That’s a cool picture, right? It’s a neuroscientist holding a real human brain! Now, pick up a piece of paper, and crumple it into a ball. Do you see any similarities…? You should!
According to a study published in Science last July by Brazilian scientists Bruno Mota and Suzana Herculano-Houzel, the mammalian brain grows and folds just like a sheet of paper, following the same mathematical pattern. Now, the researchers’ results may end centuries of debate about how the mammalian brain became so wrinkled.
To retrace the steps leading to Mota and Herculano-Houzel’s findings, the surface of the human brain – along with the brains of some other large mammals - is covered by an intricate pattern of gyri (ridges) and sulci (valleys), making it quite lumpy compared to most animals’ brains. As with many other biological traits, the reason for our folded brain lies in evolution. With folds, each part of the brain is closer together, so sending information-carrying signals from one region to another requires less distance and time, making the process more efficient.
Although neuroscientists knew that gyri and sulci develop in the human brain during the third trimester of pregnancy, the precise forces behind their formation were controversial among brain researchers, according to an article in Scientific American describing Mota and Herculano-Houzel’s study. The first hypothesis was based on brain size. Essentially, some researchers thought that the larger the brain, the more folds it would have. However, this did not explain mammalian brains like the manatees’, which is much smoother than the similarly-sized brain of the baboon. Other mammals, like the dolphin, also had brains that seemed “too wrinkled” for their size. The second hypothesis was based on the number of neurons, claiming that more folded brains contained more neurons. However, neuroscientists found exceptions to this claim, too. For example, the wrinkled cortex (outer layer) of the human brain has three times as many neurons as elephant cortex, but human brains are only half the size with less folding. Until Mota and Herculano-Houzel, this variability led scientists to believe that the processes underlying brain folding were unique to every animal.
However, Mota and Herculano-Houzel’s “eureka moment” came when they realized that the mathematics responsible for crumpled paper and a crumpled brain are exactly the same! Both objects are exposed to environmental forces that cause them to take the most stable form possible. For paper and brains, these forces are controlled by two factors: Thickness and surface area. A thicker piece of paper – and a thicker brain – each have fewer folds with less material hidden inside each wrinkle. By contrast, pieces of paper - and brains - with greater surface areas have more folds due to their larger size.
To understand how brains become folded, it is important to remember that they are subject to forces from all directions during development. First, gravity pushes down on the growing brain, just as fluid within the brain is pushing outwards. Each new cell in the brain also applies its own outward force as the entire organ expands. These forces combine with the properties of thickness and surface area to create the amazing network of folds in the human brain, and those of other large mammals like the dolphin.
Moving forward, Mota and Herculano-Houzel suggest that their “crumpled paper” model will impact the work of many other neuroscientists. They predict that their results will be especially important for researchers who study the cells that produce neurons and those who are working to understand structural disorders of the brain where the folds do not form correctly.
Chudler, E. H. (2015a). Cells of the Nervous System. Retrieved from https://faculty.washington.edu/chudler/cells.html
Chudler, E. H. (2015b). Glossary of Neuroscience Words. Retrieved from https://faculty.washington.edu/chudler/gloss.html
Henderson, T. (2015). The Meaning of Force. Retrieved from http://www.physicsclassroom.com/class/newtlaws/Lesson-2/The-Meaning-of-Force
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Mota, B., & Herculano-Houzel, S. (2015). Cortical folding scales universally with surface area and thickness, not number of neurons. Science, 349(6243), 74-77.