final stage of functional organization of the brain’s
processes and mechanisms. The numbers of neu-
roreceptors on the surfaces of neurons also
increase dramatically, particularly those for
DOPAMINEwhich is the primary brain NEUROTRANS-
MITTER for functions of cognition and behavior.
This axonal growth makes the adolescent brain
especially vulnerable to the neurotoxic effects of
ALCOHOLand drugs. Damage that disrupts axonal
growth during adolescence often has complex and
permanent, consequences for brain function dur-
ing the rest of life.
The Elderly Nervous SystemThe brain remains capable of carrying out its func-
tions across the lifespan unless injury or disease
interrupts. But by old age the likelihood of injury,
disease, and general health problems is higher
than any other stage of life. Many diseases com-
mon in old age affect the brain and nervous sys-
tem even when they primarily involve other body
systems. CARDIOVASCULAR DISEASE(CVD), for exam-
ple, may change the BLOODflow to the brain.
HYPERTENSION(high BLOOD PRESSURE), one of the
most common forms of cardiovascular disease, is
the leading cause of STROKE; and stroke is the lead-
ing cause of disabling brain injury. DIABETESdam-
ages the delicate blood vessels that nourish the
peripheral nerves, most notably damaging sensory
perception—the ability of distant body parts such
as the feet to send PAIN signals to the brain
(peripheral NEUROPATHY). Chronic CIRRHOSIScreates
widespread metabolic imbalances in the body that
alter brain functions from cognition to motor
movement (hepatic ENCEPHALOPATHY).
The likelihood of neurologic disease also
increases with age. Conditions such as ALZHEIMER’S
DISEASE, PARKINSON’S DISEASE, HUNTINGTON’S DISEASE,
DEMENTIA, CREUTZFELDT-JAKOB DISEASE(CJD), TREMOR
DISORDERS, and ORGANIC BRAIN SYNDROME seldom
develop in people under the age of 50. As many as
half those age 85 and older have Alzheimer’s dis-
ease, however. Progressive neurologic conditions
that begin earlier in life, such as MULTIPLE SCLEROSIS,
tend to exhibit the most severe of their symptoms
as age advances. Such changes are generally irre-
versible.
Neurons, like other cells, die throughout life.
Researchers believe such cell death is a form of
culling that helps the brain maintain its efficiency.
By old age the cumulative effect of this cell death
results in decreased brain tissue. Imaging proce-
dures such as MAGNETIC RESONANCE IMAGING (MRI)
show the ventricles (spaces) are larger in the brain
of a 70-year-old person than a 30-year-old person.
The numbers of sensory nerve receptors in the
body (peripheral nerve structures) also begin to
decline, reducing to some extent the sensory input
that reaches the brain.
But these changes are not sufficient, in them-
selves, to significantly diminish the brain’s func-
tions. Indeed, recent research suggests the brain
gets “smarter” with age, developing shortcuts and
efficiencies in the ways that it processes informa-
tion. Many people reach age 80 and beyond with
relative good memory, cognition, and independ-
ence. Though physical changes do take place in
the brain with aging, researchers believe neuro-
logic deficiency is notinherently a normal dimen-
sion of aging.
See also CEREBRAL PALSY; COGNITIVE FUNCTION AND
DYSFUNCTION; FETAL ALCOHOL SYNDROME; MEMORY AND
MEMORY IMPAIRMENT.Alzheimer’s disease A progressive, degenerative
condition that causes irreversible loss of cognitive
and memory functions. The hallmarks of the dis-
ease are the diminished production of acetyl-
choline, a NEUROTRANSMITTERessential for cognitive
function, and the formation of amyloid plaques
(abnormal protein deposits) and neurofibrillary
tangles (resulting from another protein, tau),
within the BRAIN. These formations interfere with
normal NEURON communication and literally
“scramble” NERVE signals. Alzheimer’s disease is
typically a condition of aging, primarily affecting
people age 70 and older, though an early-onset
form of the disease may strike people who are in
their 40s or 50s. Early-onset Alzheimer’s disease
tends to progress more rapidly. More than 4 mil-
lion Americans have Alzheimer’s disease.
The causes of Alzheimer’s disease remain
unclear, though GENE MUTATIONis emerging as a
leading candidate. In the 1990s researchers corre-
lated mutations in the apolipoprotein E (apoE)
gene on CHROMOSOME 19 with the tendency,
depending on the ALLELE(form of the gene) inher-
ited, to develop Alzheimer’s disease in old age.Alzheimer’s disease 221