Every day, thousands of people are put under general anesthesia as a routine part of surgery.
Ever since the first public demonstration of surgery under general anesthesia in 1846, these drugs have allowed doctors to perform life-saving operations that would otherwise be impossible.
And yet we still don't fully understand exactly how they send us into oblivion without totally shutting down our brains.
A new study published in Nature Neuroscience brings us a bit closer to understanding the mechanisms that allow general anesthesia drugs to suspend our consciousness temporarily.
It compares the medicine's effects across a range of animals, from worms to fish to humans.
It seems no living thing is impervious to general anesthesia's effects, not even plants or paramecia.
This suggests there's something universal in the bodies of living things that this class of drugs is acting on.
To test that idea, a team led by neuroscientist Andrea Luppi of the University of Oxford examined the neural activity of six vastly different kinds of animals when they were awake versus under general anesthesia.
"Focusing on effects that are consistently shared across multiple anesthetics and species allows us to exclude physiological or methodological confounds specific to any one drug, species, or imaging modality, and instead triangulate on potential neural underpinnings of what they share: breakdown of responsiveness to the environment," Luppi and team write.
Their datasets came from humans, macaques, marmosets, mice, zebrafish, and nematodes (roundworms), which, based on when their last common ancestor lived, represent 700 million years of evolution.
The drugs used to induce general anesthesia differed depending on the species, but even still, consistent patterns turned up in their brain activity.
In the mammals, neural activity was measured via fMRI, which uses blood flow to approximate which regions of the brain are active over time.
The zebrafish and nematodes were genetically modified so that their firing neurons literally lit up as calcium rushed in, allowing the scientists to see these animals' neural activity more directly.
"Through this unbiased, data-driven approach, we identified an evolutionarily conserved dynamical profile of anesthesia, indicating that what is common across species and anesthetics is not only the behavioral response to anesthesia (isolation from the environment), but also anesthesia's effect on specific features of neural activity," Luppi and team write.
Across all species, neural activity between brain regions became less coordinated when they were under anesthesia, suggesting these regions were still functioning, but not interacting as much as they would when you're awake.
Neural activity also seemed more chaotic over time, with anesthesia appearing to "induce a breakdown in the relationship between past and future of neural activity," the authors explain.
When we're awake, our brain activity tends to follow certain sequences: these predictable patterns fell apart under anesthesia.
The probability of these key differences in brain activity occurring across all six species just by chance is "vanishingly small," according to anesthesiologists George Mashour and Zirui Huang, who were not involved in the study but wrote an accompanying article in Nature Neuroscience's News and Views.
"These findings provide compelling support for a final common pathway of anesthesia: spatiotemporal isolation of local neural activity, in which individual circuits lose their ability to sustain, propagate and integrate information across time and space," Mashour and Huang write.
"During anesthesia, consciousness is disrupted not because the brain 'turns off' but rather because its activity becomes temporally and spatially fragmented."
Since this mechanism of oblivion is shared not only by humans, but by animals as distantly related to us as fish and roundworms, these findings may even have implications for our understanding of animal consciousness more broadly.
The research has been published in Nature Neuroscience.
This article was fact-checked by Carly Cassella and edited by Carly Cassella. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.