Consciousness, the Brain, and Behavior
237
for treating insomnia and jet lag, it has not yet proven effec-
tive as a sleeping pill. It has, however, been shown to induce
lower body temperature, a key event in falling asleep.
In addition to these neurotransmitters, over 30 other
chemical substances that affect sleep have been found in
blood, urine, cerebrospinal fl uid, and brain tissue. For exam-
ple, interleukin 1, one of the cytokines in a family of inter-
cellular messengers having an important role in the body’s
immune defense system (Chapter 18) fl uctuates in parallel
with normal sleep-wake cycles. However, none of these sub-
stances has been confi rmed as a physiologically important
factor in inducing sleep.
Coma and Brain Death
The term
coma
describes a severe decrease in mental function
due to structural, physiological, or metabolic impairment of
the brain. A person in a coma exhibits a sustained loss of the
capacity for arousal even in response to vigorous stimulation.
There is no outward behavioral expression of any mental func-
tion, the eyes are closed, and sleep-wake cycles disappear. Coma
can result from extensive damage to the cerebral cortex, damage
to the brainstem arousal mechanisms, interruptions of the con-
nections between the brainstem and cortical areas, metabolic
dysfunctions, brain infections, or an overdose of certain drugs,
such as sedatives, sleeping pills, narcotics, or ethanol.
Patients in an irreversible coma often enter a
persistent
vegetative state
in which sleep-wake cycles are present even
though the patient is unaware of his surroundings. Individuals
in a persistent vegetative state may smile, or cry, or seem to
react to elements of their environment. However, there is no
evidence that they can comprehend these behaviors.
But a coma—even an irreversible coma—is not equiva-
lent to death. We are left, then, with the question: When is a
person actually dead? This question often has urgent medical,
legal, and social consequences. For example, with the need for
viable tissues for organ transplantation it becomes imperative
to know just when a person is “dead” so that the organs can
be removed as soon after death as possible.
Brain death
is widely accepted by doctors and lawyers
as the criterion for death, despite the viability of other organs.
Brain death occurs when the brain no longer functions and
has no possibility of functioning again.
Figure 8–6
A model showing how alternating activity within neurons in the
brainstem and hypothalamus may infl uence the differing states of
consciousness. The changes in aminergic and cholinergic infl uence
are discussed in the text.
Figure
8–6
physiological
inquiry
Explain why some drugs prescribed to treat allergic reactions
cause drowsiness as a side effect.
Answer can be found at end of chapter.
Hypothalamus with
circadian and
homeostatic centers
NREM sleep
REM sleep
Waking
Activation
of the thalamus
and cortex
Histamine
GABA
Histamine
GABA
Activation
of the thalamus
and cortex
Acetylcholine
Brainstem nuclei that
are part of the reticular
activating system
Norepinephrine
and
serotonin
Norepinephrine
and
serotonin
Acetylcholine
Suprachiasmatic
nucleus
Thalamus
Posterior
hypothalamus
Brainstem
nuclei of
the reticular
activating
system
Preoptic
area
Figure 8–7
Brain structures involved in the sleep-wakefulness cycles. Cells
within the thalamus are infl uenced by output from other areas and
generate rhythmic fi ring patterns that produce the EEG.
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