452
UNIT 3
Regulation and Integration of the Body
12
synaptic activity at the surface of the cortex, rather than by action
potentials in the white matter.
Each of us has a brain wave pattern that is as unique as our
fingerprints. For simplicity, however, we can group brain waves
into the four frequency classes shown in Figure 12.18b. Each
wave is a continuous train of peaks and troughs, and the wave
frequency, expressed in hertz (Hz), is the number of peaks in
one second. A frequency of 1 Hz means that one peak occurs
each second.
Te amplitude or intensity of any wave is represented by how
high the wave peaks rise and how low the troughs dip. Te am-
plitude of brain waves reflects the synchronous activity of many
neurons and not the degree of electrical activity of individual
neurons. Usually, brain waves are complex and low amplitude.
During some stages of sleep, neurons tend to fire synchronously,
producing similar, high-amplitude brain waves.
Alpha waves
(8–13 Hz) are relatively regular and rhythmic,
low-amplitude, synchronous waves. In most cases, they indicate
a brain that is “idling”—a calm, relaxed state of wakefulness.
Beta waves
(14–30 Hz) are also rhythmic, but less regular
than alpha waves and with a higher frequency. Beta waves
occur when we are mentally alert, as when concentrating on
some problem or visual stimulus.
Teta waves
(4–7 Hz) are still more irregular. Tough com-
mon in children, theta waves are uncommon in awake adults
but may appear when concentrating.
Delta waves
(4 Hz or less) are high-amplitude waves seen
during deep sleep and when the reticular activating system
is damped, such as during anesthesia. In awake adults, they
indicate brain damage.
Brain waves change with age, sensory stimuli, brain disease,
and the chemical state of the body. EEGs are used for diagnosing
epilepsy and sleep disorders, and in research on brain function.
Brain waves whose frequency is too high or too low suggest
problems with cerebral cortical functions, and unconsciousness
occurs at both extremes. Because spontaneous brain waves are
always present, even during unconsciousness and coma, their
absence—a “flat EEG”—is clinical evidence of brain death.
Homeostatic Imbalance
12.5
Almost without warning, a person with epilepsy may lose con-
sciousness and fall stiffly to the ground, wracked by uncon-
trollable jerking. Tese
epileptic seizures
reflect a torrent of
electrical discharges by groups of brain neurons, and during
their uncontrolled activity no other messages can get through.
Epilepsy, manifested by one out of 100 of us, is not associated
with, nor does it cause, intellectual impairment. Genetic factors
induce some cases, but epilepsy can also result from brain inju-
ries caused by blows to the head, stroke, infections, or tumors.
Epileptic seizures vary tremendously in their expression and
severity.
Absence seizures
, formerly known as
petit mal
, are mild forms
in which the expression goes blank for a few seconds as
Higher Mental Functions
During the last four decades, an exciting exploration of our “in-
ner space,” or what we commonly call the
mind
, has been going
on. But researchers in the field of cognition are still struggling to
understand how the mind’s qualities spring from living tissue and
electrical impulses. Souls and synapses are hard to reconcile!
Because brain waves reflect the electrical activity on which
higher mental functions are based, we will consider them first,
along with the related topics of consciousness and sleep. We
will then examine language and memory, an area of ongoing
research that is of particular interest to our aging population.
Brain Wave Patterns and the EEG
Define EEG and distinguish between alpha, beta, theta,
and delta brain waves.
Normal brain function involves continuous electrical activity of
neurons. An
electroencephalogram
(e-lek
0
tro-en-sef
9
ah-lo-
gram), or
EEG
, records some aspects of this activity. An EEG is
made by placing electrodes on the scalp and connecting the elec-
trodes to an apparatus that measures voltage differences between
various cortical areas
(Figure 12.18a)
. Te patterns of neuronal
electrical activity recorded, called
brain waves
, are generated by
Visual
impulses
Reticular formation
Ascending general
sensory tracts
(touch, pain, temperature)
Descending
motor projections
to spinal cord
Auditory
impulses
Radiations
to cerebral
cortex
Figure 12.17
The reticular formation.
This functional brain
system extends the length of the brain stem. Part of this formation,
the reticular activating system (RAS), maintains alert wakefulness
of the cerebral cortex. Ascending blue arrows indicate sensory
input to the RAS. Purple arrows indicate reticular output (some
via thalamic relays) to the cerebral cortex. Descending red arrow
represents motor output involved in regulating muscle tone.
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