Researchers at University of North Carolina, Chapel Hill, have discovered that dendrites (the branch-like projections of neurons) not only relay information from one neuron to the other, but also process information, thus enhancing brain’s computing power. What was once considered “passive wiring” is now set to change long-standing models of functioning neural-circuitry in brain, and they may be of assistance to researchers to better understand neurological ailments. Spencer Smith, PhD, an assistant professor at UNC said- “Suddenly, it’s as if the processing power of the brain is much greater than we had originally thought. 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. That’s what this finding is like. The implications are exciting to think about.” His research team’s observations have been published in the journal Nature dated October 27.
It’s in axons where the neurons conventionally produce electrical spikes, but several likewise molecules supporting axonal spikes have also been found in dendrites. Earlier research on a dissected brain tissue revealed that dendrites employed those molecules to produce electrical spikes on their own, but it was not clear whether the normal brain function could make use of these dendritic spikes. In a series of experiments that followed, Smith’s team nodded a positive to that doubt, stating that dendrites effectively act as mini-neural computers, actively processing neuronal input signals on their own.
The experiments spanned two years and was across two continents, from senior author Michael Hausser’s lab at University College London to Smith’s own lab at the University of North Carolina. They employed patch-clamp electrophysiology to link a microscopic glass pipette electrode filled with a physiological solution to a neuronal dendrite in a mouse’s brain. The plan involved directly ‘listening’ to the generated electric signals. Smith said on the process- “Attaching the pipette to a dendrite is tremendously technically challenging. 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. You just go for it and see if you can hit a dendrite. Most of the time you can’t.”
Smith made his own two-photon microscope system to ease up the process. Once the pipette touched the dendrites, the team recorded the electrical signals that were generated from individual dendrites within the brains of anesthetized and awake mice. As the mice were exposed to visual stimuli on a computer screen, the team observed an unusual pattern of electrical spikes in the dendrite. They further observed that the spike occurred selectively, based on the visual stimulus, suggesting that the dendrites processed information about what the mouse was seeing.
To gather visual evidence, the team filled neurons with calcium dye, which gave out an optical read of the spiking. These revealed that dendrites sparked the signals while other regions of the neuron didn’t, indicating that the spikes generated were a result of local processing within the dendrites. Smith stated- “All the data pointed to the same conclusion. The dendrites are not passive integrators of sensory-driven input; they seem to be a computational unit as well.”
Smith’s team now plans to further explore their discovery and its various applications, particularly in situations like the Timothy syndrome where integration of dendrite-signals may go haywire.