REM Sleep
Jerome M. Siegel, in Principles and Practice of Sleep Medicine (Fourth Edition), 2005
WHAT IS REM SLEEP?
REM sleep was discovered by Aserinsky and Kleitman in 1953.1 They found that it was characterized by the periodic recurrence of rapid eye movements, linked to a dramatic reduction in the amplitude of the electroencephalogram (EEG). They found that the EEG of REM sleep closely resembled the EEG of alert waking and reported that subjects awakened from REM sleep reported vivid dreams. Dement identified a similar state of low-voltage EEG with eye movements in cats.2 Jouvet then repeated this observation, finding in addition a loss of muscle tone (atonia) in REM sleep and using the name paradoxical sleep to refer to this state. The "paradox" was that the EEG resembled that of waking, whereas behaviorally the animal remained asleep and unresponsive.3 Subsequent authors have described this state as "activated" sleep, or "dream" sleep. Recent work in humans has shown that some mental activity can be present in non-REM sleep but has supported the original finding linking our most vivid dreams to the REM sleep state.1
Most early work was done in cats, and it is in the cat that most of the "classic" signs of REM sleep and their generating mechanisms were discovered. Figure 10-1, top, shows the principal electrical signs of REM sleep. These include the reduction in EEG amplitude, particularly in the amplitude of its lower-frequency components. REM sleep is also characterized by a suppression of muscle tone (atonia), visible in the electromyogram (EMG). Erections tend to occur in men and clitoral enlargement in women. Thermoregulation largely ceases, and animal body temperatures drift toward environmental temperatures, as in reptiles.4 Pupils constrict, reflecting a parasympathetic dominance in the control of the iris. These changes that are present throughout the REM sleep period have been termed its tonic features.
Also visible are large electrical potentials that can be most easily recorded in the lateral geniculate nucleus.5 These potentials originate in the pons, appear after a few milliseconds in the lateral geniculate nucleus, and can be observed with further delay in the occipital cortex, leading to the name ponto-geniculo-occipital (PGO) spikes. They occur as large-amplitude, isolated potentials appearing 30 or more seconds before the onset of REM sleep, as defined by EEG and EMG criteria. After REM sleep begins, they arrive in bursts of 3 to 10 waves usually correlated with rapid eye movements. PGO-linked potentials can also be recorded in the motor nuclei of the extraocular muscles, where they trigger the rapid eye movements of REM sleep. They are present, in addition, in thalamic nuclei other than the geniculate and in neocortical regions other than the occipital cortex. In humans, rapid eye movements are loosely correlated with contractions of the muscles of the middle ear of the sort that accompany speech generation and that are part of the protective response to loud noise.6 Other muscles also contract during periods of rapid eye movement, briefly breaking through the tonic muscle atonia of REM sleep. There are periods of marked irregularity in respiratory and heart rates during REM sleep, in contrast to non-REM sleep, during which respiration and heart rate are highly regular. There does not appear to be any single pacemaker for all of this irregular activity. Rather, the signals producing twitches of the peripheral or middle ear muscles may lead or follow PGO spikes and rapid eye movements. Bursts of brainstem neuronal activity may likewise lead or follow the activity of any particular recorded muscle.789 These changes that occur episodically in REM sleep have been called its phasic features.
As we will see later, certain manipulations of the brainstem can eliminate only the phasic events of REM sleep, whereas others can cause the phasic events to occur in waking; yet other manipulations can affect tonic components. These tonic and phasic features are also expressed to varying extents in different species, and not all of these features are present in all species that have been observed to have REM sleep.