411
CHAPTER ELEVEN
Nervous System II
Delta waves
have a frequency below 4 cycles
per second and happen during sleep. They origi-
nate from the cerebral cortex when it is not being
activated by the reticular formation.
Brain wave patterns can be useful for diag-
nosing disease conditions, such as distinguishing
types of seizure disorders (epilepsy) and locating
B
rain waves
are recordings of fluctuating
electrical changes in the brain. To obtain
such a recording, electrodes are positioned
on the surface of a surgically exposed brain (an
electrocorticogram, ECoG) or on the outer sur-
face of the head (an electroencephalogram, EEG).
These electrodes detect electrical changes in
the extracellular F
uid of the brain in response to
changes in potential among large groups of neu-
rons. The resulting signals from the electrodes are
amplified and recorded. Brain waves originate
from the cerebral cortex but also reflect activi-
ties in other parts of the brain that inF
uence the
cortex, such as the reticular formation. The inten-
sity of electrical changes is proportional to the
degree of neuronal activity, so brain waves vary
markedly in amplitude and frequency between
sleep and wakefulness.
Brain waves are classified as alpha, beta,
theta, and delta waves (±
g. 11²).
Alpha waves
are
recorded most easily from the posterior regions
of the head and have a frequency of 8–13 cycles
per second. They occur when a person is awake
but resting, with the eyes closed. These waves dis-
appear during sleep, and if the wakeful person’s
eyes open, higher-frequency beta waves replace
the alpha waves.
Beta waves
have a frequency of more than
13 cycles per second and are usually recorded in
the anterior region of the head. They occur when
a person is actively engaged in mental activity or
is under tension.
Theta waves
have a frequency of 4–7 cycles
per second and occur mainly in the parietal and
temporal regions of the cerebrum. They are nor-
mal in children but do not usually occur in adults.
However, some adults produce theta waves in
early stages of sleep or at times of emotional
stress.
brain tumors. Brain waves are also used to detect
brain death,
in which neuronal activity ceases.
An EEG that lacks waves (isoelectric EEG) veri±
es
brain death. However, drugs that greatly depress
brain functions must be excluded as the cause
of the flat EEG pattern before confirming brain
death.
11.6
CLINICAL APPLICATION
Brain Waves
connect the CNS to the skin and skeletal muscles, so it over-
sees conscious activities. The
autonomic nervous system
(aw
to nom
ik ner
vus sis
tem) includes f bers that connect
the CNS to viscera such as the heart, stomach, intestines,
and various glands. The autonomic nervous system controls
subconscious actions.
Table 11.8
outlines the subdivisions
oF the nervous system.
11.6
PERIPHERAL NERVOUS
SYSTEM
The
peripheral nervous system
consists oF the nerves that
branch From the CNS, connecting it to other body parts. The
PNS includes the
cranial nerves
that arise From the brain and
the
spinal nerves
that arise From the spinal cord.
The PNS can also be subdivided into somatic and auto-
nomic nervous systems. Generally, the
somatic nervous
system
consists oF the cranial and spinal nerve f
bers that
Alpha
waves
Beta
waves
Theta
waves
Delta
waves
50
µ
v
1 sec.
FIGURE 11F
Brain waves record F
uctuating electrical changes in the brain.
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