Pop quiz: How is the human brain different from that of a mouse or rat?
Sure, OK, it's bigger — there's no getting around that. But as regular readers of this blog know, people and mice are very similar genetically, and yet we're clearly a tad bit smarter than your average furry vermin. So does size matter when it comes to brains? Or do humans have a little something extra going on upstairs?
"We have not really been able to understand why the human brain is so much more capable than that of any other animal," said neuroscientist Maiken Nedergaard, who led a recent study seeking to discover the answer, in a press release announcing his findings. "Some people have thought that it's simply that a bigger brain is a better brain, but an elephant's brain is bigger than a person's, for example, but it's not nearly as powerful."
True. We are smarter than elephants. But it turns out Nedergaard and his colleagues at the University of Rochester Medical Center discovered something else: the crucial role played in human brains by astrocytes, a type of cell long assumed to simply support neurons. The study appears on the cover of the March 11 issue of the Journal of Neuroscience.
"There aren't many differences known between the rodent brain and the human brain, but we are finding striking differences in the astrocytes. Our astrocytes signal faster, and they're bigger and more complex," added first author Nancy Ann Oberheim, in the press release.
In one of the most thorough studies yet of the astrocyte, Oberheim and her colleagues found a previously unknown form of the cell that exists in the human brain but not the rodents'. The cells are also about two and a half times bigger in humans than in rodents, and about 10 times as active.
"It may be that humans have a much higher brain capacity in large part because our astrocytes are more sophisticated and have more complex processing power," said Nedergaard. "Studies in rodents show that non-neuronal cells are part of information processing, and our study suggests that astrocytes are part of the higher cognitive functioning that defines who we are as humans."
Until now, the comparatively flashy neuron - which fires electrical signals from the brain to the rest of the body — got most of the attention in medical and pharmacy schools. But the tide may be turning: Astrocytes are 10 times as plentiful as neurons but don't fire in the same way; when scientists tried to track them with traditional techniques, they witnessed no electrical activity.
But Nedergaard devised a new calcium-measuring laser system to pinpoint astrocyte activity, because the cells use calcium to send signals to the neurons, indicating that astrocytes are fully involved in powering the brain. "The view is slow to change, but scientists are coming around," Oberheim said. "Astrocytes are now acknowledged as active participants in brain function and sensory processing."
Just don't tell the mice.
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