Chapter 12
The Central Nervous System
455
12
In most patients with narcolepsy, an emotionally intense ex-
perience can also trigger
cataplexy
, a sudden loss of voluntary
muscle control similar to that seen during REM sleep. During
cataplectic attacks, lasting seconds to minutes, the patient re-
mains fully conscious but unable to move. Obviously this can be
extremely hazardous if the person is driving a car or swimming!
Cells in the hypothalamus that secrete peptides called orex-
ins (the peptides mentioned above as a wake-up chemical; also
called hypocretins) are selectively destroyed in patients with
narcolepsy, probably by the patient’s own immune system. Re-
placing the orexins may be a key to future treatments.
Conversely, drugs that block the actions of orexin and pro-
mote sleep may treat
insomnia
, a chronic inability to obtain
the
amount
or
quality
of sleep needed to function adequately
during the day. Sleep requirements vary from four to nine hours
a day in healthy people, so there is no way to determine the
“right” amount. Insomniacs tend to overestimate the extent of
their sleeplessness, and some come to rely on hypnotics (sleep
medications), which can exacerbate the problem.
True insomnia oFen reflects normal age-related changes, but
perhaps the most common cause is psychological disturbance.
We have difficulty falling asleep when we are anxious or upset,
and depression is oFen accompanied by early awakening.
Sleep
apnea
, a temporary cessation of breathing during
sleep, is scary. ±e victim awakes abruptly due to hypoxia (lack
of oxygen)—a condition that may occur several hundred times
per night.
Obstructive sleep apnea, the most common form, occurs
when the loss of muscle tone during sleep allows excess fatty
tissue or other structural abnormalities to block the upper air-
way. It is associated with obesity and made worse by alcohol and
other depressants. Aside from weight loss, effective treatments
are either a mask that blows air in through the nose, keeping the
airway open, or surgery to correct the problem.
Check Your Understanding
15.
When would you see delta waves in an EEG?
16.
Which two states of consciousness are between alertness
and coma?
17.
During which sleep stage are most skeletal muscles actively
inhibited?
For answers, see Appendix H.
Language
Language is such an important function of the brain that it in-
volves practically all of the association cortex on the leF side in
one way or another. Pioneering studies of patients with
aphasias
(the loss of language abilities due to damage to specific areas of
the brain) pointed to two critically important regions, Broca’s area
and Wernicke’s area (see areas outlined by dashes in ²igure 12.6a).
Patients with lesions involving
Broca’s area
can understand
language but have difficulty speaking (and sometimes cannot
write or type or use sign language). On the other hand, patients
with lesions involving
Wernicke’s area
are able to speak but
In young and middle-aged adults, a typical night’s sleep starts
with the four stages of NREM sleep and then alternates between
REM and NREM sleep with occasional partial arousals. ²ol-
lowing each REM episode, the sleeper descends toward stage
4 again. REM recurs about every 90 minutes, with each REM
period getting longer. ±e first REM of the night lasts 5–10 min-
utes and the final one 20 to 50 minutes (²igure 12.19b). Conse-
quently, our longest dreams occur at the end of the sleep period.
Just before we wake, hypothalamic neurons release peptides
called
orexins
, which in this situation act as “wake-up” chemi-
cals. As a result, certain neurons of the brain stem reticular for-
mation fire at maximal rates, arousing the sleepy cortex.
±e slow theta and delta waves of deep sleep are the result of
synchronized firing of thalamic neurons that is normally inhib-
ited during wakefulness by the RAS of the pons. Some pontine
neurons of the reticular formation control the transition from
NREM sleep to REM sleep, and others suppress motor activity,
inducing paralysis. A large number of chemical substances in
the body cause sleepiness, but the relative importance of these
various sleep-inducing substances is not known.
Importance of Sleep
Why do we sleep? Slow-wave (NREM stages 3 and 4) and REM
sleep seem to be important in different ways. Slow-wave sleep is
presumed to be restorative—the time when most neural activity can
wind down to basal levels. When deprived of sleep, we spend more
time than usual in slow-wave sleep during the next sleep episode.
A person persistently deprived of REM sleep becomes
moody and depressed, and exhibits various personality dis-
orders. REM sleep may give the brain an opportunity to ana-
lyze the day’s events and work through emotional problems in
dream imagery.
Another idea is that REM sleep is reverse learning. According
to this hypothesis, accidental, repetitious, and meaningless com-
munications continually occur. Dreaming eliminates them from
our neural networks so the cortex remains a well-behaved and
efficient thinking system. In other words, we dream to forget.
Alcohol and some sleep medications (barbiturates and oth-
ers) suppress REM sleep but not slow-wave sleep. On the other
hand, certain tranquilizers, such as diazepam (Valium) reduce
slow-wave sleep much more than REM sleep.
Whatever its importance, a person’s daily sleep requirement
declines steadily from 16 hours or so in infants to approximately
7½ to 8½ hours in early adulthood. It then levels off before de-
clining again in old age. Sleep patterns also change throughout
life. REM sleep occupies about half the total sleeping time in
infants and then declines until age 10, when it stabilizes at about
25%. In contrast, stage 4 sleep declines steadily from birth and
oFen disappears completely in those over 60.
Homeostatic Imbalance
12.7
People with
narcolepsy
lapse abruptly into REM sleep from the
awake state. ±ese sleep episodes last about 15 minutes, can oc-
cur without warning, and are oFen triggered by a pleasurable
event—a good joke, a game of poker.
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