Using a new 3D reconstruction technique, scientists have mapped a long neuron that wraps around the entire outer layer of the mouse brain. A neuron is a message-carrying cell that's the basic unit of the nervous system; this long, wrapping neuron, described as resembling a "crown of thorns," may help explain how the physical processes of the brain give rise to consciousness.
This crowning neuron, one of three traced by researchers, begins in the claustrum, a thin sheet of cells thought to be the seat of consciousness in both mice and humans. It's unique in how far it stretches across the brain; with the other claustrum neurons, it connects the outer parts of the brain that take in sensory information and govern behavior. That may suggest it's wiring all of those parts together, combining all the brain's inputs and outputs into what we'd call consciousness.
It's a bedeviling question: How does neuronal firing within the brain, a complex but finite process, create something as rich and multifarious as human experience? Philosophers, neuroscientists, and other invested parties have staked out claims, but in reality we don't yet have an answer. There's even debate over whether brain activity does lead to consciousness. These kinds of arguments, you can imagine, rapidly turn head-spinning. So mapping the incredible complexity of the brain at least expands our sense of the landscape, even if we don't know how all the pieces work together.
In humans, the claustrum is one of the brain's most densely connected regions. But tracing individual neurons has been difficult, making it hard to tease apart exactly how they wire the brain. In mice, the claustrum's been well studied and is made up of only a few types of cells, making it much easier to study. Previously, researchers had to inject dye into individual cells, then cut the brain into thin slices. (The slicing being another reason to use mice brains.) By pinpointing the dye in each slice, they could laboriously trace the path, though rarely through the entire brain.
The new technique required genetically engineering mice so that a drug activated certain genes in their claustrum neurons. The mice received a small dose of the drug, producing a fluorescent green protein throughout a few activated neurons. The mouse brain was then sectioned into 10,000 slices and reassembled in a computer program that created the 3D reconstruction.
It's a fascinating picture, though one that doesn't yet prove anything. Rafael Yuste, a neurobiologist at Columbia University in New York City, told Nature that the technique could help identify different cell types in the brain, which may lead to a better understanding of their varying functions. He stresses just how fundamental the understanding would be: "It's like trying to decipher language if we don't understand what the alphabet is." With this discovery, we might be one more tantalizing step closer to getting a glimpse of that alphabet.