Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Gray Matters

16.06.2004


Even after a century of research, the workings of brain cells remain somewhat mysterious. But USC scientists have uncovered new clues into how neurons process information


Bartlett Mel, an associate professor in the USC Viterbi School of Engineering.
Photo/Gia Scafidi



Researchers from USC and the Technion Medical School in Israel have uncovered new clues into the mystery of the brain’s ultra-complicated cells known as neurons.

Their findings — appearing in this month’s issue of the journal Nature Neuroscience — contradict a widely accepted idea regarding the “arithmetic” neurons use to process information.


“It’s amazing that after a hundred years of modern neuroscience research, we still don’t know the basic information processing functions of a neuron,” said Bartlett Mel, an associate professor in the USC Viterbi School of Engineering and contributing author of the journal’s article.

“For lack of a better idea, it has always been thought that a brain cell sums up its excitatory inputs linearly, meaning that the excitation caused by two inputs A and B activated together equals the sum of excitations caused by A and B presented separately.”

“We show that the cell significantly violates that rule,” Mel said.

The team found that the summation of information within an individual neuron depends on where the inputs occur, relative to each other, on the surface of the cell.

To understand the team’s work and the significance of its findings, it helps to know a little more about a brain cell.

All of the information processing that take place in the brain is managed by a web of neurons. These living cells come in a variety of shapes and sizes, often resembling trees or bushes.

A neuron receives input from other neurons at thousands of sites — called synapses — scattered across its surface. Each of the synapses generate a small local voltage response when it is activated.

According to the classical view of the neuron, synaptic responses flow down the cell’s branch-like dendrites, which act like electrical cables and accumulate at the cell body. If the overall voltage response there is sufficient, an electrical spike is fired, carried down the cell’s axon and communicated to hundreds or thousands of other neurons.

“Recent evidence suggests the story is not quite that simple, though,” Mel said. “The input signals may interact with each other in the dendrites and may be profoundly transformed on their way to the cell body.”

“In particular,” Mel added, “individual branches of the dendritic tree can, under certain circumstances, generate local spikes that greatly amplify synaptic responses locally within the dendritic tree.”

The team set out to establish the “arithmetic” used by the neuron to combine its many synaptic inputs, focusing on the pyramid-shaped neuron that makes up the bulk of the brain’s cortical gray matter.

The experiments were conducted in Haifa, Israel by Alon Polsky, lead author of the paper and graduate student at Technion, and Jackie Schiller, contributing author and co-principal investigator.

Using slices of cortical brain tissue from rats, Polsky and Schiller located individual pyramidal neurons, filled them with dye for visualization purposes (cells are otherwise transparent) and, using extracellular electrodes, stimulated the cells very close to their dendritic branches.

While recording the voltage at the cell body, the team would deliver shocks through one or two stimulating electrodes directed to different locations in the dendritic tree, for example, to the same or different dendritic branches.

They would then compare the voltage response at the cell body as the two inputs were activated first separately and then together.

“The powerful thing about [Schiller’s] method is that you can see where you’re stimulating because the dye grows a little brighter wherever synapses are activated,” said Mel, who worked with the team remotely from USC by collaborating on the experiment design and data analysis.

“You can direct the stimuli to very specific spatial locations on the cell and start to look at what a difference location makes. That old real estate phrase ‘location, location, location’ holds true for neurons as well.”

The data showed that three different scenarios could occur when two electrodes A and B were used to stimulate the same dendritic branch:

• If the total response to the two inputs (electrodes A and B) falls below the branch’s local firing threshold, the summation looks linear - A plus B.

• If the two inputs are just strong enough that together they cross the local threshold, the summation looks superlinear — more than A plus B.

• If each individual input is strong enough to cross the local threshold by itself, the summation is sublinear — less than A plus B.

Mel explained the last point in this way: “If two people are trying to build a fire together and they each have a match, the fire isn’t going burn twice as bright or twice as hot thanks to the second match, once it’s already been started with the first. The second match is irrelevant.”

In contrast to summation of inputs delivered to the same branch, the researchers found that summation of inputs on different dendritic branches always looked linear — like lighting two separate fires.

The findings support a 2003 modeling study carried out in Mel’s lab, in which he and graduate student Panayiota Poirazi predicted that pyramidal neurons would behave in this way. This was the first experimental test of those predictions.

“So, we now think of the neuron in terms of a two-layer model,” Mel said. “The first layer of processing occurs within separate dendritic branches. Each branch independently adds up the inputs to that branch, and then applies its own local thresholding non-linearity.”

“In the second layer of processing,” Mel added, “the results from all the different branches are added together linearly at the cell body, where they help to determine the cell’s overall firing rate.”

While the results are promising, the team is certain this is not the final word on the pyramidal neuron.

“Undoubtedly, this is still too simple a model,” Mel said. “But the two-layer model is a better description, it seems, than to assume that the neuron is simply combining everything linearly from everywhere. That’s clearly not what these data show.”

According to Mel, one additional complexity that must eventually be dealt with is that synaptic inputs arriving at the most remote part of the neuron — called the apical tuft — may interact in subtle ways with inputs arriving on the basal dendrites, closer to the cell body.

“We’d now like to see if we need to extend the two-layer model in to a three-layer model,” Mel said. “It may be that the basal and apical dendrites each behave as we’ve been saying, but when they interact with each other there’s an additional nonlinear interaction that occurs between them.”

Mel emphasizes that the “arithmetic” rules he and his colleagues found in pyramidal neurons may not apply to all neurons in the brain.

“There are other neurons that have different shapes, inputs, morphologies and ion channels,” he said. “There might be a dozen different answers to the question, depending on what neuron you’re looking at.”

While much more work lies ahead, new imaging techniques, lifelike models and modern laboratory procedures are making the task of understanding the brain’s complicated neurons a whole lot easier.

In the end, Mel said, the lessons learned from individual neurons will be crucial to advance researchers’ understanding of the brain as a whole.

“We tend to view the brain as a computer,” he said. “If we want to figure out how this computer works, we must first know how its separate parts function.”

Gia Scafidi | University of Southern Californi
Further information:
http://www.usc.edu/uscnews/story.php?id=10305

More articles from Life Sciences:

nachricht Switch-in-a-cell electrifies life
18.12.2018 | Rice University

nachricht Plant biologists identify mechanism behind transition from insect to wind pollination
18.12.2018 | University of Toronto

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Data storage using individual molecules

Researchers from the University of Basel have reported a new method that allows the physical state of just a few atoms or molecules within a network to be controlled. It is based on the spontaneous self-organization of molecules into extensive networks with pores about one nanometer in size. In the journal ‘small’, the physicists reported on their investigations, which could be of particular importance for the development of new storage devices.

Around the world, researchers are attempting to shrink data storage devices to achieve as large a storage capacity in as small a space as possible. In almost...

Im Focus: Data use draining your battery? Tiny device to speed up memory while also saving power

The more objects we make "smart," from watches to entire buildings, the greater the need for these devices to store and retrieve massive amounts of data quickly without consuming too much power.

Millions of new memory cells could be part of a computer chip and provide that speed and energy savings, thanks to the discovery of a previously unobserved...

Im Focus: An energy-efficient way to stay warm: Sew high-tech heating patches to your clothes

Personal patches could reduce energy waste in buildings, Rutgers-led study says

What if, instead of turning up the thermostat, you could warm up with high-tech, flexible patches sewn into your clothes - while significantly reducing your...

Im Focus: Lethal combination: Drug cocktail turns off the juice to cancer cells

A widely used diabetes medication combined with an antihypertensive drug specifically inhibits tumor growth – this was discovered by researchers from the University of Basel’s Biozentrum two years ago. In a follow-up study, recently published in “Cell Reports”, the scientists report that this drug cocktail induces cancer cell death by switching off their energy supply.

The widely used anti-diabetes drug metformin not only reduces blood sugar but also has an anti-cancer effect. However, the metformin dose commonly used in the...

Im Focus: New Foldable Drone Flies through Narrow Holes in Rescue Missions

A research team from the University of Zurich has developed a new drone that can retract its propeller arms in flight and make itself small to fit through narrow gaps and holes. This is particularly useful when searching for victims of natural disasters.

Inspecting a damaged building after an earthquake or during a fire is exactly the kind of job that human rescuers would like drones to do for them. A flying...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

ICTM Conference 2019: Digitization emerges as an engineering trend for turbomachinery construction

12.12.2018 | Event News

New Plastics Economy Investor Forum - Meeting Point for Innovations

10.12.2018 | Event News

EGU 2019 meeting: Media registration now open

06.12.2018 | Event News

 
Latest News

Pressure tuned magnetism paves the way for novel electronic devices

18.12.2018 | Materials Sciences

New type of low-energy nanolaser that shines in all directions

18.12.2018 | Physics and Astronomy

NASA research reveals Saturn is losing its rings at 'worst-case-scenario' rate

18.12.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>