431
CHAPTER ELEVEN
Nervous System II
organ system begins before birth, as apoptosis, a form of
programmed cell death, carves out the structures that will
remain in the brain. This normal dying off of neurons con-
tinues throughout life. When brain apoptosis fails, disease
results. For example, the brains of individuals who die of
schizophrenia as young adults contain the same numbers of
neurons as do newborns. These extra neurons produce the
extra dopamine that can lead to hallucinations, the hallmark
of this illness.
By age thirty, the die-off of neurons accelerates some-
what, although pockets of neural stem cells lining the ventri-
cles retain the capacity to give rise to cells that differentiate
as neurons and neuroglia. Over an average lifetime, the brain
shrinks by about 10%, with more loss in gray matter than
white. Neuron loss is uneven—many cells die in the tempo-
ral lobe, but very few die in the brainstem. By age ninety,
the frontal cortex has lost about half its neurons, but this
de± cit doesn’t necessarily hamper function.
The nervous system changes over time in several ways.
The number of dendritic branches in the cerebral cortex
falls. Signs of slowing neurotransmission include decreas-
ing levels of neurotransmitters, the enzymes necessary to
synthesize them, and the numbers of postsynaptic recep-
tors. The rate of action potential propagation may decrease
by 5% to 10%. Nervous system disorders that may begin to
cause symptoms in older adulthood include stroke, depres-
sion, Alzheimer disease, Parkinson disease, and multi-
infarct dementia.
Noticeable signs of a normally aging nervous system
include fading memory and slowed responses and refl
exes.
Decline in function of the sympathetic nervous system may
cause transient drops in blood pressure, which, in turn, may
cause fainting. By the seventh decade, waning ability of
nerves in the ankles to respond to vibrations from walking
may affect balance, raising the risk of falling. Poor eyesight,
anemia, inner ear malfunction, and effects of drugs also con-
tribute to poor balance in the later years. Because of these
factors, nearly a third of individuals over age sixty-±
ve have
at least one serious fall a year.
Changes in sleep patterns accompany aging, refl ecting
the functioning of the reticular activating system. Older indi-
viduals generally sleep fewer hours per night than they once
did, experiencing transient dif± culty in getting to sleep and
staying asleep, with more frequent movements when they
are sleeping. Many have bouts with insomnia, sometimes not
sleeping more than an hour or two a night. Changing elec-
troencephalogram patterns indicate that stage IV slow-wave
sleep as well as REM sleep diminish. All of these changes
may result in daytime sleepiness.
PRACTICE
48
How does aging of the nervous system begin even before birth?
49
What are some diseases that aF
ect the aging nervous system?
50
What are some of the physical and functional signs of an aging
nervous system?
mitochondria, then inactivates norepinephrine. This may
take a few seconds, during which some molecules may dif-
fuse into nearby tissues or the bloodstream, where other
enzymes decompose them. On the other hand, some norepi-
nephrine molecules may escape decomposition and remain
active for awhile. For these reasons, norepinephrine is likely
to produce a more prolonged effect than acetylcholine. In
fact, when the adrenal medulla releases norepinephrine and
epinephrine into the blood in response to sympathetic stimu-
lation, these substances may trigger sympathetic responses
in organs throughout the body that last up to thirty seconds.
Many drugs influence autonomic functions. Some, like ephedrine,
enhance sympathetic eF
ects by stimulating release of norepineph-
rine from postganglionic sympathetic nerve endings. Others, like
reserpine, inhibit sympathetic activity by preventing norepinephrine
synthesis. Another group of drugs, which includes pilocarpine, pro-
duces parasympathetic effects, and some, like atropine, block the
action of acetylcholine on visceral eF
ectors.
Control of Autonomic Activity
The brain and spinal cord largely control the autonomic ner-
vous system, although it has some independence resulting
from impulse integration in its ganglia. For example, refl ex
centers in the medulla oblongata for cardiac, vasomotor, and
respiratory activities receive sensory impulses from viscera
on vagus nerve ±
bers and use autonomic nerve pathways
to stimulate motor responses in various muscles and glands.
Thus, these refl
ex centers control the autonomic nervous
system. Similarly, the hypothalamus helps regulate body
temperature, hunger, thirst, and water and electrolyte bal-
ance by infl
uencing autonomic pathways.
Still higher levels in the brain, including the limbic sys-
tem and the cerebral cortex, control the autonomic nervous
system during emotional stress. In this way, the autonomic
pathways can affect emotional expression and behavior.
Subsequent chapters that deal with individual organs and
organ systems discuss regulation of particular organs.
PRACTICE
44
Distinguish between cholinergic and adrenergic ±
bers.
45
Explain how the ±
bers of one autonomic division can control the
actions of a particular organ.
46
Which neurotransmitters are used in the autonomic nervous
system?
47
Describe two types of cholinergic receptors and two types of
adrenergic receptors.
11.8
LIFE-SPAN CHANGES
The redundancies and overlap of function in our nervous
systems ensure that we can perceive and interact with the
environment for many decades. In a sense, aging of this
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