Sleep—or a lack thereof—has a dramatic effect on neurons in the hippocampus.
Summary: Two studies from the University of Michigan demonstrate that sleep deprivation impacts hippocampal neurons and memory reactivation. The research found that during sleep, specific neurons in the hippocampus reactivate and replay experiences, crucial for memory formation. Sleep deprivation suppresses this reactivation and replay, impairing memory consolidation. Even after catching up on sleep, the reactivation levels never fully recover, highlighting the importance of adequate sleep for effective memory processing.
Key Takeaways:
- Sleep deprivation suppresses the reactivation and replay of hippocampal neurons, which are crucial for memory formation.
- Adequate sleep is essential for the proper reactivation and updating of memories, as evidenced by higher neuron activity during sleep compared to sleep deprivation.
- Catching up on sleep after deprivation does not fully restore the impaired reactivation and replay of neurons, indicating lasting negative effects on memory processing.
Two new studies from the University of Michigan reveal the role of sleep in memory formation, demonstrating that sleep deprivation hampers the reactivation and replay of memories in the brain.
Imagine you’re a student, it’s finals week, and you’re preparing for a big exam: Do you pull an all-nighter, or do you get some rest? As many a groggy-eyed person who’s stared blankly at a test knows, a lack of sleep can make it extraordinarily difficult to retain information.
The studies uncover why this is and what is happening inside the brain during sleep and sleep deprivation to help or harm the formation of memories.
Specific neurons can be tuned to specific stimuli.
Neuronal Activity During Sleep
For example, rats in a maze will have neurons that light up once the animal reaches specific spots in the maze. These neurons, called place neurons, are also active in people and help people navigate their environment.
But what happens during sleep?
“If that neuron is responding during sleep, what can you infer from that?” says Kamran Diba, PhD, associate professor of Anesthesiology at U-M Medical School, in a release.
A study, summarized in the Nature and led by Diba and former graduate student Kourosh Maboudi, PhD, looks at neurons in the hippocampus, a seahorse-shaped structure deep in the brain involved in memory formation, and discovered a way to visualize the tuning of neuronal patterns associated with a location while an animal was asleep.
Sharp-Wave Ripples and Memory
A type of electrical activity called sharp-wave ripples emanates from the hippocampus every couple of seconds over a period of many hours during restful states and sleep. Researchers have been intrigued by how synchronous the ripples are and how far they travel, seemingly to spread information from one part of the brain to another.
These firings are thought to allow neurons to form and update memories, including of place.
For the study, the team measured a rat’s brain activity during sleep, after the rat completed a new maze. Using a type of statistical inference called Bayesian learning, they were for the first time able to track which neurons would respond to which places in the maze.
“Let’s say a neuron prefers a certain corner of the maze. We might see that neuron activate with others that show a similar preference during sleep. But sometimes neurons associated with other areas might co-activate with that cell. We then saw that when we put it back on the maze, the location preferences of neurons changed depending on which cells they fired with during sleep,” says Diba in a release.
The method allows them to visualize the plasticity or representational drift of the neurons in real-time.
Effects of Sleep Deprivation on Neurons
It also gives more support to the long-standing theory that reactivation of neurons during sleep is part of why sleep is important for memories.
Given sleep’s importance, Diba’s team wanted to look at what happens in the brain in the context of sleep deprivation.
In the second study, also published in Nature, the team, led by Diba and former graduate student Bapun Giri, PhD, compared the amount of neuron reactivation—wherein the place neurons that fired during maze exploration spontaneously fire again at rest—and the sequence of their reactivation (quantified as replay), during sleep versus during sleep loss.
They discovered that the firing patterns of neurons involved in reactivating and replaying the maze experience were higher in sleep compared to during sleep deprivation. Sleep deprivation corresponded with a similar or higher rate of sharp-wave ripples, but lower amplitude waves and lower power ripples.
Long-Term Memory Impairment
“In almost half the cases, however, reactivation of the maze experience during sharp-wave ripples was completely suppressed during sleep deprivation,” says Diba in a release.
When sleep-deprived rats were able to catch up on sleep, while the reactivation rebounded slightly, it never matched that of rats who slept normally. Furthermore, replay was similarly impaired but was not recovered when lost sleep was regained.
Since reactivation and replay are important for memory, the findings demonstrate the detrimental effects of sleep deprivation on memory. Diba’s team hopes to continue looking at the nature of memory processing during sleep and why they need to be reactivated and the effects of sleep pressure on memory.
Photo 65461527 © Kiosea39 | Dreamstime.com
