Thanks to a handful of newly discovered neurons, the brain just became a little bit less mysterious.

Today a team of neuroscientists led by Xiaolong Jiang and Andreas Tolias at Baylor College of Medicine in Houston announced six altogether new types of brain cells. The neuroscientists came across these new neurons while conducting a census of brain cells in adult mice in a part of the brain called the the primary visual cortex, an area chiefly concerned with sight. The researchers credit their new insight to a recently developed method of slicing razor-thin slices of mature brain. The discovery is reported today in the journal Science.

"Just asking 'what types of cells make up the brain' is such a basic question... that establishing a complete census of all neuron cell types is of great importance in moving the field of neuroscience forward," says Tolias, at Baylor College of Medicine.

Most previous studies investigating the odd menagerie of brain cells have used juvenile mice, mostly because it's easier to get high-resolution pictures of their brains. But there's a problem: Brains keep maturing and complicating as they get older, and Jiang's team believes that their new-found neurons might not form until adulthood.

In their study, Jiang and his colleagues meticulously surveyed 11,000 neurons in three layers of the primary visual cortex in adult mice. All told, they found 15 types of neurons, six of which had never before been seen or described. The neuroscientists used a complex recoding method called octuple patch-clamp recordings—a way of tracking the many connections brain cells form with one another, all at the same time.

"You can almost think of the task of finding these news cell types like identifying trees and grouping them into classes like pine, cedar or oak—we're separating the groups based on their obvious differences of shape and structure," says Tolias. But there's a huge difference between grouping cells and trees. For neuroscientists, tracing the absurdly complex branch-work in even just a few neurons is an insanely arduous task. Tolias says that this cell census (remember, this is just three layers of one tiny section of the brain) took three and a half years and over 200 separate imaging experiments.

Although we call them all neurons, your brain has an enormous menagerie of brain cells. Even if we ignore the specialized neurons that attach to our muscles or sensory organs like our eyes and tongue (and forget our brains' helpful support cells, called glial cells) mammals like mice or humans are thought to have in excess of hundreds of flavors of so-called interneurons—brain cells that just connect with other brain cells. And today's six new neurons fall in this class.

Morphology of different cell types and their connectivity matrix in the neocortex. Daniel Berger, Xiaolong Jiang, Fabian Sinz, Xaq Pitkow, Andreas Tolias]

These neurons can differ from one another in a myriad of ways, such as when they'll fire up or what their genetic makeup looks like. But the most glaring difference is how they're shaped; specifically, in how they wend and branch, and which other cells they connect to. In fact, the names of many of previously-discovered interneurons become clear when you create a 3D rendering of them. For example: chandelier, shrub, and basket cells form wiry connections that look like those three objects. Today's six new neurons form a variety of shapes that haven't been seen before.

"Our brains contain billions of neurons linked through trillions of synaptic connections. Obviously, we are faced with a problem of immense complexity," explains Tolias. "However, if neurons can be classified into distinct cell types... and if we understand [their underlying] rules, it will be an important step in deciphering the wiring of the brain," he says.

It's hard to understate just how important it could be to scientists to find and describe all these new cells. It will help neuroscientists to refine the human brain's complex wiring diagram, and that circuitry map underlies almost every question we have about how our brains work, from understanding how memories are created to defining consciousness. Plus, it's thought that malfunctions of the microcircutry of individual cell types (lets say, your chandelier cells just aren't chandelier-ing) could be related to brain disorders as disparate as epilepsy or autism spectrum disorders.

"And we can safely say that there a plenty more cell types to be discovered, throughout the brain in both mice and humans. We only studied a few layers in the visual cortex of a mouse, even just in other parts of that same cortex, there could be many more cell types waiting to be discovered," says Tolias.

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