Researchers at the University of North Carolina at Chapel Hill have discovered that dendrites do more than relay information from one neuron to the next. Dendrites are branch-like projections of neurons. They were once thought to be passive wiring in the brain.
But now, dendrites are seen as actively processing information, multiplying the brain’s computing power.
“Suddenly, it’s as if the processing power of the brain is much greater than we had originally thought,” said team leader Spencer Smith, PhD.
The findings could change the way scientists think about long-standing scientific models of how neural circuitry functions in the brain.
They may also help researchers better understand neurological disorders.
Axons are where neurons normally produce electrical spikes, but many of the same molecules that support axonal spikes are also present in the dendrites.
Previous research using dissected brain tissue had demonstrated that dendrites can use those shared molecules to generate electrical spikes themselves. However, it was not clear whether normal brain activity uses those dendritic spikes.
Could dendritic spikes be involved in how we see?
The answer is yes. Dendrites in fact act as mini-neural computers, actively processing neuronal input signals themselves.
Demonstrating the action meant a series of intricate experiments spanning years and two continents, beginning in senior author Michael Hausser’s lab at University College London, and being completed after Smith and Ikuko Smith, PhD, DVM, set up their own lab at the University of North Carolina.
The researchers used patch-clamp electrophysiology to attach a microscopic glass pipette electrode, filled with a physiological solution, to a neuronal dendrite in the brain of a mouse. The idea was to directly “listen” in on the electrical signaling process.
“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 but all you can see is the electrical trace of a fish.” And you can’t use bait. “You just go for it and see if you can hit a dendrite,” he said. “Most of the time you can’t.”
But Smith built his own two-photon microscope system to make things easier.
Once a pipette was attached to the dendrite, the team took electrical recordings from individual dendrites within the brains of anesthetized and awake mice.
As the mice viewed visual stimuli on a computer screen, the researchers saw an unusual pattern of electrical signals. They turned out to be bursts of spikes, in the dendrite.
Smith’s team then found that the dendritic spikes occurred selectively, depending on the visual stimulus, indicating that the dendrites processed information about what the animal was seeing.
To provide visual evidence of their finding, Smith’s team filled neurons with calcium dye, which provided an optical readout of spiking. This revealed that dendrites fired spikes while other parts of the neuron did not, meaning that the spikes were the result of local processing within the dendrites.
Study co-author Tiago Branco, PhD, created a biophysical, mathematical model of neurons and found that known mechanisms could support the dendritic spiking recorded electrically, further validating the interpretation of the data.
“All the data pointed to the same conclusion,” Smith said. “The dendrites are not passive integrators of sensory-driven input; they seem to be a computational unit as well.”
Dendritic spikes enhance stimulus selectivity in cortical neurons in vivo
Spencer L. Smith, Ikuko T. Smith, Tiago Branco & Michael Häusser