Electrical Activity of the Brain in Relation to Anesthesia

　Electrical activity of the brain was discussed with special reference to electroencephalogram which is very familiar to anesthesiologists. The electroencephalogram records the electrical activity of the cerebral cortex which is mainly composed of dendritic or postsynaptic potentials. The differential effects of anesthetics on the electrical activity of the brain stem and thalamic reticular systems were clearly demonstrated. Commonly used general anesthetics depressed the ascending reticular activating system and hypothalamic activating system, but the degree of depression varied with individual anesthetic and with depth of anesthesia. The effect of anesthetics on the thalamic reticular system was studied in relation to recruiting response. Nitrous oxide and deep ether have a depressant effect at the level of the midline thalamus, but this is not so with pentothal and halothane. The study on evoked response also showed differential action of various anesthetics, although the electrogenesis of the evoked potentials awaits further study. From the study with microelectrodes, it has become apparent that the so-called anesthetic state is composed of extremely complex changes in the patterning of unit firing and there is little uniformity in the effect of anesthetic agents and thus little basis for a simple neuronal model of anesthetic state. The continum of cerebral depression produced by anesthetics was described and its generalized pattern was well recognized. The use of frequency analyzer and integrator provided more quantitative description of the electroencephalographic pattern during anesthesia. Although there is some controversy, the dose-response relationship between electroencephalographic levels and anesthetic concentration in the blood was found in common anesthetic agents. The averaged evoked potentials obtained during clinical anesthesia revealed that a great deal of sensory input is reaching the cerebral cortex and deep halothane anesthesia which might cause severe circulatory depression is only capable of abolishing the evoked response in the cortex. As to the effect of general anesthetics on the electrical activity of the brain, a common pattern can be drawn, but there are considerable differences between the anesthetics in their effects on the different neural mechanisms. The computer technique in neurophysiological study is a powerful addition to simple multichannel recording and might be used over a wide range to search for the action of anesthetics in various part of the brain. Hopefully, this presentation will, in some degree, advance current understanding of neurophysiological basis of the anesthetic state and will bring the disciplines of neurophysiology and anesthesiology more closely togerher.