Part 1 : Experimental results obtained on cats
The rhombencephalic phase of sleep
Two different stages can be distinguished from the records ( Fig. 1):
A. Falling asleep and Slow Phase of Sleep (SPS): This stage is marked by the appearance of spindles followed by slow waves which invade the cortex, the diencephalon and then the mesencephalic reticular formation (RF). At the same time, high voltage (300-500, microV) spikes of short duration (40-50 msec) appear in the limbic system (hippocampus, mamillary bodies and septum) (Jouvet et al. 1959a). During this stage, the animal prepares for sleep. The head is bent while the EMG of the neck muscles discretely falls. The respiration as well as the heart rate are regular and slow down in comparison with rates in the waking state. As the invasion of the reticular formation by the slow waves proceeds, the threshold of arousal obtained by direct electrical stimulation of the RF increases 20-40 per cent.
Whatever subcortical structures may be responsible for the synchronization phenomena (diffuse thalamic system, or caudal region of the brain stem�see Batini et al. 1959b), one fact appears to be certain: the spindles and the slow waves which are observed during the first phase of sleep require the presence of the neocortex, since there is no subcortical spindle or slow wave in the decorticate animal, nor behind a section of the brain stem at the mesodiencepha]ic level. Thus, the telencephalic origin of the slow phase of sleep is probable. Such a "telencephalic sleep" would express the activity of some corticofugal mechanism which has been discussed elsewhere (Jouvet and Michel 1958; Jouvet 1962).
B. Fast phase of sleep or Rhombencephalic Phase of Sleep (RPS): This phase always follows a SPS and never appears immediately after wakefulness. It starts suddenly and is distinguished by a fast corticomesodiencephalic activity of low volt age, identical to that of wakefulness, while the rhinencephalic formations show a slow rhythmic activity, identical to that which has been described during arousal at this level (Green and Arduini 1954). Such a rhythmic activity has also been recorded at the cingulate gyrus, the septum, the posterior hypothalamus, the central grey matter of the mesencephalon and the interpeduncular nucleus. At the same time, a 6-8/sec spindle-like activity appears at the level of the pontile RF. The auditory cortical and reticular evoked responses undergo a marked reduction in amplitude compared with that in the waking state or during SPS. This phase lasts about 10-15 min and is accompanied immediately and constantly by total disappearance of EMG activity in the recorded muscles. It is periodically repeated during behavioral sleep, with intervals of 20-30 min ( Fig. 8). During that phase, the posture of the animal is the same as that found in profound sleep. Postural muscles are completely atonic, the nictitating membranes almost entirely cover the pupils which are miotic, while the eyeballs are frequently shaken by short rapid movements . Movements of the vibrissae and, more rarely brief jerks of the jaws and the tail can also be observed. Cardiorespiratory variations are consistently observed: breathing becomes irregular, more superficial and quicker than during SPS, while the heart rate is slowed down, or more rarely accelerated.
That this phase of sleep is more profound than SPS is supported by the following arguments:
(i) An auditory stimulation, insufficient to produce arousal, causes the spindle and slow wave stage to reappear.
(ii) The arousal threshold measured in decibels for an auditory stimulation, is higher during this phase than during SPS.
(iii) The behavioral threshold of arousal obtained by direct electrical stimulation of the mesencephalic RF increases by 200 300 per cent compared to that of SPS (Jouvet et al. 1959b; Hara et al. 1960; Rossi et al. 1961).
This increase of the threshold of arousal, associated with the electrical activity of subcortical regions and with the total disappearance of the muscular tone and the somatic or vegetative phenomena permit the detection of the periodical appearance of RPS in animals with brain stem transection at several levels.