The entwined mysteries of anesthesia and consciousness: is there a common underlying mechanism?

THE mechanism by which general anesthetics prevent consciousness remains unknown largely because the mechanism by which brain physiology produces consciousness is unexplained. But the two mysteries seem to share a critical feature—both consciousness and actions of anesthetic gases are mediated through extremely weak London forces (a type of van der Waals force) acting in hydrophobic pockets within dendritic proteins arrayed in synchronized brain systems. Unraveling this common thread may reveal not only how anesthetics act, but also why we are conscious in the first place. What is anesthesia? Anesthesia provides immobility, amnesia, and loss of conscious awareness, although the latter—loss of consciousness—is often omitted from operational definitions. In recent years, putative sites of anesthetic action for immobility (spinal cord), amnesia (dorsolateral prefrontal cortex, amygdala), and loss of consciousness (networks involving thalamocortical and intracortical—corticocortical—loops, prefrontal cortex, and other areas) have been discriminated both anatomically and in terms of sensitivity to anesthetics. Immobility is least sensitive to anesthetics, followed by loss of consciousness and then amnesia, which is most anesthetic sensitive. (Implicit memory may occur without consciousness or movement, but at light levels of anesthetic.) Therefore, lack of movement—even though mediated by spinal cord rather than brain—in the absence of muscle relaxants is a good indicator of both loss of consciousness and amnesia. Autonomic responses are even less anesthetic sensitive than immobility and, in the absence of autonomic-blocking drugs, are thus useful (although not perfectly reliable) early warning indicators of changes in anesthetic depth. What is consciousness? Unlike other receptor-mediated pharmacologic targets, consciousness is ill-defined, cannot be measured, and generates heated debate about its very nature. Indeed, except for the “dark age” of behaviorism in psychology during most of the 20th century (in which consciousness was, almost literally, a dirty word), conscious awareness has been a prominent mystery in science and philosophy. However, many articles promising to discuss consciousness avoid the issue, e.g., using bait-and-switch techniques to describe memory, learning, sleep, or other related activities. Others deconstruct consciousness into a group of cognitive functions so that the essential feature—conscious awareness—gets lost in the shuffle. In this article, consciousness will be considered equivalent to even minimal awareness, the ineffable phenomenon of pure subjective experience—our “inner life.” Thus, conscious awareness can exist irrespective of memory, cognition, or organizational sophistication (e.g., reflective self-consciousness, higher-order thought, human—as opposed to animal—consciousness). These more complex levels, although difficult to explain, are relatively straightforward compared with the issue of why or how even a slight glimmer of any form of conscious experience occurs at all. Anesthesia offers a unique and profound opportunity to understand consciousness because it is relatively selective—many brain activities (e.g., evoked potentials, slower electroencephalography, and autonomic drives) continue during anesthesia while conscious awareness disappears. Thus, details of anesthetic mechanism may illuminate how the brain specifically produces consciousness and vice versa. This article reviews what is known about mechanisms of consciousness and anesthesia, finding that the “fine grain” of neuronal activities supporting consciousness and the molecular actions of anesthetic gases are one and the same—van der Waals London forces acting in hydrophobic pockets of coherently synchronized dendritic brain proteins. London forces are not chemical bonds but weak quantum interactions (in this regard, anesthetic gases differ in their actions from all other pharmacologic agents). Thus, the relative selectivity of anesthetic gases implies that the quantum nature of London forces may play an essential role in brain function leading to consciousness. Because consciousness is not directly measurable or observable, we begin with brain functional organization, systems, and activities known to correlate with consciousness. * Professor Emeritus, Departments of Anesthesiology and Psychology, and Director, Center for Consciousness Studies, The University of Arizona.