409
CHAPTER ELEVEN
Nervous System II
the reticular formation frees the cortex from what would oth-
erwise be a continual bombardment of sensory stimulation
and allows it to concentrate on more signiF
cant information.
The cerebral cortex can also activate the reticular system, so
intense cerebral activity keeps a person awake. In addition,
the reticular formation regulates motor activities so that vari-
ous skeletal muscles move together evenly, and it inhibits or
enhances certain spinal refl
exes.
A person in a persistent vegetative state is occasionally awake, but
not aware; a person in a coma is not awake or aware. Sometimes
following a severe injury, a person will become comatose and then
gradually enter a persistent vegetative state. Coma and persistent
vegetative state are also seen in the end stage of neurodegenerative
disorders such as Alzheimer disease; when there is an untreatable
mass in the brain, such as a blood clot or tumor; or in anencephaly,
when a newborn lacks higher brain structures.
Types of Sleep
The two types of normal sleep are
slow-wave
and
rapid eye
movement
(REM). Slow-wave sleep (also called non-REM sleep)
occurs when a person is very tired, and it refl ects decreasing
activity of the reticular formation. It is restful, dreamless, and
accompanied by reduced blood pressure and respiratory rate.
Slow-wave sleep may range from light to heavy and is usu-
ally described in four stages. It may last from seventy to ninety
minutes. Slow-wave and REM sleep alternate.
REM sleep is also called “paradoxical sleep” because
some areas of the brain are active. As its name implies, the
eyes can be seen rapidly moving beneath the eyelids. Cats
and dogs in REM sleep sometimes twitch their limbs. In
humans, REM sleep usually lasts from F
ve to F fteen min-
utes. This “dream sleep” is apparently important. If a per-
son lacks REM sleep for just one night, sleep on the next
night makes up for it. During REM sleep, heart and respira-
tory rates are irregular. Certain drugs, such as marijuana and
alcohol, interfere with REM sleep.
Table 11.6
describes sev-
eral disorders of sleep.
2.
Vasomotor center.
Certain cells of the vasomotor center
initiate impulses that travel to smooth muscles in the
walls of blood vessels and stimulate them to contract,
constricting the vessels (vasoconstriction) and thereby
increasing blood pressure. A decrease in the activity of
these cells can produce the opposite effect—dilation of
the blood vessels (vasodilation) and a consequent drop
in blood pressure.
3.
Respiratory center.
The respiratory center adjusts the
rate and depth of breathing and acts with the pons to
maintain the basic rhythm of breathing.
Some nuclei in the medulla oblongata are centers for cer-
tain nonvital refl
exes, such as those associated with cough-
ing, sneezing, swallowing, and vomiting. However, because
the medulla also contains vital refl
ex centers, injuries to this
part of the brainstem are often fatal.
Reticular Formation
Scattered throughout the medulla oblongata, pons, and
midbrain is a complex network of nerve F
bers associated
with tiny islands of gray matter. This network, the
reticu-
lar formation
(re
˘-tik
u-lar fo
ˉr-ma
shun), or reticular activat-
ing system, extends from the superior portion of the spinal
cord into the diencephalon (F g. 11.21). Its intricate system
of nerve F bers connects centers of the hypothalamus, basal
nuclei, cerebellum, and cerebrum with F
bers in all the major
ascending and descending tracts.
When sensory impulses reach the reticular formation,
it responds by activating the cerebral cortex into a state
of wakefulness. Without this arousal, the cortex remains
unaware of stimulation and cannot interpret sensory infor-
mation or carry on thought processes. Decreased activity in
the reticular formation results in sleep. If the reticular forma-
tion is injured and ceases to function, the person remains
unconscious, even with strong stimulation. This is called a
comatose state.
The reticular formation filters incoming sensory
impulses. Impulses judged to be important, such as those
originating in pain receptors, are passed on to the cerebral
cortex, while others are disregarded. This selective action of
TABLE
11.6
|
Sleep Disorders
Disorder
Symptoms
Percent of Population
Fatal familial insomnia
Inability to sleep, emotional instability, hallucinations, stupor, coma, death within thirteen months
of onset around age ±
fty, both slow-wave and REM sleep abolished.
Very rare
Insomnia
Inability to fall or remain asleep.
10%
Narcolepsy
Abnormal REM sleep causes extreme daytime sleepiness, begins between ages of ±
fteen and
twenty-±
ve.
0.02–0.06%
Obstructive sleep apnea syndrome
Upper airway collapses repeatedly during sleep, blocking breathing. Snoring and daytime
sleepiness.
4–5%
Parasomnias
Sleepwalking, sleeptalking, and night terrors.
<5% of children
REM-sleep behavior disorder
Excessive motor activity during REM sleep, which disturbs continuous sleep.
Very rare
Restless legs syndrome
Brief, repetitive leg jerks during sleep. Leg pain forces person to get up several times a night.
5.5%
Sleep paralysis
Inability to move for up to a few minutes after awakening or when falling asleep.
Very rare
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