The Neural Mechanism of the Anesthesia-Induced Change in Consciousness

Anestezinin Bilinçte Yarattığı Değişimin Nöral Mekanizmasi

In addition to the analyses made in the states of sleep and coma regarding how the brain physiologically gets into the states of consciousness and unconsciousness, the dose-driven effects of anesthetics on the consciousness are also studied.

A common study (2021), which was backed by the American National Institutes of Health and carried out in the laboratories of the MIT-Picower Institute for Learning and Memory, revealed how propofol, which is an anesthetic used on animals, affected the brain waves, and the states of consciousness and unconsciousness of macaque monkeys.

The researchers used EEG to measure how strongly the brain waves were synchronized before, during and after the propofol-induced anesthesia. The data obtained from the study show that, as the drug’s area of influence in the brain grew bigger, there were strong increments in the simultaneous activity only at very slow frequencies in the thalamus and four cortical regions, and at very slow rhythms that preserved a slow pace of neural activity. On the other hand, the electrical stimulation of the thalamus, which enables the synchronization of the brain activity at a higher frequency in the brain’s normal state, “reanimated” the anesthetized monkeys, and it reversed the electrophysiological characteristics of the state of unconsciousness created by the anesthetic.

On the study, Earl Miller, a Picower Professor of Neuroscience and senior author of the study, says “There is a non-verbal public assumption that what anesthesia does is to “shut down” the brain.” “What we show here is that propofol strikingly changes and controls the dynamics of the brain’s rhythms.”

Conscious activities like perception and cognition depend on the coordinated brain communication that is particularly between the thalamus and the brain’s surface areas or the cortex, at various frequency bands ranging between 4 to 100 hertz. As shown by the study, propofol reduces the coordination between the thalamus and cortical regions down to frequencies at around only 1 hertz.

One of the most important aspects of the recent study is that it is capable of using electrodes that can directly measure the activity of a number of individual neurons and rhythms or “spike-and-waves” in the cerebral cortex or thalamus, as it implements these dynamics on animal models. Emery N. Brown, another senior author of the study and Professor of Computational Neuroscience and an anesthesiologist at the Massachusetts Hospital, said that this is the reason why the results significantly deepened and expanded the findings on humans.

Drawing attention to the clinical importance of the study, Brown says that studies which show how anesthetics change the brain rhythms could directly improve the patient safety, because these rhythms can be easily seen on the EEG in the operating room. Finally, Brown said “The main result of the study in regards to patterns of the very-slow rhythms along the cerebral cortex, offers a model for measuring when the subjects fall unconscious after the administration of propofol, how deep they stay in this state, and how quickly they can wake up when the propofol dose runs out. Anesthesiologists, on the other hand, can use these results as a better way to take care of the patients.”

Bastos, A. M., Donoghue, J. A., Brincat, S. L., Mahnke, M., Yanar, J., Correa, J., … & Miller, E. K. (2021). Neural effects of propofol-induced unconsciousness and its reversal using thalamic stimulation. Elife, 10, e60824.