It has long been known that the hippocampus is involved in learning and memory.However, because nerve cells are limited, when they reach a certain level, they become saturated and cannot remember any more.Therefore, it has been expected that the hippocampus will have some kind of "cool-down" mechanism.
A research group at the University of Tokyo discovered that brain waves called "sharp wave ripple (SWR)" generated from the hippocampus weaken the connections between neurons during sleep and cool down the brain circuit.
The research group first recorded field excitatory postsynaptic potential (fEPSP), an indicator of the strength of connections between sleeping mice and neurons.Then, it was confirmed that fEPSP was attenuated with the progress of sleep, and that the connection between neurons was naturally weakened during sleep.
Subsequently, when an experiment was conducted to inhibit SWR that occurs during sleep, no attenuation of fEPSP was observed.That is, it was revealed that SWR weakens the connections between hippocampal neurons during sleep and cools down the hippocampal neural circuits. Mice that had been inhibiting SWR for 7 hours remained highly excitable in their brain circuits, and even though they had enough sleep, they became in a state of lack of sleep.
Furthermore, using genetically modified mice that can distinguish between neurons involved in memory and neurons not involved in memory, we observed the activity of neurons during SWR development.They found that only the activity of "not" neurons involved in memory just before sleep was weakened.In other words, it was inferred that SWR weakened only the connections of neurons that are not related to memory, and that memory was efficiently organized during sleep.
This result can be said to provide a clear answer to the fundamental question, "Why do organisms sleep?"
Paper information:[Science] Hippocampal Ripples Downregulate Synapses
