A unique brain signal may be the key to human intelligence

Scientists exploring human neurons directly learn some remarkable things.

Image source: vitstudio/Shutterstock
  • Most research regarding human brains is performed with rodent brains on the assumption that it may also apply to us.
  • An unusual study looked at recently resected human brain tissue that turned out to contain some big surprises.
  • Human neurons' unexpected electrical signals and their behavior shed new light on human intelligence.

Though progress is being made, our brains remain organs of many mysteries. Among these are the exact workings of neurons, with some 86 billion of them in the human brain. Neurons are interconnected in complicated, labyrinthine networks across which they exchange information in the form of electrical signals. We know that signals exit an individual neuron through a fiber called an axon, and also that signals are received by each neuron through input fibers called dendrites.

Understanding the electrical capabilities of dendrites in particular — which, after all, may be receiving signals from countless other neurons at any given moment — is fundamental to deciphering neurons' communication. It may surprise you to learn, though, that much of everything we assume about human neurons is based on observations made of rodent dendrites — there's just not a lot of fresh, still-functional human brain tissue available for thorough examination.

For a new study published January 3 in the journal Science, however, scientists got a rare chance to explore some neurons from the outer layer of human brains, and they discovered startling dendrite behaviors that may be unique to humans, and may even help explain how our billions of neurons process the massive amount of information they exchange.

A puzzle, solved?

Image source: gritsalak karalak/Shutterstock

Electrical signals weaken with distance, and that poses a riddle to those seeking to understand the human brain: Human dendrites are known to be about twice as long as rodent dendrites, which means that a signal traversing a human dendrite could be much weaker arriving at its destination than one traveling a rodent's much shorter dendrite. Says paper co-author biologist Matthew Larkum of Humboldt University in Berlin speaking to LiveScience, "If there was no change in the electrical properties between rodents and people, then that would mean that, in the humans, the same synaptic inputs would be quite a bit less powerful." Chalk up another strike against the value of animal-based human research. The only way this would not be true is if the signals being exchanged in our brains are not the same as those in a rodent. This is exactly what the study's authors found.

The researchers worked with brain tissue sliced for therapeutic reasons from the brains of tumor and epilepsy patients. Neurons were resected from the disproportionately thick layers 2 and 3 of the cerebral cortex, a feature special to humans. In these layers reside incredibly dense neuronal networks.

Without blood-borne oxygen, though, such cells only last only for about two days, so Larkum's lab had no choice but to work around the clock during that period to get the most information from the samples. "You get the tissue very infrequently, so you've just got to work with what's in front of you," says Larkum. The team made holes in dendrites into which they could insert glass pipettes. Through these, they sent ions to stimulate the dendrites, allowing the scientists to observe their electrical behavior.

In rodents, two type of electrical spikes have been observed in dendrites: a short, one-millisecond spike with the introduction of sodium, and spikes that last 50- to 100-times longer in response to calcium.

In the human dendrites, one type of behavior was observed: super-short spikes occurring in rapid succession, one after the other. This suggests to the researchers that human neurons are "distinctly more excitable " than rodent neurons, allowing them to successfully traverse our longer dendrites.

In addition, the human neuronal spikes — though they behaved somewhat like rodent spikes prompted by the introduction of sodium — were found to be generated by calcium, essentially the opposite of rodents.

An even bigger surprise

Image source: bluebay/Shutterstock

The study also reports a second major finding. Looking to better understand how the brain utilizes these spikes, the team programmed computer models based on their findings. (The brains slices they'd examined could not, of course, be put back together and switched on somehow.)

The scientists constructed virtual neuronal networks, each of whose neurons could could be stimulated at thousands of points along its dendrites, to see how each handled so many input signals. Previous, non-human, research has suggested that neurons add these inputs together, holding onto them until the number of excitatory input signals exceeds the number of inhibitory signals, at which point the neuron fires the sum of them from its axon out into the network.

However, this isn't what Larkum's team observed in their model. Neurons' output was inverse to their inputs: The more excitatory signals they received, the less likely they were to fire off. Each had a seeming "sweet spot" when it came to input strength.

What the researchers believe is going on is that dendrites and neurons may be smarter than previously suspected, processing input information as it arrives. Mayank Mehta of UC Los Angeles, who's not involved in the research, tells LiveScience, "It doesn't look that the cell is just adding things up — it's also throwing things away." This could mean each neuron is assessing the value of each signal to the network and discarding "noise." It may also be that different neurons are optimized for different signals and thus tasks.

Much in the way that octopuses distribute decision-making across a decentralized nervous system, the implication of the new research is that, at least in humans, it's not just the neuronal network that's smart, it's all of the individual neurons it contains. This would constitute exactly the kind of computational super-charging one would hope to find somewhere in the amazing human brain.

Yug, age 7, and Alia, age 10, both entered Let Grow's "Independence Challenge" essay contest.

Photos: Courtesy of Let Grow
Sponsored by Charles Koch Foundation
  • The coronavirus pandemic may have a silver lining: It shows how insanely resourceful kids really are.
  • Let Grow, a non-profit promoting independence as a critical part of childhood, ran an "Independence Challenge" essay contest for kids. Here are a few of the amazing essays that came in.
  • Download Let Grow's free Independence Kit with ideas for kids.
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Four philosophers who realized they were completely wrong about things

Philosophers like to present their works as if everything before it was wrong. Sometimes, they even say they have ended the need for more philosophy. So, what happens when somebody realizes they were mistaken?

Sartre and Wittgenstein realize they were mistaken. (Getty Images)
Culture & Religion

Sometimes philosophers are wrong and admitting that you could be wrong is a big part of being a real philosopher. While most philosophers make minor adjustments to their arguments to correct for mistakes, others make large shifts in their thinking. Here, we have four philosophers who went back on what they said earlier in often radical ways. 

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The surprise reason sleep-deprivation kills lies in the gut

New research establishes an unexpected connection.

Reactive oxygen species (ROS) accumulate in the gut of sleep-deprived fruit flies, one (left), seven (center) and ten (right) days without sleep.

Image source: Vaccaro et al, 2020/Harvard Medical School
Surprising Science
  • A study provides further confirmation that a prolonged lack of sleep can result in early mortality.
  • Surprisingly, the direct cause seems to be a buildup of Reactive Oxygen Species in the gut produced by sleeplessness.
  • When the buildup is neutralized, a normal lifespan is restored.

We don't have to tell you what it feels like when you don't get enough sleep. A night or two of that can be miserable; long-term sleeplessness is out-and-out debilitating. Though we know from personal experience that we need sleep — our cognitive, metabolic, cardiovascular, and immune functioning depend on it — a lack of it does more than just make you feel like you want to die. It can actually kill you, according to study of rats published in 1989. But why?

A new study answers that question, and in an unexpected way. It appears that the sleeplessness/death connection has nothing to do with the brain or nervous system as many have assumed — it happens in your gut. Equally amazing, the study's authors were able to reverse the ill effects with antioxidants.

The study, from researchers at Harvard Medical School (HMS), is published in the journal Cell.

An unexpected culprit

The new research examines the mechanisms at play in sleep-deprived fruit flies and in mice — long-term sleep-deprivation experiments with humans are considered ethically iffy.

What the scientists found is that death from sleep deprivation is always preceded by a buildup of Reactive Oxygen Species (ROS) in the gut. These are not, as their name implies, living organisms. ROS are reactive molecules that are part of the immune system's response to invading microbes, and recent research suggests they're paradoxically key players in normal cell signal transduction and cell cycling as well. However, having an excess of ROS leads to oxidative stress, which is linked to "macromolecular damage and is implicated in various disease states such as atherosclerosis, diabetes, cancer, neurodegeneration, and aging." To prevent this, cellular defenses typically maintain a balance between ROS production and removal.

"We took an unbiased approach and searched throughout the body for indicators of damage from sleep deprivation," says senior study author Dragana Rogulja, admitting, "We were surprised to find it was the gut that plays a key role in causing death." The accumulation occurred in both sleep-deprived fruit flies and mice.

"Even more surprising," Rogulja recalls, "we found that premature death could be prevented. Each morning, we would all gather around to look at the flies, with disbelief to be honest. What we saw is that every time we could neutralize ROS in the gut, we could rescue the flies." Fruit flies given any of 11 antioxidant compounds — including melatonin, lipoic acid and NAD — that neutralize ROS buildups remained active and lived a normal length of time in spite of sleep deprivation. (The researchers note that these antioxidants did not extend the lifespans of non-sleep deprived control subjects.)

fly with thought bubble that says "What? I'm awake!"

Image source: Tomasz Klejdysz/Shutterstock/Big Think

The experiments

The study's tests were managed by co-first authors Alexandra Vaccaro and Yosef Kaplan Dor, both research fellows at HMS.

You may wonder how you compel a fruit fly to sleep, or for that matter, how you keep one awake. The researchers ascertained that fruit flies doze off in response to being shaken, and thus were the control subjects induced to snooze in their individual, warmed tubes. Each subject occupied its own 29 °C (84F) tube.

For their sleepless cohort, fruit flies were genetically manipulated to express a heat-sensitive protein in specific neurons. These neurons are known to suppress sleep, and did so — the fruit flies' activity levels, or lack thereof, were tracked using infrared beams.

Starting at Day 10 of sleep deprivation, fruit flies began dying, with all of them dead by Day 20. Control flies lived up to 40 days.

The scientists sought out markers that would indicate cell damage in their sleepless subjects. They saw no difference in brain tissue and elsewhere between the well-rested and sleep-deprived fruit flies, with the exception of one fruit fly.

However, in the guts of sleep-deprived fruit flies was a massive accumulation of ROS, which peaked around Day 10. Says Vaccaro, "We found that sleep-deprived flies were dying at the same pace, every time, and when we looked at markers of cell damage and death, the one tissue that really stood out was the gut." She adds, "I remember when we did the first experiment, you could immediately tell under the microscope that there was a striking difference. That almost never happens in lab research."

The experiments were repeated with mice who were gently kept awake for five days. Again, ROS built up over time in their small and large intestines but nowhere else.

As noted above, the administering of antioxidants alleviated the effect of the ROS buildup. In addition, flies that were modified to overproduce gut antioxidant enzymes were found to be immune to the damaging effects of sleep deprivation.

The research leaves some important questions unanswered. Says Kaplan Dor, "We still don't know why sleep loss causes ROS accumulation in the gut, and why this is lethal." He hypothesizes, "Sleep deprivation could directly affect the gut, but the trigger may also originate in the brain. Similarly, death could be due to damage in the gut or because high levels of ROS have systemic effects, or some combination of these."

The HMS researchers are now investigating the chemical pathways by which sleep-deprivation triggers the ROS buildup, and the means by which the ROS wreak cell havoc.

"We need to understand the biology of how sleep deprivation damages the body so that we can find ways to prevent this harm," says Rogulja.

Referring to the value of this study to humans, she notes,"So many of us are chronically sleep deprived. Even if we know staying up late every night is bad, we still do it. We believe we've identified a central issue that, when eliminated, allows for survival without sleep, at least in fruit flies."

Withdrawal symptoms from antidepressants can last over a year, new study finds

We must rethink the "chemical imbalance" theory of mental health.

Bottles of antidepressant pills named (L-R) Wellbutrin, Paxil, Fluoxetine and Lexapro are shown March 23, 2004 photographed in Miami, Florida.

Photo Illustration by Joe Raedle/Getty Images
Surprising Science
  • A new review found that withdrawal symptoms from antidepressants and antipsychotics can last for over a year.
  • Side effects from SSRIs, SNRIs, and antipsychotics last longer than benzodiazepines like Valium or Prozac.
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