (apoptosis) determines which structures will
remain in the brain. Neurons continually die throughout life. When apoptosis of
the brain fails, disease develops such as in the brains of schizo-phrenics.
Their brains, in young adulthood, have the same numbers of neurons as those of
newborn babies. These extra amounts of neurons produce excessive dopamine,
which can cause the halluci-nations that are so prevalent in this disease.
After young adulthood, brain neurons of all individuals start to become damaged
and die. However, only a small percentage of total brain neurons are actually lost.
Continued learning throughout the life span occurs because the remaining
neurons can alter their synaptic connections.
By approximately age 30, neuron
death increases in speed. However, pockets of neural stem cells that line the
ventricles are still able to create cells that differentiate into neurons and
neuroglia. In most individuals, the brain shrinks by about 10% over the
lifetime, with more gray matter lost than white mat-ter. There is an uneven
loss of neurons. While many cells die in the temporal lobe, very few die in the
brain stem. By the age of 90, nearly half of the neurons of the frontal cortex
have been lost; yet, this does not mean that function is necessarily impaired.
In elderly people, the brain
becomes reduced in both size and weight, which is linked to loss of the
cortical neurons. The synaptic organization of the brain changes as the number
of dendritic branches and other structures decreases. This limits
neu-rotransmitter production. Brain neurons accumulate abnormal intracellular deposits.
Extracellular plaques may affect memory processing. When deposits and plaques
exceed normal amounts caused by aging, clinical abnormalities may occur.
Aging does cause cognitive
declines that may affect perception speed, spatial abilities, making decisions,
reacting to occurrences, and memory loss. Cognitive declines usually only
become significant when a person is in his or her 70s. Functions that do not
decline with age include fluency of speech and mathematical skills. Reversible
dementia may be caused by adverse effects of medications, poor nutrition, low
blood pressure, hormone imbal-ances, dehydration, and depression. Alcoholics
and athletes who experience head trauma experience greater loss of brain size
and weight and at much earlier ages.
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