Anesthetically Induced Unconsciousness

               Daniel Bor explores consciousness in The Ravenous Brain, but you can learn a great deal about consciousness by observing unconsciousness. Bor touches on this in The Tip of the Iceberg chapter section Unconscious Neurons Marching in Step. He describes neurons of a conscious mind like busy tourists and the neurons of an unconscious mind like soldiers in the army to illustrate the difference between neural activity in a conscious and unconscious mind. However he does not spend time describing how the tourists became soldiers, i.e. how conscious neural activity changes to unconscious neural activity. The mechanism of how anesthetics cause the loss of consciousness is not understood well.  The brain has been studied many times while under the influence of anesthesia, therefore the molecular system of the steady state of unconsciousness is understood. But, what remains a mystery is how neuronal activity is altered during the transition into unconsciousness. Researchers at Massachusetts General Hospital and MIT have identified a pattern of brain activity that appears to signal exactly when patients lose consciousness under general anesthesia, specifically propofol, one of the most common general anesthetics used today.

                Propofol works by increasing GABA, the major inhibitory neurotransmitter in the central nervous system.  We have observed that when GABA has put a damper on our brains and we are unconscious the firing of neurons becomes more uniform and develops a slow rhythmic pattern. These patterns are observed regularly, however we have not traced the transition into unconsciousness. The researchers observed simultaneously single unit local field potentials as well as intracranial electrocorticograms. This way they were able to make spatial observations with temporal resolution. They also had the patient preform a behavioral task, like the classic count backwards from ten task which one is familiar with if you’ve experienced general anesthesia, to help them determine precisely when the patient lost consciousness.

                What the researchers observed was that the transition into unconsciousness was marked by the slow oscillation of the local field potentials. Spikes in neural activity occur, however they are short intervals with longer periods of suppression in between, inhibiting the processing of information. Also these spikes in activity do not happen simultaneously throughout the brain, meaning that neural activity is disjointed both spatially and temporally. This disconnects the processing of information within a brain region and between brain regions. So the connections are still functioning and in use during unconsciousness, but they are fragmented.  Therefore the hypothesis that general anesthetics cause unconsciousness by breaking down cortical communication is supported by this research.  

Diagram A shows the increase in slow oscillation power after the patient lost consciousness. 

Diagram B shows the spike rate decrease directly after the patient lost consciousness, and then how the spike rates normalize and rise to pre loss of consciousness rates.

                     

                 This is the first study to match the onset of unconsciousness with the slow oscillation pattern. However there are some limitations to this research.  For instance they only tested the general anesthetic propofol, and their only patients were epileptics due to the invasive research method. This makes this research difficult to generalize and apply to every type of loss of consciousness for every individual.  Regardless this research is important because it suggests a specific mechanism for how anesthetics cause the loss of consciousness. Also current anesthetic brain monitors are inaccurate due to the fact that brain wave patterns are inaccurate in determining unconsciousness. This research could lead to better ways of monitoring individuals under anesthetics undergoing an operation. This could allow for better accuracy in determining anesthetic dosage and decrease the likelihood that patients regain consciousness during an operation. This research allows us to better understand how consciousness operates by analyzing how unconsciousness operates.

Resources:

Laura D. Lewis et al. Rapid fragmentation of neuronal networks at the onset of propofol-induced unconsciousness. (November 5, 2012).PNAS Journal article. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3523833/?report=classic