THE TRANSITION FROM SLOW-WAVE SLEEP TO PARADOXICAL SLEEP - EVOLVING FACTS AND CONCEPTS OF THE NEUROPHYSIOLOGICAL PROCESSES UNDERLYING THE INTERMEDIATE STAGE OF SLEEP

Paradoxical sleep in rats, cats and mice is usually preceded and somet imes followed by a short-lasting (a few seconds) electroencephalogram (EEG) stage characterized by high-amplitude spindles in the anterior c ortex and low-frequency theta rhythm in the dorsal hippocampus. The fo rmer is an index of advanced slow-wave sleep; the latter is an index o f limbic activation since it occurs during active waking and paradoxic al sleep. Barbiturates and benzodiazepines extend this intermediate st age at the expense of paradoxical sleep while concomitantly barbiturat es suppress the pontine reticular activation characteristic of this sl eep stage. During the intermediate stage, thalamocortical responsivene ss and thalamic transmission level, which are controlled by brain stem activating influences, are the lowest of all sleep-waking stages. The unusual EEG pattern of this stage is otherwise only observed in the a cute intercollicular-transected preparation. Therefore, forebrain stru ctures may be functionally briefly disconnected from the brain-stem du ring this short-lasting stage, which could possibly account for the me ntal content of a similar sleep period in humans. In spite of strong e vidence in favour of this forebrain deafferentiation hypothesis, other data indicate that the IS is in some way linked either to slow-wave s leep or to paradoxical sleep. Copyright (C) 1996 Elsevier Science Ltd.