Critical review of the major hypotheses on the mechanism of narcosis

Abstract

The many similarities between artificially induced narcosis and natural sleep are so obvious that it is not surprising that one of the hypotheses first proposed and still frequently mentioned attributes the same common mechanism to both conditions. Since ancient times, it has been assumed that normal sleep is associated with certain conditions of the circulation of the central nervous system and particularly the brain. Throughout antiquity and the Middle Ages and even in the first half of this century, it was believed that the condition of sleep was accompanied by an unusual accumulation of blood in the brain, that compressed its constituents, causing the activity of the brain to be interrupted or reduced. In short, sleep was thought to be induced by a hyperaemia of the brain. This theory appeared most closely in accord with the general experience that it is easier to fall asleep lying down than standing up, since hydrostatic laws meant that the blood would be more likely to accumulate in the head if the body were in a horizontal position. (Up until Claude Bernard, almost nothing was known of vasoconstrictors and vasodilators.) It was not until 1860 that this theory of sleep was disproved by the physician Durham [1] who showed by experiments on dogs that during sleep, the brain is no richer in blood than in the waking condition. In fact, sleep is characterized by a lack of blood and a consequently lowered volume, whereas when the animal awakens, the blood vessels of the brain expand, and the brain increases in volume.