Where does the brain store information? Usually called the hippocampus and some areas of the prefrontal cortex. However, both about the hippocampus and about the cortical zones, they usually specify that these are one of the memory centers, even if they are the main ones. Such a clarification is all the more necessary in the light of the last article in Nature Communicationswhich says that memories are distributed throughout the brain, even falling into areas that no one has yet associated with memory.
The authors of the work looked for the so-called engram neurons in the brains of mice. An engram is understood as a trace left by a stimulus; if we talk about neurons, then a repeated signal – a sound, a smell, a certain environment, etc. – should provoke some physical and biochemical changes in them. If the stimulus is then repeated, then the “trace” is activated, and the cells in which it is present will recall the entire memory from memory. In other words, our engram (“key”) neurons are responsible for accessing the recorded information, and in order for them to work themselves, they must be affected by a key signal; Obviously, such cells themselves must be able to somehow store information about certain stimuli.
We have repeatedly written about engram neurons in connection with the research of Suzumi Tonegawa (Susumu Tonegawa) and his colleagues from Massachusetts Institute of Technology – they are among the authors of the new article. Engram cells are involved in both short-term and long-term memory, help memories connect with each other, etc.; however, for the most part, such neurons were again sought either in the hippocampus or in the cortex.
In the new experiments, memory cells have already been searched for in 247 areas of the brain. Mice were taken from a familiar, safe cage and placed in another, unfamiliar one, in which they were given mild electric shocks—in other words, the mice formed an unpleasant memory of the new cage. At the same time, in some mice, neurons were genetically modified – the cells began to fluoresce if a gene necessary for recording information was activated in them. In other mice, things were a little different – their neurons began to fluoresce when an already recorded memory was activated (that is, when they were back in the “electrocell”). The glow remained for a long time, so that the brain could be pulled out, made transparent and carefully examined under a microscope.
Naturally, the brain that remembered the unpleasant experience, and the brain that had to remember this experience, were compared with the brain that did not have any trouble – in this way it was possible to determine those neurons that are really related to memory. It turned out that 117 areas of the mouse brain are involved to one degree or another in the fact that the mouse remembers the electric shock in an unfamiliar cell. But to be an engram cell, a neuron must be involved in both initial memory formation and subsequent memory activation. Initially, in these 117 zones, there were separately those in which neurons remember the stimulus for the first time, and separately those that then help to remember what awaits you in a particular situation. When the data from those and those areas of the brain were combined, it turned out that there are engram cells in about 60% of them, and in addition to the expected hippocampus, cortex and amygdala (responsible for emotions), among the engram areas were areas of the thalamus, midbrain and brain stem .
The results were rechecked in other experiments, in which the neurons of these zones were subjected to optogenetic modification – so that the nerve cells could be turned on or off by light pulses supplied to the brain through an optical fiber. As a result, it was possible to confirm that neurons from different areas of the brain really work as engram cells: if they were activated, the mouse fell into stress, although at that moment she was in a familiar and safe environment.
It was also possible to show that engram cells form something like complexes when stimulation of an engram neuron in the hippocampus wakes up other neurons in the very remote areas that were not thought to be related to memory. Moreover, in order for the memory to be the most vivid, it is necessary to turn on engram neurons in several zones at once: in other words, if you activate the “memory keys” in the hippocampus alone, the mouse will not be so afraid than if you activate them in three areas of the brain at once.
The distribution of memory throughout the brain makes it more reliable. On the other hand, it may well be that different engram cells, even if related to the same memory, perform different tasks. Still, any memory is a complex of heterogeneous information, and engram cells can be targeted at different informational components of the same memory. Perhaps, more individually, the features of our memory are connected precisely with the differences that different cells of the same engram complex, distributed over several brain areas, have.
Source: Автономная некоммерческая организация "Редакция журнала «Наука и жизнь»" by www.nkj.ru.
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