Researchers Have Discovered A Giant Neuron That Wraps Around A Mouse's Brain
In 2017, researchers discovered the longest neuron they'd ever seen, wrapping all the way around a mouse's brain. But while its length is fascinating, its origin is even moreso. This supersized neuron originates in an area of the brain that some think could be the source of consciousness.
Why we're covering this:
A "Crown Of Thorns"
Neurons are specialized cells that carry electrochemical signals throughout the nervous system. In the brain, they branch from area to area, sister neuron to sister neuron, helping different parts of the brain send messages back and forth. To map their paths, neuroscientists usually have to start with a dead brain, inject individual cells with dye, cut the brain into sections, then trace the individual neurons by hand. As you might expect, that makes it difficult to follow a neuron's entire path through the brain.
For this research, Christof Koch of the Allen Institute for Brain Science and his team engineered a line of mice so that when they were fed a special drug, specific genes in certain neurons would switch on. Only a handful of those neurons got enough of the drug to switch on the genes, but when they did, a green fluorescent protein spread throughout the whole neuron. With that, researchers were able to take 10,000 cross-sectional images of the entire brain to create a 3D reconstruction of the switched-on neurons. What they saw was incredible: three individual neurons stretched across both brain hemispheres. One of the neurons wrapped all the way around the brain, like a "crown of thorns," Koch told Nature.
The Million Dollar Question
So we've got a new way to image neurons, and evidence that some neurons stretch much further than we thought. But there's one more exciting part to this story: all three of those neurons originate in the claustrum, a part of the brain that some have suspected of being the source of consciousness. This thin layer of nerve cells sits beneath the neocortex in the center of the brain, and is one of the organ's most densely connected areas. Turning it off with electrical impulses also seems to turn off conscious thought, as the world discovered when scientists were treating a woman for epilepsy in 2014, and damage seems to do the same, as researchers saw in 2015 when studying the effects of claustrum damage on combat veterans with traumatic brain injuries.
"Koch sees this as evidence that the claustrum could be coordinating inputs and outputs across the brain to create consciousness," says Sara Reardon in Nature. Of course, that's just a hypothesis. There are limitations to how much this research can tell us—we haven't seen the same evidence in human brains yet, after all. But it sure is intriguing. It could mean the so-called "hard problem" of consciousness could one day be solved.