Ganong's Review of Medical Physiology, 23rd Edition

234
SECTION III
Central & Peripheral Neurophysiology

activity necessary to produce regular waves. However, the
rapid EEG activity seen in the alert state is also synchronized,
but at a higher rate. Therefore, the term
desynchronization
is
misleading.
Gamma oscillations
at 30–80 Hz are often seen
when an individual is aroused and focuses attention on some-
thing. This is often replaced by irregular fast activity as the
individual initiates motor activity in response to the stimulus.

SLEEP STAGES

There are two kinds of sleep:
rapid eye movement (REM) sleep
and
non-REM (NREM),
or
slow-wave sleep.
REM sleep is so
named because of the characteristic eye movements that occur
during this stage of sleep. NREM sleep is divided into four stag-
es (Figure 15–7). A person falling asleep first enters stage 1, the
EEG begins to show a low-voltage, mixed frequency pattern. A
theta rhythm
(4–7 Hz) can be seen at this early stage of slow-
wave sleep. Throughout NREM sleep, there is some activity of

skeletal muscle but no eye movements occur. Stage 2 is marked

by the appearance of sinusoidal waves called

sleep spindles

(12–14 Hz) and occasional high voltage biphasic waves called

K

complexes.

In stage 3, a high-amplitude

delta rhythm

(0.5–4

Hz) dominates the EEG waves. Maximum slowing with large

waves is seen in stage 4. Thus, the characteristic of deep sleep is

a pattern of rhythmic slow waves, indicating marked

synchro-

nization;

it is sometimes referred to as

slow-wave sleep.

Whereas theta and delta rhythms are normal during sleep, their

appearance during wakefulness is a sign of brain dysfunction.

REM SLEEP

The high-amplitude slow waves seen in the EEG during sleep

are periodically replaced by rapid, low-voltage EEG activity,

which resembles that seen in the awake, aroused state and in

stage 1 sleep (Figure 15–7). For this reason, REM sleep is also

called

paradoxical sleep.

However, sleep is not interrupted;

indeed, the threshold for arousal by sensory stimuli and by

FIGURE 15–6
EEG records showing the alpha and beta
rhythms.
When attention is focused on something, the 8–13 Hz alpha
rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the
beta rhythm.
(From Widmaier EP, Raff H, Strang KT:
Vander’s Human Physiology,
11th ed. McGraw-Hill, 2008.)

(a)

(b)

Alpha rhythm (relaxed with eyes closed)

Beta rhythm (alert)

Time

CLINICAL BOX 15–2

Variations in the Alpha Rhythm

In humans, the frequency of the dominant EEG rhythm at

rest varies with age. In infants, there is fast, beta-like activ-

ity, but the occipital rhythm is a slow 0.5- to 2-Hz pattern.

During childhood this latter rhythm speeds up, and the

adult alpha pattern gradually appears during adolescence.

The frequency of the alpha rhythm is decreased by low

blood glucose levels, low body temperature, low levels of

adrenal glucocorticoid hormones, and high arterial partial

pressure of CO

2

(PaCO

2

). It is increased by the reverse con-

ditions. Forced over-breathing to lower the PaCO

2

is some-

times used clinically to bring out latent EEG abnormalities.

FIGURE 15–7
EEG and muscle activity during various stages of the sleep–wake cycle.
NREM sleep has four stages. Stage 1 is character-
ized by a slight slowing of the EEG. Stage 2 has high-amplitude K complexes and spindles. Stages 3 and 4 have slow, high-amplitude delta waves.
REM sleep is characterized by eye movements, loss of muscle tone, and a low-amplitude, high-frequency activity pattern. The higher voltage ac-
tivity in the EOG tracings during stages 2 and 3 reflect high amplitude EEG activity in the prefrontal areas rather than eye movements. EOG, electro-
oculogram registering eye movements; EMG, electromyogram registering skeletal muscle activity.
(From Kandel ER, Schwartz JH, Jessell TM [editors]:
Principles of Neural Science,
4th ed. McGraw-Hill, 2000.)

EOG

Awake Sleep stage 1 2 3 4 REM

EMG

EEG

1 s

50 μV