Mechanisms of the rhombencephalic phase of sleep

This is an active phenomenon as it is possible to trigger off the appearance of the rhombencephalic phase of sleep by stimulating the pontile R.F. of the intact animal, provided that this stimulation occurs during the slow wave phase of sleep (Fig. 9) (Jouvet and Michel, 1960b).

In mesencephalic cats, stimulation of this same zone is also capable of triggering periods of "rhombencephalic sleep". Phases of 15 minutes duration have thus been obtained after stimulations lasting 1 to 2 seconds. After the animal has spontaneously awakened, a refractory period is observed, during which an identical stimulus determines a hypertonic phase accompanied by agitation. Therefore it is never possible to trigger several periods of rhombencephalic sleep successively. An interval of 15 to 20 minutes must be allowed to elapse before a new stimulation can produce a new phase of sleep. In some animals, sleep has been obtained by summation of stimuli of long duration (10 to 20 seconds) and in some cases a latent period of 30 to 60 seconds was observed between the end of the stimulation and the beginning of sleep. Controls have shown that these phenomena are not accompanied by significant variations in blood pressure.

Chlorpromazine (3 to 5 mg./kg.) and atropine (0-2 to o-3 mg./kg.) considerably reduce the duration of the p.p.'s or even stop them from appearing, particularly in mesencephalic animals. On the other hand, the injection of cholinergic drugs (eserine) produces longer p.p.'s although their frequency is not increased (Jouvet and Michel, 1960a).

These facts are difficult to explain as a whole through purely neuronal mechanisms. They lead us to submit the hypothesis of the existence of a neurohumoral mechanism which would "discharge" periodically during behavioural sleep but which could not be brought into play until a sufficient "stock" of neurohormones was gathered. The inhibiting action of such a mechanism on the awakening system (which is expressed by a very important increase in the threshold of reticular arousal) must be explained by a direct action of the " triggering rhombencephalic zone" on the ascending activating system since the awakening threshold rises in the intact as well as in the mesencephalic animal, which eliminates the intervention of possible cortico-reticular feedbacks. Although the periodic physiological mise en jeu of the rhombencephalic phase of sleep is not yet understood, its psycho-physiological significance seems important. Dement and Kleitman (1957) have shown that in man the oneiric activity occurred periodically during sleep and that it was accompanied by eye movements and by cortical low voltage electrical activity. These findings have been confirmed by us (Jouvet, Michel and Mounier, 1960). We have put forward other arguments which enable us to accept that oneiric activity occurs in man during the rhombencephalic phase of sleep. It is thus surprising to find that such important and mysterious psychic phenomena as dreams are dependent on the intervention of structures which are hidden in the lower part of the brain stem.

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