A pair of studies published in the same week have shown that our synapses shorten when we’re asleep, revealing more about the link between sleeping and memory.
Fans of the BBC’s Sherlock will be familiar with Mr. Holmes’ concept of a mind palace and the idea that the brain has finite space for information. This may not just be fictional speculation, and the brain’s way of dealing with this lack of storage space may well be one of life’s great pleasures: sleep.
We can all relate to the glorious feeling of waking up refreshed and reset in the morning, or even after a nap, and recent research has suggested a neurological basis for this. Two papers released concurrently this week have looked at what happens to our neurons during the period while we are asleep. They have shown using new imaging techniques that our synapses actually shrink while asleep and then looked into part of the mechanism causing this.
For a long time, the biological function of sleep has been unknown, but there has been a lot of correlation between deprivation of sleep and the decline in several cognitive functions, such as memory and attention. Now we are starting to get a glimpse of what our brains do to reset and refresh our world each night.
The first paper, by Chiara Cirelli and Giulio Tononi used thousands of images of mice brain tissue from their motor and sensory cortices, showing that their synapses, the gaps that form connections between neurons, shortened by up to 18 percent. To many, this effect may seem fairly innocuous. However, this shortening reduces the strength of a synapse, and the change in the strength is what mediates learning and memory. This change also depended on the already established strength of a synapse, stronger synapses tended to shorten less, meaning that their strength also weakened less during sleep.
Shortening synapses allows the brain to save a considerable amount of energy along with the benefit of allowing memories to be organised. The variation between the amount of shortening between weak and strong synapses allows stronger connections, storing stronger memories, to remain unchanged. While less important information is forgotten allowing the brain to be less cluttered. These processes are termed consolidation, integration and “smart forgetting” in the paper.
The second paper by Richard Huganir and Graham Diering looks at the way that this shortening occurs. They found that a protein called Homer1a is the culprit. This reduces the activity of receptor in the brain called the AMPA receptor, which is involved in learning and memory. The receptor is also involved in bringing the two neurons closer together.
While this is exciting news for sleep enthusiasts, this is just a start in unravelling the biological mystery of its purpose.
Homer1a builds up over night, and is removed during the time spent awake. Interestingly, when sleep was deprived, Homer1a was present during time awake as well, sleep deprivation on cognitive function.
While this is exciting news for sleep enthusiasts, this is just a start in unravelling the biological mystery of its purpose. The studies used mice, and so it is unclear as of yet whether the human brain will react in the same way.
Homer1a is present in the human brain as well, however the studies noted that not all areas of the brain reacted uniformly, as was demonstrated by the fact that the shortening effect varied depending on the strength of the synapse.
However, with the already strong repertoire of studies that show similarities across all mammalian brains, this is certainly groundbreaking and provides a key insight into the effects of sleep on our learning and memory.