If you work hard physically, you then feel tired and exhausted. Demanding intellectual activities have the same effect - although they are usually sitting and without significant muscle use. Why? A research team led by Antonius Wiehler from the Paris Brain Institute may have found an answer to this question. Intensive thinking causes a fabric in the brain that can disrupt the nerve metabolism and is poisonous in high concentrations. The group reports on this in the specialist magazine »Current Biology«.
Wiehler and his team had 40 test participants working on the screen. The people were shown letters every second, which they had to classify according to different characteristics – for example by color or according to whether they were vowels or consonants. Sometimes you should indicate in memory tests whether the displayed letter had been seen a short time before. 24 Participants had to cope with heavy versions of these tasks, the remaining 16 were allowed to enjoy light ones. In order to exhaust themselves to the point of fatigue, the subjects completed several thousand tests daily. This kept them busy for about six and a half hours, which is equivalent to a strenuous office job.
The longer the participants had sat on the screen, the more fought they felt and the more mistakes made them. At the end of a test day, most of them were very exhausted. And regardless of whether they had solved heavy or easy tasks.
Quick money, slow money
Every now and then the research team had the test subjects voted between two amounts of money: a small sum that was to be obtained with little effort or after a short waiting time, and a larger amount that required more effort or longer waiting. The scientists thus checked how much discipline the participants brought up. Those who struggled with difficult tasks, i.e. mentally harder, voted much more often for the small, easier reward to be maintained. A sign of fatigue and declining self -overcoming.
While doing the tests, the subjects were examined from time to time by magnetic resonance spectroscopy (MRS). This method makes it possible to measure the concentration of selected types of molecules in the nervous system. In mentally intensive participants, according to the result, the substance glutamate accumulated in the anterior cerebral cortex. The brain areas located there help to regulate feelings, and they are important for self-control as well as for action planning and control.
Glutamate is one of the most important excitatory messengers in the brain. In addition, it is involved in the detoxification of ammonia, serves as a precursor for other messenger substances and has a toxic effect in higher concentrations. Therefore, the organism must precisely regulate the glutamate balance in the brain. In too high a quantity, the substance disrupts cellular metabolism, impairs neuronal information transmission and, in extreme cases, leads to symptoms of intoxication.
The team around Wiehler suspects: If the thought organ runs at full speed, too much glutamate is produced in the front cerebral cortex. The accumulated glutamate changes brain metabolism and makes it increasingly difficult to activate the nerve cells there; This goes hand in hand with fatigue and exhaustion symptoms and restricts self-control.
Can this effect be prevented? "Not really, I'm afraid," says brain researcher Mathias Pessiglione, a colleague of Wiehler. He recommends rest and sleep to renew mental performance. "There is convincing evidence that the accumulated glutamate is broken down again during sleep.«
Fritjof Helmchen from the Institute for Brain Research at the University of Zurich, who was not involved in the work, considers the study convincing: »The fact that the glutamate balance in the brain is important and has to do with exhaustion has been suspected for a long time; The investigations by Wiehler and his group are now showing this. ”The messenger substance plays a central role in brain activity and in the nerve metabolism; His concentration must therefore always be finely balanced. It is not clear whether an increased glutamate content leads to dwindling nerve activity in the front cerebral cortex. However, the study shows that both are related. "How the system regenerates again is an exciting question," says Fritjof Helmchen, "especially whether sleep has the function of restoring the balance in the brain."
