Dendrites more electrically active than soma of neurons; perform digital and analog computations

From UCLA Newsroom:

Brain is 10 times more active than previously measured, UCLA researchers find

Dan Gordon | March 09, 2017

Neuron

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Shelley Halpain/UC San DiegoUCLA scientists discovered that dendrites (shown here in green) are not just passive conduits for electrical currents between neurons.


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A new UCLA study could change scientists’ understanding of how the brain works — and could lead to new approaches for treating neurological disorders and for developing computers that “think” more like humans.

The research focused on the structure and function of dendrites, which are components of neurons, the nerve cells in the brain. Neurons are large, tree-like structures made up of a body, the soma, with numerous branches called dendrites extending outward. Somas generate brief electrical pulses called “spikes” in order to connect and communicate with each other. Scientists had generally believed that the somatic spikes activate the dendrites, which passively send currents to other neurons’ somas, but this had never been directly tested before. This process is the basis for how memories are formed and stored.

Scientists have believed that this was dendrites’ primary role.

But the UCLA team discovered that dendrites are not just passive conduits. Their research showed that dendrites are electrically active in animals that are moving around freely, generating nearly 10 times more spikes than somas. The finding challenges the long-held belief that spikes in the soma are the primary way in which perception, learning and memory formation occur.

“Dendrites make up more than 90 percent of neural tissue,” said UCLA neurophysicist Mayank Mehta, the study’s senior author. “Knowing they are much more active than the soma fundamentally changes the nature of our understanding of how the brain computes information. It may pave the way for understanding and treating neurological disorders, and for developing brain-like computers.”

The research is reported in the March 9 issue of the journal Science.

Scientists have generally believed that dendrites meekly sent currents they received from the cell’s synapse (the junction between two neurons) to the soma, which in turn generated an electrical impulse. Those short electrical bursts, known as somatic spikes, were thought to be at the heart of neural computation and learning. But the new study demonstrated that dendrites generate their own spikes 10 times more often than the somas.

Video: Animation of a neuron firing electrical spikes

The researchers also found that dendrites generate large fluctuations in voltage in addition to the spikes; the spikes are binary, all-or-nothing events. The somas generated only all-or-nothing spikes, much like digital computers do. In addition to producing similar spikes, the dendrites also generated large, slowly varying voltages that were even bigger than the spikes, which suggests that the dendrites execute analog computation.

“We found that dendrites are hybrids that do both analog and digital computations, which are therefore fundamentally different from purely digital computers, but somewhat similar to quantum computers that are analog,” said Mehta, a UCLA professor of physics and astronomy, of neurology and of neurobiology. “A fundamental belief in neuroscience has been that neurons are digital devices. They either generate a spike or not. These results show that the dendrites do not behave purely like a digital device. Dendrites do generate digital, all-or-none spikes, but they also show large analog fluctuations that are not all or none. This is a major departure from what neuroscientists have believed for about 60 years.”

Because the dendrites are nearly 100 times larger in volume than the neuronal centers, Mehta said, the large number of dendritic spikes taking place could mean that the brain has more than 100 times the computational capacity than was previously thought.

Recent studies in brain slices showed that dendrites can generate spikes. But it was neither clear that this could happen during natural behavior, nor how often. Measuring dendrites’ electrical activity during natural behavior has long been a challenge because they’re so delicate: In studies with laboratory rats, scientists have found that placing electrodes in the dendrites themselves while the animals were moving actually killed those cells. But the UCLA team developed a new technique that involves placing the electrodes near, rather than in, the dendrites.

Using that approach, the scientists measured dendrites’ activity for up to four days in rats that were allowed to move freely within a large maze. Taking measurements from the posterior parietal cortex, the part of the brain that plays a key role in movement planning, the researchers found far more activity in the dendrites than in the somas — approximately five times as many spikes while the rats were sleeping, and up to 10 times as many when they were exploring.

“Many prior models assume that learning occurs when the cell bodies of two neurons are active at the same time,” said Jason Moore, a UCLA postdoctoral researcher and the study’s first author. “Our findings indicate that learning may take place when the input neuron is active at the same time that a dendrite is active — and it could be that different parts of dendrites will be active at different times, which would suggest a lot more flexibility in how learning can occur within a single neuron.”

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Activity in brain areas before exposure to stimulus predicts how well it will be remembered

From UT Dallas News Center:

Brain Study Uncovers New Clues on How Cues May Affect Memory

Dec. 3, 2014

Rick Addante

Dr. Richard Addante

A new study from the UT Dallas Center for Vital Longevity shows that the brain activity prior to seeing an item is related to how well it is later remembered.

In the study published online in NeuroImage, the researchers showed that receiving information about a pair of items before seeing them may affect how well they are remembered. Moreover, the researchers also found that the activity in different areas of the brain was unexpectedly related to how the information was remembered.

“If you’re interested in memory, you want to know the factors that are associated with it being worse as well as what makes it better,” said Dr. Richard Addante, a senior lecturer in the School of Behavioral and Brain Sciences and lead author of the paper. “Knowledge of these factors can lead to developing ways to help improve memory.”

The researchers used functional magnetic resonance imaging to look for activity in different areas of the brain as a participant decided which of two words or pictures would fit inside the other — for example, a dog and a house. But moments before the task, participants were shown a cue — an “X” if the items would be presented as words or an “O” if they would be presented as pictures. About 20 minutes later, outside of the MRI scanner, the participants were tested on how well they remembered the pairs of items.

“We found that the brain activity before people were presented with information predicted how well people ended up remembering that information on a later memory test,” Addante said. “What was really interesting was that brain activity wasn’t just predictive of whether they remembered the information later, but how they remembered it.”

Prior research suggested that seeing the cues would trigger more brain activity in the hippocampus and provide a better chance of remembering the information. Activity in the hippocampus during learning is generally associated with better memory. This study found the opposite.

Greater activity in the hippocampus before a participant saw the two items predicted that the subject was more likely to forget which items were in a pair. In addition, increased brain activity in the frontal and parietal cortices, areas usually associated with memory maintenance, predicted a greater likelihood that the subject would falsely identify items as previously paired even though they were not.

“We were initially only expecting pre-stimulus memory activity in the hippocampus, as indicated in previous studies, which we did,” Addante said. “But when we turned our analysis to the whole brain, we found an extensive pattern of activity that also predicted aspects of later behavior.”

Read more.