ELECTROGRAPHIC AND EVOKED POTENTIAL STUDIES DURING SLEEP IN THE CAT (THE STUDY ON SLEEP, I)

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Summary: In order to study the neural mechanism underlying the physiological sleep of the cat, especially of the stage of sleep characterized by an electrographic pattern similar to those of the arousal stage and also by disappearance of the postural muscle tone, the spontaneous electrograms of the brain as well as the evoked potentials induced by stimulation of peripheral and central nervous structures were studied in different stages of sleep by using chronically implanted cats.(1) Physiological sleep of the cat was divided into 4 stages: A (arousal), S1 (sleep with 6 to 14 c/sec. spindle burst in the neocortical electrogram), S2 (sleep with spindle burst and high voltage slow activity in the neocortical electrogram) and Sa (sleep with electrographic features similar to those of the arousal stage) stages.(2) It was difficult to differentiate between the low voltage fast activity in the neocortical electrograms during the Sa stage and those during the arousal stage.(3) The hippocampal rhythmic activity was of higher frequency during the Sa stage than in the arousal stage. The frequency became particularly high during the period of the rapid eye movements in the Sa stage.(4) During the Sa stage, spindle burst of 10 to 14 c/sec. activity was frequently observed in the caudal pontine and bulbar reticular formation with or without concomitant spindle burst in the neocortical electrogram. Spindles of 6 to 10 c/sec. were seen less frequently.(5) Evoked potentials in the specific systems induced by direct electric stimulation of the brain decreased amplitude during the S1 and S2 stages with slow background activity and showed the amplitude equal to or higher than the arousal level during the Sa stage. The enhancement of the evoked potential appeared more remarkably when low frequency repetitive stimulation was used. The amplitude of the evoked potential was particularly large during the period of the rapid eye movements in the Sa stage.(6) Evoked potentials in the non‐specific thalamocortical system such as the recruiting response were enhanced in the S1 and S2 stages and suppressed during the Sa stage of sleep.(7) Some components of the evoked potential in the mesencephalic reticular formation showed diminution of amplitude in the S1 and S2 stages and enhancement in the Sa stage, while those in the caudal pontine and bulbar level showed marked suppression during the Sa stage.(8) It was suggested that the Sa stage of sleep might be due to the inhibitory process in the lower brain stem. The hypothesis was presented that during the Sa stage, the structures lying above the mesencephalon was in the state of overactivation due to release from the ascending inhibitory (EEG synchronizing) mechanism of the lower brain stem.

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