Deciphered synaptic changes during sleep
Why do we need sleep and what happens in our brains during periods of rest? Questions that cannot be answered unequivocally to this day. Researchers at Freiburg University Hospital have now found out how the brain creates space for new information while sleeping.
Why humans and animals sleep is "still not clearly clarified," according to a message from the University Medical Center in Freiburg. Christoph Nissen, medical director of the sleep laboratory at the Clinic for Psychiatry and Psychotherapy at the University Medical Center Freiburg, was able to prove in his current study, however, that "during sleep the general activity of the nerve cell connections called synapses is reduced." The brain clears away practically during sleep and create new space for storing information. The researchers have their results in the journal "Nature Communications".
Activity of the synapses examined
As part of their study, the researchers first examined the general activity of the synapses in the brain, which is also known as total connection strength. For this purpose, a specific area in the brain, which is responsible for controlling a thumb muscle, was stimulated with the help of a magnetic coil above the test subject's head. The connection strength could be checked by this so-called transcranial magnetic stimulation (TMS). The researchers found that after sleep deprivation, a significantly weaker stimulus triggers a contraction of the muscle. This is a sign of a higher synaptic connection strength.
Total synaptic strength increases during the day and decreases during sleep
Using electroencephalography (EEG) measurements, the researchers also evaluated the different frequencies of the brain waves. Deprivation of sleep has led to a significant increase in the so-called theta waves, reports the Freiburg University Hospital. It is known from previous animal and human studies that this is another sign of increased overall synaptic strength. According to Professor Nissen, “sleep lowers the overall strength of the synapses in the brain during the day”, while “activity remains high after sleep deprivation”.
Synaptic plasticity is an important basis for learning
Most of the synaptic connections are weakened in the course of sleep, some even completely broken down, the scientists report. Only important synapses would remain or even be strengthened. In this way, the brain frees up space to store new information. This adaptability is known as synaptic plasticity and is an important basis for learning and for flexible information processing. According to the researchers, the degradation also saves "space and energy, since both are largely required by the connection points in the brain."
Lack of sleep leads to a state of satiety
When information is recorded during the day, synapses in the brain are strengthened or newly created. In the current study, evidence has now been provided for the first time that "sleep regulates the synapses again and thus creates space for new information," says study leader Prof. Dr. Nits. "So the brain cleans up in sleep," emphasizes the expert. If this process is prevented by lack of sleep, the brain will become saturated. “Synapses can then no longer be sufficiently strengthened or rebuilt. Learning and flexible information processing are correspondingly difficult, ”continues Nissen.
Overload protection mechanism
In their investigations, the researchers were also able to determine that the human organism is protected against overloading. For the first time, indications of a principle have been discovered in humans "that guarantees permanent stimulus processing, the so-called homeostatic plasticity", according to the Freiburg University Clinic. If the synapses are already maximally active due to long awake phases, new stimuli or information do not lead to strengthening, but rather to a weakening of the nerve cell connections, the researchers report. Newly arriving stimuli would then be processed normally again. "It can be assumed that practically all functions of the brain are affected, such as emotion regulation, concentration or learning," says Prof. Nissen.
Why do some people cope better with lack of sleep?
According to the University Medical Center Freiburg, the researchers “continued to find evidence that the growth factor BDNF (brain derived neurotrophic factor) plays an important role in the regulation of synaptic activity.” Although it is known that BDNF restores nerve cells after normal sleep and thus encourages learning, but a persistently high concentration of BDNF in the blood while deprived of sleep had more likely to saturate synapses. "This could explain why some people cope with a lack of sleep better than others," says study director Prof. Nissen.
Hope for new therapeutic approaches
The researchers hope that their findings can also contribute to the development of new therapeutic options, for example after a stroke or for depressive disorders. With these diseases, it is important to change the circuits in the brain. "For this purpose, a targeted influence on the sleep-wake behavior, but also other methods such as transcranial DC stimulation or medication with new mechanisms of action on plasticity can be used," said the Freiburg University Hospital. (fp)