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How does the brain work when general anesthesia is used?

Local, epidural, spinal anesthesia … are some types of anesthesia that are used to carry out different surgeries. Although the one that interests us right now is general anesthesia, since at the end of April a study was published in eLife in which it is seen how the brain works when used. And it is that we still do not know exactly how general anesthesia works to make us fall into unconsciousness, that is why this type of research is so important.

Although we know what anesthesia is, we are not entirely clear how it works in the brain. The study, as explained in Wired, was carried out by the team of Emery Brown, professor of computational neuroscience at MIT and of anesthesia at Harvard Medical School. Brown has been trying for a decade understand what happens in the brain when patients go into a state of unconsciousness. Knowing this could help you better understand how to best use anesthesia during surgeries.

Although we know what anesthesia is, we are not entirely clear how it works in the brain

Brown has been looking to know for years how neurons behave when patients are asleep. You want to be able to say, “This is what is happening. It is not a black box, ”the professor tells Wired. And once you understand how to read these patterns, and you understand the neurophysiology behind it, you can better dose drugs. You use physiology to better serve your patients. In fact, the anesthetist points out that he has been using EEGs for years to measure the brain activity of his patients.

The anesthetized brain

During consciousness, millions of neurons communicate with each other at different frequencies. These frequencies can be captured in EEGs, where we see the dialogues between neurons converted to electrical impulses “through the outermost regions of the brain, or the cerebral cortex, which is often considered the control center.”

«The brain is a very rhythmic machine“Says Earl K. Miller, professor of neuroscience at the Picower Institute at MIT, who led the study with Brown. “It does this on all frequencies, from 1 Hertz to 100 Hertz or more.”

However, once propofol, the anesthetic used in this study, has been administered, neurons shut up. This anesthetic “attaches itself to proteins called GABAA receptors, making it difficult for cells to fire electrical impulses,” the authors write in Wired. In fact, in previous studies Brown showed that propofol disrupts communication in the cortex in rodents. But now Miller and Brown wanted to go further.

This is how anesthesia behaves in macaques

Photo by Robina Weermeijer on Unsplash

To better understand how the brain behaves when we are unconscious and how anesthesia works; Brown and Miller’s team wanted to record different regions at once as an animal moved in and out of consciousness. So they decided to study this in four macaques. Among the results is that, for example, the impulses of neurons slow down between 90 and 95%. By looking at the four rhesus macaques in different states, they have been able to see how “consciousness rises and recedes.”

To do this, they implanted 64-channel microelectrodes in the four macaques. These were put into the four sections of the brain that they wanted to observe: three regions of the cortex and the thalamus. The areas of the crust chosen were the frontal, temporal and parietal lobes; which are those associated with thought, auditory processing and sensory information respectively. On the other hand, the thalamus is very small and is located in the deepest part of the brain, but it is the one that is responsible for transmitting information to the entire cortex.

The researchers began recording the electrical signals before putting the anesthesia, so they would have information with which to make the comparison. Once they put general anesthesia; The rhesus macaques only took a few minutes to enter the state of unconsciousness: “The drug reaches everywhere and does it in seconds”, says Brown. It was nothing new to him, he already counted on it. Also with the brain waves slowing down. Furthermore, we know that neurons in a healthy, awake brain have a frequency of 10 peaks per second; but under the effects of propofol, that frequency drops to once per second or less. Fortunately, they now had the deep electrodes to know exactly what was happening between neurons.

What happens between neurons?

The neurons of a person who is not under the effects of anesthesia communicate with each other by speaking at different frequencies. But when general anesthesia was administered in the Miller and Brown study, they saw that the neurons went into extreme harmony: they all lowered their frequencies and the higher ones disappeared.

To make a comparison: before anesthesia, the brain looked like a school canteen and afterwards, the murmur of the trees when the wind brushes them a little. The noise was there, but it was much lower.

Anesthesia causes brain frequencies to become uniform, throwing us into unconsciousness

The normal thing is that our brain communicates to different frequencies, which looks more like a chaos in EEGs. A healthy brain is that chaotic. But let all that noise become uniform seems to be just what leads us to unconsciousness. And that’s what anesthesia seems to do in our brain: it makes the brain’s frequencies all become uniform.

The team observed the thalamus because they believe that it is essential to reactivate the noise and bring us to consciousness. So they wanted to know what happened when stimulating him. And the researchers found what they were looking for: Despite remaining unconscious, the monkeys responded to deep stimulation. The monkeys blinked; his heart rate increased, his limbs moved, and even neurons in some areas returned to higher frequencies. «We were able to partially restore consciousness and a cortex similar to that of consciousness, “says Miller.

Here you have to make a paragraph to point out a very important thing. And is that this kind of stimulation is the one used with people who have the Parkinson’s disease and what is painless; because the brain does not process those sensations, even without the assistance of anesthesia.

The brain, key point

And what is all this for? Knowing how the brain works during the use of general anesthesia is essential. In this way, experts will be able to better tailor how to administer anesthesia and, furthermore, in the future it can be used to wake patients up after surgery. Or, better yet, reactivate consciousness after severe brain injury or coma. Wouldn’t it be wonderful to be able to make them wake up? For this, there is still a long way to go, but this is a first step to learn more about anesthesia, the state of unconsciousness it produces and the brain.

We know a lot about the brain, but we still have to learn more. This type of research, although in rhesus monkeys, will help researchers to understand this strange and complex structure that gives us much of what makes us human.

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