This won’t hurt (Image: Fuse/Getty)

Anaesthetists have been putting people to sleep for years, but how the drugs they use send you into a slumber is still largely a mystery. New research suggests it involves the obliteration of long distance communication in the brain.

A team led by Laura Lewis, at the Massachusetts Institute of Technology, and Patrick Purdon, at Harvard Medical School, were given a window into the brain during three operations for epilepsy. Each patient already had electrodes implanted into their temporal cortex, which meant that the team could measure neuronal activity as they were given propofol, a common general anaesthetic.

The team asked each patient to respond to a sound as they drifted off. At the moment they stopped responding, Lewis and Purdon saw a dramatic change in neuronal activity across the cortex. Slow wave oscillations – the brainwaves that occur in deep, non-dreaming sleep – grew almost immediately.


Stop firing

Locally, these slow waves were in sync and neurons near each other coordinated their activity to correspond with the peaks and troughs of the waves they encountered, meaning continued communication. However, the slow waves were not in sync across the entire cortex.

While conscious, different regions of the cortex fire at the same time, so neurons can communicate over long distances if necessary. The peak of each slow wave represents a moment in which that area of the brain has gone silent – and all of its neurons stop firing. A silent region cannot receive a signal from a region that is awake, so out-of sync slow waves make long distance communication near impossible.

In-sync areas were roughly four millimetres square, within which neurons continued to communicate as they did before the patient lost consciousness.

Time zones

“It’s like different brain areas are in different time zones,” says Lewis, though in this case the time zones represent fractions of a second. “When one area is awake the other is asleep.”

Clinically speaking, the more we know about how anaesthesia works in the brain, the more we can make absolutely sure patients do not wake up with memories of their operations, says Ludwig Lin, a clinical anaesthetist at the University of California, San Francisco, who was not involved in the research.

To that end, Lin hopes to see the same research applied to other general anaesthetics, including inhaled forms, to see whether there are differences in how the drugs cause loss of consciousness, and whether that loss of consciousness looks the same in the brain.

Journal reference: PNAS, DOI: 10.1073/pnas.1210907109