Tag Archives: dendrite

Dendrites generate electrical spikes too — human brain computing capacity might be 100 times larger than thought

A breakthrough study found the human brain is at least 10 times more active than previously thought after a team from UCLA proved dendrites are actually electrically active. Since dendrites make up to 90 percent of the brain’s volume, computational capacity might actually be 100 times greater than the most hopeful estimates. Yet again, science shows that the tangled mess of billions of neurons we house upstairs is of unrivaled complexity — and that we’re still scratching the surface.

Dendrites are no passive electrical conductors and instead actively generate current like the cell’s body. Credit: Shelley Halpain/UC San Diego

What is consciousness? Where does it stem from? Can you create artificial consciousness? These are the ultimate questions neuroscience hopes to one day answer and a recent paper published by a team from the University of California, Los Angeles, is bringing us one step closer by revisiting the basic building block of any brain: the neuron. This tree-like cell is made up of two key regions: the body, also known as the soma, and the dendrites, which are large extensions that look a lot like branches.

Not a conductor but a generator in its own right

Up until now, scientists used to think dendrites only had the modest role of passing down electrical signals received through synapses, which are neural junctions, through the soma. These electrical signals are transmitted by other somas, which were through to be the sole electrical current generators. But UCLA neurophysicist Mayank Mehta and colleagues showed there is much more to it.

Previously, some groups suggested that dendrites aren’t passive at all and that dendrites could also generate electrical pulses based on studies of brain slices. Observing the same effect in nature, however, is far more revealing. It’s also extremely challenging considering the delicate nature of dendrites. For instance, previous attempts to measure electrical activity in dendrites only managed to destroy the cells. The UCLA team managed to get around this issue by developing a new technique in which electrodes are placed near, and not in, the dendrites of live rodents.

Using this novel approach, dendrite electrical activity was measured for four days in rats that were allowed to move freely around a maze. The electrodes were positioned such that dendrite activity would be recorded from the posterior parietal cortex which is where movement is coordinated.

Much to everyone’s surprise, the researchers recorded five times as many electrical spikes in dendrites than in somas when the rats were sleeping and up to 10 times as much when these were actively exploring the maze.  And because dendrites are almost 100 times larger in volume this could mean the brain’s computational capacity is 100 times greater. Previously, estimates suggested the human brain is 30 times faster than the best supercomputer and simulating 1 second of brain activity requires 82,944 processors. It might actually take a conventional computer much more than that to come close to human computational raw power.

There’s much more to it.

Somas fire electricity in short bursts known as somatic spikes. These spikes are binary all-or-nothing events with little variation in voltage. As such, somas seem to be shuttle information binary much like transistors in digital computers operate which is why you’ll see many scientists refer to the human brain as some binary computing machine. The study, however, shows that dendrites not only generate similar electrical bursts to the somas, they also generate large currents of varying voltage that could be even bigger than the spikes themselves.

“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. “

Because of their singular nature, somas were thought to be the primary way through which perception, learning, and memory formation occur. This long-held belief is now toppled by these most recent findings which not only show that there’s more than one decision maker but the brain is far from binary too.

“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.”

Due to technical limitations, neuroscience has mostly focused on the cell’s body thus far. Thanks to more sensitive and clever methods, we’re beginning to explore uncharted territories in the human brain and the secret lives of neurons. Now that our understanding of how neurons compute has changed, other fields will follow from medicine (diagnostic) to computers to artificial intelligence.

Possibility of erasing unwanted memories emerges

For the first time in history, scientists from the Florida campus of The Scripps Research Institute (TSRI) have been able to eliminate dangerous drug-associated memories in mice and rats without apparently affecting other memories.

Erasing memories

Credit: © JohnKwan / Fotolia

Credit: © JohnKwan / Fotolia

If you’ve ever seen The Eternal Sunshine of the Spotless Mind (truly a remarkable movie, which I highly recommend) – you probably have mixed feelings about this, and you know just what kind of dangerous possibilities erasing memories opens – but researchers working on this research had an entirely different idea in mind. They set out to help recovering addicts and individuals suffering from post-traumatic stress disorder (PTSD), for which recovering memories can be devastating.

The way in which the human brain makes seemingly random associations and makes connections is still a blurry area – but researchers are taking baby steps towards understanding how these memories and connections form.

“Our memories make us who we are, but some of these memories can make life very difficult,” said Courtney Miller, a TSRI assistant professor who led the research. “Not unlike in the movie Eternal Sunshine of the Spotless Mind, we’re looking for strategies to selectively eliminate evidence of past experiences related to drug abuse or a traumatic event. Our study shows we can do just that in mice — wipe out deeply engrained drug-related memories without harming other memories.”

How it works

Producing a memory is a complex phenomenon which includes alteration of the structure of nerve cells via changes in the dendritic spines – small protrusions from a neuron’s dendrite that typically receives input from a single synapse. Normally, these structures change through the generation of actin, the protein that makes up the infrastructure of all cells.

dendritic spine

In this new study, researchers have inhibited actin polymerization — the creation of large chainlike molecules by blocking the molecular motor which generates them (called myosin II) in the brains of mice and rats during the maintenance phase of methamphetamine-related memory formation.

Behavioral tests conducted afterwards showed the animals lost any memories related to metamphetamine; it’s still unclear why relatively powerful drug-related memories are so fragile, but it’s believed these groundbreaking discoveries are connected to the role of dopamine, a neurotransmitter involved in reward and pleasure centers in the brain which is known to modify dendritic spines – so this method could possibly only work on dopamine-related memories.

“We are focused on understanding what makes these memories different. The hope is that our strategies may be applicable to other harmful memories, such as those that perpetuate smoking or PTSD.”

Journal Reference: Erica J. Young, Massimiliano Aceti, Erica M. Griggs, Rita A. Fuchs, Zachary Zigmond, Gavin Rumbaugh, Courtney A. Miller. Selective, Retrieval-Independent Disruption of Methamphetamine-Associated Memory by Actin Depolymerization. Biological Psychiatry, 2013; DOI: 10.1016/j.biopsych.2013.07.036

Brain

Pollution linked to memory loss

BrainIt’s pretty evident for anyone living in a big, crowded city what pollution looks like and to what degree our health is affected by it. Besides things like your lungs or skin, scientists relate in a new study published in Molecular Psychiatry, how they believe pollution can cause memory loss.

To prove their point, they confined a group of lab mice to a space in which half of their lifespan they had to live with polluted air around them. A series of simple memory and learning tests were made in between, particularly a simple maze through which each rodent had to make his escape. The mice which were exposed to pollution had a tougher time both learning and remembering where the correct exit was than those who weren’t. Polluted mice also showed depressive-like behavior and were more anxious.

Scientists this happens because of pollution which gets exposed to an area of the brain called the hippocampus, primordial for memory building. Here, the neuron’s dendrites, which are branch-like projections of the neuron that link other neurons and transmit electrical impulses through the brain, have been found to be affected. Dendrites have spines, which are the ones actually used for sending signals between neurons, however in pollution exposed mice, scientists have found fewer spines and shorter dendrites. This means worsened memory and learning.

Researchers believe this is the case because pollution is associated with wide-grade, body-wide inflammation, which hits the brain too, damaging the hippocampus.