Sleep trimming synapses
While we’re awake, your brain is forming memories. Memory formation involves a process called long-term potentiation (LTP), which is essentially the strengthening of synaptic connections between nerve cells. We also know that learning can actually cause neurons to sprout entirely new synapses.
Yet this poses a problem for the brain. If LTP and synapse formation is constantly strengthening our synapses, and we are learning all our lives, might the synapses eventually reach a limit? Couldn’t they “max out,” so that they could never get any stronger?
Worse, most of the synapses that strengthen during memory are based on glutamate. Glutamate is dangerous. It’s the most common neurotransmitter in the brain, and it’s also a popular flavouring: “MSG”, monosodium glutamate. But in the brain, too much of it is toxic.
Glutamate works as a transmitter molecule by opening channels on the cells that receive it. The channels allow calcium into the cells on the receiving end, which activates them, allowing messages to go through. But too much glutamate can cause excess calcium to build up inside the very cells that receive the message, a harmful process called excitotoxicity.
So, if our brains were constantly forming stronger glutamate synapses, we might eventually run into serious problems. One function of sleep, according to the theory, is to protect the brain against excitotoxicity or other “synaptic overload” problems by pruning the synapses.
If the brain is essentially removing the “extra” synaptic strength formed during the previous day, it must do so in a way that preserves the new information. One possible mechanism for this is synaptic scaling.
After some of the neural connections into a given cell, or “inputs,” become stronger, then all of the synapses on that cell could be weakened. This would preserve the relative strength of the different inputs, while keeping the total inputs constant.
It’s as if each neuron were a cup, and each synapse corresponds to a different liquid. During the day, memories form and certain synapses get stronger, which means pouring more of those particular liquids into the cup. At night, synaptic scaling pours some of the mixture back out, bringing it back to the baseline level without changing the relative proportions of the mix.
We know that synaptic scaling happens in the brain, but it’s not yet clear whether it has anything to do with sleep. This is an area of ongoing research.
While synaptic scaling seems to treat each neuron like a cup to be kept from overfilling, the effect of sleep on for the brain overall may be more like disk defragmentation, according to this idea.
After heavy use, hard disks tend to get “fragmented.” This is because when data gets stored, it is written to wherever there happens to be free space on the disk. This makes it inefficient to keep track of it all as files may be split and written in many different places. A defrag consolidates the same data into a more logical order. Defragmentation is a taxing chore for the computer, so many people schedule it to happen overnight. In the same way, sleep may serve to reorganize and reconsolidate memories. The mechanics of how this defragmentation works remain unclear; synaptic scaling might be just one of several processes at work.
Defragmentation is not an exact analogy, however. The process could also be likened to archiving your emails to make room in your inbox, or compressing data into zipped files, to free up room on the disk.
(This theory is specifically about slow-wave sleep (SWS). It doesn’t try to explain rapid eye movement (REM) sleep, when dreams happen. Interestingly, some animals do not have REM, but they all have SWS. In some animals, like dolphins, only one side of the brain has it at a time, which is strong evidence that SWS, but not REM, is vital for life.)