The neural tissues arise from the ectoderm, in response to its adjacent layer, the mesoderm.
Effects of
Aging on the Nervous System
The neural tissues arise from the
ectoderm, in response to its adjacent layer, the mesoderm. Neural tissues form
and develop connections during the embryonic period. Neurons are produced along
the central canal, in the neural tube. They later move to their necessary
locations, before birth. Axons and dendrites extend from nerve cells to their
intended targets. The human brain takes many years before it is completely
developed. During fetal development, a child’s head is much larger than the
remainder of the body. As periods of growth occur, the brain develops very
quickly. After birth, up to the age of three years, the brain triples in weight
and establishes approxi-mately 1,000 trillion nerve connections. Development of
the frontal lobe increases greatly between six and 12 months of age. The
prefrontal cortex portion of the frontal lobe is the final brain area to
mature, resulting in developmental changes as late as adolescence. This area is
the location of cognitive functions that include attention, motivation, and
goal-directed behavior.
Beginning at age 30, anatomic and
physiological changes begin to affect the nervous system. After age 65, there may
be noticeable changes in CNS function and mental performance. As fatty deposits
accumulate in the blood vessels, there is a decrease in blood flow to the
brain. This can increase the chances that an affected vessel will rupture,
leading to symptoms of a stroke (cerebrovascular accident). Cerebrovascular
diseases are more common in long-term smokers or when conditions such as
hypertension, high cholesterol, or diabetes mellitus are present.
Verbal abilities often begin to
decline at approximately age 70. If no neurologic disorders are present,
intellectual performance is usually maintained until about age 80. Because the
brain processes nerve impulses more slowly, performance of certain tasks and
reaction times often become slower. Other dis-orders that affect the nervous
system due to aging include depression, hypothyroidism, and degenera-tive brain
disorders. An elderly person who exercises (both mentally and physically) often
loses fewer nerve cells in the brain. Consumption of two or more drinks of
alcohol every day reduces brain function.
The spine is also affected by
aging, and pressure increases on the spinal cord and spinal nerve roots. This
can result in decreases in sensation, strength, and balance. Peripheral nerve
conduction slows because the myelin sheaths degenerate with aging. Self-repair
of damaged peripheral nerve cells is also slower in older individuals.
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