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A Single Neuron Can Change the Activity of the Whole Brain
05-06-2009, 12:47 AM,
#1
A Single Neuron Can Change the Activity of the Whole Brain

A Single Neuron Can Change the Activity of the Whole Brain
http://www.physorg. com/news16040726 0.html
May 1st, 2009

(PhysOrg.com) -- The pulsing of a single neuron can switch a brain’s
waves from the equivalent of a big ocean swell to ripples on a pond,
according to new research from Howard Hughes Medical Institute
investigator Yang Dan of the University of California, Berkeley.

The study reveals important new information about how the brain
controls large-scale activity patterns and suggests that an individual
cell has more influence than previously thought. The findings,
published in the May 1, 2009, issue of the journal Science, could
ultimately shed light on how chaotic brain patterns can lead to sleep
disorders such as sleepwalking.

Brain cells use electrical pulses to talk with one another and guide
functions ranging from heart rate and breathing to decision-making and
navigation. Like the din of a crowd, the chatter of 100 billion
neuronal cells in the human brain creates larger patterns of activity
commonly called brain waves.

These patterns reveal the brain’s general state of arousal. For
instance, large, slow brain waves that are synchronized throughout the
brain are indicative of deep sleep. “Many neurons are doing the same
thing at the same time,” says Dan. During so-called rapid eye movement
(REM) sleep, on the other hand, different brain areas are less
synchronized, firing in smaller and more frequent oscillations. And in
an awake person, the brain broadcasts a rapid, uncoordinated pattern.

Dan and her colleagues wanted to understand how large-scale wave
patterns influence the connection between two neurons. They knew that
neuronal connections could strengthen or weaken over time, and these
changes seem to underlie learning and memory. They wondered whether
the overall pattern of brain activity altered nerve cells’ ability to
change their connection strength.

Studying anesthetized rats, they used one electrode to spur a neuron
to fire rapidly and used another electrode nearby to activate the
local neuronal connections. A third electrode was used to pick up the
larger pattern emitted by all the neurons in the area. They wanted the
overall brain state to remain constant during the experiment, but
instead found that tickling one neuron could cause the entire brain
state to change.

“Initially, this was very inconvenient,” says Dan. But then the
researchers realized that the phenomenon deserved more attention.
Looking more closely, they verified that a neuron firing at high
frequency could switch the brain from a “non-REM pattern” of activity
to a “REM pattern,” and vice versa.

The result was counterintuitive. “Every neuron makes connections to
roughly 1,000 other neurons, but most of those are quite weak,” says
Dan. A target cell won’t respond unless many, many neurons that
connect to it fire at the same time and therefore she says it’s
surprising that a single neuron could change the activity of the whole
brain. “Single neurons have more weight than we used to think,” she
says.

Dan doesn’t yet know how one cell could exert such power. The
researchers had to repeatedly and rapidly fire a cell to cause the
pattern to switch, so they might be emulating the effect of many cells
firing at once. A neuron doesn’t normally fire in that way, so it is
an open question whether the activity of a single neuron could change
overall brain pattern under normal circumstances.

The findings add a new twist to how brain patterns are established.
Researchers know that certain brain structures, such as the
hypothalamus and the brain stem, play a part in setting the pace of
global brain activity. In this study, Dan and her team were tickling
brain cells in a different area: the cortex, the thin sheet of neurons
on the surface of the brain involved in such abilities as moving and
seeing.

Dan isn’t certain how cells in the cortex might control brain state,
but she posits that signaling there could link back to the thalamus
and spur it to set up a new pattern. “We know that a lot of circuits
are involved in controlling brain state,” says Dan. “We’re saying that
cortex is also part of that loop.”
By providing new information about how brain states are controlled,
the study might ultimately lead to new knowledge about what causes
certain sleep disorders. “In sleepwalking, there is a mixed-up
boundary between slow-wave sleep and the awake state,” says Dan. “Your
muscles move, but you aren’t consciously aware of your surroundings.”
Understanding the circuitry that establishes brain states could
ultimately reveal how that mixed-up situation is established.

Next, Dan wants to study animals that are naturally awake or sleeping,
rather than anesthetized, to see if under normal conditions, a single
neuron or a few neurons really can turn the tide on the entire brain.

Provided by Howard Hughes Medical Institute (news : web)
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