Why your brain needs ‘pruning’

Believe it or not, your brain has a built-in “pruning” system, wired specifically to get rid of all the “dead wood”—or synaptic connections that are used less often—and make room for newer, stronger connections.

“Imagine your brain is a garden, except instead of growing flowers, fruits, and vegetables, you grow synaptic connections between neurons. These are the connections that neurotransmitters like dopamine, serotonin, and others travel across,” wrote Judah Pollack and Olivia Fox Cabane for Fast Company.  

And, like any well-maintained, beautiful garden, regular upkeep in the brain is essential to keep it healthy and growing. Enter the glial cell. 

“’Glial cells’ are the gardeners of your brain–they act to speed up signals between certain neurons. But other glial cells are the waste removers, pulling up weeds, killing pests, raking up dead leaves. Your brain’s pruning gardeners are called ‘microglial cells,’” according to Pollack and Cabane. “They prune your synaptic connections. The question is, how do they know which ones to prune?”

The brain builds connections through learning. But, some of the things we learn are not used often, and the connections, or synapses that were built while learning them, are inefficient. While we sleep, the brain cleans up these inefficient connections, making room for more efficient ones.  

In fact, researchers have found that the synaptic connections we use less often are actually flagged with C1q, a protein. When microglial cells detect C1q, they bond to it, and destroy the synapse.

A key concept here is that this is done in our sleep. This nightly “pruning” leaves your brain with lots of room to take in new information the next day. And this is precisely why, when you do not get enough sleep, thinking becomes harder and taking in new concepts or ideas seems almost impossible.

“Thinking with a sleep-deprived brain is like hacking your way through a dense jungle with a machete. It’s overgrown, slow-going, exhausting. The paths overlap, and light can’t get through. Thinking on a well-rested brain is like wandering happily through Central Park; the paths are clear and connect to one another at distinct spots, the trees are in place, you can see far ahead of you. It’s invigorating,” according to Pollack and Cabane.

Incidentally, this is also why naps can be so beneficial for cognition. Even during a short nap, say 10 or 20 minutes, these microglial “gardeners” can get to work, clearing away unused or inefficient connections, and leaving room for new ones to grow.

Interestingly, not only was Albert Einstein a great believer in naps, but his brain was found to contain more glial cells than the average male.

Much of the attention focused on glial cells over the past few years can be attributed to a study that appeared in Science, in which Rosa C. Paolicelli, European Molecular Biology Laboratory, Monterotondo, Italy, and colleagues studied just such synaptic pruning by microglia. This microglial gardening, they demonstrated, plays a vital role during postnatal development in mice.

According to previous research, microglia play a role in the maintenance of brain function, especially after damage induced by “ischemic, excitotoxic, and neurodegenerative insults,” after which they actively seek out and eliminate neuronal debris. But recently, researchers have begun to find that microglia also play a role in “monitoring and maintaining synapses in the uninjured brain.”

Paolicelli et al. found that microglia actually engulf and eliminate synapses during brain development. In mice lacking a chemokine receptor—Cx3cr1 (expressed by microglia and responsible for its ability to migrate or move)—they noted that this synaptic pruning was delayed, and numbers of microglia were reduced, albeit transiently, during brain development.

In these mice, the lack of microglial pruning brought about an excess of dendritic spines and immature synapses, and was associated with the presence of both electrophysiologic and pharmacologic signs of immature brain circuitry.

“Understanding microglia-mediated synaptic pruning is likely to lead to a better understanding of synaptic homeostasis and an appreciation of interactions between the brain and immune system,” concluded Paolicelli and colleagues.