Here computer-simulated images of pyramidal neurons in the cerebral cortex, revealing branching dendrites now shown to carry out sophisticated computations rather than just acting as passive wiring.
The brain may be an even more powerful computer than before thought — microscopic branches of brain cells that were once thought to basically serve as mere wiring may actually behave as minicomputers, researchers say.
The most powerful computer known is the brain. The human brain possesses about 100 billion neurons with roughly 1 quadrillion — 1 million billion — connections known as synapses wiring these cells together.
Neurons each act like a relay station for electrical signals. The heart of each neuron is called the soma — a single thin cablelike fiber known as the axon that sticks out of the soma carries nerve signals away from the neuron, while many shorter branches called dendrites that project from the other end of the soma carry nerve signals to the neuron. [Inside the Brain: A Photo Journey Through Time]
Now scientists find dendrites may be more than passive wiring; in fact, they may actively process information.
"Suddenly, it's as if the processing power of the brain is much greater than we had originally thought," study lead author Spencer Smith, a neuroscientist at the University of North Carolina at Chapel Hill,said in a statement.
Axons are what neurons conventionally use to generate spikes of electricity. However, prior research discovered many of the same molecules that support electrical spikes are also present in the dendrites, and experiments with brain tissue showed dendrites can use these molecules to generate these spikes themselves.
It was unclear whether normal brain activity involved dendritic spikes, and if so, what role they might play. To find out, Smith and his colleagues attached tiny glass pipes known as pipettes to dendrites in areas of the mouse brain responsible for processing data from the eyes.
"Attaching the pipette to a dendrite is tremendously technically challenging," Smith said. "You can't approach the dendrite from any direction. And you can't see the dendrite. So you have to do this blind. It's like fishing if all you can see is the electrical trace of a fish."
Once they successfully attached pipettes to dendrites, the researchers took electrical recordings from individual dendrites within the brains of anesthetized and awake mice. As the mice viewed black-and-white bars on a computer screen, the scientists detected an unusual pattern of electrical signals, or bursts of spikes, in the dendrites. [10 Odd Facts About the Brain]
"When we started recording from dendrites, the bursts of spikes we saw were hard to believe," Smith said. While spikes from axons "are isolated, solemn obelisks, by comparison, the dendritic spikes we saw were raucous, dynamic events, with bursts and plateaus."
The properties of electrical signals from the dendrites varied depending on the features of the images the mice saw. This suggests the dendrites may actually help the mice process what they see.
Mini computing devices
"This work shows that dendrites, long thought to simply funnel incoming signals towards the soma, instead play a key role in sorting and interpreting the enormous barrage of inputs received by the neuron," study co-author Michael Hausser at University College Londonsaid in a statement. "Dendrites thus act as miniature computing devices for detecting and amplifying specific types of input."
"Imagine you're reverse engineering a piece of alien technology, and what you thought was simple wiring turns out to be transistors that compute information," Smith said. "That's what this finding is like. The implications are exciting to think about."
All in all, "functions we thought required an entire neuron may be carried out instead by just one portion of a neuron's dendritic tree," Smith told LiveScience. "This would imply that a single neuron can act like many, many computational subunits."
However, while he said it was clear dendritic activity increases the computational power of the brain, Smith added it was difficult to quantify how much it boosted it by.
The scientists plan to further explore what role dendritic activity may play elsewhere in the brain other than vision.
"This kind of dendritic processing is likely to be widespread across many brain areas and indeed many different animal species, including humans," Hausser said. "This new property of dendrites adds an important new element to the toolkit for computation in the brain."
Although this is basic research aimed at understanding how brain circuitry works, it might help address brain disorders as well, Smith said. "There are diseases that might strongly affect dendritic spiking and thus brain function, and we can use our new understanding of dendritic spiking to explore what might go wrong in those diseases," he said.
The scientists detailed their findings online Oct. 27 in the journal Nature.