Connections Between Neurons Act As Information Filters In The Brain

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Connections Between Neurons Act As Information Filters In The Brain

http://www.medicalnewstoday.com/medicalnews.php?newsid=45056

For the first time, researchers at the Salk Institute for Biological

Studies have demonstrated that cell-cell contacts in the brain play

an active role in processing information: called synapses, these

interfaces act as precise filters that sense and amplify meaningful

information, Salk researchers report in the current issue of PLoS

Biology, available online. Connections between neurons act as

information filters in the brain.

Neurons are often considered to be the primary computational units of

the brain. But it was unclear whether the connections between neurons

actively participated in the computational process, or merely acted

to convey information.

" Our study shows that synapses not only ensure the flow of

information but actively modify their properties to help with

computations, " says Medical Investigator

s, a professor in the Molecular Neurobiology Laboratory and

senior author of the study. Lead author Vitaly Klyachko, a post-

doctoral researcher working with s explains, " Brain cells

produce a lot of background chatter. Synapses filter this random

noise and enhance relevant information. They work as very fine-tuned

filters that do exactly what you would want them to do. "

Brain cells signal by sending electrical impulses along axons, long,

hair-like extensions that reach out to neighboring nerve cells. They

make contact via synapses, from the Greek word meaning " to clasp

together. " When an electrical signal reaches the end of an axon, the

voltage change triggers release of neurotransmitters, the brain's

chemical messengers. These neurotransmitter molecules then travel

across the space between neurons at a synapse and trigger an

electrical signal in the adjacent cell.

Scientists had postulated that synapses play a major role in

information processing in the brain. But not all signals are

transmitted. Just like cell phone calls are dropped in areas of

spotty coverage, synapses drop up to 90 percent of all incoming

signals. " The unreliability of neuronal connections that presumably

convey and process information throughout the brain was difficult to

reconcile with the fact that brain as a whole is very reliable, " says

Klyachko.

Trying to get to the bottom of this enigma, the Salk researchers

relied on naturally occurring activity patterns that were recorded in

living animals from a part of the brain known as the hippocampus - a

structure critical for memory formation and learning. They used these

recorded patterns to stimulate isolated groups of neurons and

measured which signals synapses transmitted to neighboring cells and

which ones they dropped.

In the past, similar studies were commonly performed at room

temperature. Since scientists had found that results were often too

complex to interpret, the Salk researchers recorded data in warmer

conditions, slightly below body temperature. " Intuitively, I recorded

at physiological temperatures instead of room temperature and that

turned out to be the key, " remembers Klyachko. " I found that synaptic

transmission is highly temperature-dependent. "

From there it was only a small step to the discovery that the two

major types of synapses, excitatory and inhibitory ones, that were

previously thought to always work against each other, act in concert

to identify patterns carrying relevant information in an incoming

signal. s explained: " Synapses recognize bursts of neuronal

activity and turn up their strength, acting like a switch. " As a

result, meaningful patterns are amplified, while stray noise

disappears into some sort of " synaptic abyss. "

Until now, experimental evidence for a filtering function of synapses

has been elusive. " Our work is the confirmation that everybody has

been waiting for, " explains Klyachko. " It is the precisely-tuned

filtering properties of the two major types of synapses and their

collaboration that makes the information processing reliable. "

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The Salk Institute for Biological Studies in La Jolla, California is

an independent nonprofit organization dedicated to fundamental

discoveries in the life sciences, the improvement of human health,

and the training of future generations of researchers. Jonas Salk,

M.D., whose polio vaccine all but eradicated the crippling disease

poliomyelitis in 1955, opened the Institute in 1965 with a gift of

land from the City of San Diego and the financial support of the

March of Dimes.