Excitability Changes of the Reticular Ascendent Activating System During Sleep Cycle in the Rabbit

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Abstract
In unanesthetized freely moving rabbits with implanted electrodes the existence of sleep cycles was confirmed. Sleep begins with high voltage slow frequency EEG activity in the necortex, hippocampus and reticular formation. During the last 1/2-1/3 of this first phase of the sleep (lasting on the whole about 20 minutes) a gradual decrease of the excitability of the reticular activating system develops, the reticular stimulation of equal parameters becomes less effective progressively. The second phase of the sleep (lasting about 3-4 minutes) is characterized by sudden appearance of regular theta activity (4-7/sec) in the hippocampus and desynchronization in the neocortex. the reticular stimulation previously effective becomes completely unable to awoke the animal. This second phase ends in the greater part of the cycles by a spontaneous EEG and behavioural arousal lasting about 2 minutes. Then the cycle recommences in the high voltage low frequency EEG pattern (phase 1) combined at the beginning by a high excitability of the reticular activating system.

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