Ganong's Review of Medical Physiology, 23rd Edition

236
SECTION III
Central & Peripheral Neurophysiology

thalamocortical circuit hypothesized to be involved in gener-
ating rhythmic activity. Although not shown, inhibitory tha-
lamic reticular neurons are elements of this network. The EEG
shows the characteristic awake, light sleep, and deep sleep pat-
terns of activity described above. Likewise, recordings from
individual thalamic and cortical neurons show different pat-
terns of rhythmic activity. In the waking state, corticocortical
and thalamocortical networks generate higher-frequency
rhythmic activity (30–80 Hz; gamma rhythm). This rhythm
may be generated within the cells and networks of the cerebral
cortex or within thalamocortical loops. The gamma rhythm
has been suggested as a mechanism to “bind” together diverse
sensory information into a single percept and action, but this
theory is still controversial. In fact, disturbances in the integ-
rity of this thalamocortical loop and its interaction with other
brain structures may underlie some neurological disorders,
including seizure activity.

IMPORTANCE OF SLEEP

Sleep has persisted throughout evolution of mammals and
birds, so it is likely that it is functionally important. Indeed, if
humans are awakened every time they show REM sleep, then
permitted to sleep without interruption, they show a great deal
more than the normal amount of REM sleep for a few nights.
Relatively prolonged REM deprivation does not seem to have
adverse psychological effects. Rats deprived of sleep for long

periods lose weight in spite of increased caloric intake and

eventually die. Various studies imply that sleep is needed to

maintain metabolic-caloric balance, thermal equilibrium, and

immune competence.

In experimental animals, sleep is necessary for learning and

memory consolidation. Learning sessions do not improve per-

formance until a period of slow-wave or slow-wave plus REM

sleep has occurred. Clinical Box 15–3 describes several com-

mon sleep disorders.

CIRCADIAN RHYTHMS &

THE SLEEP–WAKE CYCLE

CIRCADIAN RHYTHMS

Most, if not all, living cells in plants and animals have rhyth-

mic fluctuations in their function on a circadian cycle. Nor-

mally they become entrained, that is, synchronized to the day–

night light cycle in the environment. If they are not entrained,

they become progressively more out of phase with the light–

dark cycle because they are longer or shorter than 24 hours.

The entrainment process in most cases is dependent on the

su-

prachiasmatic nuclei (SCN)

located bilaterally

above the op-

tic chiasm (Figure 15–11). These nuclei receive information

about the light–dark cycle via a special neural pathway, the

retinohypothalamic fibers.

Efferents from the SCN initiate

FIGURE 15–10
Correlation between behavioral states, EEG, and single-cell responses in the cerebral cortex and thalamus.
The EEG
is characterized by high-frequency oscillations in the awake state and low-frequency rhythms during sleep. Thalamic and cortical neurons can also
show different patterns of rhythmic activity. Thalamocortical neurons show slow rhythmic oscillations during deep sleep, and fire tonic trains of
action potentials in the awake state. Most pyramidal neurons in the cortex generate only tonic trains of action potentials, although others may
participate in the generation of high frequency rhythms through activation of rhythmic bursts of spikes. The thalamus and cerebral cortex are con-
nected together in a loop.
(Modified from McCormick DA: Are thalamocortical rhythms the Rosetta stone of a subset of neurological disorders? Nat Med 1999;5:1349.)

Thalamocortical loop

Cerebral cortex

Thalamus

EEG Single cell properties

Awake

Pyramidal cells

Tonic firing

30–50 Hz

gamma oscillations

Light sleep

Deep sleep

20–80 Hz rhythms

7–15 Hz rhythms

0.5–4 Hz rhythms

Thalamocortical cell

0.5–4 Hz burst firing Tonic firing

Transition from sleep to waking