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Category Archives: neurons

Starved brains kill memory-making to survive

"Thanks for the memories, but I'd prefer a bite to eat."
UFL.edu

As the organ responsible for maintaining equilibrium in the body and the most energy-demanding of all the organs, the brain takes a lot of the body's energy allocation. So when food is in short supply, the brain is the organ that is fed first. But what happens when there isn’t enough food to fulfill the high-energy needs of the brain and survival is threatened?

The brain does not simply self-allocate available resources on the fly; instead it “trims the fat” by turning off entire processes that are too costly. Researchers from CNRS in Paris created a true case of do-or-die, starving flies to the point where they must choose between switching off costly memory formation or dying. When flies are starved, their brains will block the formation of aversive long-term memories, which depend on costly protein synthesis and require repetitive learning.

But that doesn't mean all long-term memories are shut down. Appetitive long-term memories, which can be formed after a single training, are enhanced during a food shortage.

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Model brain with 2.5 million neurons configures itself to solve problems

Chris Eliasmith, et. al

Over the last few years, the meteoric rise of computing power has allowed us to build ever-larger collections of model neurons. Often, these models have been announced with a certain degree of hype, promising performance equivalent to that of an actual animal. In reality, simply throwing more silicon neurons at a problem hasn't brought us much closer to either understanding how the brain works or getting brain-like performance out of our computers.

In a paper published in yesterday's Science, researchers report on a new machine called (perhaps unfortunately) SPAUN, which reinforces the critical role that architecture can play in getting brain-like performance. All told, SPAUN makes do with "only" 2.5 million neurons, but it has them arranged in a specific collection of functional units with populations of neurons dedicated to things like working memory and information decoding. Although SPAUN isn't as flexible as a real brain, it handles a number of tasks about as well as actual humans.

Computers operate in binary, through huge collections of switches that can be in either "on" or "off" states. Real neurons simply can't operate the same way. Although they can have spikes of activity where a voltage change propagates down the cell body, that voltage change is transient—things quickly return to their ground state. Instead, neurons convey information through the pattern and timing of the spikes they create.

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Make a left turn here: scientists direct nerve cells with lasers

Every now and then, a result comes along that sounds like something out of science fiction. Nature Photonics played host to one of these this past weekend. In it, researchers built turn signals for neurons out of nothing more than a tiny bead and some light. It's not especially useful right now, but it's still an impressive bit of engineering.

In adult animals, neurons extend small, thread-like projections called axons for large distances away from the main body of the cell. These axons help form connections between places like the brain and the end of the spinal cord, or the spinal cord and the sensory neurons on the tips of your fingers. In the body, directing these axons to the right location is a very carefully controlled process that involves lots of signaling and adhesion molecules. Put a neuron in a culture dish, where none of these signals are present, and it will tend to grow an axon in a straight line. Not satisfied with this, the authors decided to give it a little nudge and see what would happen.

The axon's growth comes at a specialized structure at the tip called a growth cone. To redirect the growth cone, the team placed a special bead nearby, and held it in place by an optical trop created with a laser. The bead was made of materials that have two different indexes of refraction, which allowed a second laser to set it spinning (the second laser had circular-polarized light, which interacts with the two different refraction indexes on the bead to impart motion). Once the bead was spinning, it set fluid flowing near the growth cone, creating a small force that could deflect it.

Between 30 and 40 percent of the time, this was enough to cause the axon to shift direction by as much as 30 degrees from the straight line (the rest of the time, they kept going straight). It was also possible to create gates with two spinning beads that channeled the axon between them.

Being physicists, the authors created a model with shear forces and viscosity to explain the behavior they observed, and they speculate about possible future utility for doing things like spinal repair. What strikes me as a bit more likely is the prospect of using a system like this to control how multiple neurons in a culture dish form connections. It may be possible to build a neural network out of actual neurons.

Nature Photonics, 2011. DOI: 10.1038/nphoton.2011.287  (About DOIs).

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