Enjoying a good night’s sleep not only recharges our energy, but also resets our memory by allowing neurons to reconfigure themselves and thus facilitating continuous learning without exhausting brain resources.
We all know that getting a good night’s sleep gives us the energy and vitality we need to tackle everyday tasks, but a new study reveals that sleep also resets another vital function: memory. Learning or experiencing new things activates neurons in the hippocampus, a region of the brain essential for memory. Later, while we sleep, those same neurons repeat the same pattern of activity, allowing the brain to consolidate those memories, which are then stored in a large area called the cortex. But how is it possible to keep learning new things throughout life without exhausting all of our neurons?
The research was carried out by scientists at Cornell University who discovered that during certain phases of deep sleep, some parts of the hippocampus are silenced, allowing those neurons to restart. “This mechanism could allow the brain to reuse the same resources, the same neurons, to learn new things the next day,” explained Azahara Oliva, assistant professor in the Department of Neurobiology and Behavior in the Faculty of Arts and Sciences, and lead author of the study, which has been published in Science.
Tools to improve and manipulate memory
The hippocampus is divided into three regions: CA1, CA2, and CA3. The CA1 and CA3 regions are involved in encoding time- and space-related memories and have been widely studied; less is known about the CA2 region, which the study identified as responsible for generating this silencing and resetting of the hippocampus during sleep.
The researchers implanted electrodes in the hippocampi of mice, allowing them to record neuronal activity during learning and sleep. In this way, they were able to observe that, during sleep, neurons in the CA1 and CA3 areas reproduce the same neuronal patterns developed during daytime learning. However, the researchers wanted to understand how the brain continues to learn without becoming overloaded or running out of neurons.
“We realized that there are other hippocampal states that occur during sleep, where everything goes silent,” Oliva explained. “The CA1 and CA3 regions, which had been very active, suddenly became silent. It is a reset of memory, and this state is generated by the middle region, CA2.”
“This mechanism could allow the brain to reuse the same resources, the same neurons, to learn new things the next day.”
Cells called pyramidal neurons are thought to be the active ones relevant to functions such as learning. Another type of cell, called an interneuron, has different subtypes. Researchers found that the brain has parallel circuits regulated by these two types of interneurons: one that regulates memory and another that allows memories to be reset.
Researchers believe they now have the tools to improve memory by manipulating memory consolidation mechanisms, which could be applied in cases where memory function is impaired, such as in Alzheimer’s disease. They also have evidence to explore ways to erase negative or traumatic memories, which could help treat conditions such as post-traumatic stress disorder.
The finding is important because it helps explain why all animals need sleep, not only to fix memories, but also to reset the brain and keep it working during waking hours. “We show that memory is a dynamic process,” Oliva concluded.
