Scientists for the first time managed to study how brain neurons behave in an unconscious state
When you are awake, your neurons “talk” to each other using electrical signals. The frequency of these signals is the same. One group of neurons can work synchronously at a frequency of 10 Hz, the other at a frequency of 30 Hz. When you are anesthetized, this whole system loses its complexity and generates a kind of uniform low frequency “hum”.
Monitoring the anesthesia process can make surgery safer, but most anesthesiologists do not use an EEG. Emery Brown is a professor of neuroscience at MIT and an anesthesia processor at Harvard Medical School. He is also a practicing anesthesiologist, and he is worried about the lack of control over the work of the brain during anesthesia. “Most anesthesiologists don’t think about it in terms of neuroscience,” he says. Brown has been studying for ten years what happens to the brain when its “master” is unconscious. What for? To obtain data on how different anesthetics work, as well as on what happens to neurons. The professor’s desire is to receive so much information that one can say about the brain under anesthesia: “This is what is happening here. This is not a black box. ”
“Once you understand how to read patterns, understand the neurophysiological processes behind them, you can dose your drugs optimally,” says Brown.
In a study published in April in the online edition eLife, the professor’s team described the use of electrodes to study neurons that are deep in the brain of monkeys under anesthesia. This work showed for the first time how individual neurons in different areas of the brain respond to sedatives: their activity is reduced by 90-95%. Moreover, by studying the work of neurons in different “modes” of brain activity, the team began to better understand how consciousness disappears under the influence of anesthesia, and how it manifests itself again.
Any thought that appears in your brain literally passes through it. This is due to chains of millions of neurons that communicate with each other. “The brain is a great rhythm machine,” says Earl K. Miller, professor of neuroscience at MIT. The professor worked with Brown on this study. “Thought triggers neurons to work at all frequencies, from 1 Hz to 100 Hz or more,” he says. EEG, or encephalogram, shows the cross-connection of neurons collectively triggering waves of electrical signals in a wide variety of areas of the brain, including its cortex, which is usually considered as the control center.
In the course of this process, the “dialogue” between neurons and consciousness is manifested. “Sights, sounds, sensations are all a single whole, thanks to which we get a unified experience of what we do, how we feel, what we think about at a certain moment,” says Miller. This, he said, is consciousness. The exact process of how neuronal activity leads to individual perception and thinking is not yet fully understood. But one method of learning is to observe what happens to neurons when they are “turned off.”
Anesthesia turns off neurons. Propofol, a common anesthetic used in research, interacts with proteins called GABA receptors. As a result, cells almost lose the ability to exchange electrical signals.
In the early stages of research, scientists studied rodent brains and human EEGs. It turned out that propofol disrupts neural connections in the cerebral cortex. But this was not enough for the authors of the project. Brown and Miller decided to monitor the activity of different areas of the brain, including “turning off” consciousness and “turning on.” To do this, they began to use implanted electrodes to understand how individual neurons change their activity. In a new study, researchers implanted 64-channel microelectrodes in four rhesus monkeys. They were installed in three areas of the cortex and thalamus. The first three areas are the frontal, temporal, and parietal. They are associated with thinking, hearing and touch.
After the electrodes were installed, the scientists injected the monkeys with propofol, and then watched them fall asleep under the influence of anesthesia. The activity of neurons decreased by about 10 times. If the majority of neurons previously transmitted signals with a frequency of 10 Hz, then after anesthesia, the frequency dropped to 1 Hz. This situation was observed at almost all points.
According to scientists, during periods of activity (including sleep), brain waves are much more chaotic than during anesthesia. In a passive state, the activity of neurons can be compared with a uniform “hum”, as if in a large dining room full of children, they stopped communicating in groups and simply began to mumble monotonously. One of the researchers compared propofol with a sledgehammer, which “knocks out the brain, putting it in a low-frequency mode.”
Above we talked about three areas of the brain. As for the thalamus, the fourth area where the electrodes were inserted, Miller and Brown believed that it was especially important in terms of recovering consciousness. According to one theory, it synchronizes the different rhythms of the cerebral cortex. If the thalamus does not work or does not work properly, the brain “out of sync” and there are no more coherent thoughts. Scientists decided to artificially stimulate the thalamus during macaque anesthesia to see if signs of conscious activity return.
The second series of experiments began. The thalamus was stimulated with weak electrical signals comparable to what people get when they treat Parkinson’s disease with deep brain stimulation. It is painless. Immediately after the start of the stimulation, the monkeys blinked. Their heartbeat quickened and their limbs moved. Neurons in some parts of the brain increased their activity from 1 to 3 pulses per second. In general, this state was close to the state when the animals were conscious, although they were still under the influence of a strong anesthetic. After the termination of stimulation, the activity disappeared – though not immediately, but after a few minutes. Scientists concluded that they were able to partially restore the consciousness of animals.
In general, Miller and Brown are not aiming at the study of consciousness and its nuances. Their task is simpler – to make anesthesia safer by allowing anesthesiologists who use EEGs to more accurately control the dosage of drugs to patients. Perhaps, in the future, it will be possible to use electrical stimulation of the thalamus to activate the consciousness of people during operations – for patients with serious brain injuries or who have fallen into a coma.
Miller also hopes the results of the study will help uncover one of the biggest mysteries of neuroscience: how our brains and the complex interactions between neurons enable us to gain consciousness.