Brain Is Ten Times More
Active than Previously Measured, UCLA Researchers Find
By Dan Gordon
By Dan Gordon
Los Angeles, March 9, 2017 -- A new UCLA study could change
scientists’ understanding of how the brain works — and could lead to new
approaches for treating neurological disorders and for developing computers
that “think” more like humans.
The research focused on the structure and function of
dendrites, which are components of neurons, the nerve cells in the brain.
Neurons are large, tree-like structures made up of a body, the soma, with
numerous branches called dendrites extending outward. Somas generate brief
electrical pulses called “spikes” in order to connect and communicate with each
other. Scientists had generally believed that the somatic spikes activate the
dendrites, which passively send currents to other neurons’ somas, but this had
never been directly tested before. This process is the basis for how memories
are formed and stored.
Scientists have believed that this was dendrites’ primary
role.
But the UCLA team discovered that dendrites are not just
passive conduits. Their research showed that dendrites are electrically active
in animals that are moving around freely, generating nearly 10 times more
spikes than somas. The finding challenges the long-held belief that spikes in
the soma are the primary way in which perception, learning and memory formation
occur.
“Dendrites make up more than 90 percent of neural tissue,”
said UCLA neurophysicist Mayank Mehta, the study’s senior author. “Knowing they
are much more active than the soma fundamentally changes the nature of our
understanding of how the brain computes information. It may pave the way for
understanding and treating neurological disorders, and for developing
brain-like computers.”
The research is reported in the March 9 issue of the journal
Science.
Scientists have generally believed that dendrites meekly
sent currents they received from the cell’s synapse (the junction between two
neurons) to the soma, which in turn generated an electrical impulse. Those
short electrical bursts, known as somatic spikes, were thought to be at the
heart of neural computation and learning. But the new study demonstrated that
dendrites generate their own spikes 10 times more often than the somas.
The researchers also found that dendrites generate large
fluctuations in voltage in addition to the spikes; the spikes are binary,
all-or-nothing events. The somas generated only all-or-nothing spikes, much
like digital computers do. In addition to producing similar spikes, the
dendrites also generated large, slowly varying voltages that were even bigger
than the spikes, which suggests that the dendrites execute analog computation.
“We found that dendrites are hybrids that do both analog and
digital computations, which are therefore fundamentally different from purely
digital computers, but somewhat similar to quantum computers that are analog,”
said Mehta, a UCLA professor of physics and astronomy, of neurology and of
neurobiology. “A fundamental belief in neuroscience has been that neurons are
digital devices. They either generate a spike or not. These results show that
the dendrites do not behave purely like a digital device. Dendrites do generate
digital, all-or-none spikes, but they also show large analog fluctuations that
are not all or none. This is a major departure from what neuroscientists have
believed for about 60 years.”
Because the dendrites are nearly 100 times larger in volume
than the neuronal centers, Mehta said, the large number of dendritic spikes
taking place could mean that the brain has more than 100 times the
computational capacity than was previously thought.
Recent studies in brain slices showed that dendrites can
generate spikes. But it was neither clear that this could happen during natural
behavior, nor how often. Measuring dendrites’ electrical activity during
natural behavior has long been a challenge because they’re so delicate: In
studies with laboratory rats, scientists have found that placing electrodes in
the dendrites themselves while the animals were moving actually killed those
cells. But the UCLA team developed a new technique that involves placing the
electrodes near, rather than in, the dendrites.
Using that approach, the scientists measured dendrites’
activity for up to four days in rats that were allowed to move freely within a
large maze. Taking measurements from the posterior parietal cortex, the part of
the brain that plays a key role in movement planning, the researchers found far
more activity in the dendrites than in the somas — approximately five times as
many spikes while the rats were sleeping, and up to 10 times as many when they
were exploring.
“Many prior models assume that learning occurs when the cell
bodies of two neurons are active at the same time,” said Jason Moore, a UCLA
postdoctoral researcher and the study’s first author. “Our findings indicate
that learning may take place when the input neuron is active at the same time
that a dendrite is active — and it could be that different parts of dendrites
will be active at different times, which would suggest a lot more flexibility
in how learning can occur within a single neuron.”
Looking at the soma to understand how the brain works has
provided a framework for numerous medical and scientific questions — from
diagnosing and treating diseases to how to build computers. But, Mehta said,
that framework was based on the understanding that the cell body makes the
decisions, and that the process is digital.
“What we found indicates that such decisions are made in the
dendrites far more often than in the cell body, and that such computations are
not just digital, but also analog,” Mehta said. “Due to technological
difficulties, research in brain function has largely focused on the cell body.
But we have discovered the secret lives of neurons, especially in the extensive
neuronal branches. Our results substantially change our understanding of how
neurons compute.”
The study’s other authors are Pascal Ravassard, David Ho,
Lavanya Archarya, Ashley Kees and Cliff Vuong, all of UCLA. Funding was
provided by the University
of California
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