IT IS strange to think that we still don’t know what our brains are made of. Yet, despite decades of research and the development of high-tech scanning techniques, that is the truth. Sure, we know the basics. The average human brain weighs approximately 1.4 kilograms and has the consistency of soft tofu. It is made up of two general cell types: neurons, which do the thinking, and glia, which support them. But beneath this simple description lies a mind-boggling complexity – a complexity that continues to surprise even neuroscientists.
Everyone knows neurons, the cells that send electrical impulses between different areas of the brain. Your brain contains around 86 billion of them and they come in many different varieties depending on their shape, function and properties. But there is still a lot we don’t know about them. Take , which were discovered in 2018. Named for their shape, the cells seem to damp down the electrical activity of other neurons. Intriguingly, there is no equivalent cell in mouse brains, despite the fact that mice tend to have analogues of other human brain cells. We don’t really know much about what they do in the human brain, says at the Allen Institute for Brain Science in Seattle, Washington, who was part of the team that discovered the cells.
Glia, which in the human brain, are even more difficult to pin down. Their name is derived from the Greek word for glue and they were long considered to be nothing more than glorified structural support for neurons. We now know that glia play some key roles in brain function. For example, one type called oligodendrocytes produces the insulating myelin sheaths that help neurons transmit electrical impulses. , meanwhile, are the brain’s immune cells: they rove around looking for signs of inflammation and other damage and then congregate to clear it. And star-shaped astrocytes help to regulate blood flow in the brain, among other things.
Advertisement
In September 2023, the picture was complicated even further. at the University of Lausanne, Switzerland, and his colleagues described that appears to act as a hybrid between neurons and glia. “The border between neurons and glia is much more flexible than initially thought,” he says. “Our discovery shows that some astrocytes also have molecular and functional features of neurons.” The cells his team discovered seem to be involved in spatial memory, which, for example, allows you to navigate a familiar route. But we are still a long way from fully understanding their role in the brain, says Volterra.
Yet this variety of cell types is just the tip of the iceberg. In March 2023, researchers at the Allen Institute reported results from studies in which they looked at 4.1 million cells from across the brains of mice. They identified an astonishing 5200 different types of cell based on their genetic activity, which indicates a cell’s function. The vast majority – some 5000 – were neurons, with glia making up the rest.
Allen Institute researchers and others have since shown that the same technique can be used to map cell types in the human brain. In October 2023, the , comprising more than 3300 types of cell. It is the most comprehensive analysis to date, examining the whole human brain rather than just regions of it. Although the resolution is far lower than for the mouse research, Hodge is hopeful that a complete cell atlas is possible. But, she adds, we are still very far from knowing how all these cells connect to each other to create a human mind. “This will take many more years of research to fully understand,” she says.
