For
the first time, a new study has observed that cerebrospinal fluid washes in and
out of the brain in waves during sleep, helping clear out waste.
Recently, Medical News Today reported on a study that found
that specialized immune cells are more active
in the brain during sleep, busy performing maintenance work.
Researchers know that sleep is
important — not just in terms of allowing the brain to reactualize, but also
for "making space" for "cleaning" processes to take place.
However, many of the mechanisms through
which this clearing out of brain waste takes place during sleep remain unclear.
Now, researchers at
Boston University in Massachusetts have found that during sleep, the fluid
present in the brain and spinal chord — called the cerebrospinal fluid — washes
in and out, like waves, helping the brain get rid of accumulated metabolic
"trash."
"We've known for a while that
there are these electrical waves of activity in the neurons. But before now, we
didn't realize that there are actually waves in the cerebrospinal fluid,
too," study co-author Laura Lewis explains.
A complex synchronization process
The new study — the results of which
appear in the journal Science — included 13
participants ages 23–33 who agreed to undergo brain scans while asleep.
The process was a tricky one. The
participants had to wear EEG caps that allowed the researchers to measure
electrical activity in their brains while lying in an MRI machine, where they were
meant to sleep.
However, sleeping in this location can
be difficult, as MRI machines are very noisy. "[I]t turns out that [the
participants'] job is actually — secretly — almost the hardest part of our
study," says Lewis.
"We have all this fancy equipment
and complicated technologies, and often a big problem is that people can't fall
asleep because they're in a really loud metal tube, and it's just a weird
environment," she notes.
Despite these challenges, the
researchers managed — probably for the first time — to monitor the activity of
cerebrospinal fluid in the participants' brains during sleep.
They saw that
cerebrospinal fluid appears to "synchronize" with brainwaves, which
likely helps remove brain waste. This waste includes potentially toxic proteins
that may otherwise form buildups that can impair the flow of information
between neurons.
These findings, the researchers add,
could also shed fresh light on the underlying mechanisms in conditions such
as Alzheimer's disease, in
which toxic protein plaques play a key role in memory loss and other cognitive
impairments.
They also explain that normal aging may
be associated with poorer self-cleaning in the brain. With age, human brains
tend to generate fewer slow waves, which may reduce blood flow in the brain, as
well as cerebrospinal fluid pulsations.
"It's such a dramatic
effect," emphasizes Lewis. "[Cerebrospinal fluid pulsating in the
brain during sleep] was something we didn't know happened at all, and now we
can just glance at one brain region and immediately have a readout of the brain
state someone's in."
'What are the causal links?'
Going forward, Lewis and colleagues aim
to answer a few intriguing questions. First, they would like to recruit an
older cohort for their next study, so as to find out if and how natural aging
affects cerebrospinal fluid's work in the brain.
Then, they would also like to establish
how brainwaves, blood flow in the brain, and cerebrospinal fluid manage to
synchronize in order to "flush out" the waste.
"We do see that the neural change
always seems to happen first, and then it's followed by a flow of blood out of
the head, and then a wave of cerebrospinal fluid into the head," says
Lewis. However, she adds that many unknowns still remain.
The team believes that when neurons
switch off during sleep, they need less oxygen, which leads to blood draining
from the brain. This, in turn, means that pressure in the brain also drops, and
so the cerebrospinal fluid needs to increase, so as to maintain normal pressure
in the absence of blood.
"But that's just one
possibility. What are the causal links? Is one of these processes causing the
others? Or is there some hidden force that is driving all of them?
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