Differential Susceptibility of Afferent Pathways to Anesthetic Agents in the Cat

Abstract

The effects of nitrous oxide, ethylene and cyclopropane on somatic afferent evoked potentials in the posteroventral lateral nucleus of the thalamus, the peri-aqueductal reticular formation of the midbrain, the deep pretectal region of the midbrain, and the caudal paramedian reticular nucleus of the medulla oblongata were studied in 46 cats immobilized with muscle relaxants. The time of onset to maximal effect, percentage decrease in amplitude of the evoked subcortical potentials and the duration of depression incident to the administration of these anesthetic gases in doses approximating those in clinical use, showed quantitative correlation with the clinically observed potency of nitrous oxide, ethylene and cyclopropane in that order. The anesthetic agents were shown to act differentially on four somatic afferent systems of diverse fiber size and characteristics of transmission suggesting that the anesthetic effect may be determined by factors in addition to the numerical preponderance of synapses in multineuronal systems. The evidence presented suggests more specific susceptibility of small fiber systems to the action of anesthetic agents.