The Central Nervous System
Although eventual shrinking of the brain is normal and ac-
celerates in old age, alcoholics and professional boxers hasten
the process. Whether a boxer wins the match or not, the likeli-
hood of brain damage and atrophy increases with every blow.
And everyone recognizes that alcohol profoundly aﬀects both
the mind and the body. CT scans of alcoholics reveal reduced
brain size and density starting at a fairly early age. Both boxers
and alcoholics exhibit signs of mental deterioration unrelated to
the aging process.
Check Your Understanding
What functional type of neuron is derived from the alar
plate? From the basal plate?
Premature babies have problems controlling their body
List several causes of reversible dementia in the elderly.
For answers, see Appendix H.
±e human cerebral hemispheres—our “thinking caps”—
are awesome in their complexity. But no less amazing are the
brain regions that oversee our subconscious, autonomic body
functions—the diencephalon and brain stem—particularly
when you consider their relatively insigniﬁcant size. ±e spi-
nal cord, which acts as a reﬂex center and a communication
link between the brain and body periphery, is equally impor-
tant to body homeostasis.
We have introduced a good deal of new terminology in this
chapter, and much of it will come up again in the remaining
nervous system chapters. Chapter 13, your next challenge, con-
siders the structures of the peripheral nervous system that work
hand in hand with the CNS to keep it informed and deliver its
orders to the eﬀectors of the body.
Growth and maturation of the nervous system continue
throughout childhood and largely reﬂect progressive myelina-
tion. As described in Chapter 9, neuromuscular coordination
progresses in a superior-to-inferior direction and in a proximal-
to-distal direction, and we know that myelination also occurs in
±e brain reaches its maximum weight in the young adult.
Over the next 60 years or so, neurons are damaged and die, and
brain weight and volume steadily decline. However, the number
of neurons lost over the decades is normally only a small per-
centage of the total, and the remaining neurons can change
their synaptic connections, providing for continued learning
Although age brings some cognitive declines in spatial abil-
ity, speed of perception, decision making, reaction time, and
working memory, these losses are not signiﬁcant in the healthy
individual until aFer the seventh decade. ±en the brain be-
comes increasingly fragile, presumably due to less eﬃcient cal-
cium clearance in aging neurons (as we have seen, elevated Ca
levels are neurotoxic). A fairly rapid decline occurs in some, but
not all, of these abilities.
However, mathematical skills, verbal ﬂuency, and the abil-
ity to build on experience do not decline with age, and many
people continue to enjoy intellectual lives and work at mentally
demanding tasks their entire life. ²ewer people over 65 than you
might think demonstrate true dementia. Sadly, many cases of
“reversible dementia” are caused by prescription drug side ef-
fects, low blood pressure, poor nutrition, hormone imbalances,
depression, and/or dehydration that go undiagnosed. ±e best
way to maintain your mental abilities in old age may be to seek
regular medical checkups throughout life.
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±e brain provides for voluntary movements, interpretation and
integration of sensation, consciousness, and cognitive function.
Early brain development yields the three primary brain vesicles:
the prosencephalon (cerebral hemispheres and diencephalon),
mesencephalon (midbrain), and rhombencephalon (pons,
medulla oblongata, and cerebellum).
As a result of cephalization, the diencephalon and superior brain
stem are enveloped by the cerebral hemispheres.
Regions and Organization
In a widely used system, the adult brain is divided into the
cerebral hemispheres, diencephalon, brain stem, and cerebellum.
±e cerebral hemispheres and cerebellum have gray matter nuclei
surrounded by white matter and an outer cortex of gray matter.
±e diencephalon and brain stem lack a cortex.
±e brain contains four ventricles ﬁlled with cerebrospinal ﬂuid.
±e lateral ventricles are in the cerebral hemispheres; the third
ventricle is in the diencephalon; the fourth ventricle is between
the brain stem and the cerebellum and connects with the central
canal of the spinal cord.
±e two cerebral hemispheres exhibit gyri, sulci, and ﬁssures. ±e
longitudinal ﬁssure partially separates the hemispheres; other
ﬁssures or sulci subdivide each hemisphere into lobes.
Each cerebral hemisphere consists of the cerebral cortex, the
cerebral white matter, and basal nuclei (ganglia).