Neurophysiological Studies on Nitrous Oxide and Diethyl Ether Anaesthesia.

The spontaneous electrical activities and alterations of the activation threshold of the neo-, palaeoand archicortical systems were investigated during nitrous oxide and ether anaesthesia. In the neocortical system the threshold for hypothalamic stimulation, induced spindle bursts, recruiting response and thalamic arousal reaction were also examined. The results obtained were as follows: (a) Nitrous oxide anaesthesia. (1) Changes in the pattern of spontaneous activity were more marked in the neocortex than in the limbic system. Slow waves with increased amplitude appeared in the neocortex but spindle bursts were scarce. The patterns of deep sleep with spikes was not observed in the limbic system. (2) The neocortical system was affected; the thalamocortical reverberating circuit showed impaired activity without any elevation of the threshold of the hypothalamic activating system. (b) Ether anaesthesia. The changes of electrical activity of the three systems were classified in relation to the depth of ether anaesthesia. During light ether anaesthesia the activation threshold of the hypothalamic activating system was elevated, while that of the reticular neocortex activating system was hardly affected. During deep ether anaesthesia the thresholds of the hypothalamic and reticular activating systems were significantly elevated. Waves of increased amplitude were observed in the hypothalamus, amygdala and hippocampus in contrast to slow waves in the neocortex after prolonged deep ether anaesthesia. High amplitude waves of about 10 c/s appeared in the amygdala and hippocampus at the onset of ether inhalation.